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A Systematic Approach to 931 Diagnostics

 
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ideola  



Joined: 01 Oct 2004
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Location: Woodstock IL

PostPosted: Thu Apr 09, 2009 2:37 am    Post subject: A Systematic Approach to 931 Diagnostics Reply with quote

This documentation is not quite as specific or detailed as the troubleshooting guides at Clark's Garage for the 2.5L engine, but it is an initial attempt at developing a general diagnostic regimen for 931's.

The exact sequence of diagnostic steps may need to change depending on your specific problem; however, I've tried to put these in some sort of logical order, starting with diagnostic steps that are cheap and easy to execute with basic tools, and progressing into more involved, more difficult diagnostics that require specialized tools. Many of the steps are designed to eliminate variables in order to help with root cause analysis. I would also highly recommend taking this outline and mapping it into a spreadsheet or paper-based matrix to use as a checklist, as well as a tool for recording results.

This should be considered an addendum or follow-up to Vaughan's 931 Resuscitation - how to resurrect a long-dead turbo thread, which is REQUIRED READING for any 931 owner.

Additions, comments, corrections, cross-references WELCOMED.

Prerequisites
  • Bypass Overboost Switch
    TOOLS: piece of wire, crimping tool, female spade connector, eye terminal connector
    DESCRIPTION: It's not uncommon for this 30 year old switch to fail. When it does, it interrupts power to the fuel pump relay, which of course prevents fuel flow. The switch should be eliminated as a variable during diagnostics to be sure it isn't interfering with fuel delivery. This is quite simple to do by unplugging the lead, and grounding it. Make a simple jumper with a male spade adapter on one end and a ring terminal on the other, and find a convenient point to ground it in the engine bay. I used the ground point on the alternator, which is close by and conveniently sized. Many 931 owners leave this switch permanently bypassed. YMMV.

  • Timing Belt
    TOOLS: 27mm and 19mm wrenches, 24mm socket, torque wrench
    DESCRIPTION: Because the 931 engine is an interference engine, the timing belt should be replaced if its age and mileage is not conclusively known. Visual inspection is not sufficient. At a bare minimum, basic timing should be reset, and the tension re-adjusted to eliminate it as a variable. Set crank to TDC, set cam sprocket to TDC, noting that the TDC mark on the sprocket is a small dimple on the BACK of the sprocket, i.e. facing the windshield.

  • Fuel Filter
    TOOLS: various line wrenches, screw driver
    DESCRIPTION: The fuel filter should be replaced if its age and mileage is not conclusively known. Visual inspection is not sufficient. Many North American 931's come to their new owners with the fuel filter installed backward. Make sure the flow is correct. The high pressure pump in the CIS system will damage the filter if installed backward, and can cause intermittent fuel delivery problems that can be difficult to isolate. This is a cheap part, it's easy to replace, so why not eliminate it as a variable out of the gate. Strongly recommend the use of proper line wrenches and a penetrating oil to avoid rounding the hard fuel line connections.

  • Ignition Control Module
    TOOLS: sandpaper, emory board, needle files, dental picks
    DESCRIPTION: The ignition control model is bolted to the inner fender well on the left hand side of the car, directly behind the head light bucket. On series 1 cars (1979-1980) it is a small silver Bosch box with a six pin connector. On series 2 cars (1981-1982) it is an even smaller black electronic module mounted to a heat sink. In either case, the connections can become corroded because they are fairly exposed to the elements. Cleaning the connections on the harness and the module is highly recommended. The exact tools and methods will depend on which unit you have. A good coating of di-electric grease on the terminal ends is good preventive maintenance.

  • DITC Connections (series 2 only)
    TOOLS: sandpaper, emory board, needle files, dental picks
    DESCRIPTION: The DITC (Digital Ignition & Timing Control) unit is mounted on the central hump directly behind the radio and center console. It is notorious for developing bad connections at both the IAT (Intake Air Temperature) harness and the primary harness, which controls both ignition advance and fuel delivery. The larger rectangular connector is also notorious for unseating itself due to vibration over time, causing a wide range of sporadic poor running conditions. As a matter of routine maintenance, this connection should be checked at least twice annually for proper seating. Clean all pins on the harnesses and module itself as with any electrical connection. On my series 2, I had an additional problem where the very ends of the black plastic harness were deteriorating and leaving small, crumb-like deposits of plastic on the module, such that the harness could not be properly seated, and would wander off within a day or two of driving. This was almost impossible to see in situ. I only discovered it after removing the DITC module and inspecting it in good light. I used a dental pick to remove the "crumbs", and upon reinstallation, I used two heavy-weight rubber bands doubled up around the module and harness to help prevent it from unseating. No problems since.

Spark
  • Spark Plugs
    TOOLS: spark plug socket, wire brush, carb cleaner, lighter torch
    DESCRIPTION: First of all, make sure you have the correct heat range, and by all means, you do NOT need any fancy platinum or multi-tipped plugs. The standard Bosch copper WR6DS plugs are BEST. Stick with these or the NGK or Champion replacements. Remove ALL FOUR plugs. Check the electrodes as per Haynes. In summary, white powder tips means too lean, black powder tip means too rich, black oily tip means oil-fouling typically due to valve stem seals or bad rings. Also, check for consistency across all four. If one or two appear to be "steam cleaned", then you probably have a bad head gasket. If one or two are fine but the other two are oil fouled, you could have bad valve stem seals on the two fouled ones. Common sense stuff. Also, make sure they are gapped properly. Specs are .7mm =/- .1mm. If you don't have new or spares to replace fouled plugs, have some carb cleaner, rags, and a wire brush handy. Spray 'em down, clean 'em with the brush, and then for good measure, use a torch-style lighter to flame clean the ends.

  • Plug Wires
    TOOLS: spark tester
    DESCRIPTION: Always check for good spark from the main coil wire FIRST. Then proceed to check each individual wire. Procedure is adequately described in Haynes. It may also be a good idea to check each lead for continuity using a multi-meter.

  • Ignition Distributor Orientation
    TOOLS: 13mm wrench
    DESCRIPTION: In order to properly reindex the ignition distributor, set the crank and cam sprocket both at TDC. Remove the cap. The rotor in the ignition distributor should point to a small scribe mark on the lip of the distributor housing, just to the right of the electrical connection. This is the basic ignition timing setting.

  • Rotor and Cap
    TOOLS: hands
    DESCRIPTION: Check the cap and rotor for carbon build up, wear, and cracks. Replace as required.

  • Firing Order
    TOOLS: hands
    DESCRIPTION: The ignition distributor rotates in a CLOCKWISE direction. Once you have confirmed proper distributor orientation, make certain that your plug leads are attached in the proper firing order: 1-3-4-2, clockwise around the distributor. FYI, if you put these on backwards, i.e. counter-clockwise, the car will run, but only on two cylinders. Make sure it's right!!!

  • Ignition Timing (series 1 only)
    TOOLS: timing light
    DESCRIPTION: Series 2 cars cannot have the ingition timing set because it is controlled by DITC. For series 1 cars, proper timing is 20░BTDC at 2000 RPM at operating temp.

Air
  • Air Filter
    TOOLS: hands
    DESCRIPTION: Inspect and replace as required. Verify proper sealing of air box, and check for excess oil in air intake, which could indicate excessive blow by at the rings, or a full / clogged air oil separator.

  • Compression Test
    TOOLS: compression tester
    DESCRIPTION: Mainly, you're looking for consistency across all four cylinders. Battery MUST be fully charged in order to get accurate readings across all four cylinders. Also, disconnect coil lead, and hold gas pedal to the floor while cranking during compression tests. Specs of tolerance for compression test is in Haynes and FSM, but the main concern initially is consistency within a certain range (I think 10%) across all four cylinders. NOTE: in my experience, if the car has not actually run for some time, the compression readings will tend to be much lower because the rings will lose their ability to seal properly after long periods of disuse.

  • Leak-Down Test
    TOOLS: leak-down tester, air compressor
    DESCRIPTION: If you're compression test is less than satisfactory, a leak down test can help you identify where you're losing compression. Lots of available knowledge online, but the "Cliff Notes" version is: Air coming from the dipstick indicates rings; air coming from adjoining spark plug holes indicates head gasket, air coming from oil filler cap indicates valve stem seals, guides, or seats.

  • Boost & Vacuum Leaks
    TOOLS: screw driver, carb cleaner, flashlight, patience
    DESCRIPTION: On a ~30 year old 931, you should plan to replace ALL under hood rubber lines. I'll post later with exact lengths, but you can order silicone hose in 3.5mm, 14mm, and 19mm ID from several suppliers. Also, there are 7 rubber elbows on a 931 that are prone to cracking and leaking. I'll have to post back later with the exact locations, but you will need a total of 7 elbows.

    The entire charge path must also be gone over. In addition to all of the same sources of vacuum leaks as on an NA, there are several additional (and even more critical) considerations on a forced induction application. Turbo outlet to lower charge tube: replace the red gasket and confirm that the lower charge tube bracket is in place and properly secured; these are known to shear off the bolts and go missing; the lower charge tube is just and interference fit, and under boost will lift off of the turbo outlet if the bracket is loose or missing; if yours is missing you can purchase an aftermarket replacement here. Turbo recirculation valve: this is the funky square housing molded into the 931 cold side of the turbo; the paper gaskets fail over time and will leak air, causing all kinds of air metering problems, as well as boost loss; not easy to repair in situ, but doable. Lower charge tube to upper charge tube gasket: not prone to failure, but cheap and easy to replace. Upper charge tube to throttle body: on a series 1, this is a rubber, ovoid tube that is quite likely badly distorted and pretty much shot; consider replacing with aftermarket silicone "silly pipe"; on a series 2, there are two gaskets at this interface, a red internal gasket and a black external gasket; replace both. Boost signal line: these lines are prone to cracking and failing over time at either end where the flex line joins the hard line; check carefully and replaced with a good used donor or fabricate a new and improved one. Throttle body butterfly: the bushings are known to leak after 30 years, may require having your TB rebushed.

    The exhaust path is also critical for maintaining proper boost. Exhaust manifold cracks: especially the series 1, but even the series 2 manifolds can develop cracks, usually on the #1 or #2 runners; I personally believe this is due to a combination of stresses induced by any log style manifold from exhaust pulse accumlation (which would occur at runner 2), plus high heat gain and improper warm down (i.e. abuse from previous owner). J-pipe: the J-pipe connection at the wastegate has no provision for flexing, and I believe thermal expansion causes the joints to eventually crack and fail; inspect both connectino points for cracks, replace with a good used donor if necessary, and consider removing the horizontal straight section with a flex joint.

Fuel
  • Fuel Pump Relay
    TOOLS: piece of wire, crimping tool, two male spade connectors
    DESCRIPTION: Because there are other factors that can affect the proper functioning of the relay, be prepared to bypass it using a jumper connecting pins 30 and 87. This is particularly true of DITC equipped cars. For many of these test, you will want to have the relay pulled and the jumper in place. Vaughan describes this pretty well in his resuscitation thread.

  • Tach Signal
    TOOLS: multi-meter
    DESCRIPTION: As described in Vaughan's resuscitation thread, the fuel pump relay on a 931 takes input from the tach signal, so this must be eliminated as a variable.

  • Fuel Pump Voltage
    TOOLS: multi-meter, test light
    DESCRIPTION: 30 year old wiring can develop high resistance, especially on longer runs, as with the leads that provide power to the internal and external fuel pumps. After bypassing the relay, confirm that you have good 12V signal at both pumps before proceeding any further. Check and clean all grounds, and in addition to voltage, check current draw using a test light.

  • Fuel Accumulator
    TOOLS: screw driver
    DESCRIPTION: A leaking fuel accumulator can result in all sorts of fuel delivery issues, including the notorious hot start problem. Follow the procedure in Haynes or FSM to make sure your accumulator is functioning correctly.

  • Fuel Pump Flow
    TOOLS: container suitable for collecting fuel, various wrenches and sockets
    DESCRIPTION: Follow procedure in Haynes, FSM, or Bosch CIS manual to verify that you have roughly the correct amount of fuel flow PRIOR to the fuel distributor. This approach will isolate any problems or clogs upstream of the fuel distributor so that you can sort them out first before tackling the more delicate and difficult aspects of the CIS system itself.

  • Fuel Injector Spray Pattern
    TOOLS: container suitable for collecting fuel, various wrenches and sockets
    DESCRIPTION: Follow procedure in Haynes, FSM, or Bosch CIS manual to verify correct spray pattern and fuel delivery through injectors. If injectors are clogged, I highly recommend sending them to Witchhunter for cleaning as opposed to the home-brew methods often discussed, quite simply because they come with a guarantee. Of course, replacing with new is ideal...but spendy.

  • Air Flow Meter Arm Movement
    TOOLS: screw driver, carb cleaner
    DESCRIPTION: Following the procedure described in Haynes or FSM, very CAREFULLY check to make sure that the AFM arm is moving freely but with resistance as the procedure indicates. A good spray of carb cleaner can be helpful as well.

  • CIS System Test
    TOOLS: CIS Tester, various wrenches and sockets, rags, catch pan
    DESCRIPTION: As I recently learned, the procedure for checking 931 CIS system is slightly different than for NA cars. The basic tests are similar, but the 931 WUR has load enrichment feature that should also be tested. The only good description of this test is in the FSM volume specific to the 931. You MUST use the correct CIS tester, the Hoffman Bosch CIS Pressure Tester, SKU# ZX123617U, available from JC Whitney usually for ~USD$60. This is an indispensable tool that any serious 924/931 owner MUST have.

  • Other CIS Components
    TOOLS: multi-meter, container suitable for collecting fuel, various wrenches and sockets
    DESCRIPTION: There are several other components of a properly functioning CIS system that have various electical, mechanical, fuel, and air/vac related functions. These include the Cold Start Valve (CSV), the Thermo Time Switch (TTS), the Auxilliary Air Valve (AAV), the Frequency Valve (FV), the Vacuum Limiter Valve (VLV), the Intake Air Temperature sensor (IAT), the Coolant Temperature switch (CLT), and the O2 sensor (on North American models). Tests for each of these components is described in Haynes and FSM. Too much detail to repeat here, but your regimen should include a test matrix to go through each of the mechanical and electrical functions of each of these components to verify proper operation. Most of these will not prevent the car from running, but if they're not functioning correctly, they will make it difficult to start or properly tune.


General Tuning
  • Throttle Position Switch(es)
    TOOLS: multi-meter, screw driver
    DESCRIPTION: On series 1 cars, there are three micro-switches that affect load enrichment. On series 2 cars, there is a single TPS mounted on the bottom of the throttle body. In either case, FSM provides a procedure for checking proper positioning as well as electrical connections. Haynes is insufficient for TPS diagnostics.

  • Throttle Cable
    TOOLS: flash light, screw driver, spring hook
    DESCRIPTION: Don't carelessly rule out the mechanical throttle linkage. There are several known issues with the cable, pedal, or throttle cam binding and not operating correctly. Make sure these are all properly adjusted and functioning such that the throttle plate is opening properly.

  • Ignition Advance and Retard (series 1 only)
    TOOLS: vacuum gauge, timing light, 13mm wrench
    DESCRIPTION: FSM provides a procedure for checking the proper function of the vacuum-activated ignition advance-retard functionality. Haynes is insufficient for 931 ignition distributor diagnostics.

  • Cam Followers
    TOOLS: feeler gauges, 3mm allen wrench or special tool 8005/2038, various wrenches, sockets, screw drivers
    DESCRIPTION: For 931's the cam follower settings are: cold: .10mm intake, .40mm exhaust; hot: .20mm intake, .45 exhaust. Remove the valve cover. Manually rotate the crank until the #1 intake lobe is point just after the cam oiler tube. In this position, the #4 exhaust lobe should also be pointing up. Check the #1 intake follower and adjust as necessary; then the #4 exhaust. Rotate the crank approximately 180░ until the #3 intake lobe is pointing just after the cam oiler tube. You can now check #3 intake and #2 exhaust. And so on until you've checked all clearances. I've found that it's helpful to draw a grid in which to record my findings, with a row for each cylinder and a column for intake and exhaust, respectively. I've also found that it's a good practice to double check all clearances if you've changed them. Repeat the process until the clearnances all show proper settings after two consecutive turns of the crank.

  • Valve Stem Seals
    TOOLS: valve spring compressor, valve stem seal removal tool, torque wrench
    DESCRIPTION: This job can be done with the head in situ, provided you have the correct valve spring compressor tool and the valve stem removal tool. I'll try to find links and post back later with references to the correct tools.


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Last edited by ideola on Tue Aug 21, 2012 6:48 am; edited 2 times in total
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ideola  



Joined: 01 Oct 2004
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PostPosted: Tue May 05, 2009 3:02 am    Post subject: Reply with quote

I can't believe this hasn't garnered any responses yet...
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CÚdric  



Joined: 27 Aug 2004
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PostPosted: Tue May 05, 2009 4:49 am    Post subject: Reply with quote

Hm, my temp sensor connection must be further investigated. My knocklite give me a sligth warning at boost, over 0.5 bar, so I havent dared going full yet before checking my ignition. How much do the temp sensor change timing ? What else can it be do you think ?
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ideola  



Joined: 01 Oct 2004
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PostPosted: Tue May 05, 2009 4:51 am    Post subject: Reply with quote

You have an IAT on a 1980 931???
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CÚdric  



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PostPosted: Tue May 05, 2009 5:32 am    Post subject: Reply with quote

ideola wrote:
You have an IAT on a 1980 931???


oops, i forgot to say it has been converted to a s2 engine (sounds so easy, but was a bit tricky to combine two harnesses and make it look like it were factory installed)
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ideola  



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PostPosted: Tue May 05, 2009 6:47 am    Post subject: Reply with quote

I'm not as intimately familiar with the S2 cars, so I don't know how much the IAT affects timing. However, if I had to guess, I would say that in its failure mode, it would retard the same 7░ that the ignition timing bypass wires do (on the LHS inner fender).
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CÚdric  



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PostPosted: Tue May 05, 2009 7:36 am    Post subject: Reply with quote

I did unplug that one, it made the car a bit slower to get on boost, but the ligth still flashed but after some more boost. Do you know if there is anything in the system that can advance the timing if its not working correct, malfunctioning boost retard maybe?
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ideola  



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PostPosted: Tue May 05, 2009 9:18 am    Post subject: Reply with quote

Again, not the world's expert on DITC, but I from what I understand, the crank sensor relies on resistance for correct reading, so if there are typical corrosion issues going on, you could be getting incorrect readings on the reference sensor. I believe FSM has a procedure for testing this, but I don't have the 1981-82 931 FSM on hand. Obviously, the other culprits could be the ignition module itself, or more likely, the multi-pin connector at the DITC box. I found a post by Smoothie recently that described taking the multi-pin connector apart and squeezing the female connectors inside very carefully with needle nose pliers. I performed this on my DITC-equipped car (along with another round of cleaning and dielectric greasing), and so far, it seems to have eliminated all of my DITC issues.
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Rasta Monsta  



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PostPosted: Tue May 05, 2009 9:24 am    Post subject: Reply with quote

Have you completely debugged the knock sensor? I seem to recall that people have had problems getting them set up on our little rattle cans.
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CÚdric  



Joined: 27 Aug 2004
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PostPosted: Tue May 05, 2009 6:07 pm    Post subject: Reply with quote

When i bougth the knockwarning thing i bougth a couple. The other one made it on a nother 931, with the same sensor at the same place and same calibration he didnt get any warnings. A friend of mine has a wide band lambda, might be an idea to mount it and se what the fuel system do under load.
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tuurbo  



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PostPosted: Wed May 06, 2009 2:52 am    Post subject: Reply with quote

Also, in addition to the boost signal line, the wastegate rubber piece can wear out.

I think too for the compression test, isn't it true that the wet and dry tests inform differently? Like, assuming a dry test, if compression is low in a cylinder, and you squirt some oil through the plug hole, and take the compression again - if the readings are higher the second time around, it means the rings and/or cylinder is worn. No change in the compression readings means the cylinder has a bad valve. Is that correct?

For plug wires, checking the main coil wire and the individual plug wires can also be done in the dark where stray sparking can show itself.

Something should be said about how to diagnose vacuum leaks in terms of the behavior of the car, sounds, methods for determining vacuum is the cause of (whatever) problem. Most of what you've said there is a regime for preventing leaks, true, but more could be said on how to discover them (diagnosis). So you need to flesh out the phrase 'gone over'. Vacuum rears it's head quite often.

Another place where vacuum messes up is down below where there's a rubber bellows that attaches to the CIS, and the (I think) a hose that comes into that area for smog.
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ideola  



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PostPosted: Wed May 06, 2009 4:46 am    Post subject: Reply with quote

Good points, tuurbo.

In my (somewhat limited) experience, dry compression readings on an engine that hasn't run in a while are almost useless. When I was resurrecting the 941 after it sat for 15 months, the dry compression readings would have indicated need for replacing the rings. After getting it to run, compression is just fine. So we should probably elaborate on that in the regimen.

Regarding vacuum leaks, one could almost write an entire book in and of itself regarding the vacuum and boost leak issues on 924/931! In all seriousness, it would be good to go thru and document all of the points conclusively. Vaughan's resurrection post comes close, but still lacks some of the specifics. Certainly worth addressing in the next revision.
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PostPosted: Wed Oct 14, 2009 5:07 am    Post subject: Reply with quote

Printed, laminated, and bound this and the Resuscitation post!!! Very nice thread for new Porsche 924 owners!

Also, tip on dry/carbon'ed rings is to pull the plugs and pour some kerosene down the holes and let it sit over night. The kerosene will break down the carbon build up and free stuck rings almost every time. I used that trick when I it tested bad when dry, then fired right up (a Fiat Spyder) for the first time in 8 years and it had good compression after that.
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Rich H  



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PostPosted: Wed Oct 14, 2009 5:25 am    Post subject: Reply with quote

Bloody hell - sticky thins please!!

Good work Dan!
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PostPosted: Sat Nov 05, 2016 9:03 am    Post subject: Reply with quote

this is great, thanks Dan, wish I would have come across this post a long time ago.
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