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931 Resuscitation - how to resurrect a long-dead turbo

 
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924RACR  



Joined: 29 Jul 2001
Posts: 8036
Location: Royal Oak, MI, USA

PostPosted: Sat Jul 21, 2007 11:58 pm    Post subject: 931 Resuscitation - how to resurrect a long-dead turbo Reply with quote

I wrote this up for a friend who just got a 931 that is being resurrected, as he's new to 931's; figured it'd be a good addition to the section. So what follows is basically a list of how to take a non-running 931 and see about getting it started.

A few critical things to start with. First off, these are interference engines, not like the 924NA. On the plus side, they still use the same $8 timing belt, and you can still set the tension by hand, with the old quarter-twist method, but you will want to do those first thing when you're getting to resuscitating it. Consider also replacing the water pump while you're in there; easy to do, and they're inexpensive on these cars.

Next item will be the fuel system. First, expect the gas tank is full of nasty junk; drain and rinse completely. Also remove the fuel filter, blow out the lines with compressed air, and install a new fuel filter. Note that the correct fuel flow direction is to the REAR of the car. Most cars come to us with the filter installed backwards. I recommend removing the in-tank pump (36mm wrench or socket required) and cleaning it's intake screen off, not to mention that this is the best way to flush the rust out of the tank. It'd be a good time to check resistance on it, make sure it's not dead. You don't want to have to drain the tank a second time!

Refit the pumps, add a little gas not a lot, and see if you can get the pumps to work and make noise by jumping the fuel pump relay. But not too much noise, of course, or they're dying. Mainly that'd be the external one making noise and dying. Naturally, no noise, you'll want to check for power and ground - the pump grounds back inside the trunk between the taillights, if that's the problem - and resistance of the pump itself, in case it's dead. Fuse number, as I recall, is #1 or #2 on the aux fuse panel (small fuse block near the main fuse/relay board).

If you do have noise, then you can check and see if you've got fuel up top at the fuel distributor. Most likely, the fuel distributor and.or warm-up regulator may be dead, but at least at this point you can, if you're getting fuel up front, see if it's getting to and through the injectors. You should be able to hear a high-pitched whine if you have the fuel pump on, and push down on the air flow meter plate - that's the fuel going through the injectors. Don't do it too much, as you'll wash down the cylinder walls and fill the oil pan with fuel!

If you're not hearing that whine, your fuel distributor may be gummed up. You can check further by pulling an injector and seeing if it'll spray into a bottle when you lift up on the plate. If this is the case, you may be able to free the center metering plunger with a little compressed air injected into the top center port, but this doesn't always work. If not, you'll have to get a rebuilt one; currently the best deals going seem to be at www.SpecialTAuto.com, a Delorean specialist who also handles Porsche fuel injection bits; he's got prices for rebuilt Bosch CIS components far below anything else I've seen out there. I just received and installed a rebuild warm-up regulator (WUR) in my '81 931 from him, and it was perfect, flawless.

Once you've got fuel to the injectors, you will likely want to check that WUR, using the CIS pressure gauges, and make sure that you're both getting the proper fuel delivery pressure from the fuel pump (around 5-6bar), and that the WUR is throttling that back properly (to control pressure between 2-3 bar). Often the WUR fails in such a way that it doesn't regulate pressure, resulting in control pressure equal to fuel delivery pressure, which means improper mixture control - usually too lean, bad for an NA, and horrible for a turbo! Again, if a rebuild is needed, I recommend SpecialTAuto.

Once these are set, you can see about starting the car; at this point, I still recommend using the fuel pump relay jumper, as there are other things that can keep that from working (beyond the relay itself being NG) and you don't want to be distracted by those just yet - you just want to get the car running now. So crank it, see if it runs, see if you have a spark if needed. The 81-82 cars have a more complicated ignition system which can result in problems at this point; in my case, the 81 had an inop crank trigger sensor. Problems here usually show up on the tach when cranking, in that the needle swings wildly or doesn't move at all. Beyond that, the '79-80 Turbos are substantially different in the ignition system than the '81-82's, which are all I've messed with, so that'll be best left for another discussion.

So if you're getting fuel through the steps described above, and you've got spark, the car should basically be ready to start.

Next step will be going back and making sure the fuel pump relay works. Easiest thing to start with is dropping it back in once the car runs, and seeing if it'll restart. If not, check first for a ground on the relay. They ground with a small pin in the top corner; pull the relay board forward enough to see the wire colors, it'll be a small brown wire. This doesn't go to ground!

It grounds and turns the relay on with a switch in the intake duct near the throttle body; this switch kills fuel in case of overboost. Naturally, for safety's sake, they fail in the open position. The switch is easy to test with a multimeter, just see if it's got conductivity between the terminal and it's threads. If it doesn't, you can jumper a wire from that plug to ground, and see if the fuel pump relay works now - just to check your results. If that was the only problem, the relay should now work and the car should start. The switch is readily available from parts suppliers or the dealership, as it's now superseded to a 993 or 996 Turbo part.

If the switch is good, replaced, or jumpered around and still the relay won't work, you need to check the tach signal; the relay also uses an input from the coil to confirm that the engine's spinning in order to receive fuel (crash safety consideration. This circuit is as I recall the solid green wire, and also goes to the tach and the green wires at the top of the coil. So if the engine will run with the pump relay jumpered, but not with the relay in place (and the overboost switch is functional or bypassed), then see what the tach is doing. If it is jumping when the engine cranks or working with the engine running, clearly there's a break in the circuit (or a poor/corroded connection)... likewise, if it isn't working, neither the tach nor relay are functioning, clearly there's a break elsewhere in this circuit... time to break out the multimeter and check connections.

After all the above is checked out, the car should be able to run properly/normally. Next thing at the top of my list would be to check cooling function; first check that the fans can and do work. I'd want to know that they both kick on with ignition on and the rad fan switch jumpered; I'd also want to know that they both turn on when the AC is turned on (as they should). At very least, this will mean that I can manually turn on the fans during the next step - drive the car, warm it up, get it hot, and see if the fans come on automagically. If you don't hear them by the time the water temp clears the 3/4 mark, you'll want to turn the AC on and force the fans on to cool the engine.

Fan switches come in 3 different flavours, 75, 85, and 95C. I think turbos used as daily drivers should have the 75C switch, they generate so much heat. Mine seems to have the 95 in it right now, and by the time the fans kick on automatically, I usually already have turned the AC on to avoid the temp spike (usually after blasting down the highway then coming to a dead stop). BTW, AC retrofit to R134a is doable and useful, but that's a discussion for another day.

So at this point you get to evaluate the effectiveness of the cooling system; you'll be best off if you preemptively replace all the hoses and flush the system with a radiator flush product from the local parts store - the rad often gets gunked up. On refilling, use the phosphate-free/silicate-free orange stuff.

Finally, running condition... I won't get into tuning them, other than to point out that these cars, being CIS, HATE vacuum leaks; it's even worse with turbos than NAs. Furthermore, there are countless little elbows that degrade over time, generate cracks, and cause odd hot-vs-cold running problems and hesitation, stumbling off-boost. I replaced all mine, some I managed to hunt down stock OEM replacements for, but not all; however there are generic cut-to-fit 3/4" silicone hose elbows that cam be made to fit and do work well. Downside is they come in rather bright colors, for the ricer crowd. You also will need all new hose clamps, as the OD is slightly bigger than the stock elbows.

I got mine from Racer Parts Wholesale: http://www.racerpartswholesale.com/silhose3.htm (scroll down to the 90 degree bends, and you'll want the 3/4"/19mm... though some may be 16mm/0.625"). I don't remember just how many are required; it's something between 5-10. You will also want to save the straight sections, as there are a few straight vacuum hoses to be replaced as well. Note also that in addition to all the ones you can see, there's one that's well-hidden, took me a long time to find and fix (at which point, this being my last vacuum leak, the car magically got smooth and driveable). This connects from underneath the intake, on a port below the #1 runner at the front of the intake, wraps around with a hard line to the front of the engine, where there's another little elbow that plugs into the turbo for the compressor bypass valve. It's extremely difficult to get to, may require some disassembly. But it was worth it.

Hope that helps, this checklist and a lot of time should help get the car up and running.
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Rasta Monsta  



Joined: 12 Jul 2006
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Location: PacNW

PostPosted: Sun Jul 22, 2007 4:26 am    Post subject: Reply with quote

Great write-up, and I wanted to emphasize the intake rubber refresh. Visual inspection is not good enough for these elbows. Assume all are bad, and replace all of them, and all vaccuum lines.

DON'T SKIMP! All components are available thru PCNA.
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Smoothie  



Joined: 01 Jan 2003
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Location: DE (the one near MD, PA, NJ)

PostPosted: Sun Jul 22, 2007 6:12 am    Post subject: Reply with quote

Cool.
-But to avoid further confusing those that drive on the wrong side of the road, right-hand-drive 931s would have the fuel filter on the right side and fuel flow is forward for them, right?
-And to avoid confusing everyone, you'd press down on the 931s' air metering plate to initiate and check fuel flow to the injectors. Of course it's lift-up for 924NAs.
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924RACR  



Joined: 29 Jul 2001
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Location: Royal Oak, MI, USA

PostPosted: Sun Jul 22, 2007 12:56 pm    Post subject: Reply with quote

Thanks for the corrections/clarifications, guys... any others? Corrections? Anyone care to add any other steps?
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scotthusky89  



Joined: 13 Jul 2007
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Location: south wales uk

PostPosted: Mon Jul 23, 2007 9:30 am    Post subject: Reply with quote

yes i drive a right hand 931 as its called the 932 over here in wales in the uk and u are correct the feul filter is on the left as u look from the front and it is flowing to the front
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Smoothie  



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PostPosted: Sat Apr 05, 2008 6:47 am    Post subject: Reply with quote

Several 931 problems with similar symptoms were solved in this thread - http://www.924board.org/viewtopic.php?t=5294
-So I'd put that on the recommended reading list for generally mysterious 931 problems.
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PeterW  



Joined: 17 Dec 2003
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PostPosted: Fri Sep 24, 2010 5:08 am    Post subject: Reply with quote

Having just finished this procedure on a "new" car that I picked up from an auction this summer (great write up BTW, it was exactly what I needed) I would only add a few things that I discovered...

1) Oil. While it's pedantic to tell someone to change the oil before starting, most will not drop the oil pan due to removing the front cross member. It's a good idea, but it's a personal decision to trade off the time/effort/gaskets/alignment (if you do it like I do normally and drop the cross member... I don' t know how to do remove the pan without) and drop the oil pan for a cleaning. Instead, this time I "flushed" using a few quarts of the cheapest, lightest weight motor oil I could find. I also turned over the engine without fuel/spark and checked that oil was getting to the camshaft. Look in the filler hole and you can see the if the cam is getting wet when turned over by the starter. There are a couple of things here, things degrade like the little plastic elbow that feeds the drip tube that lubes the camshaft and sometimes leaks, especially the feed line to the turbocharger will appear when the system is pressurized by turning over on the starter motor.

2) Coolant flush. I used clean water and a chemical flushing agent. I did this a couple of times and am still getting sludge. I'm not sure on this, but what I have found/been told on how to identify the sludge types, but "milky" is a bad thing (oil,) brown slime is most likely old antifreeze, and clean is good. Keep flushing needs to be your motto. There is no sense in replacing hoses/water pumps until you have things cleaned out for the most part.

3) Put a cheap disposable fuel filter in the return line to the fuel tank to keep the crud in the fuel system from coming back to haunt you. The fuel system circulates the fuel, and as it picks up stuff it will drop in right onto your brand new in-tank fuel pump, making it necessary to clean the fuel tank again. If you are being "smart" about trying to test the car without the in-tank pump, your pressure pump will suck up the new crud just as effectively.

<EDIT> I forgot to add - After evaluating the fuel tank, I decided to forgo using the in car tank at all and used a gas can that I modified for the the purpose. The in-tank pump needed to be replaced anyway, so I removed the actual pump bits and used the shell as a way to plug/clamp off the tank.

This car runs great after having followed the posts in this thread, and it has been sitting for about 16 years as far as I can tell. Not yet drivable (still dealing with the brakes) but as these cars age I suspect that many newbies are going to have the resurrection experience.
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ideola  



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PostPosted: Fri Sep 24, 2010 7:42 am    Post subject: Reply with quote

PeterW, nice additions, thanks!
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fwood  



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PostPosted: Wed Aug 31, 2011 9:50 am    Post subject: Reply with quote

Just as an addition that may be obvious to most-check the throttle switch with a METRIC feeler gauge and replace the throttle socket o-ring(4 bucks from pelican) Many hours of chasing all the other suggestions and I finally felt the Porsche God smile on me after these two simple steps-AND IT FEELS DAMN GOOD!!!!!
Frank Wood
82 931
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Red944ls1  



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PostPosted: Fri May 02, 2014 11:57 am    Post subject: Reply with quote

I should have read this before I posted ..great write up I will try these things.
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klec  



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PostPosted: Wed May 14, 2014 6:56 am    Post subject: Re: 931 Resuscitation - how to resurrect a long-dead turbo Reply with quote

924RACR wrote:
Note that the correct fuel flow direction is to the REAR of the car. Most cars come to us with the filter installed backwards.


If I understood - fuel filter should be installed with an arrow directed to the cabin (rear), not to the engine? Is it right? Should arrow shows on the driver?

Now in my 931 I've got 928-110-253-06 fuel filter and arrow is directed to an engine. I bought new one (MANN WK 726) and the screws on the filter allows only to mount it in the same direction - with a fuel flow from rear to front (I can change the screws).

Thanks for help!
Tom
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ideola  



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PostPosted: Wed May 14, 2014 7:40 am    Post subject: Reply with quote

If you trace the fuel lines, you will see that the line from the rear of the car comes up through the engine bay on the left hand side (by the brake booster). It passes through the firewall into the battery tray area, and connects to the inlet of the filter. Then the outlet line connects to the outlet of the filter, passes across the battery tray area and down the right hand side fender / wheel well and over to the inlet of the fuel distributor. So, yes, the flow direction arrow should point toward the windshield.
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