924Board.org

[Stuart]

Hi, first-time Porsche owner here. I recently picked up an '81 931 that needs some attention. I got it running a couple weeks ago and am starting in on fixing some of the original owner's "repairs". It seems he was trying to bypass the fuel pump relay for some reason, but it isn't clear.

In any case, I'm looking for any tips on reviving a 931 that has been sitting for 5-10 years. The fuel mechanism has been rebuilt, I just replaced the plugs, wires, cap, rotor, & air filter. I need to add coolant to it - is any particular flavor recommended?

I'm missing the nut the holed the fuel tank strap up on the passenger side. I'm sure getting a nut won't be a problem but trying to gauge the thread pitch and size is a bit of a challenge. I checked the metric hardware I had laying around without any luck. I could just go to the hardware store and buy everything close but thought someone here might be able to make an educated guess.

I appreciate any input. I've owned 20-some cars, trucks, & bikes over the year, but this is my first Porsche, first forced induction engine, and it's been close to a decade since I spent a lot of time around a socket set - I'm wading in slow.
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1981 Porsche 931

[morghen]

I hope you replaced the timing belt and did a proper job aligning the engine. Pay very good attention to this aspect. The crank mark must be aligned with the cam mark that is on the BACK of the cam sprocket.
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https://www.the924.com

[924RACR]

Pretty sure there's a thread around here to get you started...
https://www.924board.org/viewtopic.php?t=22724

I think the nut you're looking for would be M10x1.5...

Coolant: I seem to recall seeing a recommendation for phosphate-free? I prefer the orange stuff, but I believe the green stuff was all that was in use when these were new...
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Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

[chuck21401]

The Porsche parts diagrams are fairly detailed. Porsche has them on their website but others do as well. This indicates that it's a self-locking nut which is probably a good idea.

http://www.autoatlanta.com/porsche-parts/hardparts.php?dir=924-80-85&section=201-00

re: The fuel mechanism has been rebuilt

What do you mean by that?

re: it's been close to a decade since I spent a lot of time around a socket set

You may need swivel sockets and extensions with this one. lol. I've had to expand my tool collection since buying my car.
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1981 Porsche 931 w/S1 engine & g31 transmission. Water-cooled intercooler

[Stuart]

Thanks for the tips.

The timing belt kit is at the top of my list, I was waiting to put in an order for a kit till I got a little feedback. I'm adding a water pump & coolant lines to that order. This isn't my first interference engine, I just didn't want to change the timing belt before I knew if it would even run. A visual inspection showed it seems in serviceable condition, but I'm not going to put the car to pavement before swapping.

The Bosch fuel distributor was rebuilt, new O-rings and such as I understand it. I didn't open it up. It seems pulling and cleaning the injectors as well as blowing out the lines and a new fuel filter should be on my list as well.

It seems pretty clear that replacing all the vacuum lines is a must-do. It looks like most of them are 3.5mm. Living in the states I can either buy 7/64", 5/32", or spend a lot more on 3.5mm silicone lines. I've been leaning towards buying some 7/64" tubing as it's only a little narrower, easier to source, & less expensive. Any thoughts on this?

Is there supposed to be some sort of clamp that holds the oil separator together? Mine seems to be open with no obvious means to secure it, is there a clamp or similar that holds it together?
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1981 Porsche 931

[924RACR]

The oil air separator should be one piece - but it's common for them to come apart. I epoxied mine together, it seems quite happy - just be sure to remove all oil and grease from the mating surface,
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Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

[Stuart]

[Stuart]

I've got it back in pretty decent shape, but I ran into a strange problem once it warms up. It makes a loud howling sound. Not at all related to engine speed, so not a belt or pulley. Not related to vehicle speed because the tires aren't moving.

https://photos.app.goo.gl/JxYcMijZ5RroutYv9

This is with the engine off after it had run for 5-10 minutes. No sign that it overheated. It did eventually stop. My first guess is something to do with the cooling system. It did this both before and after I replaced the water pump.

I'm thinking it might be a stuck thermostat, but in my experience that isn't a common issue. This car already had worn out plugs & wires and a bad boost sensor on the manifold (so it's not like it was running).

Before I throw a ton of time and money at that I thought I'd see if there were any other thoughts.
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1981 Porsche 931

[Beartooth]

Radiator fan? I've noticed mine keeps running after the engine is shut off. It doesn't sound anything like that, of course, but if the bearings in the motor are dry, it could explain everything.
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1980 931 diamond in the rough

[bureau13]

It seemed to get louder as the mic was moved down near the radiator, that has to be it.

[Stuart]

I don't think it's the fans..the sound stopped and the fans kept blowing with only normal fan noise. I should have said that before, sorry about that.

I noticed there seems to be a leak on the expansion tank, so I'm going to investigate that as a possible source.
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1981 Porsche 931

[Fasteddie313]

Boost leaks can sound like that.. Air leaking from a hose? Coolant system pressurizing? Bad head gasket?

Check for a constant bubbles in the coolant tank with the cap off that don't ever stop..
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80 Turbo - Slightly Modified