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n4bbq · Joined: 03 Jul 2003 Posts: 10 Location: Dahlonega, Ga

First a little history:

This 931 was running fine when it was 'retired' after I bought a new truck. It has been sitting for a few years and needs a little TLC.

The first thing I have done is replace the plugs, injectors, and all rubber that I found to be dry rotted. I have replace the fuel filter and it has a new air filter as well. With this in mind, I'll proceed to my question.

Basically this car will start quick, then starve for fuel (or it seems) and dies. After I complete 'the starting process' that I list below, the car will idle ok and run great above 3000 rpm for the most part.

Now for a description of the starting process:

To start the engine, turn on the switch, adjust the driver's side mirror (for some reason it has to have you use something on the electrical system like a mirror or window), the fuel pump charges the line, and crank it. It starts as quick as any new car, then starves for fuel (or it seems) and dies. As it begins to die, I open the throttle completely until the engine dies.

I repeat this process for 5 to 20 times, if the engine ever picks back up during the open throttle step, it will then idle fine and it is drivable.

After a warm up period, the car runs great above 3000 rpm, but from
2000 to 3000 it is a mess. It will backfire, flood out, etc. And sometimes, it runs as well as it did when I stopped driving it, but it always has to be started as listed above.

The rear fuel pump is very noisy, so my next step will be to remove
and clean the 'filter' that is there in the tank. I had an 80 model years ago and I seem to remember having to clean a filter there once.

Any pointers will be great.

924RACR · Posted: Thu Jul 03, 2003 9:43 pm Post subject:

I know what you mean about the mirror!!! LOL I discovered that about my car the other day! I wonder why, gotta be something wrong with the wiring.

However, that's not the cause of your running problem. Sounds like your mixture is way off. One of mine was doing the same. I had to richen (I think) it up a lot to get it to run. Above 3k RPM, you're building boost, and are getting the mixture enrichment from that. Below, the basic mixture setting must be right. Try richening it up a lot, like a half turn, or two or three, till it runs. Right now, it's only running on the shot from the cold-start injector.

Assuming you haven't got a massive vacuum/boost leak. Check that! Though it may be because you replaced rubber that was previously leaking, and now it's all out of whack.

You might want to check the idle setting too, when all else is done.
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'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
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CMXXXI · Joined: 05 Nov 2002 Posts: 1939 Location: Vicksburg, MS

I'm going to suggest an easy check first - is you fuel filter installed correctly? My next suggestion would be to run a CIS pressure check. I'm not the resident expert by any means, but I have never advocated "dinking around" with the mixture until exhausting the other possibilities. Drain the tank (you say you will check the in-tank pump anyway), fresh fuel, some techron, give the car some exercise as best you can. I'm thinking WUR...
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'79 Eurospec 931

project931 · Joined: 10 Dec 2002 Posts: 95 Location: scranton, pa

im going to agree. sounds like a warm up regulator to me also. the audi 5000 turbo's have to same one so try one of those it might be cheaper and easier to find
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positivesale · Joined: 12 Nov 2002 Posts: 126

Common problem.. should be the WUR. But it's a pain to get at. I think the 81 and 82 are the same but 80 is different. Not sure if 81 and 82 are different. When I took a WUR apart once, it was actually interesting inside. A bunch of hard thin rubber gaskets... you could rebuild it if they sold the gaskets, but I don't think anyone makes the gaskets. There might have been springs in there too, adn other gizmos.
I remember Jon Furst and I were discussing the differences once, and I think the 81/82 has an additional hose on it that adds extra fuel when the boost kicks up. So the 1980 WUR might be compatible with a 1981 car if you plugged or figured a way to bypass that line. Obviously the 1981 was an improved design though.

I'm going to do some part number smacking around in PET and see what I can come up with about the 1981/82 and 1980 part numbers.

Ok here we have it:
the 81 and 82 931 use the same part. 80 different. 931 part number, so if the audi is the same one, that's possible.. I have found 931 part numbers that were just the same as audi parts with different numbers.
931 606 105 00---1980
931 606 107 00---1981 and 82
How about a 924 regulator on a 931.. probably incompatible since the fuel meter is different?
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82 931

Lizard · Posted: Mon Jul 14, 2003 3:02 am Post subject:

the 931 1980 WUR has a vaccum line on the side to enrichen the mix as the RPMs increase, I am pretty sure the 1980 does as well. as for a 924 NA wur working, the car would probably run but not well and it would probably lean out ALOT in the higher RPMs
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924 turbo · Posted: Mon Jul 14, 2003 7:56 am Post subject:

According to the '81 model supplement, the WUR was changed to use a different temperature switch to improve warm start behavior. The control pressure modification curve may be different as well, but I don't have the '80 model supplement and my factory manuals are unaccessable at the moment.

I agree that the symptoms sound like a WUR problem.
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Jon Furst

CMXXXI · Joined: 05 Nov 2002 Posts: 1939 Location: Vicksburg, MS

First, 931 & 924 WUR's are different. All the 931 models have an additional chamber on the base which is hooked up to a vacuum line to enrichen the mixture at higher RPM, and are almost twice the price of the 924 model. '79 & 80 models use one part number, 81 & 82 use another. The pressure curves are slightly different at different ambient temperatures, but I believe the resistance value in the heaters are the same. The photo below has a 924 WUR on the left, and a 931 on the right and in the background.

(Click for larger photo)

FWIW, the 924 model WUR has a completely different pressure curve.
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'79 Eurospec 931

positivesale · Joined: 12 Nov 2002 Posts: 126

I am guessing the AUDI turbo regulators are configured differently, but who knows maybe the internals are all the same. It's 5 cylinders, so I would think the AUDI one would have slightly more fuel addition.
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82 931