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Wayne 962's Avatar
Frustrating 959 Problem -> Need suggestions...

Okay, let me start by saying that I am aware of the proper diagnosis procedure for the engine, and it's quite involved. I'm hoping to get some ideas on what could be the problem with the system before I start from step zero and check *everything*.

Here's the problem. When we came back from SEMA, I took Tom Woodford (the fellow who helped me write the Engine Rebuild Book) out for a drive in the 959. I drove first, and then I let him drive. Before I let him drive, I noticed that the gas tank level was low, so we filled up with some 91 octane at the local Chevron (this station closed for "maintenance" a few days later). After about 2-3 miles of driving, we got back to the shop, and the car stalled as I was putting it away.

Next time we took the car out, it wouldn't start without feathering the gas, and giving it a little bit of throttle when starting it up. It's been the same way ever since. Here are some more details:

- I ran the tank down to 1/2 full, then filled up again with 91, and then some 100 octane from Willow. I also added some of the Swepco fuel system cleaner that we sell. So far this has had no effect.

- The car ran pretty fast at Willow - the car has plenty of power, particularly at the top end. If there is a power loss at the top end, it's news to me.

- When you let off of the throttle and let the clutch out (pedal in), the engine rpm drops really, really low, and then bounces up to about 900 or so. Occassionally, the car stalls if the idle gets too low, and particularly if the car is cold. This dropping down below normal idle happens 100% of the time since this problem started.

- I have swapped out and replaced the idle control valve (ICV) with 2 other ones, and this has no effect on fixing the problem.

- The fuel injection system is not showing any fault codes. This in particular is odd because I did run the car briefly without the ICV hooked up (it ran worse). I would have thought that this would have tripped a fault code. Maybe it has to run for a certain length of time before the code would have been tripped.

- No obvious vacuum leaks, although I have not yet put my vacuum gauge on there to check (that will probably come tomorrow). Still, I doubt that there are any leaks because the upper operating range seems good, and the boost levels are good as well.

- The car runs very rough at cold idle, and if you get on the throttle, you can hear it missing when you rev it to about 2000 rpm. When warm, the car seems to have lots of power. When the car is warm and idling, the idle seems to miss slightly (much less apparent than when cold).

- Oil cap on tight.

It may be a clogged injector from grime in the bottom of the tank, but that doesn't seem likely to me either. I will replace the fuel filter regardless (ordered one today - it's the same as the 928). I don't really think it's a temperature sensor, because the engine runs rough at warm too.

Any thoughts from outside the box? I have a gut feeling, it's something real simple, instead of something real complex...

Wayne R. Dempsey, Founder, Pelican Parts Inc., and Author of:
101 Projects for Your BMW 3-Series 101 Projects for Your Porsche 911 How to Rebuild & Modify Porsche 911 Engines 101 Projects for Your Porsche Boxster & Cayman 101 Projects for Your Porsche 996 / 997
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Old 11-26-2007, 05:12 PM
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I would have thought ISV (ICV) too, had a problem exactly like that with a 964 that was solved with a replacement. But you have already checked that, so I am out!

I have to admire a guy who asks the rest of us for advice on a truely unique machine!

I wish I had an answer.

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Old 11-26-2007, 05:17 PM
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Did you steal my Zeniths? This sounds really familiar

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Old 11-26-2007, 05:19 PM
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Not exactly outside the Box since you also may have suspected it…

Wayne, from your description my take is that the engine is running on the lean (will not be noticed at high RPM) side. I would think false air (not metered) may be the culprit…

Spray some ether or carburetor cleaner near the intake manifold ports and see what happens... (?)


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Last edited by Jascha; 11-26-2007 at 05:35 PM..
Old 11-26-2007, 05:23 PM
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green wire from hell???

reminds me of my 911SC acting up. I took the shotgun approach though. Injectors, injector seals, fuel distributor, and even went to msd. Also did the fuel filter, accumulator. It ended being the green wire.

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Old 11-26-2007, 05:28 PM
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Pure guess, but would the fuel pressure regulator be diverting too much fuel away at high pressure i.e. low rpm?

If it was a flow limitation such as a blocked filter I would expect you to have a misfire or lack of power at high rpm, not at idle. If the pressure regulator was opening too far when the pressure got too high and you weren't developing sufficient fuel pressure in the fuel rails then I can see that it might give you starting and idle problems.

The problem I see with this is that I can't see how you would get normal fuel pressure back again unless the regulator sticks open once open or returns too slowly...

Like I said this is a pure guess as I don't know how the pressure regulator works.
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Last edited by Aerkuld; 11-26-2007 at 05:45 PM..
Old 11-26-2007, 05:36 PM
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Oxygen sensor?
Old 11-26-2007, 05:44 PM
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I would think you should start with the ICV valve and you've removed that as the issue. Next I would try the fuel filter. Since you know it started after addition of new fuel and you've got that station being closed I think you can't rule that out. I agree, it does sound like you've got a lean condition or a fuel issue. Once you've checked with your vac tester you'll be in better shape in regards to determining if that is the problem.

But since it was running fine prior the fuel change, you've either got dirt at the bottom of the tank that has gotten in your fuel system OR bad fuel. You're probably right, it is something simple.
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Old 11-26-2007, 05:55 PM
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i had a motronic-engined BMW that did something almost identical including temperature sensitivity, surging idle with occasional dying, and i did the shotgun approach over 2 years (including cylinder head, ICV, vacuum lines, wires, plugs, distributor, coil, cold start injector, fuel pressure regulator, O2 sensor, ECU etc) and the problem finally went away when i plugged in a new main relay.
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Old 11-26-2007, 05:58 PM
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Backdate it to CIS.

Seriously - just a guess: spark. Have you run the engine in the dark to see if its throwing blue light from the plugs. Don't know much about the 959 engine.
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Old 11-26-2007, 06:01 PM
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Well, at the risk of redundancy, that sounds a lot like a vacuum leak.
Old 11-26-2007, 06:22 PM
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Sounds a bit like what I'm going through with my 86,, my first question is what is the A/F indicate?
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Old 11-26-2007, 06:26 PM
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sounds a LOT like the vacuum leak that I just fixed on my 911. Actually it was more like lots of little leaks that added up to one huge leak. The biggest leaker was the vacuum line between the heated air vent from the oil tank to the throttle body. The vacuum line at the thermo valve at the back of the throttle body was split at the end, which caused at varying leak, depending on ambient air temperature. When the weather got colder, the leak got worse. Mine ran ok at high rpm too. It was low rpm that was the pits.
Old 11-26-2007, 06:32 PM
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Wayne, Some of the items I would look at:
- TPS: If it's not signaling ilde to the DME, the idle algorithm will not kick in.
- Vacuum leak: The engine is getting unmetered air (AFM/MAF only). The 959 may be using a speed density/MAP system, so a vacuum leak may not affect it. However if a MAP sensor is in use, a altitude sensor might be used as well. Either MAP or altitude sensor can cause this issue.
- When cold, the DME enrich the mixture, a faulty eng temp sensor (reading rich) can cause the lean condition when cold.

- A faulty O2 sensor (sending the incorrect data to the DME - Reading a incorrect rich condition here) will cause the DME to lean the mixture. If the car has a test plug (round 5-pin plug), usually there is a procedure to set idle, where 2 pins are shorted. This disables closed loop and the idle control algorithm. If possible, run it in this mode and let us know what you find.

- A clooged injector(s) will cause a lean condition as well. However, it becomes a serious issue under boost (lean AFR).

Hope this helps, feel free to drop me a note if I can be of any help.

Old 11-26-2007, 06:34 PM
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Typical 959 problem they all do that when driven hard...Just Kidding, good luck finding that gremlin !
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Old 11-26-2007, 06:43 PM
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+1 on a vacuum leak or lean when cold condition.

IMO: Wayne . . . as far as knowing what kind of power your 959 is supposed to develop, I think you should try to find someone with a sorted example and drive it as well, just for comparison. I'd be happy to let you borrow mine, but Im in Colorado, and I've got it in storage for the winter season right now.
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Old 11-26-2007, 06:49 PM
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Nobody has mentioned the distributor yet. It could have had an issue while spooling up the rpm's and either freed up or threw a spring,if it has one. Does the distributor have vacuum advance/retard? If it has vacuum retard, try unplugging the line from the distributor and plugging that and fire it up. Also check timing to spec.
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Old 11-26-2007, 06:55 PM
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My guess.. water in your fuel. I've had it a few times where a bad tank of fuel will plug up a filter. It was so bad for a while that i was running a racor filter on myt 4 runner and 1 bad load of fuel did plug it up on a 600 milr road trip (the filter was new when I left and I had to replace it duriing the trip).

Try putting in some methyl hydrate (methanol) to dry up any water in the tank.
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Old 11-26-2007, 06:56 PM
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cylinder head temp sensor??? at least that prevents my 87 911 from starting ;-)
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Old 11-26-2007, 06:57 PM
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Turbo boost places all vacuum lines and intercooler tubing under pressure, and can pop them off or split the lines, especially if the rubber gets old. I'd make sure all the rubber vacuum lines were fresh, and zip tie them to the nipples for insurance.
Old 11-26-2007, 07:01 PM
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