bonus12

It will be a huge accomplishment to finally fix this problem! Anyone who helps me finally fix it will be my hero. I had an old thread on this but this will be my last one. I have a lot of updates here.


Scenario:

• 1986 944 turbo
• Currently stock except for autothority parts but problem will happen when totally stock too
• Problem only happens after sitting with engine off a few hours or longer (happens every time over night)
• Seems to idle just fine but when revving, even a little, there is a misfire + smell of fuel at exhaust + cloud of white smoke
• Harder the rev, bigger the cloud and smell
• Gradually smooths out 100% in 10-40 seconds every time it happens
• No noises such as loud exhaust leak
• Starts up right away every time
• Car runs great otherwise with truly no other running issues at the moment
• Fuel economy is about 20 mpg on the highway
• Started happening over 6 months ago. Has gotten a little better after all the work I've done. Not sure why or what could have caused it

Here are the things I’ve done:

• Painstakingly addressed vacuum hoses/connectors/o-rings except AOS seals (vacuum within spec now). Addressed turbo pressure hoses too
• ICV tested and works – I cleaned it while I had it opened to give it fresh o-rings
• Addressed all tune-up items: spark plugs (gapped), cap, rotor, plug wires (tested good), air filter, fuel filter
• Replaced coolant temp sensor (one with square top)
• Replaced head gasket a few years ago – no oil / coolant mixture
• There is normal oil loss
• No coolant loss whatsoever
• Replaced intake and exhaust manifold gaskets
• Installed newly refurbished factory injectors
• Tested fuel pressure according to Clark’s garage - within spec
• Tried a 3.0 BAR Bosch FPR – did not change the problem
• Air : fuel ratio normal after problem goes away
• Tested cylinder compression – within spec
• Adjusted wiper arm on AFM – have never adjusted the mixture screw on the AFM
• Cleaned AFM, TPS, and injector electrical contacts
• Set idle according to Clark’s garage
• Tried manipulating FQS – no change to problem
• TPS adjusted (clicks when throttle cam is moved about 0.5mm)
• Replaced O2 sensor

I’ll tell you, my 951 runs better than ever with all these improvements, but there is still this nagging problem! Again, it goes away every time after 10-40 seconds. Speed reference sensors are old and not replaced – someone once recommended replaced them – not sure about this. Have not addressed electrical grounds.

MAGK944

It sure sounds like water is getting in your fuel while it’s sitting. That would cause the white smoke, the misfire (due to unburnt gas) and It would stop once the water was burnt off and then run normally until it sits again. The question then becomes, how/where is water getting into your fuel?

Edit: When the engine is turned off the coolant pressure remains high while it sits, just a guess but worth investigating.

bonus12

Thank you for your reply, MAGK944.

I don't think that's it. Let's say the engine cools overnight and in the morning I run the engine until the problem clears up, let's just say a whole minute. At this point, the coolant fluid is still cold, certainly not much above ambient temp, but the problem will come back after several hours of sitting. This will repeat. Not to mention, I've had 0 coolant loss in a few thousand miles, at least as far as I can tell by looking into the reservoir. However, it will be easy to test this by leaving the coolant cap off while I do the above for a day.

Is it possible there is water getting in the fuel some other way?

MAGK944

That was my thinking before the edit I added. Fuel pump, check valve, emissions valves, tank.

Have you checked if your charcoal canister is full of water, maybe it could be sucked into the fuel system by vacuum from there. Just throwing it out there for discussion.

Tom M'Guinn

Just spit-ball'n too, but if water were in the tank, it would sink to the bottom and be the first thing sucked in, which could cause the white smoke and sputter until the water is agitated into the gas and/or the motor is hot enough to deal with it. Once the car cools off, the water settles at the bottom again.

Or, despite changing it a few years ago, you are seeing early signs of a HG failure. Combustion gas gets into the coolant jackets maxing its pressure, then when you shut off the motor, the pressurized coolant leaks into the cylinder which has to burn off in the first few minutes after start up.. Eventually you'd see cooling issues if that was it, but you might not notice the reduction in coolant if trapped air is taking the place of the lost coolant. You might pull the plugs when cold and use a flashlight and your nose to see if there's coolant in any of the cylinders. You might also put your overflow tube in a plastic bottle for now to see if it is spitting anything out, especially after driving hard.

The white smoke could also be oil from valve guides, turbo seals, etc. If bad enough, I suppose that could cause a stumble too. Have you looked at your plugs for signs of oil? Coolant/water in the exhaust looks much more like steam, whereas oil has a distinct smell and lingers in the air much longer. Can you tell which it is?

bonus12

The smell of the exhaust is definitely fuel when I rev the engine to get a cloud . I don't smell sweet coolant and, having had a BHG before, I know how the smoke lingers longer. I guess it possible, but definitely smells of fuel. And no coolant being lost or added after so long (thousands of miles) is an important thing here, so that's a relief. I check the oil and coolant level all the time.

Temperature is solid and stable, mostly around the lower mark on the gauge when driving. I'll definitely look more into the fuel getting contaminated. However, whenever I've changed injectors, which was a lot recently, I would allow the fuel pressure to empty into a cup instead of spilling all over, and I never saw any fluid separate, or any water in there. But maybe I missed a tiny bit. Like I said I will seriously looking these ideas.

T&T Racing

Has the AOS seals been checked? To test if a problem, remove AOS outlet vacuum hose and plug the hose. Then start the engine and see what happens.

bonus12

I haven't checked the internal seals. I assume they're old and worn. Do you mean the control line to AOS hose, #19? I replaced that elbow hose but never the AOS seals.

(Auxiliary air valve / vacuum limiter / elbow hose) (931-110-183-01)

Tom M'Guinn

When you changed the DME temp sensor, did you confirm the signal was getting to the DME as designed? Those sensors use the same connectors as the injectors and speed/ref sensor and are equally prone to fraying under the rubber boot. Mine did. If you pull the connector off the temp sensor when the engine is warmed up, does anything change? Lot of car pump out white condensation smoke when first started up especially when cool out, so that "might" be a red herring unrelated to the stumble?

dzaprev

I had a head gasket that was starting to go that did that. But it idle ruff on start up til it cleared the coolant out.

bonus12

I'm making a list of all these things to check. Meanwhile, here is a pic of NGK BPR7ES plugs. The worst one, on the left, is the one nearest the firewall. They are in order, and I didn't include the one nearest the radiator because I was switching it around and it doesn't tell a true story. But the missing one would be like the one in the right in the pic.

To be clear, the plugs are not causing the problem. New plugs don't help and so I'm just posting this for analysis.

Anyway, I see crusty and chalky deposits ranging from white to light brown. If that's right, that points to oil or fuel additives?

bonus12

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Chalt

I bought a thirty-something dollar endoscope camera that connects to my iphone via bluetooth off Amazon when I was experiencing a miss at startup. Mine was clearing up too after a few minutes of running. Now the camera isn't like the greatest thing ever, but it works (video and still pictures). At the low cost I paid for the camera I have found it's a worthy tool for my toolbox.

Well, here is what I found:

There was a tiny tiny amount in #2. Cylinders 3 and 4 looked dry.
Here is a picture of #4 for comparison:


I'm not telling you that you have the problem I had. But a cheap camera is useful.

I hope you'll post the answer when you find out what is wrong, and good luck.

T&T Racing

The AOS is not in this diagram. The AOS has a yellow cap and where oil is added to crankcase.

T&T Racing

The only spark plug that looks normal is the far right one. The other 2 are not normal and the threads show excessive oil on the threads near the electrode. This could indicate valve guide seals have failed letting cylinder head oil to leak into cylinders on engine shutdown and during intake stroke. Excessive oil in cylinder on startups would produce white smoke and fouled plugs until the electrode and gap ground come up to temperature to clear wet deposits.