FRporscheman

Hey everyone, I read a ton of old threads before posting this and never found a problem quite like mine. I bought this 85.1 944na 6 weeks ago with a hot start problem and a bad clutch. I fixed the clutch and many other little things, but the hot start problem remains. The PO mentioned vapor lock.
Symptoms: The car turns on easily and drives great while it's still cold. The idle is stable at around 1000rpm. It has the typical vibration at idle when cold that goes away when warm. It also has the "944 shudder". After being on enough to warm up (like driving about 20 miles), the idle still sounds and feels fine, but when you press the throttle there are popping and coughing noises from the exhaust, and the rpm won't go over about 5000. It just stumbles like it wants to go over 5000, but something's not letting it. The car stalled only once from bogging down, but it was a while ago. MOST of the time, especially recently, it will keep running smoothly at low rpms, but it sounds like sh*t and hesitates / stumbles from 3800 to 5000 then wont pass 5000rpm. Once you turn it off, you can't turn it on for about an hour until it cools.

I have a fuel pressure gauge hooked up at the fuel rail, and here are my readings:
cold idle at 28psi, hot idle at 36psi, when revving the pressure goes up a little, then when de-revving the pressure goes down about 5psi and then stabilizes at idle. When turned off, the fuel pressure remains above 0psi for over an hour. In fact, the fuel pressure rises about 5psi in the first 10 minutes it's off, then it starts to decrease. Wierd?

I have already changed the fuel pump relay, the fuel pump check valve (which was leaking my fuel pressure very quickly), fuel filter, and I've confirmed that my fuel pump is good, new timing and balance belts and rollers, cleaned injectors using carb cleaner and air compressor, new cap and rotor. The fuel pressure regulator and damper both raise the fuel pressure by a good amount when the vacuum lines are taken off, so I assume they are good, but I did this test when cold... When I did the clutch job I cleaned the ground a little bit (I didn't know then that grounds were so important... but they are a little cleaner nonetheless, and quite tight). My theory is the DME... What else could be causing this problem? THANKS!

Mongo

the DME may be most likely on its way out soon. See if you can borrow one and do a labotomy on the car over the weekend or when you get the chance. If it still doesn't work, your ignition wires could be bad or even grounding out on the hood. Checked the coil, cap and rotor yet too?

I hope you can find your problem soon...I understand how stressful problems on these cars can be

Andy

FRporscheman

Thanks for the lightning fast reply. I'm gonna sneak into my mom's 944 on the weekend when she's not looking and "borrow" her DME for a while to see if it fixes my car. She never lets me swap parts to eliminate possibilites, but a DME is expensive. The cap and rotor are new, and the wires still have good insulation and the correct 3000 ohm resistance... I haven't checked the coil. The car will turn back on if you try IMMEDIATELY after shutting it off, but if you wait more than a couple minutes, you gotta wait an hour.

Another thing you should know: the temperature goes up to about half way, then the cooling fan comes on and cools it down to the left yellow block on the gauge. That seems like over-cooling to me... could be a bad thermostat or bad sender... but could this be involved with the hot start problem...?

FRporscheman

bump

Chris_924s

Fuel Pressure Regulator. Check that as well.
Welcome to Rennlist!

FRporscheman

When I disconnect the vacuum line from the fuel pressure regulator it makes the fuel pressure jump up to about 39psi... I assume that it therefore is good... is it possible that it's bad even though it appears to function?

I noticed that when it gets to where is starts hesitating, when it hits about 5000 rpm, it sort of bounces down to 4000 then recoils up then back down again... it's a pretty fast flickering of the rpm. Could the reference sensor be broken? I cleaned them when I did the clutch job.

billybones

sounds like a sticking fuel injector to me.. try running a bottle of fuel injector cleaner thru it.. I have never put a gauge on my rail. but I would think that it should not go up with revs unless you were putting pressure into the injector thru the compression stroke.. Just a thought.

FRporscheman

Thanks for the feedback. I just cleaned the injectors literally a few days ago, and they all functioned properly, opening and closing with voltage applied and not applied.

FRporscheman

Today I installed a supplementary ground wire from the battery negative to the fuel rail mount bolt on the cam cover. I checked the resistance for all the sensors (water temp, air temp, aux air valve, TPS, both speed sensors, and all 4 injectors too), in both hot and cold conditions, and they all checked out OK except the air temp sensor, which read 1100 ohms when hot - is this out of spec? The haynes book only gives a spec for cold (1450-3300 ohms). The hesitation at 5000rpm is still there and the car wont start back up until it's had at least 30 minutes to cool off. I can't think of what else could be wrong... What is a cold start valve, what does it do, and could that have anything to do with the problem? Tomorrow I'll to that bernz-o-matic method of checking vacuum lines, and then I'm gonna work on my diesel benz wagon... it has SIMPLE problems to fix!!

FRporscheman

I adjusted the wiper in the AFM to contact a fresh part of the resistane track and the problem *seems* to be gone! I'm going to drive the car extensively tomorrow to really put it to the test, and I'll let y'all know. I hope this is it!

But, could an out-of-tune AFM cause hot start problems anyway?

FRporscheman

After driving a lot, I found that the problem remains. I don't have to wait as long as before, though. If I try to start it up like 10 minutes after shut-down, it just cranks. Then 5 or 10 minutes later it starts up. It seems like 20-30 minutes is the required wait.

I start school in 2 weeks and I need to drive this car daily! Can someone help me out? How would I cure vaporlock, as I think that is the problem.

FRporscheman

Anyone got an idea?
Thanks everyone

TheMAXX

I don’t know the answer so this is just a guess but how about the O2 sensor? Could that cause the stumble problem?

You asked about the cold start valve isn’t it kind of like a substitute for a choke. I thought that was all in the DME. I would think if the DME were getting a reading (false I would assume) that the car is still cold it would try to make the fuel mixture too rich for a warm engine. This could also cause your stumble problem I am guessing.

So how does the DME know when the engine is warm enough to change the fuel mixture back to normal. Maybe this is what you need to look for. Maybe it isn’t getting the “I am warmed up” signal.

I am not an expert, I am just trying to think logically. Let me know if this helps at all.

PorscheDoc

Have you thought about vacuum leaks yet?

MHT

I dont think vapor lock is your problem, the fuel pressure in the lines will stop the fuel from boiling, you get a vapor lock situation when the pressure drops and the heat in the engine compartment boils the fuel in the rail and injectors. check to see if you have spark and an injector pulse while the car is not starting. Hook up an old spark plug to an ign wire and lay it on something metallic on the engine and crank, you should see a strong spark, take a screwdriver and put the tip on one of the injectors and the handle to your ear and have someone crank the car and see if there is any clicking. If there is no spark and no inj pulse I would suspect a problem with one of the ref. sensors, assuming that you have power to the DME at this point. If there is an inj pulse but no spark, the problem could be inside the ECU or could be a coil or coil wire or possibly a bad rotor. If there is spark but no signal to the injectors, the problem could be in the wiring harness or in the ECU. Good luck and let us know what you find.