Car is an early 944 that I have been putting back together for about 2 years now. The car has all new hoses, vacuum lines, belts, plugs, wires, seals, water pump, Oil cooler seals, fuel pump, FPR, fuel filter, fuel lines, resealed fuel tank, rebuilt injectors, F9 priming DME relay, etc. I have had it running for about a year now after solving some issues with a dirty/rusty fuel tank. I have driven the car about 50 miles over the past few weeks after putting the interior back together. The fuel in the car is a few months old at most (I put a few gallons in this weekend too) so I do not see it being an issue with my starting issues. I flushed some fuel into a bottle out of the rail and it was sediment free and fairly normal looking. The car had been starting without issue 80-90% of the time and settling into a good idle but it seems lately it starts and stumbles before dying 3-4 times (idling at 300 rpm) but usually by the fourth try or so the idle will raise to normal and it will operate fine from that point on. I have read through alot of threads on here and pelican as well as went through the Clarks garage info for this issue but am still having problems. I checked the fuel pressure on the rail today because I suspected it might be a fuel supply issue (FPR, check valve, ?) and got about 37 so that does not seem to be an issue. What is the next stage in diagnosis that I should try? The routine I am seeing now is it takes three or four failed attempts at starting where the car idles very poorly before dying, before it eventually "catches" and operates as it should. It could be all in my head but it feels like the problem is getting worse too. I do not notice any noticeable smoke (white, black, or blue) at startup or during driving.

 

Check the temp sensor to the ECU per Clarks Garage test procedure.
Check fuel flow from each fuel injector by removing fuel rail and placing each injector into a identical volume jar
Crank engine for several seconds. Observe fuel volume in each jar. If not all the same, have fuel injectors tested and cleaned. SouthBay Fuel Injector Service can do an excellent job. By

 

I was going to comment in the fuel direction also. If you can do a visual test like mentioned above and get some solid evidence then injectors is a place to start. Fuel pump check valve would be worth looking into. I would think that would be something more hot start oriented but then again I suppose empty fuel line either hot or cold would make the start up fussy. If it was me I would just pull the injectors and get them cleaned up. That being said I used a outfit in San Antonio called Fuel Injection Specialties. It was a hundred dollar bill plus a few cents for what ever. You can find them online .They have been there a long time and have the machine to do the job and rescreen seal Etc. If you want to send them to me Ill drive them in and drop off for ya. It took about a week turnaround but I told them no rush and got the units back sealed individually. I go to SA every 2 weeks sometime once a week so no big deal on my part. We could meet in lampasas if you want and Ill hand them off for a burger on Tuesday!

 

The injectors were rebuilt/cleaned/flow tested when I bought the car. I pulled them myself and did the battery/carb cleaner test last year when I was diagnosing the fuel tank problem. I am not saying they couldn't have gone bad or are not gummed up now but the car has very little run time since then and seems to run fine once it can get past the sputtering idle.

Here is a link to a vid I took of the problem about two weeks ago.
The car had just done the stumble to idling normal transition about 2 minutes earlier and had ran for 30 seconds without issue before I shut it off and tried again resulting in this vid where it did it again. It now typically dies after struggling to idle for 4-5 seconds about 3-4 times in a row before running perfectly on the next attempt.

 

Was doing some further digging on other sites for similar problems and of course someone mentioned having a FPR with a very slight diaphragm leak that would become more of an issue, the longer the car sat between starts...hot starts back to back were not an issue but letting it sit for minutes/hours/etc. would make it more difficult to start as enough fuel would accumulate in a sense flooding the engine. The FPR I am 99% sure was replaced right before I bought the car but of course that has been two years ago now and I guess it could be failing. There is not noticeable smoke on startup that I have seen like excess fuel burning.

 

A couple ideas:

DME temp sensor
Fuel pump check valve
Idle Air Control Valve (usually these can be removed, tested and cleaned)
Throttle Position Sensor
AFM (repositioning of the pickups)

How to check/test all of the above is documented on Clarks

 

Good input from the previous post but ya the FPR would be worth looking at. Am I correct the vacuum/line off the back side would allow inspection for a leak.

 

Ideal would be to put a fuel pressure gauge on it. A defective FPR would read pressure on the high side, but pressure reading can also tell you if your pressure is too low (bad pump/filter or restriction) and based on how quickly the fuel pressure drops with the car off, can tell you if your pump check valve is working. It can also help identify any issues with the pump if you are getting fluctuating pressure readings such as voltage fluctuations to the pump.

But yes, often the diaphrams go bad and leak fuel into the vacuum line.

 

I put a gauge on it a few day ago...about 37 psi.

 

Assuming thats the pressure while running, its a tad high (should be around 29 +-3psi), what was the 20 min leakdown pressure? Should be at about 14.5 psi. Jumpered relay (pump prime) pressure should be 36 psi +-3psi

Would be curious to see if you had a bit of fuel in the FPR vacuum line - you could have a small/partial tear in the diaphram.

 

That was just the primed fuel rail pressure before starting. I have the F9 tech priming relay which primes the system at the first step of the ignition (on) before you fire the engine.

 

Agreed on the gauge test. You can go thru the normal start up and warm up. Monitor the pressure then shut down and see what the residual pressure is over a specified amount of time. I have no idea of the specs but should be easy enough to find. On a CIS car super important. Ill assume that it should have some static pressure and slowly drop off. If you fire up with the gauge on then shut down and by the time you get around to see what you have or it drops within a couple of minutes then Ill assume thats no good. Again Im working off previous knowledge of injection systems.

 

Changed out the fuel check-valve with a spare I had on hand just to rule that out...no change. The car would run normal on about the 5th start with the other attempts displaying the usual stumbling/dying scenario. I let the car sit for a few minutes and tried again and again it would just barely run at 200 rpms or so for about 15-20 seconds at least until like before it idles up and runs like normal from there on out. I pulled the vacuum hoses on the FPR and there was no fuel, smell, or residue. I will go through the rest of the possible problems (AFM, DME temp, etc.) later this week hopefully if I have time.

 

Did you leave the fuel pressure gauge on the motor and watched the residual pressure.

 

The most common fuel pressure regulator failure is from age related stiffening of the rubber diaphragm, which causes the valve to not fully open (causing elevated pressure or spikes) or fully close (allowing pressure decay after shut down) against the fixed spring pressure. Yes they can leak externally or thru the diaphragm, but this is not commonly the first symptom of failure.
While your car is stumbling, listen to each of the injectors with a scope or long screwdriver pressed against the injector and your ear to verify signal.