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If you can, since you have the gauge set up, I would make a DME jumper, so that you can run the fuel pump without the engine cranking, and get a better idea about the fuel system, without the other systems also being "in play". The setup that I like has a fused positive wire and a momentary switch so that you can control the power to the pump yourself. I don't think that you want to run this test and have the fuel pump just run continuously.
From all the parts you described above, the fuel system seems OK.
You're working on spark now, and the noid light seems that you've got that, but maybe there are tests to be run on other parts of that system, as well. For example, check your grounds again, as that's how the DME fires the injectors (by grounding them, I mean).
Finally, there's the air portion of what you need for a motor.
It's a pain, but I think that if you're systematic in what Clark's Garage suggests, you'll narrow things down. In the end, it may indeed be the DME, but that's really hard to check without a spare to swap in, or another working 944 that you can put your DME into for testing.
when I drained the fuel line I connected a small 12v battery to the pump and purged the return line before draining the tank for the umpteenth time. At some point refilling the tank and draining the rail at the connector.
What I'm saying is I'd much prefer to lay on the ground then try to get under the dash again. My knees and my lower back are pretty well used up.
That all aside im willing to check whatever could be the issue but at this point the fuel system appears to be functioning properly.. the only thing I have not checked is pulse width and I would need a reference to compare with my o-scope readings. Seems much easier to buy a dme for 300 and swap in.
I'm currently working on the AFM, trying to figure out what the hell this flathead screw is in my AFM.
I know it's supposed to be a cover and under it a 5 mm allen air adjustment screw but it's stuck in there with an o-ring I can feel it, it does thread up and down though..AFM has a build date of 1981..
I don't know about the AFM; messing with it gives me the heebie jeebies.
One thing that occurred to me, rereading what you've gone through, could there be a problem with your fuel return line? I'm trying to think through what would happen if your fuel was not making the full loop back to the tank. That almost surely can't be the issue, given that your fuel pressure looks OK.
The other thing that I don't think that you've mentioned is anything about your crank position sensor; have you looked at that? If it's original, I'd put it on the list of suspects.
AFM adjustments don't bother me I have a dyno shop who can help tune it to stoich and I can also install a wideband if needed. Wouldn't be my first time.
The screw had no affect, in or out.
Could be crank sensor, is there a cam sensor? I am reading about a speed sensor sometimes. Not sure how that can affect idle. This whole ordeal makes me think about carburetors and ITBs...
Not that I have ANY experience in this area, but I note the following:
1) Fuel Pressure while pump running good at 34-35 PSI.
2) The engine is running, so the injectors open.
2) Fuel pressure holding at 15 PSI when off, so the injectors close and hold pressure, though not at 35 PSI.
From this, we can conclude the following:
a) Fuel pump and fuel pressure regulator working.
b) Neither the fuel pressure regulator, nor the fuel pressure damper, are leaking into the intake as the fuel pressure would quickly drop to zero when stopped.
c) The injectors are not leaking.
So, why is it too rich?
x) The injectors leak at high pressure.
y) The injectors have an issue, either too slow closing (unlikely) or are the wrong injectors.
z) The o2 sensor is bad, telling the ECU to make the mixture richer.
From years of working on 944s/924s and other cars, please don't start down the rabbit hole of throwing parts at it. (You mentioned just buying a new DME without knowing if that is the problem).
Clark's Garage will help you test methodically. Write stuff down as you test, or document it however you want. But document. Document dates, readings from test equipment, or just written notes about what problems you are experiencing, what you did to test, and what the results were. Maybe a spiral notebook or a diary type journal.
When you document, you will be able to point to hard facts rather than your memory. After you start going in circles, you can look back and read your notes with dates and descriptions, and you'll be miles ahead of someone who jumps all over the place trying this and that.
Another comment you said it's frustrating not being able to buy 944 parts at your local parts store. These cars are old. Even when I had a nine year old 924, the only place I could get parts besides the dealer was mail order or a German car parts house in Houston (BAP/GEON). In a city as big as Houston (4th largest city in the US at 2.2 million in city proper and 7.3 million in greater Houston area), only one parts store had any parts for my 924, not counting the Porsche dealer.
I know it's hard to be patient, diagnosing, chasing leads, waiting for parts ... but these cars demand patience, or you will learn patience by the end of working on one. Take your time and enjoy the learning experience that it is. You'll be a better mechanic than when you started.
One more thing, I find it useful when I document, to be detailed ... description of problem or test, date, time, temperature outside, humidity, raining or no, voltages (if testing that), pressure readings, working on a car that just ran 30 minutes ago or hasn't run since ______ date. You will forget a lot that you have done later. You can't really over-document.
I am working on a few AC systems on different vehicles (I'm not a mechanic by trade), but I have been documenting fairly well, and it has helped me track problems over time and pointed me towards diagnoses easier than if I never had written anything down. I find it especially helpful to have readings from things when they are working, like fuel pressure readings, compression readings, o-scope readings from speed and reference sensors, etc. Later, when something goes wrong you can have baseline readings from when it was working.
Last edited by Chalt; 07-01-2023 at 12:53 AM.
Reason: Added info
I think I stated in the first post that I sent the injectors off to RC injector service and they came back with a perfect flow rating.
I'm beginning to think its an electronic timing problem. Like a crank sensor... if you listen to it run in the injector noid light videos they sound about the same as the fuel pressure test. Let me upload or find that one https://youtube.com/shorts/3Mjk05jJ9eY?feature=share3
I really hope I found it. when I put the new spark plugs in I inspected the wires, took off of the cap, and looked at the rotor and everything looked fine..but right now I just tested everything with a multimeter and cylinder 3 which looked the wettest - has zero continuity. I make more continuity through my body on my craftsman multimeter. The cap makes zero continuity until I scrape the probe at the contacts inside. And the same goes for the rotor and the wires.
Well I drove all around so cal about 200 miles and I replaced the wires, cap and rotor and there's almost no change which is just blowing my mind.
I rushed under the hood with it sputtering and I tried to give it throttle by hand (first time under hood with it running at all) and it moved a few hundred rpm then died.
After it died, the fuel pump kept running. I think thats incorrect. What would keep it running after it died? The computer does that right?
I'm not positive, but it sounded like the injectors were spraying wide open too. I tried to take a video . Its coming
The wire harnesses are getting old now.
Without a wiring diagram handy.... I don't know for sure, but possibly (probably?) the injectors are pulled to ground by the DME to fire.
If so, and there's a short to ground, that'd hold the injectors wide open.
As for the fuel pump continuing to run... got nothin' for that.
The wire harnesses are getting old now.
Without a wiring diagram handy.... I don't know for sure, but possibly (probably?) the injectors are pulled to ground by the DME to fire.
If so, and there's a short to ground, that'd hold the injectors wide open.
As for the fuel pump continuing to run... got nothin' for that.
with the noid light plugged in when the engine stalls the injectors are not told to fire the noid light shuts off. Yet the fuel pump continues to run
Originally Posted by Jfrahm
Did you check the AFM and DME temp sensor ohms at the DME connector end?
would either of those make the fuel pump run after the engine stalls?
06-30-2023, 08:20 PM
