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Hello Everyone, I wanted to bow my head and humbly ask for help with my otherwise great 83 Black on Black 944. The car has me completely stumped. I bought the car as non running project car and started to sort it out. The car had a few problems initially one of which was that it would cut out with application of throttle, if you cracked the throttle, it would rev up to a point cut out, die down to a lower RPM, cut back in and rev-up, then cutout again and repeat...sort of a rev....pause.....rev....pause etc. cutting out about 2-3K and cutting back in about 1500RPM. I knew I had to fix this but I put this problem to the side as the bigger problem was a fried clutch and leaking head-gasket.
I did an engine swap with a used-tested engine and replaced the clutch, did a water pump and belt service, so I know the timing is correct. I also setup the gaps on the speed and reference sensor set while the bell housing was off. Got the engine in and still the hit and miss running. And when I say hit and miss, I mean like you cut the ignition off and on.
Since that time I have checked and verified the following with no correction to the hit and miss.
Fuel pump - checked OK - pressure at injector manifold 30psi idle and 40-45 with throttle.
Fuel filter - replaced both external and in tank screen
Fuel pressure regulator - was leaking a bit of fuel in the vacuum lines - replaced with a new one
Fuel injectors - replaced with new
Ignition - new plugs, wires, cap and rotor - ignition isn't cutting off during the hit and miss....verified with ignition light and has HOT spark.
New Battery and cleaned ground cables.
Replaced AFM with rebuilt one - no change in symptom or problem
Swaped DME with tested rebuilt unit - no change in symptom or problem
Changed DME harness with a different one,
New speed and reference sensors
I've checked the speed and reference sensors at the DME plug and get correct static Ohm readings and correct dynamic voltages
I've tested with and without TPS connect (was a different one with the replacement engine) no change
Tried unplugged TPS and no change in hit/miss (worse idle (higher) when unplugged).
Changed the temp sensor - no change
The car starts right up and settles into a great idle and will idle all day long, slight throttle crack and it revs, big throttle moves and it hit/misses. With all the new parts it is running great up to the cut-out. I have tried hooking up a noid light to one of the injectors and can see it pulse dim/bright in relationship to the cutting out, so it leads me to believe the injectors stop firing. I also just started the car with the alternator unhooked to ensure it wasn't an electrical noise problem, no change what so ever.
I'm going to bypass the alarm module tonight to rule that out. Any help would be dearly appreciated.
Google Clark's Garage Fuel Test Procedures. The fuel pressure at idle should be about 35 psig and when blipping the throttle, the pressure increases about 2 psig.
If the stated fuel pressure in your OP is in 45 psig at throttle, that is too high. This higher fuel rail pressure provides too rich a fuel mixture that can cause the hesitation.
My 944 race engine experience d the same situation and my fuel pressure regulator had failed closed. The thread states a new fuel pressure regulator was installed. Is it the correct one? Maybe the fuel damper valve is not functioning properly or the vacuum lines connecting the fuel damper and fuel pressure regulator are leaking or not connected to the intake
Hi...thanks for the reply. I double checked the pressure at idle I'm getting 30psi on the gage and above idle getting up to maximum of 39-40psi. Given gage error and viewing angles these align with the specs at idle (29psig +/- 3psig). The 40-45 i noted was during the cut in/out cycles and per Haynes with no vacuum it should be around 39 psi However I thought about your post and verified what I was seeing. I think I was just seeing gage needle bounce during the cutting in and out cycling. Here's what I did to test the theory. I first disconnected and plugged the vacuum line to the pressure regulator and tested at 39psi - no difference in cutting out. I then hooked up my vacuum hand pump and pulled a vacuum on the pressure regulator to get a fixed fuel pressure of 30 psi, again no change in cutting in and out. I tried midway around 35psi again no change.
I then got a noid light and hooked it up to one of the fuel injectors and noted that it dimmed in cycles that matched the cutting out...I think what I'm seeing is a loss of injector pulses causing the cutting out. Therefore I'm thinking it is an electrical problem. I'm in process of removing the relay panel to clean the under dash grounds in hope it's a grounding problem limiting how much current gets through the injector circuit. Who ever designed the rat nest of wires down there is a sadist, what a nightmare just to get to the grounds.
Hi....the problem existed with the old and new fuel injectors both. It starts and idles better with the new ones, no surprise. Yes I swiped a complete DME harness with fuel injector wiring and it made no difference. I pulled the relay panel down last night and I can see green corrosion on the ground cluster in the far back left corner. I'm going to clean and address that today.
Go to Pelican Parts and find the electrical schematics for the wiring harness to the DME. The grounds at the fuse panel does not interact with the DME fuel injector pulse circuit. To verify it, remove a fuel injector plug from on injector. Rotate the ignition switch to energize the DME. With a volt meter, measure the voltage to ground from each of the 2 pins. The voltage should be 12 volts for each pin to ground.
With the engine cranking and running, the DME pulse one of those two pins from 12v to a "generated ground" which is a ground wire connected to the bell housing. So the switching of this connector from 12v to 0 volts or ground while the connector stays at 12v generates the pulse. At higher rpm, the pulse occur more frequently
Maybe there is unmetered air leakage into the intake manifold, especially all the connections into the J boot. Does a hose have a crack in it or disconnected from its source?
Hi all, I wanted to share with you what was wrong and how it was corrected. When I first got this car the AFM had it's cover missing and the wiper arm was bent and crooked. So one of the first things I did when I got the replacement engine installed was to replace the AFM with a rebuilt unit from one of the online Porsche parts resellers, an AFM that was remanufactured by "Fuel Injection Corp". After chasing gremlins for a couple of months, I kept thinking what could be causing my injectors to shut down off idle......It haunted me. I chased wiring harness, DME, grounds, injectors, etc....nothing fixed it. Just for grins, I bought a second hand used AFM to try and it solved the problems I was having with the car. I put the used AFM on and instantly it was fixed. The rebuilt unit was the culprit, as it is either defective or the wrong unit for my application. Strange thing is the cover has the right Bosch part number but the "Fuel Injection Corp" part number isn't the one they show on their website. Their website lists their part number for my year/model as 412028 however as the picture below shows, this unit as the number 28921 on the label. So what's up? I don't know but I"m going to write them and let them know how this hosed me up for months, and give them my personal thanks.
Thanks to everyone who took the time to help me and I appreciate it very much.
11-22-2018, 08:26 PM
83 944 Cutting Out - Driving me crazy
