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Fuel Rail Pressure drop under load

  • 1983
  • 944
26 380
Fuel System
New Jersey

I've got an 83 944 that i've been trying to get back in running order after it sat for quite awhile (the inspection sticker is from 91). I've done a ton of work to it the last few months but I've been battling this one issue for quite awhile. 

When you put a load on the engine, it will go to a point and then it will start stumbling and cut out. The engine does not stall or stop running, it basically runs like its running out of gas, but as soon as you let off the gas it will go back to idle and run perfectly. I was able to get fuel pressure gauge on the rail today and what seems to be happening is it gets to a point and the fuel pressure drops into the teens. <a href="">I took a video to show the pressure drop.</a> When I get to a certain point the pressure drops but as soon as I take my foot off the gas the pressure comes right back. 

Just to make this a little weirder,today I made some changes to the vacuum hoses (yanked all the hoses that go to the evap system and capped any openings), and afterwards the car ran great. Took it out for about an hour and half ran perfect the whole time, revved all the way and everything. I stopped and shut the car off for about 30 mins, started it, probably idled for about 5-10 minutes then I left and the car went right back to doing what its been doing for months.

Here's the long list of things that have been replaced/checked (twice in many cases):

First off timing belt, balance belt, pulleys, water pump, and thermostat have all been replaced. Never been much of a gambler.

All vacuum hoses are new silicon, or factory replacement, including the J-boot. I do have a small vacuum leak at the AOS (not the engine seals but an internal AOS o-ring). Throttle body seals have been replaced as well as AOS seals.

Fuel Pump is new (aftermarket, not sure of brand), as well as filter (mahle), injectors(rebuilt from pelican parts), and DME relay(ATE I think). In tank screen was cleaned and a full can of seafoam has been run through the car, with new 91 octane fuel.
Ran a fuel pressure/delivery rate check per clarks garage, engine off and idle pressures were spot on and delivery rate was well over spec.
Edit: Also when the car was running I pulled the vac line off the pressure regulator and the fuel pressure jumped to the high 30's.

Throttle Position Sensor checked and passed (per clarks garage procedure)

AFM & AFM temp sensor checked out ok (per clarks garage procedure)

DME Temp sensor checked ok (per clarks garage procedure)

Cap, rotor, ignition coil, plugs and wires are all new (Bosch)

Reference Sensors are both new and have been adjusted (bosch, clarks garage procedure)

Oxygen sensor has been replaced (Bosch) and was not soldered in (per clarks garage instructions)

Engine and battery grounds have been cleaned.

If anyone has suggestions I'm all ears. Today was the first time I got to take the car out and really drive it and I want more!


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