I am having an absolutely insane issue with my 88 vert, my car will only idle with the fuel pump off, no power no ground completely disconnected, whenever I turn the fuel pump on or plug it back in it will chop idle, sputter, backfire, dump black smoke and completely shut off if I dont hold the throttle, but it'll idle perfectly with the fuel pump completely disconnected.

It will not rev up when the fuel pump is off it will immediately drop and stall but will hold idle completely fine

When the fuel pump is on it will catch and chop up revs or cut and stall, it will not hold idle at all.


If anyone has any ideas, please let me know. I haven't done anything to it to cause this. It's been sitting for a couple months but it's pretty modified but it is still a NA S4 with a stock ECU.

 

1) check your fuel pressure with the pump on and off, likely some pressure in the tank is circulating fuel somehow.
2) you likely have an engine control sensor issue, like AFM that is telling the engine it needs way more fuel than it really does. a broken coolant temp sensor can also cause the engine to run rich. with the fuel pressure low or nonexistent the injectors can stay open enough for the engine to run semi normally with perhaps no or near 0 pressure but any throttle inputs should cause it to bog and stall out.

 

That's makes alot of sense, i do notice a very weird "hiss" that comes from around the front rotor secondary injector that will immediately stop if I unplug the AFM, it's just strange because I've done nothing to the car as it's been sitting for a little over 2 months because I snapped a axle, but beforehand it was running perfectly fine.

 

i would probably recommend checking the AFM signal while doing a sweep of the flapper to see if the voltage is linear and appropriate. there is tests for the AFM in the FSM. could also be something wedged inside it and holding the door open, a visual inspection with the air filter out should answer that.