Puzzle: vacuum leak, injector crud, or O2 sensor fail?

[ksa8907]

Im curious of your fuel trims. Negative numbers mean the ecu is trying to lean the mixture and positive mean it is trying to enrich the mixture.

So you have a problem with negative fuel trims indicating a rich condition. The ecu is detecting too much fuel or too little air as compared to the amount of fuel it expected to need.

I would suspect bad injectors or leaking injector o-rings.

Map sensors are cheap and personally, i would replace that asap to rule it out.

[Chrysler kid]

It could also be a dirty fuel strainer or weakened fuel pump

[ksa8907]

Originally Posted by Chrysler kid

It could also be a dirty fuel strainer or weakened fuel pump

Remember, negative trims mean too much fuel.

[California98Civic]

The fuel pump tested normal.

Would a dirty strainer fail intermittently? Would injectors or their o-rings leak intermittently? After two or three rainy days in which normal function resumed, the problem has suddenly emerged agajn and with a vengeance. Return of the sun, warmth, and dry conditilns has meant a return of my fuel trim wackiness.

Part of the difficulty is knowing I cannot necessarily count on the sensors, meaning that the ECU might be cutting fuel based on false data from the MAP Sensor and/or the O2 sensor, right?

Bah! I gotta start testing stuff. Thanks all.

[user removed]

Have you checked every ground. Grounds usually fail when it gets dry. Fuel pressure good?
All testing must be when it is in failure mode, obviously.

Have access to another CPU?

I had a customer with a 75 Z car. " I drove all the way to Jacksonville Fla, where the car died. It restarted after a while, ran rough, until I revved it up a few times, blew some black smoke, then ran fine ever since."

I had the 5 most likely suspects in inventory, installed them all, then removed one at a time over the next 6-8 months and it never acted up again until I reinstalled the original CPU.

Maybe next time it messes up spray some water on the engine compartment and see if it clears up with the added humidity.

Run an additional ground wire from the cpu to the negative batt terminal.

regards
mech

[user removed]

Also spray some water on and around the plug wires at night,see if you see any sparks, indicating bad plug wires.

regards
mech

[Chrysler kid]

I would check your exhaust manifold. Is it still the stock one?

[pgfpro]

Originally Posted by California98Civic

The fuel pump tested normal.

Would a dirty strainer fail intermittently? Would injectors or their o-rings leak intermittently? After two or three rainy days in which normal function resumed, the problem has suddenly emerged agajn and with a vengeance. Return of the sun, warmth, and dry conditilns has meant a return of my fuel trim wackiness.

Part of the difficulty is knowing I cannot necessarily count on the sensors, meaning that the ECU might be cutting fuel based on false data from the MAP Sensor and/or the O2 sensor, right?

Bah! I gotta start testing stuff. Thanks all.

This sounds silly but what is your ground wire like (condition) on the intake???

[2000mc]

In the process of replacing the fuel pressure regulator, is there a wire harness or connector that you move, or even touch? If so try to figure out if / which wiring that could effect fuel trim is there. Then with the engine running and monitoring fuel trims and anything specific to what might be in the harness there, move the harness around / push/ wiggle connectors.

[California98Civic]

Car ran so poorly by the time I got home today it would shudder as I accelerated, fuel trims got as lean as -29%, and in the driveway I was able to idle it until it stalled. The computer took the car out of closed loop and ran it in open loop. So I got out the mitivac vacuum gauge and my scan tool, figuring I would measure vacuum mechanically and via the MAP sensor in inches Hg simultaneously. But first I tried three suggestions from this list: I hunted unsuccessfully for a ground wire on the intake; I sprayed water onto the hot engine to mimic rainy day humidity (no improvement); and I wiggled/pushed every wire and connector I could remember moving or brushing in previous efforts at repair. I also wiggled a couple others in the EVAP canister (I think).

That wiggling suggestion worked! The car runs perfectly normally again.