Abnormally lean AFR

[California98Civic]

"Black & Green" (a 1998 Civic DX) has developed a lurching stumble associated with extremely lean AFR readings on the UG. At about 30 or 40% load at low speed this evening with the car fully warmed, I saw short-term AFRs bounding around -34% ... and then when it hit -45% I cut the then stumbling engine and coasted to a light. This problem has emerged gradually in the last week or two. It became really obvious a few days ago. And it is progressive. Yesterday longterm AFR was -9.4%, so I took off the WAI to see if it was the culprit. Stumbling continued. And today AFR has gotten steadily leaner. Longterm AFR stood at -10.9% when I pulled into the driveway.

Cold or warm, the car will do this stumbling thing that seems to be associated with the lean AFRs.

Injector failing? Vacuum leak? Fuel pump?

No CEL has lit up, and there are no pending CELs either, but I'm taking it off the road for the weekend to figure this out before I get stuck on the side of the road.

Thoughts?

EDIT: At idle, with no throttle, the AFR gets steadily leaner, 0%, -5% -12% -28% ... I observed that sequence within a few seconds at a stop sign.

EDIT 2: pulled each plug they all seem fuel fouled, and cylinder 1 and 3 smelled of gasoline in the cylinder still (especially #1)

[darcane]

How are you determining AFR with the Ultragauge? It can report commanded AFR but that normally reports on a scale of 0-1.999, not a percentage.

[California98Civic]

Maybe it is more accurate to say "fuel trim"? The UG displays a fuel trim figure based on readings from the O2 sensor. It displays as, for example, -10.9, and the manual says to read that as a percentage change from 14.7:1

[Baltothewolf]

O2 sensor getting tired? Idk. Maybe you melted something with your WAI.

[user removed]

Check to see if the fuel rail will hold pressure. Could be the pressure regulator is leaking into the intake manifold through the vacuum port (bad diaphragm). The 02 sensor is trying to lean the mixture out but the fuel leaking past the regulator diaphragm is overwhelming the feed back system with unmetered fuel.

If the fouled plugs are closest to the vacuum port to the pressure regulator then that is more evidence to support that conclusion.

Disconnect the vacuum hose to the regulator and look for leaking fuel coming from the regulator diaphragm. Turn the ignition on to run the fuel pump, but you might not want to start it.

If you find that my scenario is correct then you might consider changing the oil and cleaning the plugs. You could reuse the filter.

regards
mech

[darcane]

Originally Posted by California98Civic

Maybe it is more accurate to say "fuel trim"? The UG displays a fuel trim figure based on readings from the O2 sensor. It displays as, for example, -10.9, and the manual says to read that as a percentage change from 14.7:1

Ah, OK, that makes more sense.

Fuel trim doesn't directly tell you what the AFR is doing, it tells you how long the fuel injectors are being held open relative to how long they should be held open for the current conditions (load, air flow, intake temp, etc).

So, a negative fuel trim means that the O2 sensor is reading rich, so the computer is cutting fuel to compensate for it.

[user removed]

The 02 sensor is detecting the massively rich mixture that is fouling the plugs. The ECU is driving the injector duration cycle shorter trying to compensate for the raw fuel being sucked into the mainfold. It would probably run better with a failrly massive vacuum leak. Clamp off the vacuum supply to the fuel pressure regulator and see if it runs better.

regards
mech

[California98Civic]

Originally Posted by Old Mechanic

Check to see if the fuel rail will hold pressure....

Thanks, as ever. I went out and rented the pressure tester based on this advice but probably can't do anything tonight with it. But I checked the FSM and the regulator is closest to the number 4 cylinder, which means it is as far from the 1 and 3 as it could get.

Originally Posted by darcane

...So, a negative fuel trim means that the O2 sensor is reading rich, so the computer is cutting fuel to compensate for it.

So then in the scenario OldMech describes above, the computer is radically cutting back how long the injectors open because the O2 is detecting too much fuel due to pressure regulator diaphram leaking into the intake manifold. There is clearly unburnt fuel in the #1 and #3 cylinder (I can smell it when I pull the plugs out).

Thanks! I'll run tests tomorrow and report back.

[user removed]

I think you are reading the ECU trying to lean out the mixture based on the 02 sensor reading being way too rich. The "trim" is telling you that it can not lean itself out enough to get the 02 sensor readings anywhere near close to specs.

The rich mixture is confirmed by the condition of the spark plugs. If you clamp off the vacuum source to the fuel pressure regulator you will eliminate the unregulated fuel entering the manifold and it should clear up fairly quickly.

Keep us informed of any changes.

regards
mech

[user removed]

Quick test, if possible, just pull the vacuum line off of the regulator and turn on the ignition and see if fuel is coming out of the vacuum line. Better done with the engine off.

regards
mech