Abnormally lean AFR
 

"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)

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.

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

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

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

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.

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

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.

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.

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

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

Clamp off the vacuum line to the regulator and see if it runs better. It won't run perfect since the regulator requires vacuum to make it run lean enough but even with that it should run a lot better than with raw fuel running into the manifold.

The plug not fouling worse nearest the regulator could also depend on how close it is to the idle air circuit.

regards
mech

https://www.youtube.com/watch?v=mSxpDCr4WCo

lousy video

regards
mech

Rented the proper gauge from O'Reilley and the fuel pressure regulator failed the test specified in the FSM. I bought a new regulator for $80 (with tax) and installed it. I took it for a test drive, and the engine ran normally. My DD is back in action for tomorrow morning's commute. :)

Thanks all, and especially OldMech, who nailed this diagnosis perfectly.

Kudo's to OldMech for "...tele-commuting..." diagnostics, par excellance!

Glad it worked out but don't forget to make sure the oil is not diluted. We used to check it by smell and when it was real bad you could light the oil on the dipstick on fire and it would burn.

If it's close to time just drop it and refill with fresh oil.

Your tank mileage will suffer severely.

On the first FI Z cars in 1975 Nissans system was licensed from Bosch. They had a flap air flow meter and all the injectors fired at the same time (not sequentially). Two things would flood that engines out in a very short time.

Open circuit in the water temp sender+massively rich engine died almost instantly. Jump the connection to the sender to daignose.

Fuel pressure regulator diaphragm ruptured, ran very rich but usually would keep running if you flogged the crap out of it.

Saw a live show of the Jersey Boys tonight, tickets courtesy of the two step daughters Christmas present. Great show for those who lived in that era.

regards
mech

Nice work Mech:thumbup::thumbup::thumbup:

Undoubtedly, the fuel pressure regulator change fixed the problem [EDIT: fixed it for a little while]. For two weeks almost everything has been normal, but slightly longer starter motor turning before she fires-up. And I think I was in a little denial, because all the previous issues that started this thread just came back this afternoon with a vengeance. Suddenly, the car sputters, the short term fuel trim leans-out steadily during a pulse at 75-80% load. It will get as lean as -28% short term fuel trim in a single pulse. Does that every time, over and over. Go into neutral and a new symptom is that it will sometimes stall. Stalling events sometimes are preceded by an idle speed that is unsteady: rpms 180 and then 500 and then 900 and then 700, 900, 800, and then stall or... you get the idea. Sometimes it does that and then keep running. The one thing that never returned quite to normal after replacing the fuel pressure regulator was the starting cycle, which required slightly longer cranking almost every time. Pre-priming the fuel pressure with the key only sometimes seemed to solve the problem. I have been meaning to look into it, but there are always a few things with this car, and I gotta live and work too. Haha! Back to testing...

Any ideas?

Perhaps the WAI is the issue, since that's when the problems began.

If so, then it must be because of air restriction. the IAT temps are not higher in an absolute sense from temps I have seen for years. Today was still a little under 130 degrees. I have seen 145+ with previous WAI configurations. But those were bigger diameter tubing. This is the first that is more restrictive than stock. But even if it is the WAI... why take weeks for a problem to form. Wouldn't the engine react quite quickly?

Perhaps it causes some damage that takes a few weeks of driving to get worse.

Idk, but if it was me I'd remove it and see what happens.

Recheck the fuel pressure, could be a #$%^&*()_ pos replacement part. Check the plugs to see if they are dark.

regards
mech

Did that previously... and no change

Yeah, that makes sense. Try again...

james

Well if it hasn't been on, then that's obviously not the problem. :cool:

Testing today. Got much different pressure readings. With regulator vacuum hose off and pinched. Car not running. Not warm. I primed the pump and got 10-20psi, when 38-46psi is the spec. Two weeks ago, this test showed 42psi repeatedly.

So then I reattach the vacuum line. The FSM says pressure should climb slightly, but it didn't. However, two or three priming cycles with the key and it climbs close to the normal range.

Also unlike two weeks ago, the pressure quickly drops off, if I get it even close to normal, whether the vacuum line for the regulator is connected or not.

What the hell?

It could be a bad fuel pump, odds are bad regulator, replace and retest. Plug check?

regards
mech

I ran the test again just now, and now I get no fuel pressure at all in the first test. Zero. I can hear the pump prime. Could a failing pump create the symptoms I have seen? too much fuel causing trim to get leaner, then too little fuel causing a stall?

Seems I have successfully tested fuel pressure gauge failure. The gauge is not working. I can run the car while the pressure gauge shows zero fuel pressure. Lol. Gotta go get another. This car is parked for a day or two more. Time to bum a work ride for tomorrow morning!

Geez, we had a Belden Drum Doktor (brake drum only lathe) at my old shop that was probably older than I was and it worked fine, lots of equipment around here dating back to the Arsenal of Democracy of 70+ years ago. Glad I don't have to deal with the frustration of parts AND tools I can't trust.

regards
mech

I disconnected and returned the aftermarket fuel pressure regulator too. I'm confused about whether the pressure gauge ever gave me an accurate read, which could mean the OEM regulator was fine and the gauge lead me to think it was not. Because I could not test the replacement regulator, I don't conclusively know if it was defective. So there remain three possibilities to explain the symptoms: (1) bad regulator, (2) bad regulator and a secondary problem too, or (3) the regulator was fine but something else entirely was responsible for the symptoms. Frustrating how many variables and unknowns there still are. Oh well. Scrounge the rides I need!

I wouldn't say this if you had a fuel trim code, but sometimes the extra info is more like extra noise. About a year ago I had an unstable idle, saw my fuel trim sitting around -15%. Looked for something causing me to be running rich, but I found and repaired a vacuum leak. That fixed idle issues, drove / ran fine, but fuel trims moved up to -22%. I still don't know why my fuel trims are so far off.

Yeah I hear you. I'm grateful for the advice. Looks like Honda tech is going relatively silent on these older Hondas, now that the k-series is well established, with a good aftermarket culture, and wide sales. Too bad for me. Thanks!

After another lost day to a second non-functioning fuel pressure gauge rental kit, I got to retest with a new test kit today. I think I did my first test of the fuel pressure incorrectly. The FSM does not say, but I now think I needed to be running the car when I tested the regulator. With my second new regulator installed, I ran the engine and tested. All pressure normal. Longterm fuel trim slowly returned to normal. I cannot retest the OEM regulator, but I also can't say for sure it failed the test I thought it failed. I also replaced the fuel filter, since it is maybe 4 years old and I had the assembly partly dismantled anyway.

I'm going to drive the car, with the new, more restrictive, WAI removed so I can see if the problem will return.

If the problem returns, I will know to check for vacuum line leaks too.

James