'91 First CEL at 69k (stock), code #44

[e-rod]

Well I guess it was only a matter of time for it to happen to me, I was just hoping not this early into my ownership (less than a month)! The car is a '91 NA1, bone stock and in amazing condition. Single owner, have all the service records since 1994, just got it back with a clean bill of health from my trusted Acura dealership who has a dedicated tech for these cars.

The other night I was coming back home on the highway and got the dreaded check engine light. Pulled off to check fluid levels and everything looked good, no noticeable oil or coolant leaks. Let her rest for about 10-15 minutes and started her back up, no issue so I babied her home staying under 3k rpm.

Used the service connector I checked for the blink codes, 4-4 "Rear Fuel Supply" issue. Thinking back, it makes sense since the light came on while I was trying to accelerate on the highway. So perhaps my fuel pump/filter are starting to go, causing issues delivering fuel. I'm already on track to have them replaced in the coming weeks, not too concerned there.

This morning is the first time I started her up since, with the back glass open to get a better view/listen. The engine sounds more "chattery" to me and sitting inside feels slightly rough compared to what I remember. Granted I don't have much time with the car yet to know its usual behaviors, and maybe it's in my head now... and this is a cold start compared to running warm. It might also be the ECU is in some kind of safe mode still, as it smells a bit more rich than I remember. I know O2 sensors are something which go bad on these cars, and the owner did replace one of the rear ones some years back.

Perhaps I am a bit of a worry-wart, but my biggest fear is the engine blowing up because of me doing something dumb (or neglect). The "clacky" sound of the engine idle makes me think valves or injectors, and in my head I'm damaging the poor thing the more I run it (even just idle) which may not even be the case.

I already have an appointment next Monday with my service center for the PCV valve + hose, fuel filter, spark plugs, and MT fluid. Is the car in a state where it's safe to drive it ~15 miles to there, or should I just have it towed?

Should I reset the ECU via the clock fuse and see if it relearns fuel trims or leave it be? I want to get to the bottom of the issue for my own sanity but I also don't want to do anything I shouldn't so I appreciate all and any advice!

 

[Old Guy]

It might be a fuel pump or filter or a fuel pressure regulator issue; but, I would expect those to cause problems on both the front and the back cylinder banks. A fuel pressure test is relatively easy and always a good first step to find obvious problems with those three candidates. Since it was a heavy load issue, you might want to check the fuel pump resistor and the associated wiring to make sure that the resistor is getting by-passed at high engine load. The resistor generates heat and the wiring can get cooked on older cars.

Given that it is only 44, I am thinking perhaps it is an injector(s) problem on the back or a back O2 sensor again. If you also had a code 46 (fuel metering) then the fuel pressure, MAP sensor, injectors and O2 sensors could all be in play. If you had an OBDII car it would be easier to figure out what might be going on because you could look at the O2 sensor voltages through the OBDII data port. That might give you a ready indication of a dead O2 sensor. You could also look at fuel trims. Trims are not accessible in pre OBDII ECU; but, O2 sensor voltages are accessible - with a fair amount of effort. You have to create an intercepting wire harness and have access to a data logger or oscilloscope to directly measure the sensor voltage at the sensor plug. That is a fair amount of work and requires stuff that most people don't have.
If you had a wideband O2 sensor with a tail pipe sniffer

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you could shove it up the tail pipes and check your AFRs to find out whether they were drastically out of whack. If the AFRs on an engine that is up to temperature (in closed loop) are hanging around 14.7 then things would appear to be mostly OK. Certainly no potential for engine damage.

As a first step, I suggest you reset the ECU and see if the error reappears just to confirm that it is not a one-off. That is kind of service manual standard procedure. In the case of the 44 you start the engine, let it come up to temperature and then 'fast idle' it at 3000 RPM for two minutes. If the problem still exists that should initiate the CEL again.

If the CEL returns, I am thinking its injectors or O2 sensor. If the injectors are original / never been cleaned a trip to RC Injector Services to have them cleaned would never be a bad thing. To be definitive about the O2 sensor you would have to do the voltage measurements. I expect most people skip that and just replace the O2 sensor to rule it out as a cause.

Since you mention what appears to be a noisy engine, if the valve clearances have strayed that can affect engine operation as can a timing belt job with a 1 tooth out problem. Make sure that your engine noise is not sticky LMAs because no amount of valve adjusting is going to fix that and sticky LMAs are primarily an annoyance as opposed to a performance issue (although they might become a wear issue at some point). If the timing belt is moderately fresh then you might want to have the service guys check the valve clearances if there is no record of that ever being done (if a TB is in the near future then wait until TB work is being done).

 

[docjohn]

If you are on original injectors you are likely running lean..

 

[asylum]

High chances that it is just the O2 sensor. I would not touch anything else before.
Clacking sound when cold could be the LMA's sticking if they have not been already changed for the later spring type. Something to do during next timing belt replacement.

 

[e-rod]

Thanks everyone for the advice, I greatly appreciate it. Gone ahead and ordered the Denso O2 sensors for all four (234-4065, 234-4093, and 234-4209). Will have them replaced soon out of caution.

I reset the ECU this morning and let it warm up and re-learn idle from a cold start (~40*F this morning). The clicking noise is absolutely the LMAs, I saw Kaz's video and have the exact same noise. Have ordered the upgraded spring LMA kit for the future belt + WP service.

Did a fast idle at 3k rpm for about 60 seconds and the CEL didn't come back on. Will take her for a ride around town shortly to see if it comes back under a little more spirited driving.

The one thing that still stands out to me is a slight "grating" sound when the rpms fall, near idle speed. It's more noticeable if the revs fall faster (like a quick sudden lift versus gradual). I've attached a video with the audio (you can hear the LMAs at idle at the end).

I've come to learn that this engine is more "noisy" under normal operation than the F22C I'm used to but I don't want to miss something that needs attention.

Good part is it seems like whatever it is can still be driven around and taken in for service as planned.

EDIT: After doing some more driving around it almost sounds like there's a potential exhaust leak. I'm hearing a kind of wispy/airy sound when lifting throttle that isn't what I remember normally being there. I noticed a minor backfire (still original cat/exhaust) if I rev it to 4k-5k rpm quickly and lift throttle, so I do think it's still running rich. Unfortunately the 91 models don't have any misfire detection to throw codes if that's happening. No check engine light has returned.. yet.

It drives fine though, I don't feel any hesitation or otherwise bog at low rpm. The engine sounds fine otherwise and idle doesn't seem overly rough. I guess, like any problem, I wonder why it seemed to happen suddenly but with a 30 year old car... what can you do. Will have my tech take a look.

 

[Old Guy]

Thanks everyone for the advice, I greatly appreciate it. Gone ahead and ordered the Denso O2 sensors for all four (234-4065, 234-4093, and 234-4209). Will have them replaced soon out of caution.



EDIT: After doing some more driving around it almost sounds like there's a potential exhaust leak. I'm hearing a kind of wispy/airy sound when lifting throttle that isn't what I remember normally being there. I noticed a minor backfire (still original cat/exhaust) if I rev it to 4k-5k rpm quickly and lift throttle, so I do think it's still running rich. Unfortunately the 91 models don't have any misfire detection to throw codes if that's happening. No check engine light has returned.. yet.

Since your car is a 1991 (and since you are obviously running an original ECU) it is not OBDII and you have ordered two unnecessary O2 sensors. The pre OBDII cars only have two O2 sensors, the primary sensors on the front and back cylinder banks. It is only the OBDII cars that got the extra two secondary sensors on the catalytic converter exits. The front sensors you ordered should be OK. I think the C30 and C32 used the same front sensors up until 1999. The secondary sensors are going to be returns.

Heads up that I think the primary sensor on the front bank is particularly nasty to extract from the cast iron exhaust manifold. That is one you may want to leave to a shop because they may need heat from a torch to extract the old one.

Backfire is when the engine exhaust exits through the intake system, typically because of a severe ignition / valve timing problem. You don't need misfire detection code to identify a back fire because it is serious and sometimes engine fatal. I expect you do not have backfire. Afterfire is when the fuel mix explodes in the exhaust manifold and is that annoying pop pop pop loved by the tuner boyz. In that particular case it is trailing throttle over run caused by changing the ECU firmware to continue to dump fuel into the engine when the throttle is closed, particularly on shifting. Because your noise occurs on throttle lift during shifting you might be getting some after fire / trailing throttle over run. That is a symptom that might be more consistent with an injector that is continuing to spray fuel rather than shut off completely when you let off the throttle. If the throttle position switch is worn and is not giving the ECU the 'I am closed message' the ECU will not go into fuel cut off during trailing throttle. Checking the correct operation of the throttle switch as per the service manual is fairly easy. Fixing it if it is out of spec - not so easy.

 

[e-rod]

Thanks again for the informative reply, seems I am learning a lot quickly. I should be able to return the unneeded O2 sensors, no worries there. Afterfire is the correct term here as to what is happening. The noise definitely comes from the rear of the car, near the exhaust area and not engine area and is that typical "pop" sound. It only happens when letting off the throttle, and not when driving casually.. I almost have to intentionally let off immediately at higher rpms to make it pop.

Given the initial #44 code indicating rear fuel supply and trailing throttle issue, I am getting a suspicion it might be injectors. I have the service records from the original owner for 26 years, and I did notice that the car did seem to sit quite a bit in the last years of his ownership (driven <200 miles in some years). I have heard that a sitting car is bad for injectors, which may be why they seem to be questionable at only 70k miles. Then again, it is 30 years old and maybe age is a factor as well not just mileage.

The one remaining thing is the wispy/airy sound I hear as I drive around (low speed), seeming to come from the rear of the car. If I had to guess it sounds like air is escaping somewhere in the exhaust system as if some piping is loose or has developed a hole. The strange part about all of this is how suddenly it happened and it seems to be two (seemingly unrelated) issues of fuel delivery and the exhaust system.

EDIT: Actually thinking about it more, it may be more related than I initially thought. If there is some leak between the catalytic converter, it may be causing some air to get back into the exhaust system which is enough for afterfire to start happening (more ambient O2 entering).

Will let the tech know when I take it in Monday, useful notes for sure!

 

[Old Guy]

Using an official Acura parts data base such as this one is probably the most reliable method of finding the correct parts for your model year.

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You may or may not choose to purchase from them; but, at least the data base should give you the correct part number which you can then use to double check your purchase. Aftermarket vendor websites are less reliable with their parts look up functions.

 

[e-rod]

Well it looks like things are adding up! The rear O2 sensor actually separated the core from the inner metal shroud and popped out of the exhaust manifold area. Explains the code and exhaust leak at the same time…

Fortunately was able to get the old O2 sensor piece removed without any damage to the manifold or threads. Replaced both front and rear primary sensors to be on the safe side.

Was doing spark plugs anyways so ran a compression test, all cylinders are 200-205 psi. Engine running great there.

Went ahead and did PCV valve, hose, and fuel filter. Got some new Honda MTF in there and a new paper air filter to top it off.

She’s purring like a kitten now. I’m still laughing to myself about the O2 sensor thing, in hindsight it makes complete sense why the two issues appeared at the same time!

Now to drive around and remember why this car is amazing!

 

[Old Guy]

Well it looks like things are adding up! The rear O2 sensor actually separated the core from the inner metal shroud and popped out of the exhaust manifold area. Explains the code and exhaust leak at the same time…

A two for one fix!