Last fall I reported on a mystery engine hesitation combined with engine code 232 ("compensating for lean mixture at cruise"). Eventually it turned out to be the ECU. I replaced it with a used 561 that seemed to solve the problem. For a while.

Recently my gas mileage has started getting a little worse, and the exhaust note is starting to sound like a Chevy Cavalier. The engine sounded this way before I changed the computer--afterwards, it had a huskier, deeper tone. I checked a few days ago, and Mr. OBD wasn't showing any engine or ignition codes.

Is my foot just getting heavier, or is the current ECU starting to croak on me?

Thanks for any help you can provide.

Doug Harvey

"...exhaust note is starting to sound like a Chevy Cavalier."

My neighbor has two of these. One sounds just like you describe, an iroc-z24 or something. Never heard a Volvo sound like that. Aren't you thinking cat or resonator? Oxygen sensors and AMMs die slowly sometimes. Both could go out of range of one ECU and yet be "seen" by another ECU, possibly making a swap of the computer seem to be conclusive.

What is a little worse, mileage wise?

Have you measured the oxygen sensor's output?

Could you swap in your old 561 to see if you can repeat the code?

But to specifically answer your question to the best of my ability regarding slow ECU failure, I've not ever heard of it, but it is certainly possible.
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Art Benstein near Baltimore

"...exhaust note is starting to sound like a Chevy Cavalier."

"My neighbor has two of these. One sounds just like you describe, an iroc-z24 or something."

I was thinking of the opposite end of the scale. My car is sounding real tinny and weak, like a leaned-out wimpy Cavalier 4-cylinder. When my ECU failed last year, that sound was one of the symptoms.


"What is a little worse, mileage wise? "

Was 21, now 19. Automatic, suburban driving. I'm using the AC a little now (in Texas it's already getting warm...) But not all the time, and not running against a real backpressure. That couldn't be the culprit, could it?


"Have you measured the oxygen sensor's output?"

Hmmmmm. It's old (unknown mileage--I've had this car 38k, and that item didn't appear in any of the PO's service records). May be time for a replacement anyway. Would a weak OX sensor cause bad mileage? The car starts and runs perfectly. I hate to do anything that might get it riled up.


"Could you swap in your old 561 to see if you can repeat the code?"

I got the codes with the old one before I replaced it. No codes yet with the current one. I'll check again today.

Thanks for any help you can provide.


Doug Harvey

Roger on the tinny sound. Like a clogged cat? Like a muffler baffle loose? Like one of those kazoo noises coming out of Civics with clear taillights? The neighbor's Cavalier sounds like the exhaust is tuned and there isn't much muffler. I believe if I heard what I think you are describing, I'd disconnect the cat at the headpipe (gasket and 8mm hardware needed) and do some snooping. Maybe I'd be able to tell by the way it revs if I just removed a serious restriction, or maybe I'd just cause my neighbor to come out and offer to help me put it back together.

That would be a new one on me if an ECU, or any other engine management device for that matter caused it to sound weak and tinny. But, rather than cause, an ECU code could easily be the symptom of a clogged cat or restricted exhaust, in theory.

The reason behind my suggestion to swap in the old ECU is it is very little effort to re-verify there's a difference in them. 19mpg does not sound to me like "limp home". I've set my ECU in limp-home and watched the injector pulse width more than double - around 7ms if I recall from the armchair. This is more like 12mpg and black exhaust pipe 'n' plugs. Run too long like that and you will most definitely need to remove your lambda sensor to clear it of soot.

Speaking of the sensor - age and mileage have little to do with when to replace them. If not abused they can and do last the Volvo's life. But, they're easily abused, and the abuse is a little harder to recognize, so lots of folk just take the easy way out and tell you to replace it at xxx miles. Too expensive for me to do that. Yes, a bad one will definitely affect mileage -- it is the engine's only gauge of how it is tuning the mixture. Just as the ECU measures the sensor, so can you, with a voltmeter.

One of the "abuses" I recently encountered was a tranny leak. The tailshaft bushing and seal allowed ATF to contaminate the outside of the oxygen sensor. A voltage reading quickly led me to the problem -- and an afternoon of gasket removal.

Good luck, and don't hesitate to use the AC, it should handle the load with ease.
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Art Benstein near Baltimore