What would cause a car to run better after a reset?
February 13, 2017 12:17 PM   Subscribe

I've been chasing an issue with my car (1998 Infiniti Q45) for a few months. The basic symptoms are rough idling (in drive, stopped at a light for example, the rpms will dip erratically), loss of power (the engine seems to "skip" when I put the gas down too far or accelerate up a hill). What could be wrong?

I discovered accidentally that if I disconnect the battery for a few minutes the car suddenly runs great, like a brand new car all of a sudden for maybe ten miles of driving. Then it starts missing and eventually goes back to being as bad as it was before. I've been working on my own cars for a long time but I'm stuck. I get no error codes except knock sensor. Is it a coil? O2 sensor? Why does disconnecting the battery (resetting the computer) temporarily fix the problem? Where do I even start with this? I've also noticed that my exhaust smells bad where I don't think it did before. I read that could be from a bad catalytic converter. Would that cause the other symptoms? How could I check that?
posted by nzero to Travel & Transportation (8 answers total)
You need to have a mechanic hook the car to a diagnostic system to pull and read any stored error/fault codes. Even if the check-engine light isn't on, there's probably something throwing a code. It could be anything making your car run bad.

Do you do regular maintenance on the car?
posted by Thorzdad at 12:30 PM on February 13, 2017 [1 favorite]

I'm reading it with an obd2 scanner and the only codes I get are the knock sensors. Check engine light is not on. Yes I do regular maintenance.
posted by nzero at 12:34 PM on February 13, 2017

Btw, the idea of taking it to a mechanic is where I'm headed if I can't figure this out. I guess posting it on here is my last attempt to solve it myself in case someone might know what it could be.
posted by nzero at 12:37 PM on February 13, 2017

It really could be anything. A bad sensor. Plugs. Bad gas. Clogged fuel filter. Failing MAF sensor. And on and on, up to and including a failing engine management computer.
posted by Thorzdad at 12:49 PM on February 13, 2017

A faulty knock sensor (or some other sensor, but since you're getting knock senor diagnostic codes you might as well start there) could well be giving a false reading, causing the engine to react to/try to compensate for the bad info. Resetting the battery probably reverts the ECU back to the default settings, until over time the bad readings from the sensor reestablish themselves and start the cycle over again.

Alternatively, maybe the knock sensor is doing it's proper job... Unless the mechanic can get better diagnostic codes than you can, the only way I know of to test either hypothesis is to swap out the knock sensor and see what happens - if the problem clears up, bad sensor; if it doesn't, the cause lies elsewhere.

(If a bad catalytic converter was causing an engine issue (for instance, due to unexpected back pressure or lack thereof), it seems like the symptoms would be constant, but that's only a guess.)
posted by Greg_Ace at 1:04 PM on February 13, 2017 [3 favorites]

Holy crap Greg- I think you might have solved my problem lol. I was just looking at the shop manual for about the hundredth time after reading your link when I realized that the knock sensor codes aren't for the knock sensor detecting a problem as I had been assuming, they are for faulty knock sensors. *smacking forehead*
posted by nzero at 1:33 PM on February 13, 2017 [2 favorites]

I discovered accidentally that if I disconnect the battery for a few minutes the car suddenly runs great,

To expand on what Greg says - the ECU 'learns' over time. It initially goes off a textbook 'when a happens, b should be the case' and it used those values to set all the fuel/air/timing/spark stuff. It then modifies that information in that look up table (basically) as the car wears or things change to keep the engine within the right band of tolerance. If a sensor is out by a factory tolerance of 4% (say), over time it will just take any reading as 'sensor reading+4%' to account for the error.

(disclaimer, extraordinarily simplistic example, not to be chewed on by en electrical engineer at great length for hours because I wasn't accounting for the manufacturing tolerance of thermoset plastics, and it is actually 3.167577976%).

This is why after you replace an air flow meter, for instance, you reset the ECU. Because the engine has been compensating for the degrading readings from the failing component and so needs to start from zero again to learn the new component. You need it to learn the component quickly, not just assume the readings are bad like the old one was.

In your example, those base values are being scuppered by a bad sensor. Which is why it runs better on the 'book' values than the information it is getting back from the engine as it runs. All the extra information it was getting was faulty and it got confuzzled.
posted by Brockles at 2:06 PM on February 13, 2017 [7 favorites]

I had my spark plug wires replaced last year, it made an enormous difference in now the car ran. Unbelievable, really. My mechanics had skipped replacing them, even in major tune ups.
posted by Oyéah at 4:27 PM on February 13, 2017 [2 favorites]

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