Possible reasons for engine idling higher, and "auto revs?"
September 17, 2013 6:02 PM   Subscribe

Carfilter: Car had been running fine all summer. Saw a bit of this yesterday, more during today's commute. Engine idling slightly higher and it revs itself at rest. Otherwise, it runs normally under load.

It's an old B16A3 in an '95 manual transmission Honda del Sol. Normally, it idles at ~850rpm. Now, sometimes at rest, the engine idles at around 1100rpm but spontaneously fluctuates up to 1250rpm over a couple of seconds, backs down to 1100rpm over a couple seconds, fluctuates up to 1250rpm over a couple of seconds, &c.

Sometimes it'll go back to idling steadily at ~850rpm.

Otherwise, the engine behaves and sounds "normally" when under load. Have not noticed any dramatic difference in gas mileage or smell or torque. Mostly city driving with variable traffic.

Have not changed octane or source of fuel recently. Currently just under half a tank of gas. I don't think that this behaviour correlates with engine temperature/warmed-up-ed-ness. There's an aftermarket radiator that keeps the engine at or below 50% on the gauge no matter how adverse ambient/driving conditions have been. Ambient temperature has moderated but its not extreme (averaging 20'C instead of 25'C) but average engine temperature ~30% instead of ~45% on the gauge. About 5000km since last (synthetic) oil change, next recommended change at 7000km.

What could be some possible reasons for this sudden behaviour, is this a sign of imminent demise, and is the situation likely to become worse? I'm not sure if this behaviour is always replicable.

posted by porpoise to Travel & Transportation (6 answers total)
This thread has a number of possible culprits. Check your coolant level, check for vacuum leaks, followed by TPS sensor and then more expensive and/or annoying things.
posted by Slap*Happy at 6:25 PM on September 17, 2013

Is the check engine light on? I would say it can be few different things.

The first thing i would suspect is a vacuum leak, which isn't really helpful since there are LOT of potential leak areas. Check all the rubber vacuum lines on the intake to make sure they are connected and also for dryness/cracking.

Have you ever changed the O2 sensors? If not it is a pretty good idea on general principles for a car of this age and not too expensive.

I had a similar problem with a new Nissan Juke that was a cracked charcoal canister (this one will throw a check engine light).

And then all the other sensors, mass air flow, throttle position sensor and so on.

It is unlikely to cause this problem but have you checked the rotor/distributor and spark plug wires and spark plugs? these are pretty easy to check and replace and if original on the car (or even more than 5 years old) should be replaced anyway.
posted by bartonlong at 6:31 PM on September 17, 2013

Cheap things to try:
-Vacuum leaks - spray carb cleaner along vacuum lines, listen for engine changes.
-Throttle - Remove the snorkel (you can run without the air cleaner for a little bit at rest, no worries) clean the throttle blade with carb cleaner, wipe that fucker down.
-Pull the distributor cap. File off the grit at the contacts and on the rotor. Or replace the rotor and cap, and hell, the plugs too. Cakewalk.
-As mentioned above O2 sensor.

I have a 94 that still runs pretty well.
posted by notsnot at 8:09 PM on September 17, 2013

Best answer: In general, examine the accessible parts of your engine, and look for connections that have been knocked loose or dislodged, cracked or broken, or simply left open. Check your belts and belt tension.

Check your PCV valve, and make sure that it is properly seated and not stuck open. This can behave as a vacuum leak.

Check your oil fill cap, and make sure that it is well seated.

Check all your vacuum lines, very gently, because it's a disappointment to crack a brittle vacuum line or connector *while* looking for a vacuum leak.

Check your Idle Air Control Valve. They get a little gunky, and if it is stuck so that it cannot close all the way, the engine will always get extra air just like a vacuum leak, and the fuel injection will compensate by adding more fuel. Actually, don't check it: just remove the IAC, spray it out with IAC cleaner, and remount it, and see if that fixes it.

Check the throttle linkage and the throttle spring. Metal can get tired after 18 years.
posted by the Real Dan at 8:30 PM on September 17, 2013

Does a car that old have a Mass Airflow Sensor (MAF)? Because, the symptoms sound like a classic case of a MAF in its death-throes.
posted by Thorzdad at 5:49 AM on September 18, 2013

Response by poster: The IAC was an absolute mess. Thanks the Real Dan! That's what I suggested they look at first, after a little googling.

The shop cleaned it up but said the rest of the engine has a lot of carbon buildup, and the guy is soft-selling me Terraclean decarbonation for $180.

I'm thinking about it...
posted by porpoise at 7:52 PM on September 19, 2013

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