I was out of town so had my car sitting in an airport parking garage for 10 days. Driving home last night, it was doing something it wasn’t when I left on my trip: it was revving in cycles whenever I was stopped (say at a stop sign).
By this I mean that the RPM’s would spike from 1 to 2 over and over and over, every two seconds, when I stopped. It literally sounded like I was lightly gunning the engine or tapping the accelerator when I was at a standstill, except that I wasn’t. The car was doing this completely on its own.
The engine light didn’t come on and the car (a Honda 5-speed) otherwise ran as expected and without incident, but this was different and I wasn’t sure if continuing to drive like this before taking it to a mechanic might exasperate a potential problem.
On my Saturn it was due to an intake manifold gasket leak. I’m guessing it caused the fuel mixture to fluctuate and the computer was trying to compensate for it.
That’s usually a high mileage issue. Let a mechanical monitor the computer while the car is running or at least go to an auto parts store and let them pull the codes for free.
The term we use for this symptom is “surging.” The most common cause on modern engines is a vacuum leak, such as from a torn intake manifold gasket, a disconnected or split vacuum hose, or a faulty PCV valve (among others).
Normally I wouldn’t expect the car just sitting to lead to this. Maybe the garage hosts mice who like to chew vacuum hoses? ( – but not unheard of.)
A quick check for a vacuum leak is to spray something like starting fluid on suspected areas to see if the RPM changes. Keep in mind that is a flammable substance. A little squirt here and there rather than large amounts.
As Gary T said, anything causing a vacuum leak could be the culprit. Cracked or disconnected vacuum hoses are good things to look for.
could also be the idle air control (IAC) valve, or the electrical circuit controlling it.
as Gary T said a vacuum leak could be in play too, but I would tend to expect a steady high idle instead of surging up and down.
Hondas in general use speed-density engine management; they calculate the amount of air flowing into the engine by measuring the intake air temperature and the manifold air pressure. a vacuum leak can cause surging like that because the PCM interprets the increase in manifold pressure as the throttle being opened a bit more, and happily adds more fuel to go with the air being sucked in through the vacuum leak.
Did a quick errand and it’s revving even higher now. Before it was only surging when I was at a stop, but now in low gears it’s revving into the 2.5 range while on the road. And it sounds worse, too. So off to the mechanic I go tomorrow.
– but not unheard of.)