I have had the same issue with my car for the past few years. Basically, the RPM fluctuates erratically when it is warmed up. For example, if I have driving for a while (let’s say 30 minutes) and shut off my engine, then start it without giving it time to cool down, it begins to act up. Same thing happens if I start it for the first time of a hot day.

The symptoms I experience when it is acting up, are like this. If it is left idle, the RPM sits below normal…maybe around 500 RPM. If I press on the gas and hold it steady at anything below 1500 RPM, it holds pretty steady. If I press on the gas and try to hold it steady at anything above 1500 RPM, it just seems to fluctuate out of control. Sometimes the engine light comes on.

Now, to get it back to normal, I have to let it cool off for a while (sometimes have to wait hours depending on how cold it is outside…in the winter, the problem occurs less frequently). Again, during the summer on hotter days, the problem occurs more frequently. For some reason, it also goes back to normal if I am driving at higher speeds with less frequent stops (see next paragraph for details)

When the problem does begin and I continue driving without letting it cool down to get back to normal, my battery slowly gets weaker and if I keep going long enough, my lights begin to dim and eventually the battery dies…resulting in me having to either recharge the battery or replace it completely). The weird thing is, this usually only happens when driving at city speeds (60 KM/hr or less) and where there are frequent stops (traffic lights). If I drive at higher speeds (100 KM/hr) like on highways/freeways with no stops, it will eventually correct itself as I am driving.

So far, I have replaced the following parts (not necessarily because they were related to the issue, but because they needed replacement): alternator, air filter, engine swap, fuel filter, battery (replaced a few times since problem started). I change oil when it is due for an oil change.

Since the problem occurs more frequently on hotter days, I assume it somehow has to be temperature related, but my knowledge of cars is very limited. If anybody has any suggestions or may have seen this problem before, I would definitely appreciate some feedback. Thanks!

Weird problem, expensive too if it has killed a couple of batteries. Sounds like a bad connection to me. I can't think a bad reading from one of the sensors would cause the alternator to quit charging or something else drain the battery. I would focus on the wiring to the alternator including the heavy wire that goes to the battery or solenoid. You might even study under the hood when the RPM is fluctuating. The engine moves slightly. It could be causing a wire to short out, dropping the voltage, reducing the spark, making the RPM drop. Wiggle the wiring, lightly tap on things, see if anything you do changes the way the engine is running.

I had a problem like this: If I was driving in the city and stopped at a red light, the engine would stay on higher RPM until I punched the gas pedal and released it quickly, then it would drop below normal. Also it would jump from like 500-1000 RPM at a red light, at it's own will. What I did was had a mechanic clean the throttle body (its a Civic 90 DX - fuel injected), and the problem was gone. Now I know your problem sounds a bit different, and is more batterry oriented, but you might want to try this.

Kresho

Labman, thanks for the advice. I’ll have someone check the wiring to see if they can find anything. I’ve been meaning to replaced the wires to the battery to see if that might be the case, but I just never got around to doing it.

CroCivic91, I actually experienced the same issue you had, when I had my older engine. It didn’t happen all the time though. After the engine swap it no longer did that. I had a friend experience the same thing with his 91 prelude too. I’ll definitely suggest to a mechanic to clean the throttle body and see what they say.

Anyhow, thanks a lot for your suggestions. I’ll keep you guys updated if I ever get this issue resolved. Also, if you or anybody else has any other suggestions, anything at this point would help a great deal. Thanks.

Do keep us posted.

On a side note, I just ran into this page - http://www.integra.orcon.net.nz/tps - you might find it interesting.

This type of battery drain sounds like a burnt voltage regulator. A part which gets worse in heat. It is usually located inside the Alternator. An easy way to check it is attach a voltmeter to the battery and turn the car on all the way. Then watch the voltage: if it continues to climb beyond ~13Volts, or fall off within say 5 minutes. Then it's a 95% chance your voltage regulator is burnt, take the alternator to an auto parts store. Have it checked to confirm and then replace the whole unit. Good luck. :)

-Drew

I recommend testing/replacing the Ignition Control Module (ICM) and coil. Since the Civic is over 10 years old, I would replace them. The ICM is the likely culprit.