My 1990 Civic STD, DPFI, runs well, but at times the idle runs low. Half the time it idles around 750 rmps, but the other half it is at 500. Sometimes, for no reason, it will dip to around 300, but only for a second or 2. This does not happen frequently, happens maybe once every 2 weeks. I don't know what causes it, it happens randomly. When it is at 500, the car usually is shaking, but not too bad. When it is at 750, it is really smooth. I tried different types of driving, but the lower idle issue is totally random. I've been looking at other forums and have done the following:

- adjusted timing by using my timing gun and rotating the distributor. The timing marks line up good.
- adjust the idle adjustment screw via the Honda manual (it worked for a little while, then the idle started dropping again)
- cleaned IACV with CARB cleaner
- Changed IACV with another one from the junkyard
- adjusted my throttle cable
- Seafoamed via the vacuum line
- Seafoamed via the gas tank
- checked my PCV and cleaned it.
-New distributor, wires, and spark plugs about 5 months ago (NGK plugs and wires)
- Passed California smog about months ago. Had the same problem at the time. I asked them if there were any vacuum leaks, but they said it was fine.

Again, this only happens half the time I am driving. I can go to a stop light, it will idle normally. I can then go to another stoplight, and it will drop down to 500. There are no CEL lights, just shuddering from the car when it goes to 500. When it dips to 300, it feels like it is about to die, but surges back to 500 instantly.

I just want to see if there is a way to fix this. I don't want to be stranded at a stoplight. Do you think that a good cleaning of the throttle will fix this?

 

Not that I think it would solve your problem, but when you did the timing, did you disable the ignition advance first (jumper wire in service connector) ?

 

Thanks for the comment. Yes, when I did the timing, I put a piece of wire in the connector in the passenger side foot well, per the service manual. The funny thing is that I did not get a CEL light when I did it. I used 20 guage wire and connected it into the 2 prong connector.