I have a 1990 Accord DX manual. I am having a problem with a very rough idle. I have changed the cap and rotor, plugs, wires, pcv valve, adjusted the valves, cleaned the whole egr system, cleaned the intake, checked all the vacuum lines, and bleed the coolant system and nothing has changed. The timing belt has also been changed recently too. My question is would the alternator cause all the idling problems if it is going bad. I'm just tired of trying to find the problem and nothing working. The rough idle only comes when the fast idle closes, and that is when the car is fully warmed up too.

 

Have you checked/cleaned the iacv?

 

yes and cleaned the fast idle and the other valve that is part of the fast idle

 

because the volts go way down and the idle gets worse when i turn the headlights on or the heater on

 

Because of those symptoms it may be a good idea to get the battery and alternator tested, as well as checking all ground straps

 

ok sounds good but if the alternator is going bad it would cause i rough idle though right?

 

It can, so can a bad ground strap. Funny story that happened to me once, I had a broken ground strap and didn't realize it for a while but I did notice my stereo shutting off above a certain volume so I pulled the console apart quick (in a store parking lot no less) to find the problem. After 10mins of checking wire pins for a bad connection I gave up and put it back together and went inside to shop. Came back out and my car stalled out on me and was idling like garbage and got even worse when accessories were on. I ended up diagnosing it right there in the parking lot, I checked all the fuses and found nothing at first until I spotted the strap. Touched it to the rad support where it had broken and the idle was fixed, when I let go it idled bad again. Turns out while it was broken it seems my car was using the stereo antenna as it's ground and when I had put it back together I had left the antenna disconnected since I don't use the radio anyway. I was amazed nothing bad happened in those circumstances, but it shows just how important ground straps are.

 

If you test for ohms from the negative battery terminal to the main ecu ground bolt there should be less than 1 ohm.