I need help to figure out why my car idles itself up and down seemingly only when the a/c is on. It may do it while off too but not as bad. At idle it seems like it's gonna stall but doesn't. If I hold it at any rpm it goes up and down about 500-700 at a pretty steady pulse, and while driving I can feel the car jerk forward and backwards no matter what gear, speed etc. The damn things runs though and doesn't stall out or nothing just this annoying friggin pulsing and rpm fluctuation. The battery voltage is fine, tune up is about a year old new plugs, wires, cap and all. It's driving me nuts and i can't afford to pay some mechanic too much money to diagnose it.. so any help/ideas would be greatly appreciated.

It's a 98 civic ex manual. Only mods i've done is short ram intake, header, & cat back. Exhaust work was done about a month before this started happening.

Thanks in advance guys, I know someone will be able to help me figure it out.

 

Idle air control valve - or- vacuum leak.

 

thanks gary i'll check for vacuum leaks tonight and get the iacv.

btw.. i have the stock exhaust parts from my car so if my muffler is what you are looking for take it off my hands.

 

No problem!

I forgot to tell you to check your plugs and wires too.

I know it sounds VERY unlikely but my would run like crap only when it was under a load. Look under the cap and look for corrosion...

I replaced the plugs and wires and it fixed the issue.

Start with the cheapest stuff first.

 

HOLY HELL! the iacv is $200 special order at pep boys!! ebay is still 100 bucks. It is most under load and hardly noticeable at idle so I'm gonna do the plugs, wires and cap this weekend when I do my oil change cuz they are much cheaper! Plus it's been a year since I did them and I drive hard. Any suggestions on good plugs, wires and cap other than stock pep boys replacements? Hope that fixes the problem and I save some money.

 

Why dont you just clean the IACV...its really easy...And it only costs 5$ to clean it...(which is the price of a cleaning spray)