I've had this problem for a while now, but I don't know where to start. Mechanical timing is spot on as well as the Ignition timing, Sprayed cleaner all over intake manifold and found no leaks, coolant has been bled multiple times. I'm gonna replace the head gasket in a few months as well as replace all of the intake gaskets while I'm at it, head gasket has been leaking a bit, confirmed via block tester, fluid developed a green tint to it, also valve seals are leaking on cylinder 3, might as well replace those also, all in hoping of solving this issue if it is indeed a vacuum leak I cannot find. I let a mechanic change my timing belt and water pump before this happened, engine had a normal idle before, it did idle low, but not high like now, the engine ran really rough when it idles below 1000 RPM for some reason, unlike now it's smooth idling at 1200-1500 RPM, that's if it's even idling lol since I think he messed it up. I think he messed with the idle screw, since I saw it all scratched up, told me to get a new throttle body and IACV. The car was running really bad when he gave it back, even worse than before! He broke a spark plug causing a bad misfire, he left the throttle open slightly because It's the only way it would stay on, when in park or neutral at normal temp it would not start, or it will start then die, but as soon as I pop it into drive or reverse the idle will climb. Here's a video of it, if anyone has any ideas or clues please let me know! I will really appreciate it, I need to get this fixed asap. 1994 Prelude Si Automatic transmission.


Don't mind the dash buzzing lol. Also notice the sucking noise grows louder when put in drive or reverse, also the "pop" noise coming from the intake as the idle surge is coming down.

 

Check your base idle first. See if you can get it down to 600. If you cant then you either have a vacuum leak or your iacv is bad.

Then do a compression test to see how your combustion chambers are sealing.

Report back all your results.

 

The car will idle high if it's cold........ until it reaches optimum temperature.

 

I did a compression test and the results are 170 all across. I actually tried 2 IACV's and both had no change, even tried my friends ECU and nothing. Gonna have to try to set the base idle now.

 

Check for proper ignition timing and a clogged cat after you address all the intake stuff.

 

Its going to depend on what your base idle check turns up.

And with those compression numbers you dont have a head gasket leak.

 

That's what I thought, but it starts to overheat and the temp fluctuates after 15-20 min of driving. Fans work, new water pump, replaced it cause it started to leak, new OEM Tstat, and new radiator and air has been bled, and still runs hot. Block test fluid developed a hint of green, I was gonna say can a high idle like mine cause it to overheat? Since it is always idling at like 1200-1400, but it use to run hot when the idle was normal, I'm thinking cracked head, at least I;m hoping it's the head gasket, gonna find out when I pull the head off get it pressure checked.

 

Start by removing your Idle Air Control Valve (IACV), cleaning it thoroughly and replacing it. If does not solve your problem, remove and clean your Fast Idle Thermal Valve (FITV) and replace it. I had a similar problem with up and down idle when in neutral. Hope this helps.

 

Its possible you have a pinhole leak in the block thats leaking into the coolant.

This has happened before but its rare.

You might want to use an infrared temp gun and check the temp of the block to see if the gauge is reading correctly.