Bad idle when warm?

[97fwdtsitalon]

I cant figure out whats causing this for anything. I have no boost leaks, I've deleted the fiav and replaced my temp sensor. The car will run awesome with no problems until it gets up to operating temperature. Once its warmed up it revs back n forth from 1200rpm to 2000rpm. It even does it when theres a load on the engine. I have no idea what to check for now, please help! I also changed the ECU and still have the problem.

 

[gofer]

Hard to help when you have nothing in your mods list in the profile.

 

[97fwdtsitalon]

Shouldn't make much difference but im running a evo3 16g, safc 2, cometic head gasket, ARP head studs, FMIC. Also, according to the safc my TPS is still working fine.

 

[97fwdtsitalon]

No ideas?

 

[gofer]

What intake do you have? Check for boost/vacuum leaks and once you fix them or find out 100% you have none you might have a bad ISC.

 

[cutty2fast]

mine was doing fine but now its doin it again its the o2 sensor

or at least that was what mine was doin then i found a bad ground to the o2 look into that

 

[97fwdtsitalon]

The car now idles perfectly fine. My throttle cable was a lil too tight, BUT, once i go over 2% throttle it does the same thing. I try to hold the throttle at 1200rpm and it just goes from 1200-2000rpm back and forth. Ive tried unhooking sensors while holding the throttle open slightly and nothing stops the surge. I also sprayed starting fluid on everything i could think of. i cant find any vacume leaks anywhere. when i do a boost leak check it will hold steady for quite a while. The car is not drivable at low speeds or just cruising.


I will try to check the O2 sensor when i get home from work tomorrow. Where is it supposed to be grounded to? I will try anything at this point.

 

[gofer]

You need to check for vacuum leaks at the vacuum hoses, PCV-valve hose, brake booster hose, intake manifold gasket, throttle body gasket, damaged diaphragm in the BOV, damaged diaphragm in the wastegate actuator, or a damaged diaphragm in the power-brake booster.

If no leaks are found cover the BISS hole with your finger on the throttle body, if the engine speed drops you need to replace the o-ring on it. BISS O-Ring If you cover the hole and nothing happens try adjusting the BISS all the way in (reduces air into the IM) and all the way out (increases air into IM) which might fix the idle surge if its just a simple adjustment thats needed.

If your ISC shorted to ground internally it can burn up your ECU's ISC driver, you should probably pull it and check for damage. Visually check your ECU and look for damage similar to this picture ----> Burnt Up ISC Driver If it is burnt up replacing the ECU won't fix it, you'll have to put in a good ISC as well because if you don't you'll just burn up another ECU. If the ECU looks great then test the coils on the ISC, heres a link to a "how-to" video...
ISC Coil Test Video

It could also be your Fast Idle Air Valve (FIAV) and the only way to make sure its good is by replacing it with a known good one or getting a FIAV block-off plate. FIAV Block Off Plate

The engine uses timing to control idle as well, you might check and make sure your base ignition timing is where it should be.

Good Luck and post up your findings...