RPM breaking up when engine gets hot, need help

[07GtS197]

Do you have an aftermarket throttle body? If so, I’d do a base idle reset like others have said. In my experience, these cars have the idle set way too low making the iac do all the work creating idle issues. If the throttle blade is open slightly to allow the engine to idle around 500 rpm’s or so with the iac unplugged it’s always fixed my idle issues.

 

[clayton]

Do you have an aftermarket throttle body? If so, I’d do a base idle reset like others have said. In my experience, these cars have the idle set way too low making the iac do all the work creating idle issues. If the throttle blade is open slightly to allow the engine to idle around 500 rpm’s or so with the iac unplugged it’s always fixed my idle issues.

I recently just installed a brand new bbk TB along with new TPS, restrictor plate and IAC motor and set base idle and TPS voltage right before replacing the ICM.

 

[Terrorist 5.0]

Ne

I recently just installed a brand new bbk TB along with new TPS, restrictor plate and IAC motor and set base idle and TPS voltage right before replacing the ICM.

Never do multiple mods at once like that. Unless you mean you drive the car with these mods and you know they didn’t affect anything.

 

[96blak54]

^^^ This is great advise! One can really chase their tail trying to figure out what has caused an issue.

 

[clayton]

I reolaced the ignition control module and it fixed the problem with my rpms breaking up and misfiring. But now it replaced that problem with a hugh idle that I cannot get rid of. The car starts fine and idles normal but as soon as you press in the clutch, put it in gear and go to take off it idles high and sticks at a 1300 rpm idle until you shut the car down. Ive reset base idle a hundred times along with reset tps voltage, my TB is not sticking, im running a new IAC and restrictor plate, I still cannot get rid of the high idle. What blows my mind is that it idles great when start up. Then raises on its own and stays there. And playing the guessing game and replacing every single part on this car just to fix one problem and create another is getting old fast.

 

[GTamas]

Sounds like your car is doing well in open loop after start, but when it enters closed loop and starts reading sensors then the idle is high. Maybe while working in the engine bay you introduced a vacuum leak or unplugged a sensor connection?

 

[clayton]

That was my thoughts. But if that was the case, wouldnt I have the highest idle as soon as I start the car? Like I said, it doesnt happen until it gets warm. When I replaced the idle control module the only thing I unplugged was the sensor wiring harness that plugs into the idle control module which ive double checked. The only thing I tampered with that would effect a vacuum leak is my air intake pipe ( had to remove it from the TB and cold air intake to get to module) which is sealed up also. Tps is brand new and set at .98v. TB is brand new BBK. Throttle cable is brand new and not getting hung up on anything.

No matter how many times I re set the idle it still wants to climb and stay at 1300 once warmed up. Its like these cars are impossible to mod and not spend 99 percent of your time trying to get them to run the way they should. I see why so many people trash the fuel Injection and swap to carb. Not worth spending every weekend hunting new problems and hoping to drive it for 1 afternoon issue free.

 

[07GtS197]

Bbk had a reputation years ago for causing all kinds of idle issues. Maybe try swapping to the factory tb to test. And I’ve seen where there’s a vacuum leak around the tps as well. Something else to check.

 

[Terrorist 5.0]

Well if it is only when warmed up, that sounds like it isn’t happy in closed loop. I would try unplugging the O2 sensors and see if that changes anything. A vacuum test couldn’t hurt too.

I had a really weird issue with bucking only while in 4th gear and off throttle. Turns out my HVAC vacuum line was severed. Chased that for almost a week if I recall. Really confused me why it only happened in 4th gear.

You could probably find my posts on it if you deep dig enough just to see how puzzled I was lol

Also, I’m surprised nobody has mentioned a code scan yet? Unless I missed that part.

 

[weendoggy]

Also, I’m surprised nobody has mentioned a code scan yet? Unless I missed that part.

I think he has the EECiV system, unless I missed something ('95 331 SBF), and I thought they still needed the OBDI style testing and that usually needed a Break Out Box to really dive in and test, and the testing of circuits is very crude by today's standard. Even my OBDI OTC Monitor w/cartridge was a PITA. Definitely old school compared to OBDII testing. Now, you can forget all of this if he has something newer or different.

 

[clayton]

Youre 100% correct. It requires the OBDI. And even if i was able to scan it, its going to throw crazy codes regardless. The car has EGR delete, cat and muffler delete, running shorty bbk headers to H pipe then straight out. It is dyno tuned and chipped. I reset TPS again m, unhooked battery overnight and started it up this morning before work. Started up oerfect, idle at 950rpm, let it warm up for 7 to 8 minutes and when i put it in gear to back it out, it went up and stayed at 1200rpm. Pulled it right back in the garage and parked it. Im all out of ideas, might just park it for 2 weeks to regain my patience and clear my mind. Something is causing the TB to slightly open at idle when it warms up and I just cannot figure it out.

 

[96blak54]

So do you think adding a spring to the throttle linkage to help remain in closed position may help eliminating some sort of throttle drift?.......ive experience weak throttle body springs, but never a drift opening, however......ive seen stranger things Perhaps a slight throttle opening is your culprit.

When I had to add throttle springs, it was because the pedal effort was effortless and would cause me to over pedal.

 

[clayton]

So do you think adding a spring to the throttle linkage to help remain in closed position may help eliminating some sort of throttle drift?.......ive experience weak throttle body springs, but never a drift opening, however......ive seen stranger things Perhaps a slight throttle opening is your culprit.

When I had to add throttle springs, it was because the pedal effort was effortless and would cause me to over pedal.

Im not sure. I have tried to manually close the throttle linkage while the car is running and idling high ( thinking the same thing, that it drifts open somewhat). But the linkage wont budget, meaning it has to be fully in closed position. So im assuming something else is spiking the rpm? Ive smoke tested the engine with no leaks ( because i suspected perhaps a vacuum leak). But had no leaks and my gayge read a solid 14 vacuum which is about normal for my stage 1 tfs cam.

 

[clayton]

Gauge* sorry for the typo.

 

[RAU03MACH]

check all vacuum lines
you may have an incorrect plug in somewhere
you will need a vacuum line map diagram

 

[07GtS197]

I’d beg to differ, I would scan it for codes. One or two should point you in the right direction.