Engine Idle is normal then randomly increases to 1500 RPM

Mrg2783

5 Year Member
Apr 1, 2018
85
3
8
Newport Beach
My 92 mustang GT idles maybe a smidgen rough but its nearly always idleing at 800 RPM warm. I'll drive it for hours and it's fine. I have a KOER code 11 and a 90 on the cylinder balance test.

I recently swapped in ford performance 60lb injectors, a 3/8" fuel line, a 340ml fuel pump and a pro-m 80 MAF. Its pushing 10 pounds of boost on a vortech V-1, TFS 170cc o-ringed heads, gt-40 lower intake, cobra upper intake, bored .030 over at 8 1/2:1 compression, comp 294 blower cam.

It goes to dyno this Monday April 22nd as it's never been tuned since the upgrades. I'm really just wanting to make sure I have it dialed in before having it tuned.

I replaced all the vacuum hoses, brand new upper to lower gasket, brand new lower intake gasket etc. New IAC, new tps, tps is at .87, adjusted the fuel pressure to 40 psi with vacuum line unplugged from the regulator and plugged the vacuum line before adjusting, fuel pressure at idle with vacuum hooked up is about 35 psi per my jegs fuel gauge. I feel pretty confident there are no vacuum leaks as it's a random occurence and not constant.

Like I said, for the most part the car idles at 800 RPM. A smidgen rough but over all she runs solid.


If I turn the car off and restart it when the idle randomly pushes to 1200-1500 it starts perfect at 800 rpm again. Doesnt push up, bounce, anything along those lines. Sometimes it never even fluctuates when I drive it for an hour or two. Its completely random.

My question is, is it the computer doing this possibly? Because it needs a tune? Could the fuel pressure be to low? Too much fuel from the injectors? Maybe the new IAC is faulty at random?

I'm really just wanting to make sure there is nothing I need to fix as far as sensors etcetera before I get it to my tuner. I'm hoping with your wisdom and experience you can give me some insight. I dont want to go replacing parts on a whim if I dont have to, you know?

I appreciate any feedback you might have.

Thanks!
 

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My 92 mustang GT idles maybe a smidgen rough but its nearly always idleing at 800 RPM warm. I'll drive it for hours and it's fine. I have a KOER code 11 and a 90 on the cylinder balance test.

I recently swapped in ford performance 60lb injectors, a 3/8" fuel line, a 340ml fuel pump and a pro-m 80 MAF. Its pushing 10 pounds of boost on a vortech V-1, TFS 170cc o-ringed heads, gt-40 lower intake, cobra upper intake, bored .030 over at 8 1/2:1 compression, comp 294 blower cam.

It goes to dyno this Monday April 22nd as it's never been tuned since the upgrades. I'm really just wanting to make sure I have it dialed in before having it tuned.

I replaced all the vacuum hoses, brand new upper to lower gasket, brand new lower intake gasket etc. New IAC, new tps, tps is at .87, adjusted the fuel pressure to 40 psi with vacuum line unplugged from the regulator and plugged the vacuum line before adjusting, fuel pressure at idle with vacuum hooked up is about 35 psi per my jegs fuel gauge. I feel pretty confident there are no vacuum leaks as it's a random occurence and not constant.

Like I said, for the most part the car idles at 800 RPM. A smidgen rough but over all she runs solid.


If I turn the car off and restart it when the idle randomly pushes to 1200-1500 it starts perfect at 800 rpm again. Doesnt push up, bounce, anything along those lines. Sometimes it never even fluctuates when I drive it for an hour or two. Its completely random.

My question is, is it the computer doing this possibly? Because it needs a tune? Could the fuel pressure be to low? Too much fuel from the injectors? Maybe the new IAC is faulty at random?

I'm really just wanting to make sure there is nothing I need to fix as far as sensors etcetera before I get it to my tuner. I'm hoping with your wisdom and experience you can give me some insight. I dont want to go replacing parts on a whim if I dont have to, you know?

I appreciate any feedback you might have.

Thanks!
You guys with idle/stall problems could save a lot of time chasing your tails if you would go through the Surging Idle Checklist. Over 50 different people contributed information to it. The first two posts have all the fixes, and steps through the how to find and fix your idle problems without spending a lot of time and money. It includes how to dump the computer codes quickly and simply as one of the first steps. I continue to update it as more people post fixes or ask questions. You can post questions to that sticky and have your name and idle problem recognized. The guys with original problems and fixes get their posts added to the main fix. :D

It's free, I don't get anything for the use of it except knowing I helped a fellow Mustang enthusiast with his car. At last check, it had more than 250,000 hits, which indicates it does help fix idle problems quickly and inexpensively.