1995 Mustang Gt 5.0 Intermittent Idle

Nick 1972 · May 29, 2016

Hi, I am new to this site, so I apologize in advance if I am posting incorrectly.
My car is a 1995 Mustang GT 5 speed with 92k miles. Mods to car, Kenne Bell supercharger setup for 8 psi, matching mass air meter and 42lbs injectors, 255 in tank pump. Custom dyno tune with smog pump and charcoal canister delete. Cold air intake, throttle body, MSD distributor.
Problem: car's idle will intermittently surge when started (rpm from 300-1000) and you have to essentially keep revving the engine to get it to correct itself, which it does. This only occurs when first starting, and seems to do it only when the engine is at operating temperature. I have read my codes and have nothing. The IAC has been replaced but made no difference. My mass air is a Pro M and I am thinking about sending it in for evaluation but thought I might post first and see if others might have suggestions. Thanks in advance!

a91what · May 29, 2016

The open loop tune is off. This is the hardest area to tune for as the car is only truly cold once a day. The maf is hardly to blame here if it was suspect the issue would not just dissappear once up to temp. Do you have a wide band hooked up? Or data logging tool?

Nick 1972 · May 30, 2016

a91what said:
The open loop tune is off. This is the hardest area to tune for as the car is only truly cold once a day. The maf is hardly to blame here if it was suspect the issue would not just dissappear once up to temp. Do you have a wide band hooked up? Or data logging tool?

Thank you for your reply, I unfortunately do not have access to either of those items. Normally the car runs perfect hot or cold, the problem is very intermittent. I will mention one thing, it seems to occur more frequently when I get gas for some reason. I am wondering if I should do a wiggle test of all vacuum lines to rule this out. Any thoughts are certainly appreciated!

jozsefsz · May 30, 2016

When you stop and get gas, the car is nice and warm, and you shut it down for only a short time, so the temperature actually spikes quite a bit as things heat-soak. You probably wouldn't notice this if you shut it down for a longer period of time as the engine would cool off a bit. When you restart, the idle mixture is going to be a little bit richer (as commanded by the computer) as the engine approaches 240 degrees, and you'll be running open loop for a little while after restart, even though the engine's already warm (closed loop requires a number of conditions to enter, including a minimum time the engine's been running).

Your car may not want the extra fuel in that last column of the fuel base table (at temperatures approaching 240) during a hot startup. A good tuner could fix that up for you in no time. They could also extend the buzz rpm a little higher (or a little longer) which basically just does the revving for you until things clear out a bit on startup. (Yes, basically there's a function in the computer to do a little startup revving as acting funky for a few seconds at startup is a pretty normal condition).

Long story short I agree that the open loop fuel settings at high temperatures are probably a little bit off of what your engine / combo wants. I wouldn't replace parts or go hunting too much to remedy the condition, but look into adjusting the tune through however you like to have that done -- hopefully they gave you a warranty or will do a little tweaking for you at a low cost - (though it's never a bad idea to check the vacuum lines).

jrichker · May 31, 2016

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.

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 200,000 hits, which indicates it does help fix idle problems quickly and inexpensively.

Nick 1972 · Jun 4, 2016

jozsefsz said:
When you stop and get gas, the car is nice and warm, and you shut it down for only a short time, so the temperature actually spikes quite a bit as things heat-soak. You probably wouldn't notice this if you shut it down for a longer period of time as the engine would cool off a bit. When you restart, the idle mixture is going to be a little bit richer (as commanded by the computer) as the engine approaches 240 degrees, and you'll be running open loop for a little while after restart, even though the engine's already warm (closed loop requires a number of conditions to enter, including a minimum time the engine's been running).

Your car may not want the extra fuel in that last column of the fuel base table (at temperatures approaching 240) during a hot startup. A good tuner could fix that up for you in no time. They could also extend the buzz rpm a little higher (or a little longer) which basically just does the revving for you until things clear out a bit on startup. (Yes, basically there's a function in the computer to do a little startup revving as acting funky for a few seconds at startup is a pretty normal condition).

Long story short I agree that the open loop fuel settings at high temperatures are probably a little bit off of what your engine / combo wants. I wouldn't replace parts or go hunting too much to remedy the condition, but look into adjusting the tune through however you like to have that done -- hopefully they gave you a warranty or will do a little tweaking for you at a low cost - (though it's never a bad idea to check the vacuum lines).

Excellent explanation, I will check with my tuner and see what we can come up with. Thanks for your time!

Nick 1972 · Jan 20, 2018

Nick 1972 said:
Excellent explanation, I will check with my tuner and see what we can come up with. Thanks for your time!

I was able to locate the problem last fall, honestly I think I just got lucky as I started the car and it started its usual idle from 300-1000 rpm, so I just happed to be under the hood and wiggled my wiring harness to the mass air meeting and it started to idle properly. I replaced that particular piece of wiring harness and I have not had the problem since.

General karthief · Jan 20, 2018

Bout time you let us know, I was about to run out of popcorn!

1995 Mustang Gt 5.0 Intermittent Idle

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wonder how much it would cost to ship you a pair

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