Speed Density Mustang Searching For Idle

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gmsceo

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May 6, 2004
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Charleston, SC
My '88 GT speed density is doing some intermittent idle searching from 750rpm to 110rpm over the past few days. It's not a constant problem, only when cold or idling in park on occassion. If I tap the gas while it's surging, it will almost stall sometimes. It runs wonderful otherwise.

I've changed the IAC and installed Ford's idle adjustment plate, but no luck. The car only has a 65mm throttle body with matching EGR spacer, CAI, O/R Xpipe with a cat-back and the timing is at 12 degrees. The full pressure is around 35-38psi and fluctuates with the surging. Any ideas?
 
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JR's list is second to none. :nice:

I would add (not many of us are still SD, which is why I'm even posting) that it can be tough to get rid of the idle surge altogether on some SD cars.

I get it occasionally (the occasionally part has made it tough to 'slay'). I think it's some EEC burp (the right conditions, ambient temps, ECT temps, etc etc. It's generally with ECT temps at about 130* and a restart of the engine after being in a store, etc) that make it burp. It lasts about 30 seconds so I don't care all that much (the slightest application of gas makes it better, and I keep my idle at 650 RPM, so it's not tough to circumvent).

Just in case that makes you feel any better. :rlaugh:

Good luck.
 
Thanks a lot Guys. Very helpful. I notice a lot of you are saying the idle should be 650-750rpm. My idle is at 950-1000rpm all the time, even with the new IAC and idle adjutment plate between the IAC and throttle body set at the lowest idle.
 
Thanks a lot Guys. Very helpful. I notice a lot of you are saying the idle should be 650-750rpm. My idle is at 950-1000rpm all the time, even with the new IAC and idle adjutment plate between the IAC and throttle body set at the lowest idle.

That could do it. The puter might be trying to get back to the commanded idle.

Also, have you checked for any vacuum leaks? A stockish SD car should crank out 20" hg of vac at idle. A leak can cause a high idle, as well as really screwing with the computer (the vacuum signal is super-critical with SD).

Good luck.
 
Thanks for the great advise. I've checked for vac leaks but found none. I'll check the housing and setting on the tps tomorrow. .97-.99 on the tps, right?

That's right in the ballpark. The puter should baseline and ratchet as you go however. If it can't baseline, it will throw a code. Were I adjusting the TPS for a surge issue, I'd set it low - 0.85V or so. This will help with slight transient voltage issues (sometimes when cars are running, the reading is a smidge higher, etc). A setting around 0.85 keeps you well below the Tip-in threshold.

Good luck.
 
I've got it at .89 and I'm still getting the intermittent idles surges in and out of gear. I've got mods on this SD car (65mm tb, full exhaust, and CAI), so I'll probably have to just live with it knowing they don't respond to bolt-ons as well as MAF cars.
 
You guys with idle problems could same 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 post has all the fixes, and steps through the how to find and fix you idle problems without spending a lot of time and money. 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.
 
You guys with idle problems could same 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 post has all the fixes, and steps through the how to find and fix you idle problems without spending a lot of time and money. 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.

Agreed!
 
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