89 5.0 Ho 5 Spd Stalls At Idle With A/c On

Dan2017

Dan2017

Member
Aug 13, 2016
16
6
13
Texas Gulf Coast
#1
The idle was searching from about 600 to 900 rpm, I replaced the IAC, and the idle has leveled out at about 900 rpm... until I turned the a/c. The engine runs fine without the a/c, but with the a/c on the idle wants to snuggle 550, and stalls coming down from higher rpm. The engine is stock except for. 1" spacer under the plenum. Any suggestions?
 

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#6
Check out the tech/how to section of the fox 5.0 tech section, look for the surging idle check list, that also has instructions on how to set the tps, I would go through the checklist from the beginning
I think this belongs in the tech section anyway.
 
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#7
Hi! New today. Left here: https://mustangforums.com/ today, seeking a better forum. My idle problem similar, it's a '94, still EEC-IV, pretty same as '89. Runs perfect as long as it's not shut down. Then, if restarted in 10 min. or so, starts normally, up to ~ 1000 rpm, drops to ~ 500, jumps up again, down again, maybe third time dies, or evens out to normal.

Have a brand-new 5.0HO sitting in the corner, so yanked off the TB, with TPS and IAC, stuck it on the car. Same problem. Thinking ECT sensor? Any ideas appreciated! Also, at 161K, maybe original HO2S, when it goes closed-loop, maybe......but why wait until shut-down. For kickers, it doesn't happen every time, maybe 75%. If I start hot, hold rpm at ~ 1200 for 5 secs., everything normal. imp
 
#8
imp said:
Hi! New today. Left here: https://mustangforums.com/ today, seeking a better forum. My idle problem similar, it's a '94, still EEC-IV, pretty same as '89. Runs perfect as long as it's not shut down. Then, if restarted in 10 min. or so, starts normally, up to ~ 1000 rpm, drops to ~ 500, jumps up again, down again, maybe third time dies, or evens out to normal.

Have a brand-new 5.0HO sitting in the corner, so yanked off the TB, with TPS and IAC, stuck it on the car. Same problem. Thinking ECT sensor? Any ideas appreciated! Also, at 161K, maybe original HO2S, when it goes closed-loop, maybe......but why wait until shut-down. For kickers, it doesn't happen every time, maybe 75%. If I start hot, hold rpm at ~ 1200 for 5 secs., everything normal. imp
Click to expand...
Hey Imp, welcome to Stangnet.

The best thing for you to do is what is suggested above. This works for the '94-'95 mustangs as well. The surging idle checklist will help you find the exact problem.

Have you dumped the codes yet?

In the future you'll want to start a new thread when you have a question. The people that view this one only see the original poster's title and you won't get as many people to respond personally to your question that way.

Good to have you with us though.
 
#10
Dan2017 said:
The idle was searching from about 600 to 900 rpm, I replaced the IAC, and the idle has leveled out at about 900 rpm... until I turned the a/c. The engine runs fine without the a/c, but with the a/c on the idle wants to snuggle 550, and stalls coming down from higher rpm. The engine is stock except for. 1" spacer under the plenum. Any suggestions?
Click to expand...

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.
 
#14
Being new, I wouldn't have known about, nor been able to find, without searching what, stickies (?), previous step by step. Glad to hear though. Anyhoo, as I may have said, it's got brand new TB, with new TPS, IAC, so ruled that out. Bad info getting to PCM from some sensor's incorrect output at operating eng. temperature. This morning stuck brand new ECT sensor in, drove it to the store, came out 15 min. later, turned key, BOOM! Like at 5.0s, started almostr explosively, immediately hit idle speed, about 750-800, solid as a rock!

Now, please, one more question: If no CEL (MIL), might there still be codes? More knowledgeable about EEC-V than IV. Thanks! n imp
 
#16
Stickies just refers to various threads that are kept for the common problems and helps for our cars.

Joe provided you a link to the Surging Idle Checklist above.
It's highlighted in blue. Click on that link, not every issue in it will pertain to you, but going through the entire list will give you a familiarity with your vehicle that you may not have had before.

Joe (@jrichker ) is one of the more senior and intelligent members here. It would be beneficial for you to allow him some of your time.

Not all codes will cause the CEL to trigger, so, yes, you could have codes from problem areas without the CEL.
 
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