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Cardoctor71, ASE Master Technician
Category: Ford
Satisfied Customers: 652
Experience:  MasterCertified Technician with 35 years experience, Owner of two full service repair facilities
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Ford F250 Pickup 4x4 My 1991 F250 (5.8L) has a rough idle,

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My 1991 F250 (5.8L) has a rough idle, gets worse when warm, and sitting at stop. Just had the computer rebuilt, and still 'rolling' idle. It's been to Ford, for tests, and carbon cleaning, checked for everything else, got new distributor, replaced fuel filters, and at least one fuel pump, compression test, all good. Seems idle is better when very cold air, as it is kept higher, but when warm it's always got a rough idle-not steady. It sounds sick. After years of it, but otherwise running fine, I finally concluded it is California emissions.

Recently the computer really started messing up, not starting sometimes, then it would start after the fuel pump stopped buzzing. But because there were no codes in computer no one could do anything. I chauked it up to bad gas, and ran a can of Seafoam in each tank, and one in the oil. Idle would then go sky high, and would not come down. Within a few days, I was driving at night, and the headlights went out and the battery meter was jerking up and down. I had to drive home with Flashers on. Since then it was in the shop for 3 weeks getting diagnosed for shorts, when they finally decided it was the computer. Now the computer has been reprogrammed and it runs again. It's always started great, but the idle STILL sounds sick. Every time I accelerate it has just enough hesitation to be annoying. (As if it has to think about it first.) It misses, then goes.   I feel it bog before it picks up. It's been going on for years and I hate it. I don't remembering it doing this until it was 10 years old. I had the trans rebuilt in 2000 also. It there something I've missed?

Had the engine coolant temp sensor changed once in 2004. But that was also hit and miss, since it has to completely FAIL before it stores a code.
It appears to me ,you may be having issues with a bad EGR valve,They were notorious for problems on the 5.8 liter motor. If The EGR valve is ok after testing suspect problem with the idle air control valve, there was a revision in which a spacer plate was to be installed under the air control valve that greatly enhanced it's performance, there was also a new adjustable idle air control valve that was available that worked in conjunction with the spacer plate. If both of these issues have been addressed then check the intake manifold upper gasket for vacuum leaks using propane. If I have helped you kindly click ACCEPT so I can get credit for answering your question, Thanks.
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Customer: replied 5 years ago.
Still rough. They adjusted the throttle position sensor. Seemed to help some, still rough idle.