Engine Miss
Thread Starter
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Freshman User
Joined: Feb 2005
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Engine Miss
Hello all,
I have a 1990 Bronco with a 300 6 in it. It has a 185,000 miles on it. I have noticed that i have a slight miss when its at idle and when i first start off it feels like a "dead spot" in the gas pedal. I just recently did a full tune up (Plugs, wires, cap, rotor, air filter, pcv, fuel filter.) I read alot of the posts but it seems like i did almost everything they had said. Its driving me nuts and any info would be much appreciated!
-Broncocustom1219
I have a 1990 Bronco with a 300 6 in it. It has a 185,000 miles on it. I have noticed that i have a slight miss when its at idle and when i first start off it feels like a "dead spot" in the gas pedal. I just recently did a full tune up (Plugs, wires, cap, rotor, air filter, pcv, fuel filter.) I read alot of the posts but it seems like i did almost everything they had said. Its driving me nuts and any info would be much appreciated!
-Broncocustom1219
Senior User
Joined: Mar 2004
Posts: 173
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From: Southern Illinois
I had a very similar problem. It had a miss at idle and then had a "dead spot" as soon as you hit the gas, and was fine the rest of the way. It basically turned out to be a vaccum leak at the intake and throttle body. The gasket was all deformed and cracked. Also, try cleaning up the IAC valve (on top of throttle body). The gasket was $1.99 at auto zone.
Junior User
Joined: Jul 2003
Posts: 88
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From: Vancouver, WA
Welcome to FTE! 
Yep, vacuum checks are a good start. If everything seems good, do a compression check. I've got rough idle from a couple weak cylinders due to major blow-by (bad rings). You can see the vacuum gauge needle bounce/vibrate at idle on mine. I also get reverse flow in the PCV hose from the blow-by pressurizing the crankcase, which makes oil collect in the air cleaner box. Probably not your truck's issue, but something else to check if nothing else solves it. Especially given your odometer.
Oh, man I just remembered something - have you pulled your computer codes? I had an EEC-IV engine act like that once, with the off-idle bog. It was a while ago, but I think it turned out to be the MAP sensor. Codes pointed me to it.
Good luck.
Yep, vacuum checks are a good start. If everything seems good, do a compression check. I've got rough idle from a couple weak cylinders due to major blow-by (bad rings). You can see the vacuum gauge needle bounce/vibrate at idle on mine. I also get reverse flow in the PCV hose from the blow-by pressurizing the crankcase, which makes oil collect in the air cleaner box. Probably not your truck's issue, but something else to check if nothing else solves it. Especially given your odometer.
Oh, man I just remembered something - have you pulled your computer codes? I had an EEC-IV engine act like that once, with the off-idle bog. It was a while ago, but I think it turned out to be the MAP sensor. Codes pointed me to it.
Good luck.
Thread Starter
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Freshman User
Joined: Feb 2005
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Thanks very much for the info. I have a throttle body spacer and brand new throttle body that i got off ebay for 50 bucks on my 300. I also have 3" Gibson Exaust and one high flow cat (i ripped the other one off). My engine burns absolutely no oil which might be a little hard to believe with the mileage but its true. The IAC valve is new and so are all the gaskets. I ran it for codes about a year ago and the map sensor came up but when i replaced the IAC that code went away. Is it possible that it came back and thats whats wrong? Someone also told me to try new injectors?
Last edited by Broncocustom1219; Feb 2, 2005 at 09:57 AM.
