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Old 11-21-2008, 08:25 PM
jesterdog jesterdog is offline
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Rough idle-'94 F-150 4.9L

Hi guys. My engine has been idling a little rough lately. Plugs, wires cap, rotor, fuel/air filter are less than a year old. I typically stay on top of the routine stuff. The truck accelerates great. I know the timing is good because I had to set it after replacing the pickup coil in the distributor. I ran some fuel injection cleaner through it, but it didn't seem to make a difference. This has been going on for more than a few tanks of gas. It is the same on both tanks.

I got to looking at the IAC. I need to measure the reistance on it as per the manual. I unplugged it while the engine was running and it did get worse. The idle started doing more of a rythmic surging up then down, then up then down. With it plugged in, it stablizes but is a little rough.

When I say rough, looking at the engine it seems to be perfectly smooth. The engine isn't bouncing. But, I can feel it sitting in the driver's seat.

Ideas anyone?
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Old 11-21-2008, 08:46 PM
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6CylBill 6CylBill is offline
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Quote:
Originally Posted by jesterdog View Post

When I say rough, looking at the engine it seems to be perfectly smooth. The engine isn't bouncing. But, I can feel it sitting in the driver's seat.

Ideas anyone?

Your seat has Mexican Jumping Beans in the cushion.



Serious note, can you feel it in the steering wheel / breaks / accelerator?
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Old 11-22-2008, 07:24 AM
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Motorhead351 Motorhead351 is offline
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disconnecting the IAC doesnt prove much, as if the engine was in perfect condition, you disconnect the IAC, it will run worse.

codes? codes that dont trip the CEL?
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Old 11-22-2008, 03:37 PM
stractor stractor is offline
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Take a look/change at the gasket between the throttle body and the manifold. There was a bulletin about the gasket being sucked in and causing a vacuum leak. I don't recall the years the bulletin covered.
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Old 11-24-2008, 01:06 PM
jesterdog jesterdog is offline
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Yeah, I think I'll remove and clean the throttle body. I sprayed carb cleaner all over the vacuum hoses and the TB but there was no change in idle speed. So, that leads me away from the vacuum leak path. Maybe it's a dirty TB?

thanks for the input. I'll post back after the TB service.
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Old 12-02-2008, 08:17 AM
jesterdog jesterdog is offline
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Well, I removed the TB and cleaned it. Looks like the TB gasket might have failed. It is amazing how dirty a TB can get when only air goes through it. Also, the two vacuum lines that connect to the TB were comparable to old marshmellows. I replaced them as well.

Problem is resolved. It idles smooth now.

Thanks for all the help!!!
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Old 12-02-2008, 08:17 AM
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Go Back   Ford Truck Enthusiasts Forums > Performance, Engines & Troubleshooting > Ford Inline Six, 200, 250, 4.9L / 300

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1994, 49, 49l, 94, engine, erratic, f150, ford, idle, liter, missfire, rough, running

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