4.2L Idle problem - F150online Forums



4.2L Idle problem

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  #1  
Old 02-09-2006, 12:46 PM
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4.2L Idle problem

Everytime I start my truck when the engine is cold in the morning, it will start up just fine and run really smooth for about 2 minutes. Then my engine will start to shake and miss like it's about to quit.

That lasts 10-15 seconds. Then it clears up and doesn't miss a beat until the next time I start the engine cold.

I've cleaned my IAC valve (nearly spotless at 60k miles), MAF and IAT sensors.

There is no CEL or DTC'S. This has me stumped.

The only thing I can think of is the Flowmaster muffler I'm running now has crapped out. It's rusty and has a loose baffle banging around in there.

I don't think that's the problem though. Because it starts shaking at exactly the same time about 2 minutes after cold start up.

Any help?
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  #2  
Old 02-10-2006, 12:34 PM
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Can anyone help me please? Quintin? Anybody?
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Old 02-10-2006, 05:15 PM
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After about 2 minutes or so, the oxygen sensors should heat up enough, or whatever they do to start running lean as opposed to rich upon startup (to go from an open to closed loop, or vice versa.) I had them go bad on a bronco II years ago, and that was the problem. Maybe someone can give the ohm specs for an oxygen sensor, or tell how to test em using a DVOM. Anyways, something to check out.
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Old 02-10-2006, 06:50 PM
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Ok, thanks man.

Last edited by Peacemaker; 02-12-2006 at 06:22 PM.
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Old 02-15-2006, 11:02 AM
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did you get it fixed?
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Old 02-15-2006, 12:36 PM
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No, I've had a killer flu virus for about week and haven't had a chance to even think about it. I asked my Dad about the 02 sensors, and he said if I didn't have a check engine light that the 02's were ok. I did notice when I put my Mac intake on that inside the barrel of the throttle body had alot of carbon build-up in it.

I've got a can of "safe" throttle body cleaner sittin' out in the garage I'm going to use on it next chance I get. I forget the name of it "CRS" or "SRS" throttle body cleaner. I hope that helps it.
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  #7  
Old 02-16-2006, 11:44 PM
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Cold start

It is your plenum gaskets leaking. Once the engine gets a little warm then the rubber seals swell back up.
Fairly easy job and the parts are inexpensive.
Andrew
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Old 02-17-2006, 10:23 AM
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Question Quintin?

I don't think its that because my idle has started surging. That leads me to believe it's a bad 02 sensor (or both upstream sensors), or a bad TPS sensor.

I'm leaning more towards the 02 sensors because I did a seafoam treatment through the brake booster vacuum hose last November.

My engine started slightly missing two or three minutes after startup shortly after the seafoam treatment. 1500 miles later, the idle surges up about 300 rpm and back down to normal every two seconds at normal operating temperature.

I think it might have coated my 02's with some build-up because I only ran it about 4 or 5 miles to burn all stuff out. Then I parked it not letting the 02's burn off all the gunk going through them.

My engine didn't have any problems with idling until after I did the seafoam, just a pinging problem that was fixed.

There still is no SES light illuminating?

Quintin? ProjectSHO? What do you guys think?

Last edited by Peacemaker; 02-18-2006 at 10:23 AM.
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Old 02-18-2006, 07:38 PM
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IAC valve,maybe?
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Old 02-18-2006, 10:26 PM
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Sorry, I don't frequent the V6 forums as much as I should. :o

Anywho, I'm thinking leaking intake manifold gaskets and/or bolts as well. Next time it acts up on cold start, try spraying some carb cleaner around where the upper and lower intake join together and seeing if the idle smoothes out.
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Old 02-19-2006, 04:03 AM
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BOF, Idon't think it's the IAC. I just cleaned it last week and it wasn't even dirty. But I know that it could still be faulty. Wouldn't that cause an SES light? I have no light.

Quintin, if it is the intake manifold gaskets or bolts. If the bolts are a little loose, should I tighten them down snug or just replace the gasket? I'm thinking tighten them down a little first and see what happens?

Thanks for the advice guys.
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Old 02-19-2006, 08:48 AM
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You can tighten them all you want, I don't think it'll help. The vacuum leaks are actually through the bolts themselves. There's a revised set of gaskets and bolts for this concern, although at it's worst you should get CELs with 171/174 lean codes. I'd try the carb cleaner thing first so we can make sure that you do or don't have a leak before we inadvertently fix something.
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Old 02-19-2006, 02:37 PM
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Ok man. I'll do that when it warms up above 32 degrees here.
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Old 03-15-2006, 07:01 PM
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Floating an old thread, but i have a similar problem, but mine idles funny from the time i start it, warm or cold, and even after driving 100+ miles.

The Tach. needle just floats up and down... not sure if this is normal or if there's something different with my truck.

Any help will be
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  #15  
Old 03-16-2006, 01:35 PM
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Gravy, sounds like a vac leak or something.
Peacemaker, any updates? I'm still sticking with the O2 sensor theory...
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