4.9 300 inline 6 surging

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Mar 6, 2008 | 10:55 PM

oldsilver

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Joined: Mar 2008

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From: Hope,BC

4.9 300 inline 6 surging

My '89 F-150 4X4 4.9 300 finally crapped out after 325,000 km's or 200,000 miles. Have installed another from a 1992 F-150 with 110,000 km's. This replacement engine runs awesome, except at idle, at operating temp. The idle speed surges from 8 to 13 grand. I have put on 10,000 km's on this engine & mechanics can't seem to figure out the surging. Spoke to a fellow today who told me that the problem is with the original 1989 computer. He said I should have made sure to get the computer from the '92 truck that the engine came out of.
My question is: anyone here agree with him? I have owned the '89 from new & replaced the computer once about 10 years ago.
Thanks for your help.

Mar 7, 2008 | 08:58 AM

jnewkirk77

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Joined: Jul 2007

Posts: 309

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From: Owensboro, KY

Well, I don't think it would hurt to have the newer computer. That being said, I don't know exactly what differences exist between '89 and '92 with the 300 to say for sure it would be necessary. OTOH, on the rare occasion when I have swapped a motor or assisted with same, we have done the computer as well just to be safe. I'd think in your case it'd be more of an issue if the '92 had the E4OD tranny and you'd swapped that in as well. Your '89 wouldn't have had that tranny, at least not from the factory.

It sounds more to me like you may have a vacuum leak. My '87 does the same thing, and I can hear the leak, but I can't find it yet. It's been a bit too cold to trace these things anyway. Once the weather warms up, well, then I'll find it.