MAP sensor puzzle
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Tuned
Joined: Aug 2007
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MAP sensor puzzle
1988 F250 351
New crate engine with 600 miles on it. I'm still chasing the rough idle problem. Truck runs great at all other speeds.All sensors, distributor, and computer new. Compression is great. The leak down test showed less than 3 PSI per cylinder. Plenum and lower manifold tight with no leaks. All vacuum hoses are new or have been checked with no leaks.
The vacuum off the plenum is 16/1/2" at best.
Here's the puzzle. When the 16 1/2"plenum vacuum is applied to the MAP the idle surges and is irratic. When 13" to 16 1/2" of vacuum is applied to the MAP using a hand pump the truck smooths out beautifully and the vacuum at the plenum increases to 20".
Why the difference between the two vacuums? Why does the hand vaccum make the truck run like a top and the plenum vacuum create a rough lean condition?
How can I fix this!?
New crate engine with 600 miles on it. I'm still chasing the rough idle problem. Truck runs great at all other speeds.All sensors, distributor, and computer new. Compression is great. The leak down test showed less than 3 PSI per cylinder. Plenum and lower manifold tight with no leaks. All vacuum hoses are new or have been checked with no leaks.
The vacuum off the plenum is 16/1/2" at best.
Here's the puzzle. When the 16 1/2"plenum vacuum is applied to the MAP the idle surges and is irratic. When 13" to 16 1/2" of vacuum is applied to the MAP using a hand pump the truck smooths out beautifully and the vacuum at the plenum increases to 20".
Why the difference between the two vacuums? Why does the hand vaccum make the truck run like a top and the plenum vacuum create a rough lean condition?
How can I fix this!?
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jemini
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