91 F-150 302 - Engine Troubleshooting


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91 F-150 302

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sparticas44
New User

Jul 31, 2011, 12:36 PM

Post #1 of 4 (1908 views)

91 F-150 302	Sign In

Ok so my truck isn't running right and I am kinda flustered now about what it could be. My truck has a pretty bad miss in it and I can't find out whats doin it. I have changed the following: Spark Plugs, Wires, Coil, Distributer Cap, Rotor Button, and 4 of the injectors. I have run a compression test on the engine on all cylinders and they all have good compression. Now I was noticing that the truck ran rich on gasoline. So I know it's got good fuel pressure. I also noticed something odd. You can cap your hands over the throttle body where the main air duct should go. Now I was told that doing so should kill the engine due to no air getting into the engine. But this wasn't the case. It simply bogged down and ran a little smoother. I am thinking Vacuum leak. but i am not sure. Any thoughts?

Sidom
Veteran / Moderator
Sidom profile image

Jul 31, 2011, 1:40 PM

Post #2 of 4 (1893 views)

Re: 91 F-150 302	Sign In

Very possible.....

Those were good for vacuum leaks around the plenum.... Get a can of carb cleaner & spray around the intake. If it idles up or does anything then more than likely you have a vacuum leak....

sparticas44
New User

Jul 31, 2011, 1:51 PM

Post #3 of 4 (1886 views)

Re: 91 F-150 302	Sign In

Thanks. Will do that. I'll post an update if it doesn't yeild a result.

Discretesignals
Ultimate Carjunky / Moderator
Discretesignals profile image

Jul 31, 2011, 8:31 PM

Post #4 of 4 (1875 views)

Re: 91 F-150 302	Sign In

Vacuum leak will also make the engine run rich because the fuel system is speed density based. Basically the fuel system relies on the MAP sensor to determine engine load. Vacuum leak will make the computer believe the engine is under load because it is seeing a high MAP sensor voltage and inject more fuel.

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