Very weak rebuilt 4.0

[travis_smith25]

I recently purchased a 91 4x4 with a seized 4.0. I have rebuilt and reinstalled the engine but while it now runs, it has very little power and low, but steady, vacuum (14 in Hg).

The cylinders all have good compression (130-150 psi). I have pulled several plugs and they are black so I don't believe this to be a fuel delivery issue.

I have put together many motors and have never had this kind of trouble. Is there something on the 4.0 that can cause the ignition timing to be retarded?

The exhaust is new and free flowing.

Thanks for looking at this

 

[rdecker83]

Well did you reuse the old coil pack? I would have that checked at an auto parts store to see if you have a weak spark. Also is it possible that the EGR could be stuck open or that something is not connected in the wireharness. If timing is off check you sensors like you crank postion sensor and so on...It may not be much but at least its a star ehh....

 

[Doug904]

If your plugs are black then the engine is running rich and this can kill power. Also with that low of vacuum it sounds as if you have some sort of restriction. What type of air filter do you have? What condition are the cat. converters in? What is your fuel pressure?

Your low vacuum could also be caused by a rough or very low idle

Let us know and we'll see if we can help

Thanks, Doug.

 

[travis_smith25]

I've thought many of the same things... got a new coil pack, checked the whole inlet path and exhaust. The engine doesn't have an EGR. The idle is fairly smooth.

I have not checked fuel pressure though. The pressure regulators I've seen fail in the past always seemed to allow too much fuel to the engine (this would cause the black plugs) and have a lot smoke coming from the tailpipe, in addition to an overall lack of power. Do the 4.0s act this way when the regulator fails?

Thanks for the ideas!