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MattBe 09-01-2007 10:54 PM

Help - 4.0 rebuild no power!
My buddy just had his engine bored at a machine shop figuring the old tired 4.0 would come back to life and then some. After the rebuild he was supposed to take it easy, which he did. After the "break in" period......absolutely no power gain whatsoever. He also had a new cam from comp cams installed. I'm now thinking its the transmission? Can anybody give me/us some insight? Thanks!!!!!

bobjp 09-05-2007 03:03 PM

How big of an overbore? Was it even enough to need oversized pistons? How much different is the cam?

I am thinking he was expecting too much. Small overbores won't make a difference, and likely the cam was designed for specific rpms. What were his goals with the build?

MattBe 09-05-2007 06:35 PM

Hey thanks for the reply. I think it was a .40 overbore, cant remember. Basically the jeep had absolutely no power before and he figured it was pissed out (the cone air filter he had was not filtering worth **** either). The machine shop told him to expect 300 horsepower after all said and done which surprised the **** out of me, but I didnt really know. After the rebuild it is the same as before, and it was expected to atleast be better than before ya know?

Chile99 09-05-2007 07:08 PM

300 HP from a overbore. ..... Whatever that guy is smoking is illegal. The 4.0 is a strong motor but not a powerhouse I have a stroker in my YJ and do not build that kind of horsepower. Tell us about the jeep. Transmission type, tire size, gear ratio. Year of manufacture. Cam number. Perhaps we can make some intelligent sugestions. Unlike the 300 hp overbore.

MattBe 09-06-2007 10:25 PM

Bored 30 over
15 throw off the head
Says coated pistons
The cam part number is 68-232-4 from comp cams.

Cam installed::::: @110.0 intake center line, duration-intake 206, exhaust- 212... lobe lift intake-.2890 exhaust- .2990.... lobe seperation- 113.0

The 300 was a mistake, he said he probably heard 30 and added a 0...haha.
Anyway....the thing hardly has any power, and tonight I found out it has very slow throttle response. A quick foot to the floor in neutral brings the rpms up to a couple grand. Maybe that tells us a little more? Help is much appreciated, thanks!!

oldmanscherokee 09-09-2007 09:28 AM

all they did was a stock rebuild with a cam, your not gonna feel a hell of alot of difference from before, that cams not crazy enough to do much, youll just a get a little more torque and maybe a very slight lope to the idle. boring a motor out doesnt give you any noticible power. i dont know who keeps tell people this. decking the head would bring the compression up a bit but 15 its not enoguh to bring the motors already low compression up to anything good. maybe the shop didnt degree the cam up strait up?

journeyjim 09-10-2007 03:52 PM

I had "98 4.0, that had no power, found out the throtle positioning sensor was stuck a 38 percent, had it replaced and it ran strong.

MattBe 09-15-2007 07:16 PM

Thanks for the replies, GREATLY APPRECIATED!!!!! He says his throttle position sensor is in working order. I think its the transmission......
If it ever gets fixed i'll let you all know what it is!

waynehartwig 09-27-2007 09:26 AM

If it started out making good power and it sluffed off to what it is now, then a couple things could have happened...
1-timing jumped on the camshaft
2-the lobes on the camshaft wore out/improper assembly from rebuilder

Pull your valve cover and check to see that all valves are opening and closing the way they should be (you should see nearly .5" of lift on the push rod). If you do, then check your valve timing.

Also, poor ignition timing will give slow throttle response as well.... Double check your timing...

MoparXJ 04-17-2012 09:28 AM

Also check the crank sensor, you can test it and it will pass but the gap might be off

waynehartwig 04-17-2012 09:38 AM


Originally Posted by MoparXJ (Post 13424359)
Also check the crank sensor, you can test it and it will pass but the gap might be off

This was almost 5 years ago ;)

Way to use the search button I guess :cheers2:

mdm 04-17-2012 09:45 AM

What's your baseline for comparison? What was the compression in the cylinders before the rebuild? After?

Was it put on a dyno before or after the rebuild? The butt makes a lousy dyno, it won't calibrate at all. The expectations were far greater than what was achieved and that makes the butt dyno feel even worse.

The time now is 10:39 AM.

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