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Feeling weak (rebuilt 4.0, 4.56, AW4, 31s...), Why?

notamos

NAXJA Forum User
Location
Fort Collins, CO
I rebuilt the engine in my '99 at 114k miles because the previous owner let it overheat, cracked the head and trashed the bearings in the lower end. I did a stock rebuild with proper machining and quality parts.

At the time of rebuild, I replaced all sensors with Mopar sensors, new OEM O2 sensors, Dynomax cat back, Magnaflow cat, cowl intake, Mopar waterpump, stainless exhaust manifold from A1, Clearwater 630-clone cylinder head (new casting), etc. Basically, everything was replaced and I went overboard a bit...

Aside from the stock items, I am running a ZJ HD fan clutch (272310), 4.56 gears and 31" ATs and a lowish/light lift. I'm not heavy with tons of armor or anything. But, I live in CO at 6k feet.

Here is my problem... This thing just feels slow. I thought with a "new" engine (has 14k on it now), 4.56 and 31s that this thing should feel fast (for a XJ). I know the fan is WAY noisy - could the additional drag from the HD clutch cause significant power loss? I'm getting about 11.5mpg around town (with a correct speedo).

Thoughts?
 
Is this something new, or has it been this way since the rebuild? Or before the rebuild?

I'd consider checking fuel pressure and volume. Also consider checking the cam timing is correct.
 
Did you build the engine and check the tolerances? Check the compression and do a leak down. There are a lot of incompetent machine shops everywhere. One local shop mis bored an engine so a cylinder was .030 over the rest, that engine ran like garbage and smoked. Just an FYI though.
 
A few answers.

-I built (assembled) the engine. I gauged all of the bearings but did not check compression...

-It has felt like this since the rebuild. I never drove it before - only a few miles to hear the trashed rod bearings.

-I did check fuel pressure after I got it running (installed a new OEM Bosch fuel pump.

Possibly a compression and leakdown test is my next thing to check?
 
I'd get rid of the ZJ fan clutch while you're at it - especially with a new motor, there's no reason it should overheat. Check the timing as well, i'm not sure if 4.0s can only go on one way, but i know certain FI motors have it set at a certain degree and leave it. Could the camshaft be off a tooth? This could be your symptom as well, depending on HOW slow it feels. How did you install the camshaft?
 
I'd get rid of the ZJ fan clutch while you're at it - especially with a new motor, there's no reason it should overheat.

Yes, get a fan stock fan clutch.
When using one a few years ago, the power drain was very noticeable, as it was continuously spinning fast, providing extra cooling even when it was cold out and wasn't needed.
It made winter warm ups a lot longer.
I lost over a MPG using it, which was regained when it was replaced with a stock clutch.
 
I installed the timing gears/chain by getting cylinder 1 to TDC (compression) and then installing the gears with the lines pointing at each other. Off memory, I can't recall more detail than that. I was following the FSM. Is there a way to check this (possibility of being off a tooth) without pulling the cover?

I do remember I had a hell of a time getting it started the first time. Distributor was off. Maybe I need to reindex it?

As for the cam, I'll have to look it up in my file and report back.

For diag "tools", I have a timing light and Torque on my phone. Any basic tests you could recommend?
 
Did you bore the cylinders and get new pistons?
What piston rings did you use?
What gap did you set them at?
What spacing did you use?
What break-in method did you use?
 
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Did you break in the cam correctly? That is extremely important on flat tappets. No idling for the first 20 minutes and varying the rpm between 2k and 2500 for the first 20. I always use high quality 10w30 and a bottle of GM eos. Modern oil absolutely sucks for flat tappets unless you buy oil specifically marketed for them. Did you use the correct lifter paste on assembly? If it didn't fire right off the bat and bring the revs up the cam could be flat. Never use the red comp cams cam lube.
 
I installed the timing gears/chain by getting cylinder 1 to TDC (compression) and then installing the gears with the lines pointing at each other. Off memory, I can't recall more detail than that. I was following the FSM. Is there a way to check this (possibility of being off a tooth) without pulling the cover?

I do remember I had a hell of a time getting it started the first time. Distributor was off. Maybe I need to reindex it?

As for the cam, I'll have to look it up in my file and report back.

For diag "tools", I have a timing light and Torque on my phone. Any basic tests you could recommend?
Its kind of hard to mess the timing chain up if the motor is at tdc.

I had a hard time with the dist after my rebuild also. You can try re indexing it you might be a tooth off.

A compression test and leak down should probably be your next step. Take a look at the plugs while you do the test to see if one or more cylinders is running richer or leaner compared to the others.

If everything is checking out mechanically check fuel pump pressure, filter and injectors
 
Did you break in the cam correctly? That is extremely important on flat tappets. No idling for the first 20 minutes and varying the rpm between 2k and 2500 for the first 20. I always use high quality 10w30 and a bottle of GM eos. Modern oil absolutely sucks for flat tappets unless you buy oil specifically marketed for them. Did you use the correct lifter paste on assembly? If it didn't fire right off the bat and bring the revs up the cam could be flat. Never use the red comp cams cam lube.

Yup, broke in the cam correctly (exactly like you describe). Although, I did use their red lube. I ran PYB 10w30 and a bottle of Comp 159.
 
If the engine is at TDC the FSM procedure for installing the distributor should be very easy. Is this a factory original distributor or an aftermarket one? Cardone does not not differentiate between 2.5/4.0 distributors the way Chrysler did. The only difference is in the drive gear offset.

If you go the route of the compression test, don't forget to adjust the numbers for altitude.

Taking off the ZJ fan clutch seems like step 1.
 
The problem with the red comp cams lube is it runs off the loves after about a day. If the lobes were dry on start-up then one or more may be toast. If compression is low on any cylinder I'd pull the cover and check lobe lift at the lifter.
 
You seem awfully certain? Please tell me how you figured that out?

14K miles, any mechanical problems would have shown themselves long before.


How? I've become pretty good at diagnosing my own problems over the years.


Obvious mech issues aside, every poor running 2.5 or 4.0 is a result of wiring/sensors, guaranteed.


Most think there Jeep is running well, when it isn't.
 
You seem awfully certain? Please tell me how you figured that out?

14K miles, any mechanical problems would have shown themselves long before.


How? I've become pretty good at diagnosing my own problems over the years.


Obvious mech issues aside, every poor running 2.5 or 4.0 is a result of wiring/sensors, guaranteed.


Most think there Jeep is running well, when it isn't.

Because a map, tps and coolant sensors frequently cause the problems you are experiencing.

My first thought: The fuel economy is horrendous, the ECU must not be going into closed loop.

It can't hurt to verify cranking compression, and put #1 at TDC to see where the distributor rotor is pointing. But, if it starts and runs, I would not obsess over the distributor indexing.
 
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