4.0 puzzling performance issue

Hello,

I am currently facing a very frustrating problem with my 01' xj with a 4.0, 5spd manual.

I recently bought it, the car has only 30k miles, it was stored most of its life. perfect condition.

The problem is is a general lack of power and huge engine harshness in higher RPM's.
All usual maintenance items were conducted but that did not solve the problem:
- oil and oil filter
-air filter
- new spark plugs
-new oils for differentials, gearbox and transfer case
- a can of Injector cleaner for the fuel injectors
- oil and cooling systems checked for function and integrity

The symptoms are as follows:

1- repeated 0-60 of 10.5sec ( different drivers ), I am assuming an XJ 4.0 with a manual should at least be at 8 sec stock?, The engine responds to wide open throttle with harshness and some sort of hesitation with each new gear, for example 2nd to 3rd.

2- intense engine harshness at higher rpms and a "strangled" feeling above 3000RPM, the RPM climbs up slowly even when i floor it in 3rd gear for example. the engine just feels like a big grinder on the way to the red line past 4000rpms.

3- The engine shudders and vibrates when the ignition is shut off. In other words it takes the egnine like a second or two to shut off after the ignition.

4- The odd this is RPM idle is rock solid, no hiccups or fluctuation 600rpms, slight hissing sound from around the intake and water dropping out of the exhaust in the morning. No engine fault codes, nothing out of the ordinary

The frustrating thing is that my local mechanic and jeep shop say its fine!, and I am simply not convinced.My seat of pants feeling says something is wrong. I have driven a friends exact car , and it feels much faster and smoother ( stock )

Any thoughts on where to look, how to approach this?
My experience with engines says power and torque are always about air, fuel and spark timing but I'm not very familiar with how this specific engines behaves.

Appreciate some help from the experienced people here

Hello,

I am currently facing a very frustrating problem with my 01' xj with a 4.0, 5spd manual.

I recently bought it, the car has only 30k miles, it was stored most of its life. perfect condition.

The problem is is a general lack of power and huge engine harshness in higher RPM's.
All usual maintenance items were conducted but that did not solve the problem:
- oil and oil filter
-air filter
- new spark plugs
-new oils for differentials, gearbox and transfer case
- a can of Injector cleaner for the fuel injectors
- oil and cooling systems checked for function and integrity

The symptoms are as follows:

1- repeated 0-60 of 10.5sec ( different drivers ), I am assuming an XJ 4.0 with a manual should at least be at 8 sec stock?, The engine responds to wide open throttle with harshness and some sort of hesitation with each new gear, for example 2nd to 3rd.

2- intense engine harshness at higher rpms and a "strangled" feeling above 3000RPM, the RPM climbs up slowly even when i floor it in 3rd gear for example. the engine just feels like a big grinder on the way to the red line past 4000rpms.

3- The engine shudders and vibrates when the ignition is shut off. In other words it takes the egnine like a second or two to shut off after the ignition.

4- The odd this is RPM idle is rock solid, no hiccups or fluctuation 600rpms, slight hissing sound from around the intake and water dropping out of the exhaust in the morning. No engine fault codes, nothing out of the ordinary

The frustrating thing is that my local mechanic and jeep shop say its fine!, and I am simply not convinced.My seat of pants feeling says something is wrong. I have driven a friends exact car , and it feels much faster and smoother ( stock )

Any thoughts on where to look, how to approach this?
My experience with engines says power and torque are always about air, fuel and spark timing but I'm not very familiar with how this specific engines behaves.

Appreciate some help from the experienced people here

Get a high end scan tool and check PID's for tps and map sensors

I'd Check the tps. Use a volt meter and check it throughout its sweep. Make sure theres no dead spots.

*Checking the throttle position is easy. Look for a smooth sweep with an analog meter through the entire acceleration band.

*Getting it on an engine analyzer is never a bad idea. Can look at lots of things REAL TIME. Can be money well spent.

*Continued detonation after turning off the key can indicate a carbon buildup on the top end.

*What kind of plugs are you using? I've found NGK work the best on the 00-01 coil pack distributorless ignition. Don't use Bosch; quite a few threads of performance issues with them.

Good luck!

The hissing sound, sounds like a vacuum leak which could definitely explain all your problems. Might wanna check all your electrical connections while you are at it. BTW, the hesitation before it shuts down is call dieseling or engine run on...

any oil in the coolant or vice versa? the 01 heads are known for being easy to crack. Also, was it stored for a long time with fuel still in the tank? could be condensation has formed inside the tank and contaminated the fuel in the tank with water, or the fuel pump could be acting up.

id second both keith and grimms posts. but the vaccum line seems like the culprit here. this might sound a little off beat but did you open the air filter box at all and check the filter. being that it was sitting sometimes rodents can get in there and bulid a nest. if the filter is choking you can get the dealyed power and weird rpms. ask me how i know. lol. its a long shot but it only takes a second to check. i would definitely check for old gas though.

I would also look at fuel pressure, check the fuel pressure regulator and perhaps change the fuel filter.

Unfortunately there is no fuel filter that is serviceable on the 01 xj
You sure you don't have a crappy california emission model? Mine is a dog compared to my boys same year, make, and model xj except he is 49 state and I am Crapifornia....

I would be inclined to not say it is an 0331 head problem. Mine had very different symptoms before it quit. But don't leave that off the table.

You might also want to consider a clogged cat. That could explain your loss of power at higher RPMs and the harshness.

You could have a bad MAP sensor, I would search about testing that.

And as everyone else said, you could be dealing with a vacuum problem.

Good Luck.

CherBear said:

You might also want to consider a clogged cat. That could explain your loss of power at higher RPMs and the harshness.

You could have a bad MAP sensor, I would search about testing that.

Click to expand...

With only 30k on it I'd be less inclined to suspect either of these, but yes they are also possibilities.

This thread is also in the street/performance section, mods please combine.

Thou Shalt Not Cross-Post - you get disjointed advice and can miss useful logical connexions.

Meantime, follow advice previously given and report results.

How long stored and where? Don't laugh, but have you checked for exhaust or intake clogged by mice?

Have you put a vacuum gauge to it?

Hesitation at lower RPMs, "strangled" feeling & bit of run-on would seem to me to possibly point at a lazy 02 sensor. It would be causing a slightly rich condition. Otherwise, what about a slightly leaking injector?

You could test the O2 sensor response and fuel trim with a live data scanner.