XJ performance issues

Hi all.

I've been having some issues with performance. It's very noticeable when going up hills. I'm barely able to keep the speed up while going up a moderate hill. Sometimes after climbing a hill (highway) I can smell a burnt smell from under the hood.

I've done some searching, and this is what I've gotten so far:
* Compression: Equal, and well within range (maybe it took a while to build up)
* Fuel rail pressure: good
* Spark plugs looks visually good to me. They all look the same, no buildup
* Exhaust backpressure < 1PSI
* Air filter and intake is not blocked
* No error codes
* I did a sea foam treatment trough the intake

While poking around, I did find that my exhaust manifold was cracked. While replacing that, I also cleaned up the intake manifold and replaced the fuel injectors.

I feel pretty confident that I did the tests correctly. This is the first time doing a compression tests. Even though I followed the intersections, there is a chance I missed something.

One interesting thing happened yesterday that made me think more about this. With the engine off, stopped on a relatively minor incline and in first gear, the rig started slipping backwards. It was a jerky movement, like engine was being turned over. This is a new thing. Having it in gear used to be able to hold it in place. I know I'm not supposed to park without the e-brake, but that's not the point. The point is that something appear to have changed WRT the engine. This makes me think that the compression is not great.

I know that my axle gear ratio is a bit low, but I have a hard time believing that's the root cause here. I noticed this issue a while after putting the lift on, and I feel like since it's a manual I'm able to compensate for the axle ratio.

Details about the rig:
* 4.5" lift w/31" tires
* 3.07 Rear Axle Ratio
* V6 engine
* '99 with 185k miles
* Manual transmission

At this point, I'm out of ideas. The only next steps I can think of is to start over again, or just give up an take it to a mechanic.

Any suggestions what to do next?

You could have weak fuel pump that can provide enough fuel output, at idle, but can not keep up with greater demand on upgrades.

Being a 99 with a V6 is a bit of a red flag. I'd start lookin at the trans.

I just did a fuel pump capacity test, and it gave me 1/3L in 7 sec. The service manual says it should deliver at least 1/4L in 7 sec. It seems like the fuel pump is still good.

I forgot to mention that it seems to be more thirsty for fuel since the problem occurred. I've not measured the MPG, but I have a feeling it's lower than it used to be.

Ian: What should I be looking at with the transmission?

Clutch maybe. Could be the burning smell.

Catalytic converter.

also, its an I6, not a v6.

Yeah, the thought of a worn clutch crossed my mind. I don't really get the feeling it's the clutch. When I go up a hill, or try to pass, it's more that I can't get the RPM up. I would expect the RPM to be high if the clutch was slipping.

I also was expecting it to be the catalytic converter. Especially since I've been driving with a cracked exhaust manifold for who know how long. I'm thinking the crack would mess with the O2 sensors, and the engine would run rich. Which would lead to gasoline in the exhaust, that would melt the cat.

I measured the exhaust back pressure at the forward O2 sensor port. That was less than 1PSI. That is within spec. If the cat was clogged, that would be higher.

I do have a decent dent in the exhaust, in the downpipe, right before the forward o2 sensor. Searching other threads, it seems like most people agree that's from the factory. The reason I bring that up is that I measured the back pressure after this "dent".


Let me know if I'm wrong in any of my assumptions / conclusions...

TPS maybe

I just read a little bit about the TPS. The service manual says to test the TPS measuring the voltage. I did not try to do that, since I didn't see any easy way to measure without cutting the wires.

I connected a OBDII scanner. That gave me the "absolute throttle sensor position". The value when the gas pedal was pressed all the way in was 78%, and when it was fully released it read 16%. Since the service manual does not describe this test, I'm not sure if those numbers are within spec or not. 78% seemed a bit low to me... Does anyone know what these numbers should be?

You report first noticing the problem after lift & tires, correct?
And the problem is a lack of power and poor MPG?
Buddy, I hate to say it - but your problem is probably just gearing.
I had a 98 4.0 / automatic with 31's and 3.55 gears. It was a total dog going up the hills around Oakland. So bad that when I decided gears weren't in the budget, I went back to 235 size tires. Boom: MPG and power, solved.

yossarian19 said:

You report first noticing the problem after lift & tires, correct?

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It happened after I put the lift and new tires on, but not immediately after. I'm sure that's a component, but I'm not convinced that it the only cause. I agree that's it's geared too high, but I feel I'm able to compensate pretty well since it's an manual.

fuel flow rates or clogged cats wont expain the creeping when the engines off, thats a worn motor, do a compression test.

Ummm, 3.07 gears and bigger tires.....

haavar said:

It happened after I put the lift and new tires on, but not immediately after. I'm sure that's a component, but I'm not convinced that it the only cause. I agree that's it's geared too high, but I feel I'm able to compensate pretty well since it's an manual.

Click to expand...

You can compensate while you are changing gears, yeah, but you can't do a damn thing about the gearing once it's moving.

As for the Jeep's tendency to roll - somebody check my logic if I'm messing this up - you now have 3" more tire on the Jeep, increasing leverage against the axle shaft & decreasing the engine's ability to compression brake.
It's also worth noting that on the Cherokee at work, you can put the parking brake on & leave it in first: on the right hill it will still creep on you, compression noises & all. Know what? It drives just fine going up the Sierra Nevada on our way to Truckee.

I'm not saying you definitely don't have a problem but you aren't describing anything I wouldn't expect from a totally healthy Jeep with 31's & 3.07's.

lazyxj said:

You could have weak fuel pump that can provide enough fuel output, at idle, but can not keep up with greater demand on upgrades.

Click to expand...

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