Less than optimum power at higher speeds.

[SubSonic]

OK, well I finally got the clutch sorted out 75mm of spacing later...

 

Well as I said before I just replaced the crankshaft position sensor when the tranny was dropped, seemed like a good time ;) and the motor is damn near brand new (1991, H.O.) 6 months since I put it in...

 

The crankshaft pos. sensor seems to have improved the long cranking time some... (sometimes it cranks right up like my cherokee used to and other times it cranks for a little while, but nowhere near what it would do at times before.)

 

Lower speed (less than 40mph) has been drastically improved and is spot on with a buddy of mines cherokee (granted his is an 01 with an auto and 4x4, which i'd expect might slow it down a bit) Mines a 1991, 2wd. (manual)

 

But past about 50 or so it just seems like the power isn't there like it should be, it doesn't bobble or anything it just seems like it's not making a whole lot of power. It will eventually get up to any higher speed, I just feel that it takes longer than it should, and downshifting to 3rd/4th if I want it to accelerate. I have noticed a very slight rough idle that most probably wouldn't even notice, it's pretty minute but it is there. I had a 93 cherokee, so I know the 4.0 isn't the smoothest motor but I can tell when something is just slightly off. Also the gas mileage seems just under what I would expect... I do A LOT of highway driving and the Cherokee averaged well over 20mpg and it was a 4x4 manual, I'm averaging 15mpg... (I understand drag might have an effect)

 

So on my list of things that were NOT replaced when the motor was are: wires, o2 sensor, cap/rotor, fuel injectors...

 

But before I just start throwing these parts at it since money isn't unlimited I wanted to see if you guys had some opinions of a cause...

 

Thanks in advance...

[Eagle]

But past about 50 or so it just seems like the power isn't there like it should be, it doesn't bobble or anything it just seems like it's not making a whole lot of power. It will eventually get up to any higher speed,I just feel that it takes longer than it should, and downshifting to 3rd/4th if I want it to accelerate. I have noticed a very slight rough idle that most probably wouldn't even notice, it's pretty minute but it is there. I had a 93 cherokee, so I know the 4.0 isn't the smoothest motor but I can tell when something is just slightly off. Also the gas mileage seems just under what I would expect... I do A LOT of highway driving and the Cherokee averaged well over 20mpg and it was a 4x4 manual, I'm averaging 15mpg... (I understand drag might have an effect)

Are you saying that you are trying to accelerate from 50 MPH in 5th gear, and you think you don't have adequate power?

 

If that's what you're saying ... you're right. They put that shifter handle beside your right knee and gave you five forward gears so you can match the engine speed to the road speed for optimum power. Assuming stock tires are gearing, at 50 MPH in 5th gear your engine is turning approximately 1400 RPM. With a 1991 4.0L HO engine, the torque peak on your engine is around 4,300 RPM. For acceleration, you want the engine to be operating somewhere near the torque peak. Of course you aren't getting acceleration.

 

You can't compare a 5-speed to an automatic. The automatic downshifts automatically when you begin to accelerate. You should be doing the same thing with your manual.

[SubSonic]

Sorry if I mistyped what I was trying to say... No the lack of power in 5th gear is more around 75mph, where I would expect to not have to downshift in order to reach 80 or 85 in any sort of timely manner. What I was attempting to say is at higher speeds 50+ the pull (in any gear) doesn't seem to be terribly strong.

[Eagle]

Even at 75 MPH the engine is still only turning about 2000 RPM. That's more than 2000 RPM below the torque peak and the horsepower peak. Even at 75, if you punch an auto it'll downshift. There's a reason.

 

BTW -- another reason not to compare a 5-speed to an automatic is that the automatics have different axle gearing.

[kro10000]

You most likely have 3.07 gearing.... Which in one word, SUCKS... It is geared way too high, and with my with 3.07s and 31ish tires I have to downshift at the slightest upgrade on the highway

[SubSonic]

The gearing actually makes good sense. And no I was only comparing launch from first to higher speed with the auto. The highway was a comparison to my 93 Cherokee which was also a manual tranny. It had much more kick at those speed, even in fifth. Believe me I'm not expecting earth shattering acceleration in 5th (afterall it is more or less just an od gear), but I would hope that I could actually accelerate in fifth to some degree, even very slowly. However, the gearing is a definite possibilty for the difference from my Cherokee because even though it was a manual it was 4x4.

[kro10000]

...Well... The XJ should have 3.07s too, although it is possible that it doesn't... Only way to find out is to crawl under and check tags and/or count teeth