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RPM/Speed question....

J

jeepman121

NAXJA Forum User
Location
Ohio
My buddy has a 99 xj 4.0 with a fresh motor bored .40 over with 23575R15 and 3.55 gears. He has been noticing a lagging feeling. His brakes are not sticking. I have a 95 same tires and gears.... we were side by side at 45mph in 3rd gear (auto tranny) I was at 1500rpm and his was at 1850rpm??? is there an explanation for this?? he claims that my jeep seems to have more pep under a load...especially going up hills. I was thinking maybe his torque converter wasn't locking/unlocking properly. thoughts???.......
 
The first thing that I would do is to verify the accuracy of both tachs with a handheld tach. Then I would verify that the gears are truly the same (I am sure that you already have). If they both check out then maybe the transfer case? Other than that I am clueless. Good luck, post your results.
 
I know that the 99's have a more restricted head but twice the intake volume of an earlier 4.o.....would this effect anything??
 
I was thinking along the lines of whether there was a different output gear reduction in the transfer case of trans. I don't think that's the case though.
 
Check tire pressures, are the tire brands the same? tread wear? anything at all to create a difference in diameter. Im sure youve already looked into that but just being thorough. Could be torque converter lockup, you sholuld know if its locking up though if it has engine braking when coasting, right?
 
A 300 RPM difference indicates you guys are using different gears either in the AW4 or in the diff.

I would check the shift on the AW4 to be certain the intended gear selected is actually the gear it is in.

Make certain that both XJ's are infact running the same gear in the diff. Unless both of you have the XJ from new, the gears could have been changed by a previous owner.

Why not run both XJ's in 4OD for the comparison test? 300 RPM is too much of a difference.
 
Absolutely sure we both have 3.55's, both running 235-75-R15's, both a have aw-4 with np231 TC's and neither are weighed down...My buddy is convinced his takes more throttle to achieve 45 mph in 3rd than mine.....I'm still convinced his torque converter is acting up. He thinks there might be bad bearings in the tcase (i told him you should hear/feel that) am I right?? is there possibly a different final gear ratio in the tranny??
 
How many miles does he have on his fresh rebuild?

When we built the stroker for Goatmans KOH buggy, the first 3 or 4 times out you REALLY noticed drag on the engine - it was new and tight, and not worn in. The first time out, we were getting ~2-2.2 mpg at a casual pace, but had to stay on the throttle. By the time race day came around, it was FAR more peppy, and up to ~4-4.25 mpg.

Mind you, that's only talking about 120 miles of driving, but it was mostly in second gear at 4,300 RPM's.
 
Motor has about 7000 miles. We have not done a side by side...However I noticed that when my jeep is cold will run about 1750 rpm once warm it runs at 1500 rpm. (3rd gear 45mph) both of our jeeps were running at operating temp when we did the side by side.
 
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