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coasting downhill issues???

T

taki1203

NAXJA Forum User
Location
Concord, CA
sorry if this issue has been addressed previously. i'm new to the jeep scene, just bought a 99 cherokee sport (bone stock). recently while coasting down a large paved road i noticed that the tachometer would jump to about 1500 rpm then drop to about 900 rpm (and repeat over and over). on level pavement everything seems fine.

1) what causes this?
2) is this a big issue?
3) what the heck can i do to fix the prob?

thanks ahead of time, i've been reading posts for a little while now and everyone seems pretty knowledgeable and helpful.

oh ya, 99 cherokee sport, AW4 auto, about 82,000 miles, stock everything else.
 
Sounds like the Torque Converter locking and unlocking. If you're only doing 1500 PRM my guess is that you're in too high a gear for the speed you're travelling. I'm guessing you're doing about 40mph in OD? Drop it down to Third around town and save your tranny.
 
by my response, you can probably tell that i have no real idea how my jeep works (sorry im trying to learn).

my tranny is an automatic and im pretty sure that my jeep lacks over drive.
i'll check the speed nextime i head down the hill (on my way to work and school).


thanks
 
Yeah, you've got OD if it's an AW-4, and it will be an AW-4 in the Cherokee. Your gear selector is as follows:
P
R
N
(D) - this is Overdrive, and I bet the gear you automatically put the Jeep in
3
1-2
 
No worries - 98% of the folks out there probably drop their trannies straight into D when tooling around town; the problem is that the tranny tends to shift up too high, and you end up running around with the engine RPMs lower than they need to be. You end up lugging the engine when you try to accelerate from ~1500rpm, whereas if you were in Third at the same speed the engine would be closer to 2000rpm...anecdotal evidence says that's better for the long-term health of the engine as well as reducing heat in the tranny, which is better for longevity there as well.
 
You might want to check out your TPS; Throttle Position Sensor. Do a search, the high number of hits should tell you how often the TPS goes out. I noticed that a bad TPS not only made the torque convertor unlock more frequently - since it ffeds infor to the TCU (Transmission Control Unit) - but it also seemed to make the RPM's shut up higher with it unlocked.
 
Yucca-Man said:
No worries - 2000rpm...anecdotal evidence says that's better for the long-term health of the engine as well as reducing heat in the tranny, which is better for longevity there as well.
Click to expand...

I had heard before about the XJ with the 4.0 operating at maximum efficiancy at about 2000 rpm. Any technical support for this?

BSD
 
You should not need to take it out of D for any sort of around town
driving. If it is 'hunting' on a downgrade there is something wrong..

I don't find even any performance advantage to manually shifting
the 4L, except on long steep upgrades. No doubt the 4cly
is a whole different story..
 
Well, when my 98XJ with its 5 speed had around 50,000mi on it I was heading from home to allentown pa. While heading downhill on a steep 4 mile grade I had my check engine light come on. No change in how it ran, took 3 or 4 days for it to reset or cycle thru the 5 full cycles to reset it. It never did it again even on that same hill...
 
gjxj,
the tranny doesn't seem to be "hunting" for a gear, just the tachometer seems to be jumping around. like i said, i'll pay more attention to it when i go to school and post what i see and hear.

richp,
no check engine light, wish there it were on. at least then i could pull the error codes and see if the TPS (or something else) is actually bad.

if all else fails, i have a warrenty which covers vitually all of the jeep so maybe i'll use it.
 
okay update,
while driving home tonight, tried yucca-man's shifting into third recommendation and the problem still there. guess i'll try the TPS next :(
 
BSD said:
I had heard before about the XJ with the 4.0 operating at maximum efficiancy at about 2000 rpm. Any technical support for this?

BSD
Click to expand...
Yeah. Lay your mitts on one of the old sales brochures and look for the torque curve diagram. The early 4.0L engines had a torque peak at about 2400 RPM, IIRC. The first HOs raised it through the roof, then DaimlerChrysler wised up and dropped it back down into the high 2000s or low 3000s. Other factors (such as the exponential increase in drag as speed increases) aside, the most efficient speed at which to operate any engine is the speed at which the engine makes maximum torque.
 
Eagle said:
Yeah. Lay your mitts on one of the old sales brochures and look for the torque curve diagram. The early 4.0L engines had a torque peak at about 2400 RPM, IIRC. The first HOs raised it through the roof, then DaimlerChrysler wised up and dropped it back down into the high 2000s or low 3000s. Other factors (such as the exponential increase in drag as speed increases) aside, the most efficient speed at which to operate any engine is the speed at which the engine makes maximum torque.
Click to expand...

91-95 torque peak is around 4000, 96+ is at 3000, but there isn't much of a difference because the "peak" is very small. It's mostly a straigh line through the RPM range...
 
mine "hunts for gears all the time as well- especially around town, it feels like it shifts half way- if there is sucha thing- and thien goes back to original gear after kickin the rpm's DOWN and then returning to normal I've never replaced the tpc or cps
 
Eagle said:
... the most efficient speed at which to operate any engine is the speed at which the engine makes maximum torque.
Click to expand...

Maximum torque is produced with a slightly rich mixture. Maximum fuel efficiency is usually at an RPM lower than maximum torque. According to the EPA, RPM for maximum efficiency is at about 75% of peak torque.
 
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