Allpar Forums banner

1 - 15 of 15 Posts

·
Registered
Joined
·
574 Posts
Discussion Starter #1
Jeep Grand Cherokee is having a difficult time going up hills. I have driven it and when you hit the gas on the highway for "passing gear" (especially uphill), it makes a quick BUMP, and then has no power at all to pass or climb the hill. You have to drive it slow (or let off the gas some) to make it not downshift and it will gradually climb the hill or complete the pass.

That quick BUMP happens mostly if not always when going even uphill just a little bit.



Also... I notice that if we are at a drive-thru, stop light or low speed you can hear a light rumbling or sound similar to cat converter rocks underneath the truck.

I don't know if one of these things are the reason for the other, but these are the problems that the truck is having.

Thanks for reading. What do you all think is troubling this Jeep?
 

·
Super Moderator
Joined
·
21,361 Posts
If the ceramic element inside the cat has broken up, it will rattle like rocks and possibly cause an exhaust restriction when it blocks the exit of exhaust flow at the outlet. This will cause a loss of power when acceleration or uphill loads are demanded. It can also be a fire hazard.
Is the 'bump' a harsh downshift or backfire?
A 'direct-fit' replacement is usually the best way to go, but that option may be restricted in California by CARB.
 

·
Registered
Joined
·
574 Posts
Discussion Starter #3
Hmm, this is interesting....


I forgot to ention that the check engine light it not on. I don't know if this would maybe cancel out the clogged cat theory.

If it doesn't, would the truck be able to perform fine during normal driving, say around the city, but act up ONLY when highway passing or downshift for uphill is needed? I ask this because that's the only time this seems to happen.

The "bump" doesn't seem to be a downshift, since I don't think the RPMS move when it happens, but I'll check again next time to be certain. It's like a quick bump/tap. I don't know how else to describe it. However, right after that happens, it has no power to pass, or it will be realllly slow uphill. It could be a backfire, but I do not hear anything.


Thanks for the reply.
 

·
Registered
Joined
·
6 Posts
Nelly said:
Hmm, this is interesting....


I forgot to ention that the check engine light it not on. I don't know if this would maybe cancel out the clogged cat theory.

If it doesn't, would the truck be able to perform fine during normal driving, say around the city, but act up ONLY when highway passing or downshift for uphill is needed? I ask this because that's the only time this seems to happen.

The "bump" doesn't seem to be a downshift, since I don't think the RPMS move when it happens, but I'll check again next time to be certain. It's like a quick bump/tap. I don't know how else to describe it. However, right after that happens, it has no power to pass, or it will be realllly slow uphill. It could be a backfire, but I do not hear anything.


Thanks for the reply.
I would still buy or borrow a code reader and do a scan on it, beacause the check engine light could be burnt out. These symptoms sound exactly like my Gr. Cherokee when the Throttle Position sensor went out. I think its only a $20 -$30 part. I got a good one at NAPA auto with a 3 year warranty.
 

·
Super Moderator
Joined
·
21,361 Posts
If the cat element is loose and rattling, it still may be working well enough not to set a P0420- catalyst efficiency fault. It also takes a long time to set a cat code. A pending code or one-trip failure won't turn on the 'ck eng' light.
If the element rattles, I would not hesitate in replacing it, if you plan on keeping the vehicle.
Make sure that the yellow 'ck eng' light lights at key-on for a bulb check if you have doubts about the lamp being burnt out (I believe that it is a long-life LED anyways).
The TPS may also not set a fault code, even when it is failing. The TPS signal may still be believable to the PCM, but if you were to unplug it, that would set a fault code.
Not everything will set a fault code. You still may need a scan tool and professional diagnosis to form a repair plan for this.
 

·
Registered
Joined
·
574 Posts
Discussion Starter #6
Thanks for the replies!

No codes are present and did the key dance AND hooked a scanner up to it and got no codes. The check engine light works fine.

I began looking into cat converter prices and I see a listing for it having 3 cats? :huh:

Something about 2 precats and then one regular one?

Anyway, which one (if there are that many) is likely to be the one rattling. I'm going to have to look under the truck again at some point, because I only one cylinder looking exhaust component under there last time. I don't know where the precats would be.

As for the tps.... what should I be looking for if I can get a scan tool to tell me more information?


Thanks again!
 

·
Super Moderator
Joined
·
21,361 Posts
The 2 smaller pre-cats are part of the exhaust pipes off of the engine. One for the first 3 cylinders and the 2nd for the last 3 cylinders. The larger main cat is under the vehicle and more likely to rattle. Getting under the vehicle and 'thumping' it with a rubber mallet or fist should reproduce the rattle if a loose internal element is the cause.
The TPS can be tested with a volt/ohmmeter by watching the signal wire-to-ground voltage (with the key on) track steadily and evenly from about 0.65 v at closed throttle position up to about 4.70 v at full throttle. The other 2 wires to the TPS are the 5v in and ground, which should be 0.3 v or less. A good scan tool can also show this signal voltage, percent throttle opening and minimum TPS on the display.
 

·
Registered
Joined
·
574 Posts
Discussion Starter #8
Okay everyone, I'm back with some good news (somewhat).

We went in to have the brakes done on the Jeep and had the mechanic look to see if we could use a direct-fit or universal converter.

The brakes were done and the mech found the root of the rattle underneath I talked about. Turns out it was a heatshield above the main cat that was the culprit! No more rattle underneath! :thumbsup:

However, the no downshift when trying to pass is still there. I'm going to learn more about the tps, and I'll be back when I discover anything!
 

·
Registered
Joined
·
8,042 Posts
Have you ever changed the fluid and filter in the transmission? Perhaps a fresh change with Mopar ATF+4 will help it. How many miles do you have on it?
 

·
Registered
Joined
·
574 Posts
Discussion Starter #10
The truck has a little over 150km. It was bought used at around 75k and was never changed (I was introduced to this car three years later).

I followed the therory that if it hasen't been changed, don't touch it as it could make matters worse (though nothing was wrong).
 

·
Registered
Joined
·
8,042 Posts
I don't go by that theory at all and wonder how it ever got started. I bought my 99 Grand Caravan with over 150,000 miles on it and the previous owner (3rd) didn't have a clue if the fluid and filter had ever been changed and he'd bought it with 90,000 miles on it. Within a week of purchasing it, I'd changed the transmission fluid and filter, air filter, oil and oil filter, new plugs, wires and PCV valve. I've not had one problem with the transmission and yesterday, it rolled over 199,000 miles. When it hits 200,000, I plan on changing the transmission fluid and filter again. If you do it yourself, make sure you use nothing but Mopar ATF+4. Dexron with an "additive" will kill your transmission.
 

·
Super Moderator
Joined
·
21,361 Posts
Agree. ATF+4 only and note what debris is in the pan bottom and on the chip magnet. Maybe let a transmission shop road test it and offer a preliminary diagnosis and estimate?
 

·
Registered
Joined
·
749 Posts
Catalytic converter, throttle position sensor, and transmission [plus linkage] are all possibilities; but I would check the ignition timing. It's at the age where the timing chain could have slipped a cog or two. A bad oxygen sensor is another possibility and it will show up in a scan. The crankshaft position sensor could be bad and should show up in a scan too.
 

·
Registered
Joined
·
574 Posts
Discussion Starter #14
hmm, these are all interesting suggestions. Thank you.

The sensors suggested, is there anything I can learn from a home scanner? I have one that gives alot of data via home computer.

I *might* consider that transmission fluid change if I run out of options.


ImperialCrown said:
A good scan tool can also show this signal voltage, percent throttle opening and minimum TPS on the display.
I think with the scanner I have I can get these answers. What should they be normally?
 

·
Registered
Joined
·
749 Posts
I don't remember what years it was but the kick-down linkage to down-shift the transmission under heavy throttle was changed from mechanical to electrical and there was a solenoid instead of a rod and lever. If you down-shift manually does the transmission drop to the next lower gear and climb OK?
 
1 - 15 of 15 Posts
Top