Hello all, after lots of searching and looking I've not found exactly the same issue as what I have. I actually have two issues.

The first is when I start the Cherokee, (doesn't matter warm or cold) the engine stumbles for just a few seconds after I've started driving it. Then if I goose the accelerator the engine smooths out and after that it runs fine.

The second issue I have is a very occasional glitch. Every once in a while, under heavy load/hard acceleration I'll hear a small "pop", almost sounds like a light backfire through the intake. After that It will stumble/hesitate under medium acceleration. It will also idle quite rough. If I accelerate really lightly then it's fine. I can accelerate hard and the vehicle will accelerate but the hesitation/stumbling is still there but not as pronounced. I may do this for a day, or it might do it for a week. The whole time the vehicle starts strangely, it might start very quietly, meaning that it starts but the classic rev's in acceleration that is normally there isn't. Or it might start VERY loud. almost sounds like a very hard knock then it will run at a rough idle. Then i may leave home or work and start it and it will be quite hard to start, and when it does it smooths out and after that it's running fine.

I'm at a loss. I think it may be my either my TPS or my MAP sensor. I've parked the car in the garage after dark with all the lights in the garage off to see if I have shorting plug wires but no sign of sparks.

The other day it was doing the hesitation/stumbling thing again and saw a post on here where someone turned the ignition key from lock to run and back three times and it temporally fixed his problem. I tried it and it worked.

Anyone have any ideas?

 

ya need a tune up man. ill bet the upstream o2 sensor is your main culprit, but heres my list:
-plugs
-wires
-cap
-rotor
-IAC
-TPS
-MAP
-inspect the ignition coil well
-o2; up and downstream
-inspect for vac leaks
-seafoam thru the brake booster vac line; 3k rpm, pour in "waves" more-less-more, adjust throttle as needed, its ok for it to bog a bit.
-oil; rotella is great

 

Gunna agree with the upstream o2 as well. They're the main cause to most driveability issues.

 

The 02 sensor doesn't come into play until a few minutes after starting, and things have come up to temperature. So it is not normally implicated on startup, or immediate driving hesitations.

Stumbling starts, with successful light acceleration, but stumbling, hesitating, backfiring under heavier acceleration (or heavier load, such as going up a hill). Is often associated with a fuel pump, or other fuel delivery issue that shows its face when the demand for fuel is higher (start up and under load).

Given that the fuel prime trick (key back and forth) caused a different behavior, this lends even more support that there are problems with fuel delivery.

So a tune up is always good (as mentioned). But you will most likely need a fuel pressure gauge, so that you can see what is happening there. That is where I would look first.

 

Thanks guys, I'll look at getting a fuel pressure gauge this weekend, then when the issue rears it's ugly head I can determine if it is a fuel delivery issue.

I'll post my findings when I get more info but if history repeats itself it could be weeks or months till it happens again.

 

This sounds like what mine was doing two months ago. Ever had the exhaust manifold replaced? Grab the flashlight and have a peak. It's free. Even the largest of cracks won't throw a code and cause you many headaches. Prone to breaking here at the collector because of bad motor mount design:

The theory behind it is your o2 could be brand new and not work properly since its sample reading is escaping before the o2 has a chance to read. I had the buck, pop and drive followed by fuel being dumped into the engine as the o2 thinks the engine is cold and thus, runs rich.

 

Thanks NewbJeep, I do have a tiny hole in the manifold, about the size of a pencil lead. Might have to see about getting it welded or replaced.

 

Pics. Everyone loves pics

 

Ok, here is the hole:



Small hole in manifold, could this cause ignition problems?

 

A $40 OBD2 tester from ebay would help. That said, I would check out the TPS, known for just those kinds of symptoms. I don't think it's your exhaust, or your O2, because it happens before it gets warm and O2 trim doesn't start until you're in closed loop.

 

Thanks 97grand4.0, I'll look into that this weekend.