ok i have this problem, when i start my jeep up cold, well i live in palm springs so cold is 70 degrees, it runs fine for about 1 minute, then it idles rough and when i throttle it up quick with my hand it pops and sputters till i let off. when i throttle it up slow, its just as smooth as glass. now this is my 87 waggy 4.0 auto trans....
where should i start?????

 

I have the same issue on a 89 cherokee I recently purchased. The guru suggests checking 2 possible suspects. First the cat converter may be clogged. Second is to check the throttle position sensor.

 

both check ok. unhooked the cat to check it out and performed the basic check outta the book on the throttle pos sensor.
thank you, at least i got to check some stuff...

 

The "check outta the book" is probably a simple test of adjustment, but not of function. Useful for one test, but not for another. You need an analogue (sweep needle) for a "function test" - but the test itself is simple:

1. Disconnect TPS from harness
2. Find two terminals that vary in resistance when the throttle moves.
3. Leave the meter connected
4. Watch the meter while you SMOOTHLY move the throttle by hand.

You do not need the engine to be running for this, so don't bother starting it.

If you see the needle "hitch" or "skip" at any point, you have a flat spot in the TPS - which can cause symptoms like this.

You say you'd Dx-d the cat to check for clogs, and negative results, yes?

Does this problem go away when the engine is warmed up? A lot of RENIX problems (and funky idle problems on inline six engines, particarly side-draught engines) come back to vacuum leaks from manifold screws loosening. It may be useful to check torque on them - I do as part of an annual inspection. I invariably find two or three a bit loose - although usually at the bottom rear.

Might also be instructive to use your analogue meter and backprobe the MAP sensor while you "blip" the throttle and again while you run the rpm up slowly - it's also a variable resistance. While it isn't commonly subject to skipping or hitching (from "flat spots" in the sensor response,) it's not unknown.

The MAP and TPS are the two variable-resistance type sensors that come to mind for idle and throttle response issues. Vacuum leaks are also a common possiblity. (I could go into the mechanics of how and why it happens, but you're probably not interested right now. It is fairly simple, tho.) I've had TPS problems that showed a properly-adjusted TPS, so I don't consider checking the adjustment to be a thorough test anymore. As far as the function test I outlined, you're more interested in relative changes than absolute resistance values, so you may have to fiddle around with the range selector on your ohmmeter to get a good picture of what the sensor is doing.

 

thanks 5-90, yeah i will try your way tonight. also, i really do need to check all my vacuum lines and hook-ups. now i got some work to do...think i'll do a compression test too, just to see where im at....
you guys are the best

 

When you solve your problems, please post the results. I will gladly accept any additional advice on what to check on my 89.

 

My motorcycle did this when I had water in my fuel. run some Seafoam if you haven't recently and see if it helps clear it up. Who knows what crap is in fuel these days?

s

 

that was the first thing i did, then i tuned her up, then i replaced the o2 sensor. im gonna go on a vacuum leak hunt tonight, and a couple other things, maybe a fuel filter also....
i will post my findings, i know this happens alot and it would be nice to narrow it down, but all xj's are different, so we shall see....

 

I was gonna say check the fuel filter. You tried sea foam though a vacuum line or the TB as well?

 

Well I'v erespected your work after reading you builds and such so I thought you would have covered the basics first. I just wanted to make sure and you certainly have.

 

hey, theres never a bad time to make suggestions, i do appriciate all help. gonna try and work on her this weekend, busy work week...dooooooohhhhh