92 Jeep Cherokee 4.0L - Rough Idle - TPS Readings

Hello to all,

This is my first time using the scope and following the directions included in the Snap On Modis. I'm working on my 92 Jeep Cherokee with the straight 6 4.0 motor, 4WD automatic. It currently has a rough idle and a possible misfire. I've replaced all the 25 year old vacuum lines and confirmed with a smoke machine that there are no vacuum leaks.

Below you'll see TPS voltage sweep graphs that seem to show a bad spot on the TPS at about 60% throttle. The instructions in the Modis state that there should be a smooth transition from idle voltage to WOT voltage. Included are both the idle to WOT and WOT back to idle graphs.

What do you guys think? Replace the TPS is what it's looking like to this diagnostic newb!!!

IDLE TO WOT


WOT TO IDLE


Thanking you all in advance for helping me to interpret these readings.


Steven

Definitely looks like you have a bad TPS.
Does it only run rough at idle?

LarryTheCarGuy wrote: Definitely looks like you have a bad TPS.
Does it only run rough at idle?

Yes, it has a intermittent stumbling idle that I think may be a misfire, but I'm not getting any codes. It drives fine, has power, and nothing that I can see bad coming from the TPS at 60% throttle while driving it.

I agree with Larry it looks like a bad TPS. What do your fuel trims and O2 look like?

So it runs smooth, intermittently stumbles then runs smooth again?

Dave101 wrote: I agree with Larry it looks like a bad TPS. What do your fuel trims and O2 look like?

There are no fuel trim PID's in the scanner (Modis). I'm not sure a 92 Cherokee provides that information. The O2 was recently replaced and looks great. On a cold engine start it begins to switch after 1 min or so and continues throughout idle and 2500rpm, ranging from .100mv to .850mv. I do snap the throttle and see it react accordingly. I have not tried the enrichening test to confirm it's response to that yet.

LarryTheCarGuy wrote: So it runs smooth, intermittently stumbles then runs smooth again?

That's exactly how it acts while idling.

There should be fuel trim data, it might not be labeled fuel trim though, I cant remember what chrysler called it on OBD1
How often does it stumble? how do the plugs/wires look?

LarryTheCarGuy wrote: There should be fuel trim data, it might not be labeled fuel trim though, I cant remember what chrysler called it on OBD1
How often does it stumble? how do the plugs/wires look?

I'll go out and put the Modis on it and get you some screenshots of the PID's that I have available.

The plugs/wires/cap/rotor were all changed out about 200 miles ago. It was doing this prior to those being changed... which tells me those weren't the culprits.

Be right back with those screenshots.

Here are the PIDs I have available. I let the engine get warm, but probably not up to full operating temp. The only ODD PIDs that I see are the Ignition 1, 2 and 3.

Answering your question about how often it stumbles.. it's random and it happens every 3-5 seconds, sometimes more often, sometimes less. It's very intermittent.

I don't have a misfire count PID, so I can't go by that...

I'm struggling to understand the entire problem so bear with me please. Your main complaint is that the Jeep runs rough at idle. Very interment but still stumbling. When you go on a test drive, the vehicle runs fine. By the first screenshot that you sent, the vehicle definitely looks like it would need a TPS, but you would have a stumbling while driving, or at idle doing a snap throttle. Please tell me if I am misinterpreting the symptoms. I am wondering if you possibly could have more than one issue????

Dave101 wrote: I'm struggling to understand the entire problem so bear with me please. Your main complaint is that the Jeep runs rough at idle. Very interment but still stumbling. When you go on a test drive, the vehicle runs fine. By the first screenshot that you sent, the vehicle definitely looks like it would need a TPS, but you would have a stumbling while driving, or at idle doing a snap throttle. Please tell me if I am misinterpreting the symptoms. I am wondering if you possibly could have more than one issue????

Thanks for responding Dave.

Yes, the jeep runs rough at idle and has an intermittent stumble, or what feels like a misfire (while at idle). ON a test drive, it drives fine. I do not notice any hesitation or issue while test driving. On a snap throttle, I definitely notice a bog down on the very first second of the snap, probably as the TPS reaches that bad point. I've already ordered a new TPS based on the sweep wave form.

There very well could be a second issue here. I just recently received the secondary ignition adapter I needed to take a look at those waveforms, so I'll be working on that ASAP. I did perform a fuel injector balance test and all are exactly the same at 33psi after initiating the test from a resting pressure of 37psi... so a 4psi drop across all 6 injectors. The running PSI is 32psi with the vacuum fuel regulator hooked up and 40psi with the regulator disconnected (per the FSM). So all within spec there.

The other weird part is I have no fuel trim PID's in any of my scanners. I tried the Snap On MT2500, Solus Pro, and MODIS. All I have available across all 3 scanners are what you see in the previous screenshots provided earlier in this thread.

Not sure Im understanding your problem, when you say a stumble is it like an intermittent miss or just a sorta rough idle?
How many miles are on the engine?

BTW for the most part all snap on scanners will show the same data, if something is missing its best to try a different brand tool.

LarryTheCarGuy wrote: Not sure Im understanding your problem, when you say a stumble is it like an intermittent miss or just a sorta rough idle?
How many miles are on the engine?

BTW for the most part all snap on scanners will show the same data, if something is missing its best to try a different brand tool.

It's more like an intermittent miss. Picture the engine idling smoothly... then you'll see it jump as it stumbles, then it'll purr for a while, then jump again and so on.. there is no pattern to it.. it's random stumble events. To me it seems like a misfire, but to your trained brain it might mean something else.

I just scoped all 6 cylinders looking at the ignition wave forms and I did not see any miss anywhere. I snapped the throttle and the wave forms all looked normal. I'm going off of the explanations of ignition wave forms that were taught in the Scanner Danner Premium videos as well as the pictures of them in the book. So I believe I'm good there.

What you are describing sounds more like an IACV problem or vacuum leak.

Andy.MacFadyen wrote: What you are describing sounds more like an IACV problem or vacuum leak.

Thanks Andy. I also ordered a new IACV a couple days ago along with the TPS sensor. I was getting a weird reading on the IACV when I scoped it. The IACV has 4 terminals, labeled A through D. The Snap On guide stated to check the wave forms of terminals A - D and then B - C. It stated the two terminals should be switching from their resting voltage of what I saw .063 volts to 12 volts. When I scoped A - D, that's exactly what I saw (see below for example) both were switching places with each other. Then when I scoped B - C, they weren't switching (swapping places with each other), they were synced up and following the same pattern. Since this doesn't follow the Snap On guide and I believe something was also wrong with the IACV, I went ahead and ordered a new one since it was pretty cheap. I hope this conclusion was correct. What do you think?

Pins A & D with what looks like proper switching


Pins B & C with what doesn't look like proper switching

I'm not sure about the IAC but those readings look suspect but I'm not sure. Want to go in another direction since we know we have good fuel and spark. Have you done a relative compression test just to rule out a low cylinder?

Dave101 wrote: I'm not sure about the IAC but those readings look suspect but I'm not sure. Want to go in another direction since we know we have good fuel and spark. Have you done a relative compression test just to rule out a low cylinder?

Thanks Dave. No I haven't done a relative compression test yet. I have a low amp probe on the way and once arrived, I will perform that test.

I agree on the change of direction though.. thanks