Jeep Enthusiast Forums banner

Bucking, Sputtering, Wierd idle IAC motor, TPS, Im stumped??

9K views 42 replies 10 participants last post by  cp3 
#1 · (Edited)
Ok to start i had another post on here about some problems with my jeep. Its a 98 jeep grand cherokee laredo 4x4 I6 4.0 engine. A few months ago i had some problems with my jeep bogging down and acting like it was starved for fuel almost anytime. It kept throwing a P0138 code, turns out it was the wiring on the wiring harness it has since been repaired. The probs stopped for a week or so and now they are back. My jeep started bogging down again and acting like it was out of fuel. It happens mostly now when i pull off from being stopped, not in park just sitting at idle. And really bad in hot weather. Yesterday when i was pulling off from a red light it started bucking and jerking again it hadnt done this in almost a week. Now while it is bucking, jerking, bogging down etc. i can feel and hear a clicking or wierd noise kind of like a clicking thump if that makes any sense lol that is underneath the gas pedal. Then when i let off the gas pedal for a minute it stopped and caught up, as it always does. Then drove fine the rest of the way home. Then last night i was sitting in a drive thru and all of a sudden my jeep started revving up on its own and my foot was on the brake but the jeep was trying to take off and kind of lurching forward. None of the other instruments in the dash panel change except the rpm when it either bogs down or revs up while im sitting at idle its always only the rpm that does anything. I have read that it is most likely the IAC or the TPS is this true. If so which one is it most likely to be?? I am so tired of trying to find the real problem it wont do it when anyone important that could tell me what is going on drives it. It is driving me crazy!!! I cant really go that far im too afraid that it will either take off on its own or not go fast enough and get me hit. What should i do??? Any help is greatly appreciated!!! Im lost at this problem:brickwall
 
See less See more
#3 ·
thats what i thought but my mechanic friends say that could be one of the problems. i think that the downstream o2 sensor needs to be replaced but everyone kept saying that it wasnt the sensor, it was the wire harness. but even after it was fixed it threw the code once (the p0138) but hasnt since i had the codes cleared with that machine the week before last. it just messes up w/o the cel coming on. and the problem is intermittent. i just wonder if it could be a combo of both or all three. i could be wrong since i know nothing about cars, but why would it constantly throw that code if there isnt something wrong with it. i do have a minor exhaust leak that i know i need to get fixed.
 
#8 ·
Does this bucking etc happen shortly (2-3mins) after starting the engine and lasts only ~ 2mins? This might be due to an aging upstream (not the downstream) O2 sensor. It's easy to determine if it's indeed caused by the O2 sensor, just disconnect it. The connector is attached to one of the engine oil pan studs on the driver's side, approximately above the axle. The 4.0 engines seem to be effected quite often by this issue (ZJs and WJs). With the O2 disconnected you WILL get a Check Engine Light. Don't worry that won't cause any harm, just don't plan any 1000mi road trip with a disconnected O2 sensor. A few days of driving like that won't hurt anything. If the bucking goes away, you diagnosed a failing (aging) O2 sensor. Replace with MOPAR only (not Bosch!!!) or you'll have the same issue with a new one!!! Let us know if this solves your issue.
 
#10 ·
Yes it usually does i dont go too far out of town due to the probs it does do it other times also but not always usually its only after a few mins after i leave my house my boyfriend lives about 6 miles away and it does it about halfway to and from his house almost always when i come out of a stop at a stop sign or stop light. it doesnt do it for a whole two mins. if i let off the gas pedal it will slowly stop sputtering and get back to normal but if i keep pushing the gas pedal nothing changes and it wont go any faster than 10-15 mph. a while back the only time it did it to my bf he said he could push the gas pedal all the way to the floor while it was bucking, sputtering etc. and not going but 10 mph. he says its the fuel pump, but everyone keeps saying the pump either works or it doesnt there is no in between
 
#14 ·
Also as a side note, fuel pumps CAN degrade without completely failing. I haven't had experience with it in a Jeep, but I have had it happen three times with Chevrolet. And considering it's the same company that makes the fuel pumps for both...
 
#15 ·
thats what my bf said and i have a friend whose wife drives nothing but jeep and he said that hers did the same thing only hers died a lot quicker. i may have them pull off the gas tank and check the fuel pump and make sure its ok as well as check the tps, iac valve, and both o2 sensors. i figure that i am bound to find it. and alldata has helped me quite a bit. it just that so many probs have the same symtoms and its hard to pinpoint the acutal prob causing the conditions i am having. thats what is sooooo confusing.
 
#19 ·
just so you know I replace my 02 sensor,cat, tps, IAC, and fuel filter and yet it would still sputter after it warmed up. i ended up replacing the PCM and everything is back to normal. It took about a month to finally realize the PCM was faulty as it would not throw codes and if I disconnected the battery, it would run great until it learned it's evil ways.
 
#20 ·
Ya my CEL came back on last night again and, i would bet money that its the stupid P0138. I hope that i dont have to replace the PCM. I hope its just the 02 sensors. I had thought it was my PCM a while back due to crazy electrical probs, but i think that was cause my battery cable was a little loose since it stopped after it was tightened just the other symptoms stuck around. I have been looking at prices for the PCMs and they are quite high for a new one...if you dont mind my asking where did you find yours? i just hope its not that serious, but i will do what i have to do i guess.
 
#22 ·
So does anyone know how to test the pcm?? do they do that anywhere or do i have to take it to the dealer...or is it even possible. the mechanic out in the shop where i work that drove it today after we disconnected the upstream o2 sensor is leaning torwards the pcm. i did find some re-mans on ebay....i was just checking up on em i am hoping thats not it though i hope it is just the 02 sensor prob. so if anyone knows how or where to test the comp. please let me know...
 
#24 ·
Its not the downstream O2 sensor all that sensor does is tell the PCM what shape your CAT is in by monitoring exhaust after it leave cat will not affect running I've had code tripped for downstream but the upstream was bad you unplugged the sensor and the engine goes into closed loop mode gives engine a preset set of values so it can run but the settings are not optimal and does not guarentee it will run perfect I'll bet it needs replaced
 
#25 ·
it wouldnt surprise me lol since all of the car companies are going bankrupt... so what have you found out anything on yours yet?? after i unplugged the upstream one today it had a little hesitation upon takeoff. but so far no actual bucking, jerking, or surging or clicking noise in the gas pedal. it seems to still run rough but thats surely because the o2 is unplugged. i also have an exhaust leak near the manifold so i am going to drive it like that one more day and see what happens. i hope thats it i have an alldata account and some of the causes it lists for the p0138 i could do without. the most expensive being the pcm or the fuel pressure regulator which from what i understand is a single unit so you have to buy the whole pump. i have had a lot of people tell me that if its not the o2 sensors it could be the fuel pump, or the pcm. there is just so much stuff that causes probs. like that. so lets hope its just little stuff for both of us.
 
#32 ·
Kolak is a member on here and an absolute GOD at finding replacement and upgraded parts for all things Grand Cherokee.

PM him on here.

[EDIT] Saw a post from him in another thread. Here's his info. He's a vendor on this site.

Kolak Performance and Offroad
Member: Specialty Equipment Market Association
Telephone: 480 998-3661
kolak@aol.com or kolak@kolakperformance.com
 
#42 ·
After having this same problem on my 94 with a 5.2 I changed EVERYTHING that people have been talking about on here the TPS, New Tranny, New fuel filter, New fuel pressure regulator. Sometimes it would run great then other times just very sluggish and especially between about 1000 to the 2000 rpm mark. It didnt matter if it was hot or cold just very frustrating and intermittent.

Well I started to suspect a vacuum leak. While monkeying with the vacuum lines and wiggling them I got a surge and thought I was on to it. It wasn't the vacuum after all.

It was the wires that go into the TPS and when I would wiggle them it would surge and even cut off the engine. I unplugged it and Ohmed out the wires while my ohm meter was plugged into and wiggled the wires and I didnt get any Resistance drop. I figure out that the pin that comes out from the ground part of the TPS was not making a good contact with the connector. When it was not making a good solid contact I would get the problem.

You can fix this one of two ways.... you can replace the entire plug assembly to one where the female end of the connectors make a better contact or you can take up the slack in the now too loose female end with a small piece of wire inserted into the tooled out female end (lol) and now it wont be like a hotdog in a hallway and will make a good ground. Plugged it all back in and now runs like it should with full power all the way through the spectrum.

This worked for me so you may try this!! Its a connection and grounding problem not an actual prob with any of the devices.

if your grounding and connections in that TPS are not solid it spikes all up and down sending the wrong voltage to the rest of the injection system.
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top