Low RPM cut out

Lots of threads regarding rough idle & bad running jeeps. None that I've read seem to describe what I'm experiencing.

I have an '87 Cherokee "Laredo", 4.0 5spd. Sat for 5 years in the desert before Some work was done to get it running again, then the jeep was sold to me because the guy didn't want to pay back fees for registration.

The engine idles fine. At low RPMs/idle if i punch the throttle it will hesitate & stutter/choke for a less than a second before revving up. Otherwise, there are no problems what so ever. I don't have to baby it, i just can't punch it.

Saturday i was finally able to drive it. Back-registration was all caught up & was taking it to the smog shop to take care of that so I could get brand new tags. I noticed that when coming off a stop sign at low RPM, the jeep would start to bog down. Push the clutch in, throttle up to around 3k RPM & it takes off no problem. I don't have a tac so i can't say for sure, but I'd call it around or less than 2k RPM is when it bogs out under load.

Another thing - if i was cruising down the road (again, around 1.2-2k rpm) and started climbing a hill, start to give it more throttle for the climb - it'd start to cut out and run rough. Ofcourse at this point i down shift, get my RPMs up & get past it.

I just replaced the spark plugs (Champion Platinum), plug wires look new, cap & rotor are in great shape (look new), the fuel injectors Look new, air filter is clean. My fuel gauge reads 1/4 tank, sometimes bumps to 1/2 tank briefly - regardless of how much fuel I have (as i found out saturday morning after running to the gas station). I don't know if this may be part of the problem, tho i doubt it. I don't believe the fuel pump is an issue, if it were I'd imagine I'd have problems @ high rpm, and not Only with hard throttle @ low RPM.

Could this be caused by a faulty sensor, or bad pressure? I already have to pull the fuel pump/sender/regulator to replace the gasket on the tank (since i also found it leaks right there when i fill the tank, thus insta-failing the smog check). If this is the fuel pump & i need to replace it, great! I'll do that all at once rather than going back to do it again, if not..... Crap! Lol I'm not real sure where to go with this.

This Is a toy vehicle, not a DD. Won't kill me if I can't fix this in one day. Sorry so long winded, and thanks reading!

I would break out the DVM/OHM meter and start testing. I would think your TPS is out of adjustment but test everything including the EGR.

You need the RENIX FSM but Lunghd has a good site devoted to the RENIX testing procedure's

Alright... Guess it's my turn to start testing sensors! Thanks for the response

Start with testing the TPS. An analog volt meter is best. Test for bad spots in the stroke, look for spots where the analog meter needle jumps back and forth, instead of smooth change in voltage.

Then the MAP sensor, check for a vacuum leak in the vacuum hose to the MAP sensor.

Then the CPS and the O2 sensor. Check them all live. Use the back probe method for the MAP, O2 and CPS. The TPS can be tested with an ohm meter, power off, disconnected for the noisy signal worn internals problems.

I don't have an anolog volt meter, but I do have a digital meter. I followed the instructions in my Haynes manual. Checked TPS, Probed the "middle wire" and the ground. The wires on the right side of the connector. Got Nothing. I checked vs the grey wire on the left side (grey is "middle" wire on the right side) and I get a solid 4.8 volts. I am sure I had good contact, I could see bare wire. I tried shoving a paper clip into the back end of the plug, incase I wasn't getting a solid connection. Still nothing. No voltage at all. I figure I must be doing something wrong, but I'm going to replace the tbs regardless so I know for sure it's not the problem.

I did test the grey vs blue wires @ the plug with an ohm meter. No throttle gave me 2, which ranged up to 5 @ full throttle.

I did have a vacuum leak, I guess... That large flex hose (that's a lot like a dryer exhaust hose) was disconnected. Maybe that added to the problem.

DezertTitan said:

I don't have an anolog volt meter, but I do have a digital meter. I followed the instructions in my Haynes manual. Checked TPS, Probed the "middle wire" and the ground. The wires on the right side of the connector. Got Nothing. I checked vs the grey wire on the left side (grey is "middle" wire on the right side) and I get a solid 4.8 volts. I am sure I had good contact, I could see bare wire. I tried shoving a paper clip into the back end of the plug, incase I wasn't getting a solid connection. Still nothing. No voltage at all. I figure I must be doing something wrong, but I'm going to replace the tbs regardless so I know for sure it's not the problem.

I did test the grey vs blue wires @ the plug with an ohm meter. No throttle gave me 2, which ranged up to 5 @ full throttle.

I did have a vacuum leak, I guess... That large flex hose (that's a lot like a dryer exhaust hose) was disconnected. Maybe that added to the problem.

Click to expand...

Are you talking about the large flex hose that runs from the exhaust manifold to the front of the air box? That is only a heat riser, not really needed in warmer climates. Make sure you are testing the ECU side of the TPS and not the TCU side(2 connectors for a automatic equipped XJ)

Yup that's the hose! Glad to hear it's not that important. Temps are running high 50s in the am & prob 70s in afternoon. Nice weather.

My jeeps a 5spd. Anyways on the back of the sensor is 6 wires. On the left is grey/black/blue. On the right it's black/grey/blue, top to bottom. On the left side I get constant 4.8 volts. I was told that's reference. On the right, I don't get any voltage reading at all. I disconnected the plug & tested the blue-grey wires (right-side) & it showed 2ohms, moved up to 5 ohms & full throttle.

Since you have manual tranny the 4 pin connector on the TPS is not used. The three pin connector is the one you need to test.

http://www.lunghd.com/Tech_Articles/Engine/Basic_Sensors_Diagnostics.htm

To save $$s someone put an automatic TPS on your manual Trans rig, which is OK, but you need to ignore the square connector wires.

Well mine has 6!? Lol now I'm lost! I got home too late to get anything done tonight, so I'll work on it tomorrow hopefully. I'll post pics as well cause now I'm really confused. I was going to buy the TPS, but O'Reily's & Pep Boys both quoted $120 so I said no thanks! Rock Auto has prices ranging 30-80. I'll try testing the other sensors. I'll throw pics up too.

You have 6 what?

Should be six wires, 3 to a square four pin connector which is not needed on yours, and 3 to a flat 3 pin connector. Are all the connectors gone?

That's what I have, and I believe both connectors are being used... Pretty sure infact. I'll verify tomorrow after work. It's 1130 here & I have to get up @ 530 for work. I'll take pics to help show what I'm talking about. Hell if I get off my ass early enough I may snap a few with my phone before I leave & post them.

Aside from low-end problems, the engine runs fine. No CEL light either. Don't know if that helps at all...

DezertTitan said:

That's what I have, and I believe both connectors are being used... Pretty sure infact. I'll verify tomorrow after work. It's 1130 here & I have to get up @ 530 for work. I'll take pics to help show what I'm talking about. Hell if I get off my ass early enough I may snap a few with my phone before I leave & post them.

Aside from low-end problems, the engine runs fine. No CEL light either. Don't know if that helps at all...

Click to expand...

There is no CEL on a RENIX, when the fuel mileage tanks out or it starts running bad then its time to start testing!

Oh really? Good to know! Here's the pics, sorry if they come out crappy full size. Took them with my phone & had to upload to photobucket from there.

The Automatic transmission TPS is cheaper than the Manual transmission version. Your jeep may have been automatic at one time? The square connected is not used right now on yours, as yours is a manual Transmission, so focus on the flat, three pole connector only, it talks to the ECU, and needs testing and calibration.

Ah ok. That explains why I couldn't get a reading out of the other connector at all. If my jeep was an auto at one time (maybe I should research the vin) that would explain why the square connector isn't left hanging unplugged. Unless the computer is from an auto jeep... Well I'm OMW home now, gonna stop off @ harbor freight & see about picking up an analog volt meter to test with per your suggestion. I'll hunt down a thread where it explains how to adjust the TPS (I'm assuming I'll be pulling it off the throttle body to do that). Thanks for all the help I really appreciate it!

LOL, reread thread 8! Before you pull the TPS!!!

http://www.lunghd.com/Tech_Articles/Engine/Basic_Sensors_Diagnostics.htm

Use a digital meter to do the TPS calibration, use the analog meter to test for noisy, jumpy needle bad spots in the TPS travel from idle to WOT.

haha I bookmarked his page for the info. I figure at some point (most likely sooner than later.... ) I'll be running each one of those tests on my jeep Harbor Freight didn't have any analog volt meter's in stock, only a bunch of cheapo digi meters, and a few nicer ones... Had to take care of some stuff around the house before I started gettin greasy, I'll be runnin out there in a minute to start checking on this and doin a couple other things to the jeep.

Ok...now that I'm testing the CORRECT wires...

B+ to A- = 4.84 -regardless of throttle position.

C+ to A- = 4.28 @ closed/ 0.62 @ WOT

C+ to B- = 0.54 @ closed/ 4.19 @ WOT.

I realize the test is C-B, and my volts are off, but I went ahead and tested the other wires as well.

Loosen the two mount screws and twist the sensor CW, or CCW till it reads

.82 to .85 volts on C+ to B-

"C+ to B- = 0.54 @ closed/ 4.19 @ WOT. "

Click to expand...

Adjusted. I set the tps @ .84. The top mounting screw were barely snug, bottom one was maybe finger tight... Wonder if that was part of the problem. On initial start up I had a high idle, but I think that may had also been caused by me not shutting the key "off" after adjustment & before start up. Once I turn the key to the off position, it fired up the second time with a nice idle.

Now I have a fuel leak @ the return (regulator?) @ the end of the rail. The engine still hesitates. I also managed to break one of the vacuum lines - the larger one, top left @ the tree. Yay. Gotta fix that now to see if that helps resolve this. Gotta love old brittle plastic!