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90 XJ 4.0 Lost Power And Barely Idles

S

Stumper92

NAXJA Forum User
Location
Smithfield, Va
Here's the story (I appologize in advance for the length of the writeup):

I had the Jeep at an event a couple weekends ago. The Jeep ran great until Sunday morning. When I started the Jeep that morning it idled fine. I allowed it to warm up before I moved it. As I left the campsite I noticed that the engine would hesitate a little under load. It was hesitating as if the plug wires were arching, so I figured as I drove and the moisture from the heavy dew the night before burned off, the engine would smooth out. However, as I continued to drive the missing progressively got worse until the Jeep was at a point where it would barely run. Since I trailer the Jeep to events anyway, I loaded it on the trailer and brought it home. My first guess was the fuel pump was dieing. So this weekend I connected a pressure guage to see if I am losing fuel pressure. I found I am getting about 35 - 40 psi while the engine is running. What I did notice is that as soon as I shut the engine off the fuel pressure bleeds off within 10 seconds. I connected to guage to my 94 XJ to use as a reference and found I am getting the same fuel pressure but when I shut the engine off the pressure does not bleed down. Knowing the injection systems are different (90 is a Renix and 94 is HO) my question is should the fuel pressure be bleeding down when I shut the engine off? If fuel delivery is not the issue, what else could be the problem? I tuned the engine up a few years ago and the engine has at most 100 hours on the tuneup. Any thoughts or ideas would be appreciated.

Regards.
 
You could have some crappy injectors for sure.
Could also be a faulty MAP sensor or the hose to the MAP sensor from the throttle body. That hose can get a hole in it, melt shut, or come disconnected at the TB.
As far as injectors go, the original injectors were very prone to leaky fuel through a crimp in their bodies and causing engine fires. Might be a good time to upgrade to a 4 pintle injector set from www.precisionautoinjection.com.
 
You may also want to check the IAC. Remove it from the throttle body and clean the passages with throttle body cleaner. If you did not replace the IAC when you tuned up the Jeep I would just replace it now. Also, did you adjust your TPS when you tuned up your Jeep? Make sure you did not knock a ground loose. Renix Jeeps are extremely ground sensitive. Ensure all three grounds under the hood are clean and tight.
 
Just how heavy was that dew? Did you check for moisture under the distributor cap?
 
x2 on the moisture. i had a similar issue when fo rsome unknown reason i decided to wash the jeep????????? i know, i know. buti fired it up and it ran rough and i figured the same itll burn off well it didnt it got worse, i got to the show and pulled the cap off and the water had calcified , i cleaned it off a was able to drive it home with no issues.
 
My full size Bronco 351 liked to do that all winter long--PITA when you were in a hurry in the morning.

Did it with the stock distributor and cap, and it still did it with an MSD distributor and cap. Go figure.
 
When I tuned up the Jeep I didn't clean or replace the IAC motor. I have not touched the TPS either. The Jeep had been running fine for the last few years and I haven't done anything other than normal oil and maintenance. I did check under the distributor cap when the Jeep first started acting up. Everything was nice and dry. I am going to do some more troubleshooting maybe tomorrow evening. You guys have some good suggestions. Keep them coming :). I will keep everyone posted. Thanks for the help.
 
I too have a 90 XJ at it had similar problems and it ended up being the TPS (Throttle Position Sensor). The website above tells you how to test the TPS with a voltmeter, I would suggest doing that, mine was changing voltages wildly. I do believe there was a recall on the TPS for that year. So try the dealer and they might put a new one on for free. Good luck.
 
Just an update on this thread:

I have been on the road a lot with work lately so I haven't had much time to look at the Jeep. I did, however take a little time Sunday and this is what I discovered

- The engine runs really rough the first second or two after startup and then smooths out
- Typically the engine will idle fine as long as I do not give it any throttle. If I open the throttle quickly and hold it open at any amount (say 1/4 throttle or so) the engine will stumble and sputter for a few seconds and then smooth out at whatever RPM I have the throttle open to.
- The stumbling seems to come and go, when the Jeep is in gear. It will run fine for a minute or two and then start hesitating and in case cases stall out.
- With the engine running, I disconnected the vacuum line from the MAP sensor and the engine would either shut down completely or almost stall. Sort of a crude way of testing the sensor but I didn't have a lot of time and I didn't have the reference material posted here to use for testing.
- I disconnected the plug from the TPS and connected up my multimeter and read between each of the three conductors while rotating the throttle. The resistances between each lead varied smoothly with no signs of dead spots or wild variations in resistance values.
- I did not do any voltage testing yet.
- I swapped out my coil wire just to see if maybe the wire was either shorting out had high resistance in the cable. Changing the cable out did not fix the problem.

I don't feel like I have done enough testing to really rule either of the sensors out totally yet. As I said I just had a little bit of time and thought I would play around with it a little. I am open to any suggestions. Hopefully I will have time to look at it a little closer this weekend. This is an interesting problem and I would like to document everything as much as possible for future reference. I like to test my troubleshooting skills every now and then.
 
Ok, the TPS is adjustable.

First, remove the single connector from the TPS (manual) or the 4 wire connector (auto) key ON, engine OFF, measure voltage on the TPS connector between "A" positive and "D" ground--mark it down, that is your REFERENCE VOLTAGE.

Second, reinstall the connector to the TPS. Now, back probe the connector (use a couple of small paper clips with a straight leg, push it through so it contacts the wires), wires "B" positive and "D" ground, key ON, engine OFF. This voltage needs to be 17 percent of REFERENCE VOLTAGE for a manual transmission OR 83 percent of REFERENCE VOLTAGE for an automatic transmission. Adjust to those values, if you can't replace the TPS and test again.
 
Its been a while since I had to time to work on this issue. Work always seems to get in the way of things. So here's a summary of what I have done so far:

* Swapped out the MAP sensor with a known good replacement. It had no effect on the condition.

* Connected a hand vacuum pump to the EGR valve to verify it was functioning properly. When appling vaccum to the valve the engine would stall as it should. Not a perfect test of the EGR, but I think I am convinced it is not causing the problem.

* Ran an additional ground cable (temporary) from the engine block, directly to the battery to ensure a good ground. Still no effect.

* Tested the TPS per the above information (Thanks Joe) I did adjust the TPS voltage from 4.30VDC to 3.80 VDC. This too did not help.

* I retested the fuel pressure to be sure I did not overlook something since it had been a while since I worked on it last. Fuel pressure was right around 40psi.

* One observation I did have. Based on the link above, there was a suggestion to ground pin D of the TPS directly to battery ground. However, when I tested impedence from pin D to battery ground it was infinite. To ensure this was correct, I tested the same connection on a Jeep that is running properly and got the same reading. Upon testing the voltage at pin B to battery ground, I found it to be one volt higher than from pin B to pin D. Based on what I found, I do not think pin D is supposed to be actual ground but rather the zero reference for the ECU. If this is the case, it may not be a good idea to connect to directly to battery ground. If I am incorrect please someone let me know.

The issue has actually gotten quite worse and seems to be getting worse the more I run it. It now runs very rough when I first start it and then clears up a little for maybe 15 seconds and then the idle begins to get irratic until finally the engine shuts off. I have a spare ECU and am considering swapping it out to see what happens. I am open to any other ideas or suggestions. Other than the TPS, MAP sensor, or the EGR what else could cause these type of issues? I am beginning to wonder if I have an intake leak, although I would think it would idle extremely high all the time. Thanks again for all the help so far. I will continue to post as I troubleshoot further.
 
Has anyone ever seen a crank sensor cause this problem? Its been my experience cranks are usually good or fail completely. I have a spare and would swap it if there is a chance it could be the problem.
 
Update:
I had a little time to work on the Jeep this evening. I checked the distributor cap again to make sure there were no cracks. I had a spare ECU so I swapped it out just to eliminate the possibility that it was the problem. Still no luck. I plan to pull the plugs tomorrow to see if I may have an injector that has gone bad. I did go through my grounds and cleaned/ verified they are all good. Any other suggestions would be welcome.

Regards
 
I worked on the Jeep a little more tonight. I changed out the plug wires and distributor cap. I also pulled all six plugs to see their condition. I found all plugs to look the same. Based on what I am seeing (a blackish brown coating) there is most likely some oil burning, however not excessive. I cleaned the plugs and reinstalled. None of these changes helped. I bought a fuel filter and will change that out tomorrow. Anyone have any other ideas or suggestions?
 
Hey 1990JEEPXJ. I was just reading the same thing on another thread. I have a couple spare coils laying around so I will swap it tomorrow and try that. I have been through most everything else. Thanks for your input. I will post up the results tomorrow.
 
Another update to the thread.

* I think I have the main issue resolved. It appears to have been the ignition coil. Thanks 1990XJ for the tip. I swapped in a spare coil and the missing and sputtering cleared up.

* Even with the above fix I was still not able to get the Jeep to idle. As soon as I would take my foot off the accelerator, the Jeep would stumble and shut down. I noticed that each time I would start the Jeep, after a few seconds I would hear this buzzing noise from somewhere on the intake. I disconnected the vacuum line from the EGR valve and started the Jeep and it idled perfectly. I connect the line and the engine stumbles and stalls again. There is very little vacuum passing through the EGR solenoid. It would appear to be enough to open the EGR... but apparently it is. I am thinking that maybe the return spring on the EGR diaphragm may have broken or is weak. I will order a new EGR valve and install it to see what happens.

* With the above issues resolved, I thought I was out of the woods... Not so easy. I started the Jeep and began driving around the fields by my house. It ran perfectly with a smooth idle and good acceleration. Then suddenly it began to miss-fire for a few seconds and then shut down. It would not start for two minutes or so. The engine would turn over but no fire. I could hear the fuel pump run each time I turned the key. This happened twice. The second time I went to get a spare spark plug to test whether I had spark or not. By the time I got back, the Jeep started with no problem. I drove it around for another 30 minutes and it never missed a beat. Any suggestions? I am going to change the fuel filter and run a can of injector cleaner through it, although I don't think it is going to solve the problem.


Thanks again to all those that gave advice on this thread. I apologize for my lengthy posts, but I want to document the issues detailed as possible so that it might help the next person with similar problems. I am going to continue to troubleshoot and will update this thread as I go.

Regards.
 
Last edited:
Just an update to this thread. I think I have found a couple problems they may have all lead to missing and idle issues. I replaced the ignition coil and that cleared up the hesitating and missing considerably. However I still had issues with rough idle and stalling at idle. I had previously used a vacuum hand pump to test the EGR valve. When I applied vacuum to the valve, the engine stalled as expected. So based on that I assumed the EGR valve was working properly. But as I was running out of ideas, I decided to pull the vacuum line off the EGR and leave it off. Low and behold, the idle cleared up. There is maybe only a slight amount of vacuum leaking past the solenoid, but apparently enough to open the EGR slightly. After plugging off the line the engine runs much smoother most of the time. But I am still getting some hesitation from time to time. Tomorrow I am going to change the fuel filter and add injector cleaner to the fuel tank. I am also thinking that maybe the return spring in the EGR is very weak or broken and possibly the valve may still be leaking or unseating from time to time. Anyone ever have any such issue with an EGR valve? I will post up my results tomorrow. Thanks again for all the suggestions and help. I think I may be wrapping up this thread soon.

Regards
 
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