1990 MJ issues

[Rept1lefreak]

1990 MJ 4.0

 

Ok so this summer has been filled with failed attempt after failed attempt to make my MJ run right again.

 

Issues started after a round of wheeling where the face of my sending unit cracked and I was leaking gas everywhere. So after she sat in a shop for a couple weeks while they tried to find a sending unit or a pump that they could make sit right in the tank, she was "ready". I picked her up and drove her about 25 minutes to an offroad event where just outside the gates she died. After messing with her for the weekend it seemed like when she got hot she would just shut off. Toying with several options and having other issues the following is a list of parts i put on to make her run right:

 

-Fuel pump

-02 sensor

-fuel pressure regulator

-Ignition coil

-ignition module

-thermostat (160*)

 

So I don't really know where to turn next. I can probably run her straight on a highway for a full tank of gas and hot, but as soon as i get to city driving and stop and go, thats when she runs rough and wants to or does stall out. Any other Ideas on what I can do so I can run my rig daily again?

 

[Eagle]

That's what sidelined my '88 MJ. I have about decided it has to be the coil and/or the ignition module ... so I'm a bit disappointed to see that those didn't help you.

[cruiser54]

Check your CPS output voltage. At lower RPM like idle the signal can be too weak when the CPS heats up.

 

Renix CPS Testing and Adjusting

 

 

 

Renix CPSs have to put out a strong enough signal to the ECU so that it will provide spark.

 

Most tests for the CPS suggest checking it for an ohms value. This is unreliable and can cause some wasted time and aggravation in your diagnosis of a no-start issue as the CPS will test good when in fact it is bad.

 

The problem with the ohms test is you can have the correct amount of resistance through the CPS but it isn’t generating enough voltage to trigger the ECU to provide spark.

 

Unplug the harness connector from the CPS. Using your voltmeter set on AC volts and probing both wires in the connector going to the CPS, crank the engine over. It won’t start with the CPS disconnected.

 

You should get a reading of .5 AC volts.

 

If you are down in the .35 AC volts range or lower on your meter reading, you can have intermittent crank/no-start conditions from your Renix Jeep. Some NEW CPSs (from the big box parts stores) have registered only .2 AC volts while reading the proper resistance!! That’s a definite no-start condition. Best to buy your CPS from Napa or the dealer.

 

Sometimes on a manual transmission equipped Renix Jeep there is an accumulation of debris on the tip of the CPS. It’s worn off clutch material and since the CPS is a magnet, the metal sticks to the tip of the CPS causing a reduced voltage signal. You MAY get by with cleaning the tip of the CPS off.

 

A little trick for increasing the output of your CPS is to drill out it’s mounting holes with the first drill bit that just won’t fit through the original holes. Then, when mounting it, hold the CPS down as close to the flywheel as you can while tightening the bolts.

 

 

Revised 11-29-2011

[cruiser54]

Also, have you cleaned the throttle body and checked the TPS adjustment?

[Rept1lefreak]

Cruiser, I have not cleaned the throttle body, checked the TPS or CPS so I will look into those things.. Thank you

[Zenobian_84]

I wouldn't install that 160 degree thermostat if I were you. Go with the stock 195 degree one; as anything lower will probably create some kind of problem down the road, or just mask existing problems already there. Do you have an "Open" or a "Closed" system for your engine cooling?

 

Would probably start with the thermostat and giving the radiator a good flushing, irregardless of the engine setup. You'd be surprised at how many chunks of built up gunk can come out. As stated before, It wouldn't hurt to do the TB cleaning, as well as adjusting the CPS. :cheers: