Lewis E

Some stuff I forgot. I have a fuel pressure gauge installed on the fuel rail. It shows 34 when running. Is that ok? The pump and filter are new. The cat and O2 sensor are new and the coil is about a year old. It's an MSD coil which I would expect to be high quality but you never know. New Champion coppers, quality cap and rotor, MOPAR wires. I noticed this as well when I hook my hand held scanner up.......no codes but the RPMs are all over the place. It goes from 0 up to 3000 or so...not the engine, just the display on the scanner. I don't know what to make of that.

I didn't realize I used that huge font on my original post....sorry 'bout that.

You know, I had read through that one. I can't figure why mine is so consistent though with the 15 min good runtime on my original sensor and no good runtime on any of the new ones. I did see someone on that thread said they cured a similar issue with a new camshaft sensor...I have a newer distributor in mine and I think I have a new sensor to put in there in my collection of goodies. I'll try to get that in there in the next couple of days to see what happens.

Sorry if this seems long winded. I have a 1991 4.0, AW4, 2WD all stock with 65k miles on it. It’s in really good shape and has no rust…not even on the floors. It starts and runs fine for about 15 minutes and then starts dying and coming back like you would expect a failing crank position sensor to cause. Dies for like a half second at a time, tach drops then it comes back to life getting progressively worse as the engine warms. The other gauges and lights are unaffected. Eventually it’s dying and bucking so much I have to stop. If I let it cool for 30 minutes or so I can get another 10 or 15 minutes runtime. Always starts and runs fine when cold. I think my runtime is getting less as time goes by and eventually something will fail altogether. My fuel pressure is about 34 when running. So, anyway, I’m thinking it’s the crank sensor so I set out to get a MOPAR one….ha, not for a 1991. They can still be found for 97+ so, thinking they are most likely the same electrically, I get a 97+ MOPAR one and a new plug for my harness. I put the new one on and fire it up….within 30 seconds I have the same symptoms but with a cold engine now….WTF. Well, as the new MOPAR sensor was not U.S. made, I figured I either had a dud or the 97+ ones are not the same electrically after all. I went out and got myself a new “Standard” brand one for my specific year and made an adapter to plug into my new connector on my harness. Now I can use either type. Put my new one with the round connector for my model year on and fired it up. Same symptoms again with a cold engine. Since I was doing this work in a friend’s garage I thought, crap, I gotta get home somehow, so I threw the original, good for 15 minutes, one back on using my adapter. What do you know, it’s running good for 15 minutes at a time again. WTF. Couple of days later I get yet another two new sensors one old style and one 97+ one and they both seem bad too even when cold. The old, original one still runs good for 15 minutes. This basket full of new crank sensors I have can’t all be bad, can they? What in the heck am I missing here? There is obviously something else bad but why does it run good with my old CPS for 15 minutes but not good at all with any of the new ones? Could there be some other piece of the puzzle that for some odd reason is more happy with my old CPS? Anyone seen anything like this?

Yeah, with the XJ one I had about a 10 gallon tank. The one from MTS fit very well. I don't have an original MJ one to compare it to but it went in OK. I spoke with one of the MTS guys about selling just the rheostat for the 91 and up XJ so I could do the swap I did without buying the entire other unit....they wouldn't do that. Didn't want to discuss it with me really. Not sure why they aren't making what I made and selling 'em to us....it works perfectly.

I didn't have an original. My truck had an XJ unit in it when I got it.

I bought the 86 to 90 one from MTS with pump and one for a 91 XJ from MTS with no pump. I drilled out the rivets for the rheostat or Potentiometer (whichever that thing is) and swapped the XJ one over onto the 86 to 90 MJ sending unit. It works perfectly. A little expensive but I was tired of not knowing how much gas I had and MTS has been hinting at a unit for HO MJs for what? a year now?