Crank Position Sensor Bad or Not?

[Lewis E]

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?

[89 MJ]

You may want to read through this thread. A guy with the same year truck seems to be having a similar issue.

 

 

[Lewis E]

Quote

You may want to read through this thread. A guy with the same year truck seems to be having a similar issue.

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. 

[Lewis E]

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

[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.

[ghetdjc320]

Sounds like a possible coil issue or cam sensor  but id have a few more questions. 
Have you replaced the distributor at all recently? A failing cam sensor also causes very similar issues as it triggers and times your fueling sequence. As for the FP, it’s 39psi without vacuum attached. Check the pressure without vacuum attached and see what you have. Have you checked engine vacuum as well? There are several more possibilities of issues here but let’s narrow some things down a bit and see what’s left

[Lewis E]

Quote

Sounds like a possible coil issue or cam sensor  but id have a few more questions. 
Have you replaced the distributor at all recently? A failing cam sensor also causes very similar issues as it triggers and times your fueling sequence. As for the FP, it’s 39psi without vacuum attached. Check the pressure without vacuum attached and see what you have. Have you checked engine vacuum as well? There are several more possibilities of issues here but let’s narrow some things down a bit and see what’s left

The coil is one thing I don't have a spare for. I do have a new cam sensor that I can try but I have to work on it on the street for the moment and I'm so far north that it starts getting dark at 4:30. The distributor was replaced almost a year ago because it had some play in the shaft. I got one for the later models but it's no doubt Chinese so it's suspect. I did recheck the fuel pressure and by my rail mounted gauge it's 41 without vacuum and 34 with. I only have a hand vacuum pump to check for leaks. I isolated the different vacuum lines and pumped them down....holds good vacuum on all of them. It'll be dark soon plus I'm always worried that some old lady will call the Polizei to come give me a ticket for working on my vehicle on the street. I hope I can swap out the cam sensor tomorrow or Saturday....it'll probably snow or rain. 

[Lewis E]

So, I took the truck to a Jeep garage here in the area. they have some guys that have been there since the 80's so I felt like they might have seen some stuff. They went through the same thinking as me and some of you guys but they had the tools to test the ideas. They suspected the crank sensor like me but hung a scope on it and could see steady output even when the problem was occurring. Next they suspected the coil....put on a new Bosch unit but it didn't fix it. Next it seems they isolated it to a loss of fuel....swapped out the filter which they said was pretty dirty. It was only 8 months old. They dropped the tank and looked for contamination and checked out my new fuel pump and sending unit. They finally started looking at the pressure which they said was low by their gauge. They replaced my nearly new fuel pressure regulator and it seems to have fixed it. They tell me the symptoms were really odd and they were surprised at it being the pressure regulator. Hmmm.....me too so it will be a while before I really trust it. Just thought I'd post to maybe wrap this up. Thanks for everyone's input.

[ghetdjc320]

Thanks for sharing that update