2.5 engine dies after sitting for a few months

woody367

TJ Enthusiast
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Nov 6, 2017
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248
Location
Long Island, NY, United States
I have a 1997 TJ 2.5 l. It has been sitting for about 2 months and was running fine prior. I went to start it the other day and it ran fine for about 5-10 minutes and then started sputtering and missing and then shut off. Tried restarting it it would just sputter Miss and shut off again. If I let it sit for a while it'll start up and run fine but then do it all over again. Was wondering if someone might think it could be the ignition coil going bad? I just moved out of state and all my tools are at my new place and the Jeep is still up where I used to live where I no longer have any tools. So I really don't have much for running any tests. Can anyone give me any suggestions or if they think that it is the ignition coil? Thank you
 
I put fresh fuel in it and stabilizer when I parked it so I don't think it's bad fuel. I will this weekend try which others have advice to check. I have to borrow some tools code reader from a friend seeing that none of my tools are up where the Jeep is anymore. Is there any particular brand of coil or O2 sensor if it's one of them to buy? Thank you and please if there is any other ideas to check I will check them. And I don't know if the check engine light is on it wasn't on when I last looked.
 
Avoid store brands for both. NTK, NGK, or Mopar are what you want for the O2 sensor. Avoid Bosch and store brand O2 sensors. For the ignition coil go with a premium brand like Standard Motor Products, NTK, Hitachi, or Delco if you can't find a Mopar.
 
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So it run OK while the engine is cold but it starts running poorly after it starts to warm up? If so that's likely a bad upstream O2 sensor.

Hi Jerry,

Can you share a bit more here? What would cause the engine to work fine with a bad upstream sensor until it warms up? Thanks in advance.
 
Hi Jerry,

Can you share a bit more here? What would cause the engine to work fine with a bad upstream sensor until it warms up? Thanks in advance.

The engine runs in open loop (injector pulse width is based on a table lookup using the intake-side sensors) until it warms up, then goes to closed loop (upstream O2 values are used to trim the lookup table value up or down) once it warms up. A bad upstream sensor providing bad data will only affect the mixture when in closed loop.
 
I put fresh fuel in it and stabilizer when I parked it so I don't think it's bad fuel. I will this weekend try which others have advice to check. I have to borrow some tools code reader from a friend seeing that none of my tools are up where the Jeep is anymore. Is there any particular brand of coil or O2 sensor if it's one of them to buy? Thank you and please if there is any other ideas to check I will check them. And I don't know if the check engine light is on it wasn't on when I last looked.

I don't know what thought process led you to question the coil. When I've had those fail, they failed instantaneously and totally, leaving me with a crank-no start condition.

Electronic parts don't usually go bad spontaneously just from sitting for a couple of months, either. I like Jerry's idea to question the O2 sensor but my first thought is to look for any signs that a wayward rodent has chewed on the O2 sensor wire. You should have a CEL and DTC in that case though.
 
When you hook up the scanner you should be able to read the upstream sensor volts (bank 1 sensor 1) while it run. It should be flopping back and forth within the range of about 0.1 to 1V. If it's flat lined or out of range that's a pretty good indication that it's toast. I don't know how long the signal has to be bad for the PCM to throw a code but I can't rule out that it's so bad and so sudden that the engine hasn't even run long enough to trigger it. One of the PCM experts here can probably comment on that.
 
Would two months really be enough time for the fuel to go bad? Not disagreeing, just curious so I'll know in the future.

Well it should be fine in all honesty but I’m a big proponent of Occam’s Razor.
 
Hi Jerry,

Can you share a bit more here? What would cause the engine to work fine with a bad upstream sensor until it warms up? Thanks in advance.
Because the PCM (engine computer) doesn't use the upstream sensor at all when the engine is cold, it relies on a built-in programming for the air/fuel mixture. Once the engine starts to warm up the PCM switches over to using the upstream O2 sensor for setting the air/fuel mixture.
 


I started it this morning and it ran fine for about 5-10 minutes then it started sputtering and misfiring. The engine shut off and the check engine light came on. Got it started a few more times but only for a few seconds. The code that came up is p1391. Any advice would be great I think that code comes up as a cam sensor or crank sensor but I'm still wondering if it could be that upstream O2 sensor?
 
I was able to borrow a basic code reader to get the code. I have no tools available to me up here so this is very difficult. I don't have my fuel pressure tester. My thought that it might have been a coil is because I've had a lot of old equipment with points and coils that it has happened to where when the coil heats up it cuts out. But obviously the code is not pointing to that.
 
Do you have critters? Check on top of the bell housing and along the back of the upper part of the motor in case something chewed your crank sensor wires while it was sitting.
 
That can be a crank position sensor going bad...or it could be something else making the idle go to crap and then the PCM thinks the CKPS is bad because the signal becomes so erratic while it's stalling out.

Did you get a chance with the code reader to view the O2 voltage?
 
That can be a crank position sensor going bad...or it could be something else making the idle go to crap and then the PCM thinks the CKPS is bad because the signal becomes so erratic while it's stalling out.

Did you get a chance with the code reader to view the O2 voltage?

No I did not get the check the O2 sensor voltage. The scan tool that I borrowed I don't know how to really use to do that test. Any suggestions on how to do the test?