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No start when hot


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Howdy All,

I'm new to the site and new to MJ's. I just picked up a 88 2.5 4 spd 2wd last week. It is not in terrible shape, I picked it up and drove it home . The issue that I am having is that when this thing gets hot it runs crappy. If I shut the key off and try to restart it, it will not fire and i found that I have no spark and no injector pulse. Has anybody run into this issue before? I have tested the crank sensor and it appears to be good. It has good resistance and a good pulse while cranking. I picked up a test PCM from the wrecking yard and I'm going to give that a shot tomorrow. Any other ideas or experiences?

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ok, so I'm a little confused now. I tried my used pcm and got the truck still did not want to start. However, now when it doesn't start I still have injector pulse and spark. I get these results with either PCM. I found that if I floor the gas pedal I can get the truck to start. I even got it to start with the injector unplugged while testing it with a noid light. It would appear that the thing is just loading up on fuel and flooding out when warmed up. Are coolant temp sensors a common failure on these things? Or maybe a leaky injector? Not sure what to think.

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I will have to check that out in the morning. In regards to the question about how hot it is running, it is getting rather warm. My coolant recovery tank is cracked and it has been boiling over. Obviously this is an issue, but do you think it could be causing an issue with fuel delivery?

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I do agree that I need to address the cooling system asap. I only drove the truck about 3 miles home when I picked it up and have not driven it since. I am working on trying to get a hold of a stock recovery tank. (Anybody have any suggestions for finding one new?)

I guess that I am not quite seeing how it running too hot is going to affect my fuel delivery in the direction it is going. If coolant temp is seeing a high value that would go the other direction. (runs fatter when cold) I suppose that it COULD boost my under hood temp enough to cause a sensor to go open circuit. I am not too teribly familiar with this system, am I off base? I am just going from an understanding of a generic system.

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Ok, so I found a coolant recovery tank this morning, only cost me 17 big ones. I got to thinking about this last night in terms of the cooling system and thought that the ect actually could be causing an issue if it is reading cold (high resistance) all the time. I unplugged it to check it and found it to be right on the money. I checked it cold and after the truck warmed up and it is moving exactly like it should. I plugged it back in and I appear to no longer have a problem with the hot restart. Maybe some poor pin fit in the plug (it is a little scorched like it kissed the exhaust mani.) I am not saying that I am satisfied or at all ready to rely on it yet, BUT I seem to be moving in the right direction. I will continue to post back as I gather details and hopefully come to a definitive solution sooner rather than later.

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  • 2 weeks later...

OK, so I got my recovery tank the other day and got it installed. I went out for a short drive and then parked the truck and just let it idle, after about 10 minutes I noticed that I was boiling into the recovery tank again. I drove the truck back to the house and once I got moving it seemed to be fine. Seems like I may be looking at a weak water pump or a partially plugged radiator or something similar. Anybody have any ideas? At this point I am getting pretty tired of dealing with this, I have got the truck all serviced up and ready to cart my butt around but I would really like to know what is going on first. I guess I will get a radiator in/out temp and see if I have got a blockage. I am pretty certain that my fan clutch is fine. How common are head gaskets on these pigs? I am really hoping that is not something I will have to deal with.

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ok, so I'm a little confused now. I tried my used pcm and got the truck still did not want to start. However, now when it doesn't start I still have injector pulse and spark. I get these results with either PCM. I found that if I floor the gas pedal I can get the truck to start. I even got it to start with the injector unplugged while testing it with a noid light. It would appear that the thing is just loading up on fuel and flooding out when warmed up. Are coolant temp sensors a common failure on these things? Or maybe a leaky injector? Not sure what to think.

'88 Comanches didn't have PCMs. Are you referring to the ECU (Engine Control Unit, a.k.a. computer)?

 

How's the timing?

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OK, so I got my recovery tank the other day and got it installed. I went out for a short drive and then parked the truck and just let it idle, after about 10 minutes I noticed that I was boiling into the recovery tank again. I drove the truck back to the house and once I got moving it seemed to be fine. Seems like I may be looking at a weak water pump or a partially plugged radiator or something similar. Anybody have any ideas? At this point I am getting pretty tired of dealing with this, I have got the truck all serviced up and ready to cart my butt around but I would really like to know what is going on first. I guess I will get a radiator in/out temp and see if I have got a blockage. I am pretty certain that my fan clutch is fine. How common are head gaskets on these pigs? I am really hoping that is not something I will have to deal with.

 

Head gaskets aren't too common on the older ones, from what I've seen atleast. The first thing I would do, if I were you, is check to see if the radiator fins are clogged (check behind the power steering cooler if you have one of those too). I had that issue on my '90 Cherokee and it was because a lot of mud dried up in the fins.

 

If all the fins are clean and you can see light through them, then it's a flow issue. You can try flushing the system with a garden hose to see if you can get any gunk out. You can either buy the flush adapter or do it the poor mans way. Empty the system, disconnect the top rad hose, put a garden hose in the radiator and fill the system up with water, keep the hose running and turn the engine on and let it run for a while. Keep an eye on the Jeep to make sure the temp stays down while it runs. I usually let it run for about 30 mins or so checking it periodically.

 

After that, just fill the system and see where it sits then. If it still runs warm, then you will need to find what is restricting flow.

 

If could have a clogged thermostat, something cheap and easy to fix.

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