1991 2.5 doesn't like to start when engine is hot

Brandon.otey1986

Brandon.otey1986

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1991 2.5 don’t like to start when engine is hot. If you cut it off it will start right back but if it sits for 10 minutes or so it’s hard to start. I’m kind of lost as to what could be going on. If it cools off it also starts right back. Any information would be helpful thanks in advance.
 
Check the fuel pressure at the fuel rail and see if it holds pressure after the key is turned off. It should hold for 10 - 20 minutes, I don't recall the spec's but thy can be found in the reference section here. It sounds like it's low fuel pressure maybe from the pump or filter. Vapor locking is rare unless the fuel lines are up against the exhaust.
 
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I agree with flyer, mine does the same thing. If you turn the key off and on a couple times it will run your fuel pump for about 1 second, priming the fuel system back up to pressure and help with starts.
 
I feel like this modification could help with the issue too. If I get to applying it to my jeep this weekend I'll report back and let you know if it helped with mine. I'm also putting in new injectors at some point which could fix it.
 
While the jeep wouldn’t start I had good fuel pressure. I swapped the crank position sensor. I’m lost at what could be wrong it starts great in the morning and after work it’s only if you cut it off and it sits for 1/2 hr to a hour
 
50 to 100 causes
but in pure science just 3, macro reasons
bad engine, bad spark bad fueling.
when fails to start.
and the IAC is stuck closed
add 5 % throttle pedal and crank (THIS HIdes the stuck ISC
if fails, add 100% called WOt. THIS CLEARS A FLOOD.

watch scotty.
 
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Cold start, hot fail, cold start makes me think it's temperature related and one of the 50 causes could be the ECU/PCM/Whatever getting hot and failing. I'm sure there's ways of checking when hot and the engine quits. My '92 has been working so far but I see a lot of capacitor failures and home fixes if you know how to solder. Heat and electronics never mix well.

Scotty Kilmer. LoL. I watch his videos and think the guy must be rolling in the cash when he gets 100,000 views in the first week of a new video. Many years ago (1990's) when he lived in Houston he had a short segment on the morning TV news station. The car enthusiasts would gather around to watch and some of his ideas were good but many were absolutely silly and some outright dangerous. It was only about two years ago someone recommended his YouTube channel and I laughed thinking back but after watching I see he now puts out some good recommendations. I guess there was only so much you can do in a one minute TV filler segment.
 
Brandon.otey1986 said:
While the jeep wouldn’t start I had good fuel pressure. I swapped the crank position sensor. I’m lost at what could be wrong it starts great in the morning and after work it’s only if you cut it off and it sits for 1/2 hr to a hour
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Brandon.otey1986 said:
While the jeep wouldn’t start I had good fuel pressure. I swapped the crank position sensor. I’m lost at what could be wrong it starts great in the morning and after work it’s only if you cut it off and it sits for 1/2 hr to a hour
Click to expand...
Brandon, did you solve your ”hot no start” problem ?
I have an ‘88 with the same problem.
 
lost spark or not when dead the test to find this is 100 years old ,unchanged.
if spark is good, and engine is presumed good 30+ years old. then we
add air. right foot %5 throttle cranking to add air to hide bad iAC (just for the start) it may stall if foot is raised at the starts now event.
if its flooding, as in spark tips soaking in fuel (oil or antifreeze) we crank WOT, WIDE OPEN THROTTLE, THIS CLEARS THE FLOOD, WOT CRANKS CUTS THE INJECTORS DEAD. FOR THE CRANK.
AND LAST TEST FUEL , SPRAY AS MY SCOTTY POST CLEARLY SHOWS ABOVE. (instant start spray in a can sold at all autostores)
this hides dead fuel pressure problems.
lots causes do not go to the dark side and modify the jeep engine EFI system now. just do the test, and work the problem to a logical conclusion.
if spark is lost, check out why at the ASD relay. ask.

ASD cuts fuel and spark both, if it sees ASD output overloaded, even shorted 02 sensors, shorts. see wires shorting at the 02 yet a very common failure on all cars. shorted injector wires. shorted primary spark coil or any of those intermittent.

do the tests at the FAILURE< will not start.
work quickly

the 10min dead symptom, hints flooding to me but is just wise guess. base on time factor.
WOT cranking the ECU detects you doing that and cuts all fuel at the injectors.
called WOT unflood mode,

watch scotty.
 
lost compression (collapsed HLA lifter) {and lots more ways}
lost spark
lost fuel (pressure dead, too low or too high a hand fuel pressure gauge finds this)
too much fuel.
no IAC air supply it clogged as they all love to do. (IAC is Idle air controller (or speed)
ASD drops out and fuel and spark both go dead. by design..
 
Downs, my ‘88 has a 2.5. My problem is identical to what Brandon describes.
I haven’t done much troubleshooting yet. I’ve only had the jeep a few weeks and the problem has occurred only two times. Next time it happens, I will check for spark and fuel. I also have a can of starter spray that I will try.
 
Brandon hasn't been on the forums since May of 2022. I don't think he's going to show up to answer your question but he might. Maybe.

Get a fuel pressure gauge and get readings. And good 'nuff gauge is 20sih dollars on Amazon. Even cold. I had fuel pressure issues, it was way low due to a weak pump. It would start fine sometimes and other times be a massive pain to start. Having wonky fuel pressure messes things up bad for these FI vehicles. The computer doesn't know what the fuel pressure is, its fuel tables are based off of the factory spec fuel pressure and if you go outside that window the fueling gets messed up.
 
Thanks. I’ll get a fuel pressure gauge.
i was also thinking “flooding” because I could smell gas during the crank/no-start. But the last time I had the problem, I had to wait almost two hours before it started.
 
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I have a spark test light and a can of starter spray to help isolate my problem, the next time it occurs.
I have also seen on YouTube that the Crankshaft Position Sensor (CPS) is often the cause of my problem.
I plan to also carry some freeze spray or ice water so I can quickly cool off the CPS.
These three tests will be very easy to perform. If the results indicate “low fuel pressure“ might be the problem, then I will get a pressure gauge.
Any thoughts on my plan ?
 
Yeah, just buy a fuel pressure gauge anyway. It's a good number to keep track of when doing other maintenance. Can give you a heads-up on a weak pump or a failing check valve or FPR.

Starting spray is...meh. I can use starting spray to make an engine with no carb at all on the intake manifold run. Spark tester can get you going in right direction if it shows no spark.

The Crank position sensor gets blamed a lot, and there's no telling how many of those have been wastefully changed when the issue all along was bad grounds or some other issue. If you've never replaced the battery and ground cables, just do that. They can look fine externally, but be a tube full of corrosion that causes electrical gremlins. I'm not saying it's not that, I'm just saying I would troubleshoot more before going down that hole. At least the 2.5's crank sensor is pretty easy to replace.

A multimeter is your best friend, especially on Renix based systems. Even the 10 dollar one from harbor freight will get the job done.
 
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