No start after warm but fires up with foot to floor.

lapisxj · Jun 25, 2015

On our 99 XJ (stock 4.0) we have an issue with it not starting. After a short drive and the vehicle has had time to warm up (like to work or store or something) let it sit for a bit (time it takes to get groceries, grab lunch, etc), come back out and it won't start. It will crank, has spark, and has fuel pressure (mid 40's psi). When it gets in this condition I can mash the pedal to the floor and crank on it for 5-10 seconds and eventually get it to fire up. It's worth noting that we do smell fuel while troubleshooting this condition. If you shut it down and crank it right back up it doesn't seem to have any issues doing so.

We had originally suspected a fuel pump problem but after hooking a gauge up and monitoring it for the past few days I don't think that's the case. How long should it hold fuel pressure in the rail and how would I test for a bad injector?

Throttle body is clean and most of the sensors on the TB are new/been replaced.

old_man · Jun 25, 2015

Pull you dip stick and see if it smells like gas. What you describe sounds like the engine is being flooded. That would be caused by sticky injectors. Another test would be to put a fuel pressure gauge on the rail and verify the correct pressure while it is running. Turn off the engine and see how long it takes to drop.

Your statement that it starts if you hold the throttle to the floor is a key. If you do that, the computer recognizes this state and turns off all the injectors. That would allow the flood to be pumped out and the plugs to dry enough to fire.

Heat soak on the other hand is caused by the fuel boiling in the injectors after you stop. That puts a bubble that has to be pushed through the injector before it will work. It would not respond to the flooring the pedal.

I would recommend running a can of BG-44K through the fuel to clean the injectors. I run a can every 10k miles on every vehicle I own.

lapisxj · Jun 25, 2015

Fuel pressure while running normal is still in the mid 40's. The fuel pressure doesn't drop for the first few minutes while I'm watching it but I'll go see how long it takes to eventually bleed down. 5 min, 10 min???

I've just left the gauge on for now. It's a cheap HF one and the fitting doesn't push on the shrader valve so I had to pull the valve core and screw it on till my testing is over. I'd like to get a better gauge that I don't have to do that with.

I'll pull the stick too.

ehall · Jun 25, 2015

sounds like injector leaking after you shut it off

you can pull the fuel rail and lay it on a piece of cardboard, prime the pump a couple of times and see if any of the injectors leak

lapisxj · Jun 25, 2015

ehall said:
sounds like injector leaking after you shut it off

you can pull the fuel rail and lay it on a piece of cardboard, prime the pump a couple of times and see if any of the injectors leak

I've never pulled injectors before. Once the clamps and electrical connectors are undone do they just pull straight out?

lapisxj · Jun 25, 2015

Looks pretty straightforward. May have to remove the throttle linkage bracket but not bad.

ehall · Jun 25, 2015

just remove the bolts holding the rail down and wiggle everything up and out. The hardest part is getting the injectors out without damaging the o-rings

lapisxj · Jun 25, 2015

All injectors read 13.0-13.5 ohms cold.
No cel, no codes from my scan tool.
Fuel pressure at
t-0 is 45
t+5 min is ~44
t+15 min is ~40

Dragonslayer · Jun 25, 2015

Old man was on the right track: heat soak is causing bubbles to form inside the injectors.
There was a TSB (Technical Service Bulletin) issued on this for '99-'01 cars and an injector
heat insulation kit was made available. Basically, the kit was some closed-cell foam socks
that you wrapped around each injector, and kept them in place with plastic zip ties.

lapisxj · Jun 25, 2015

Interesting.

Posted for future reference... TSB 18-031-03
http://www.wjjeeps.com/tsb/tsb_wj_1803103.pdf

I don't get a misfire code for #3 but it's worth checking out further.

ehall · Jun 25, 2015

Also possible that the IAC is defective--if the computer doesn't open it during crank the engine will flood and you will have to open it manually (with your foot) to get the AFR

sidewaysstarion · Jun 25, 2015

Does your jeep crank over like it has a weak battery?

Also try running a minimum airflow test to see if the idle valve is working and the TB isn't dirty.

http://naxja.org/forum/showthread.php?t=1121429

lapisxj · Jun 25, 2015

sidewaysstarion said:
Does your jeep crank over like it has a weak battery?

Nope, cranks strong just like a brand new Interstate Megatron should.

lapisxj · Jun 25, 2015

Also, pulled the stick and no noticeable fuel smell in the oil.

trippled · Jun 25, 2015

Coolant temp sensor verified? Could stuck cold flooding the motor

sidewaysstarion · Jun 25, 2015

Try the minimum airflow test.

lapisxj · Jun 25, 2015

sidewaysstarion said:
Try the minimum airflow test.

?? how and in what way does this relate to the problem??

sidewaysstarion · Jun 25, 2015

lapisxj said:
?? how and in what way does this relate to the problem??

Dirty throttle body combined with a malfunction idle valve will lead to hard starts.

trippled said:
Coolant temp sensor verified? Could stuck cold flooding the motor

The 99 has a single coolant sensor, if it was failed and showing full cold it would show that on the gauge as well.

lapisxj · Jun 25, 2015

lapisxj said:
Throttle body is clean and most of the sensors on the TB are new/been replaced.

See post #1. That includes the IAC.

I'll try to make a note to watch the temp gauge the next time it does it.

old_man · Jun 25, 2015

Run the can of BG44K. It is a good thing to do anyway, I would bet you will see a big difference. It will normally fix a leaky injector.

No start after warm but fires up with foot to floor.

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