Trouble starting after sitting 10 minutes

[Bridgerman]

Hi Folks,

First post here, but I have been reading a lot.

Jeep: 1999 TJ 2.5 manual transmission

Here is the problem: When I drive a short distance (maybe 5 minutes) and it is not hot, still winter in Montana, the Jeep won't start unless I let it sit for 10-15 minutes, then it starts right up. When running, it is smooth and has no issues regardless of the length of the drive. I should also say that the rig passes a smog test.

Here is what was done before I purchased the rig:
Fuel pump changed (maybe regulator but it wasn't listed on the receipt)
Fuel Injectors changed
Fuel hoses changed
Fuel tank washed out
Coil changed
Throttle Position Sensor changed

Here is what I have done:
I checked the ASD relay. It cycles on for a couple of seconds when the key is turned and I can hear the fuel pump running. It is also activated when I am cranking with a no start condition. I can tell this by examining the PCM fuel injector signal. It starts at 12VDC and that is supplied by the ASD relay, so to me that seems normal. I also swapped the ASD relay and the horn relay just to see if it made any difference. It didn't.

I checked the fuel pressure. It seems very high and the guy that owned the rig before me had the fuel pump and fuel injectors changed. The fuel pressure reads about 90PSI. I did let the fuel system sit for a while and I didn't see a bleed off of the pressure which indicates to me that the injectors are not flooding the cylinders with fuel which would need to be reduced before the spark plugs could fire.

I connected an oscilloscope to the low-voltage side of the coil and to the PCM side of the injectors. When the Jeep is running, I see the signal to fire the injector followed by a signal to the coil to cause a spark.

When the Jeep is cranking and not starting, I am seeing the signal to the coil, BUT, I am not seeing the signal to the fuel injectors. This is to say that the voltage on the PCM side of the injector stays at 12VDC and I am not seeing a low pulse which would fire the injector. The PCM is not activating the injector during this crank but not start time. I used starting fluid and the Jeep wouldn't start which is weird since I am seeing a signal to the coil so I should be getting a spark. I only checked this once, so I am not 100% sure I don't get some firing on starting fluid. I can check this again.

This led me to think that perhaps it was the crank position sensor since when I disconnected the current CPS, the Jeep exhibited the same symptom, in other words, it would crank but not start. Also, no codes are being thrown. It doesn't even throw a code when the CPS is disconnected and I am cranking.

So today, I changed the CPS and the problem persists.

What I haven't done is to verify the 5VDC signal at the CPS during a crank but not start condition. I am thinking of adding some wires to the CPS connector to see if maybe 5VDC is not present during the issue. That could be a PCM problem. I also haven't changed the Cam position sensor.

Soooo...I am interested to see if anyone has any ideas that I can try before I bite the bullet and change out the PCM since they are kinda pricey,

Thanks in advance,
Bridgerman

 

[macleanflood]

Fuel pressure is supposed to be 50 psi. Technically 48. Need to determine if your gauge is wrong or the fuel pressure is too high. You could rent another gauge from an auto parts store to verify.

What pump assembly did you use?

Fix that and you'll probably fix your issues.

-Mac

 

[Bridgerman]

Thanks Mac, I thought the same thing about the fuel pressure gauge. I checked it on my tires and it read the correct PSI, so I am guessing it is correct.

I believe this rig had this problem before the fuel pump was replaced and that is why it was replaced.

The most curious thing is that the injectors are not getting a signal when the problem is happening, so while the fuel pressure could be an issue, if the PCM is not sending a signal to the injectors it almost doesn't matter what the fuel pressure is (aside from damaging some hoses). Do you know if the PCM monitors the fuel pressure and will cause a no-start condition? The pressure is that high when the Jeep is running smoothly.

 

[macleanflood]

I've never found a PID for fuel pressure.

The fuel pressure regulator is located in the top of the pump assembly, along with a permanent filter.

What brand pump did you use?

Perhaps the gauge isn't accurate across it's entire sweep.

My 60 psi tire gauge said my tires were at 4 PSI. @srimes 15 psi gauge said 7 psi.

I'd still borrow a gauge. Two pumps are probably not wrong.

What brand CPS did you use? I had luck with a Napa Echlin. Lots of folks have had luck with Mopar and Standard Motor Products.

When you crank no start the engine do you see RPM on the tach?

-Mac

 

[Bridgerman]

HI Mac,

The fuel pump was replaced before I bought the vehicle so I really don't know the manufacturer. I have the receipt for the parts and labor, but it doesn't give a brand. I am more of a DIY guy so had I done the job, I could have told you. I'll swing by the local parts store that rents tools and see what their gauge reads.

Those CPS devices are expensive. I bought one with a life-time warranty from O'Reilly's here in Bozeman. I'll check the output and see if it is sending pulses.

I'll also check the tach during cranking. I'll have to drive it around for a bit this morning to duplicate the problem. Right now, it just cranks and starts.

I really appreciate your insights and help. I'd rather not shell out $600 for a PCM if the fix is more simple.

-Bridgerman

 

[Bridgerman]

HI Mac,

The fuel pump was replaced before I bought the vehicle so I really don't know the manufacturer. I have the receipt for the parts and labor, but it doesn't give a brand. I am more of a DIY guy so had I done the job, I could have told you. I'll swing by the local parts store that rents tools and see what their gauge reads.

Those CPS devices are expensive. I bought one with a life-time warranty from O'Reilly's here in Bozeman. I'll check the output and see if it is sending pulses.

I'll also check the tach during cranking. I'll have to drive it around for a bit this morning to duplicate the problem. Right now, it just cranks and starts.

I really appreciate your insights and help. I'd rather not shell out $600 for a PCM if the fix is more simple.

-Bridgerman

Hi Mac,

Thanks for the advice. I did cross check my fuel pressure gauge with a loaner from the local parts store and they both read the same value, so my pressure was way over the specification. I went ahead and changed the fuel pump and regulator. The pressure is now 48 PSI and the problem has gone away.

My only guess is that the high fuel pressure was causing the injectors to malfunction (not open). After waiting the proscribed 10-15 minutes after shut down, the fuel pressure drains back to some nominal level and the injectors can function normally since the fuel pump and injectors come on at the same time during a key turn which will lower the pressure to allow a start.

For anyone coming to this thread in the future, here is a link to the exact same problem and his solution was to change the pump and regulator as well.

https://wranglertjforum.com/threads/88psi-fuel-pressure-hot-start-issues.73642/

Cheers!
Bridgerman