Hot Start Problem

I have had a long term problem with my truck, it is a 1996 XJ, 4.0 auto with 99k. It always starts fine when cold, but the only way to start it when hot is to give it gas!

History, last feb I replaced the fuel filter, plugs, wires, cap/rotor and air filter, ran great but problem still there. Then i cleaned out the TB and replaced the idle air control valve. The problem seemed to go away, but came back a few months later. I just had the fuel pump/sender and fuel filter replaced, nothing. I again replaced the IAC valve and cleaned the TB, nothing. My next step is to replace the throttle position sensor...if thins doesnt work I am stumpted. What would be next?

I agree that it could be the TPS, you may want to test it before buying a new one. Use the SEARCH function for more information.

Well I replaced the TPS, apparrently my 96 has a 97 style as the one I bought 1st had a different connector and i eeded to get one for e 97 to work! Anyway, I replaced the TPS and no improvement. My friend was over and said it could be a leaking injector, and it did appear that the area around one of the injectors was dirtier than the other 5. He thought that it may be leaking and that there is to much gas at initial startup since with my fuel system it doesn not drain back into the tank. Any thoughts?

That's an interesting diagnosis. I have the same problem with my 1996 which has 1997 parts on it (it was a change over year). I've got to stomp it to the floor sometimes when it is hot and she will fire right up. It's wierd. I'm running 5.0 mustang injectors so I'm doubtful that injectors alone are the problem. Someone on here said it is a coolant sensor that causes this which sounds plausible. I just havent looked at my scantool live data to see what temp my sensor is giving me. This would be the sensor on the front of the motor not the back of the motor. I'm tempted to unplug it next time I have the startup problem and see if it makes a difference.

Hallo Jeepers, Don't forget the ignition coil. It can fail also ,when old and hot. Like myself:yap:
It was a big difference on my rig, when I renewed it.:wave:

wim hoppenbrouwers said:

Hallo Jeepers, Don't forget the ignition coil. It can fail also ,when old and hot. Like myself:yap:
It was a big difference on my rig, when I renewed it.:wave:

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Hmmmm...... I was thinking about this, the only thing is that I had the hood open, ran it for a few minutes shut it off then tried to re-start a few minutes later. Wouldnt the coil not have heated up? I may still change it, at least with replacing all this stuff the jeep runs great!! (when it starts)

When you push the pedal you aren't actually giving it gas, you are giving it air. The IAC positions itself for enough air to start the engine without having to touch the pedal (in theory). The computer meters a longer injector pulse to give it a little extra fuel for start.
What usually goes wrong is the IAC gets sticky and doesn't park properly. And/or the IAC isn't getting enough voltage to park properly.
Weak battery, sticky IAC or connectors with excessive resistance or a combination of all of the above, are likely candidates. Maybe a flaky ASD relay, though the ASD would likely cause other grief to boot.
I've had good luck spraying oil down the IAC orifice with the motor running. I do this a few times a year and my no starts when hot have disappeared. I haven't had to give any pedal for a start in years. A strong, fresh battery, with good clean connections also helps. The IAC seems to need all the voltage/current it can get.

8Mud said:

When you push the pedal you aren't actually giving it gas, you are giving it air. The IAC positions itself for enough air to start the engine without having to touch the pedal (in theory). The computer meters a longer injector pulse to give it a little extra fuel for start.
What usually goes wrong is the IAC gets sticky and doesn't park properly. And/or the IAC isn't getting enough voltage to park properly.
Weak battery, sticky IAC or connectors with excessive resistance or a combination of all of the above, are likely candidates. Maybe a flaky ASD relay, though the ASD would likely cause other grief to boot.
I've had good luck spraying oil down the IAC orifice with the motor running. I do this a few times a year and my no starts when hot have disappeared. I haven't had to give any pedal for a start in years. A strong, fresh battery, with good clean connections also helps. The IAC seems to need all the voltage/current it can get.

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Good post.....

You might want to check the fuel pressure..... If you have a leaking injector your fuel pressure will bleed off after you key off the motor.... A bad pressure regulator can leak into the vacuum hose and will also lose pressure when keyed off..... A vapor lock or fuel boiling condition will also show up as low fuel pressure when you crank....

A bad coolant sensor will definately give you problems at start up.... It is one of the main things the puter looks at for air fuel mixture during start up....

A dirty tb or sticky Iac will limit your air as previously stated.... It sounds like you covered that though...

Do you get black or grey smoke out the tail pipe when it starts?

UPDATE:

It looks like I have fixed the problem, it seems to have been the engine coolant sensor gone bad. It does make sense, if the computer did not know the motor was hot, it would try for a cold start, to much fuel, pressing the accelerator pedal opens the throttle body alowing more air so it would start. This also explains why my TB got so dirty again. I will check the plugs when I get a chance as they are probalby pretty dirty now.

On a sidenote, I had to get a sensor for an 98 not 96. I dont know where the errors lie, with Autozone or my Jeep. Lesson here is to not buy anything wo bringing the original part with me!

Good. We have now learned from it for the future.
There is often a misunderstanding in sensor types and buildingyears.
From '87-'90 the sensor had a pigtail.
From '91-'95 a connector on the sensor. (oval shape) (Ohms values changed)
From '98 has the connector a right-angled shape.
How is the '96 and the '97 looks like?.:rattle:

p.s. Did you also look at the MAT (Air Sensor)?

I am not sure how this::rattle: is really necessary.
To answer your question, according to autozones part numbers, the 96 and 97 temp sensors are the same part. Very simple. I was just sharing this so others can get the right part the first time around.

wim hoppenbrouwers said:

Good. We have now learned from it for the future.
There is often a misunderstanding in sensor types and buildingyears.
From '87-'90 the sensor had a pigtail.
From '91-'95 a connector on the sensor. (oval shape) (Ohms values changed)
From '98 has the connector a right-angled shape.
How is the '96 and the '97 looks like?.

p.s. Did you also look at the MAT (Air Sensor)?

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Way to go on this guys. I see alot of problems talked about but very few posts get updated when they figure out the problem. Is this the temp sensor on the front of the engine near the thermostat? I believe the one on the rear of the engine is for the gauge only. I've got an OBD II reader that shows live data. I don't recall that my temp sensor was giving a faulty reading last time I was trouble shooting this problem but it could have been.

ARBOGASTER said:

Way to go on this guys. I see alot of problems talked about but very few posts get updated when they figure out the problem. Is this the temp sensor on the front of the engine near the thermostat? I believe the one on the rear of the engine is for the gauge only. I've got an OBD II reader that shows live data. I don't recall that my temp sensor was giving a faulty reading last time I was trouble shooting this problem but it could have been.

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Yes. The CTS is in the thermostathousing on the '91 and later Jeep 4.0L engines.
It's providing a varying voltage to the ECU. (Depending of the resistance) and is used also for switching the E fan.:yap:

it isnt just this forum where problems never seem to be solver or at least followed up on, I am on several toyota forums where the same thing happens. I didnt want to do that. And as stated, it is on the thermostat housing, a 5 minute fix to a long term problem!

wim hoppenbrouwers said:

Yes. The CTS is in the thermostathousing on the '91 and later Jeep 4.0L engines.
It's providing a varying voltage to the ECU. (Depending of the resistance) and is used also for switching the E fan.:yap:

Click to expand...