KBRANDT0813

I just got back from 2019 Easter Jeep Safar in Moab. This was my third year in a row going and probably the best one yet. However, the entire time I was out there I was fighting weird 3.8 gremlins. The first issue was vapor lock. This happened to me last year, and only a few times and only in Moab. If I was out on the trail, and turned my jeep off, it would not start back up. I had to disconnect the battery, let it sit for about 30 minutes, reconnect the battery and it would start right up. It sounded like it was not getting fuel. This happened yet again this year so I left the jeep running the entire time out on the trail. Then the next issue started with overheating. The temp gauge was reading normal, then all of a sudden the overflow was completely full with coolant running out. Then the temp gauge started going down in temp (which makes no sense) and then back up to normal again in about a few seconds. Water does not change temp that quick, so that leads me to believe it is my temperature sensor. Also, it was so hot under my hood that a thermal overload I use for a disconnect for accessories (such as my compressor) kept tripping and not resetting because it was so hot. And then another weird thing happened where I was parked on a downward incline (the rear at a higher elevation than the front) for a bout 10 minutes with the motor running, I get back in to keep going down the trail and the check engine light started flashing and the jeep sounded like it was missing. I believe it sounded like it was not getting oil, maybe because I was parked on an incline? Anyone else had any of these issues, and what did you do to fix it?

ronjenx

I thought vapor lock went out with carburetors and low pressure fuel pumps.

Your fuel delivery system should be pressurized to 52 psi, and remain nearly that when shut off. Perhaps you have problems in that area.

If everything you describe is caused by extra high under-hood temperatures, consider this: OEM Mechanical Fan Retrofit

KBRANDT0813

I thought the same thing, but it sure does seem like vapor lock. However, it only does this in Moab, I'm wondering if it has something to do with the elevation? It is weird. Runs fine, then I turn it off, and it cranks and sputters a little bit and wont start. Sometimes if I tap the gas petal it will eventually fire, but most of the time I have to disconnect the battery, let it sit and then it starts right up. I have looked at a bigger radiator and fan, however I am thinking it has to do with the temp sensor because the fan does not turn on as frequently as it usually does. The weird thing about the vapor lock, it did that last year when my cooling system was functioning fine. This year the cooling system was having the issues I described above.

jedg

I know the fuel injected XJs had this problem. To the point that there is a small market for the heat shielding to protect the fuel rail and the fuel injectors.

What confuses me is you saying that disconnecting the battery influences it starting. I would think raising the hood and letting it sit would be all that's necessary...

Mr.T

I've changed settings in the PCM to start the fan (both hi/lo speed) at lower temps, and with a smaller deadband. However, this was just based on a feeling that underhood temps were a bit high and the tronics would appreciate the airflow more than I'd miss 0.001% less mpg. Even though this is a cool idea (pun intended) there wasn't any problems before the change, even in 100+ heat going slow in 4lo.

A few thoughts on diagnosing the problem(s)

When the CEL was flashing, it was indicating "serious conditions that could lead to immediate loss of power or severe catalytic converter damage." It should have set a code too, which would be very helpful. Low engine oil pressure is a separate red lamp. Maybe a sensor is malfunctioning at higher temperatures.

On the other hand are the symptoms of overheating, the coolant bottle overflowing, etc. I'd start checking things like the percent antifreeze, if the fan starts at the right temperatures (216F low and 223F hi), if the thermostat is opening at the right temperature (195F), if the rad cap is holding pressure (quick and dirty check is to try and squeeze the radiator hose), and if the temperature sensor reading from an ODB scan tool roughly agrees with an infrared thermometer.

KBRANDT0813

So after doing a little more digging, I think it has to do with the temperature sensor. Im replacing the sensor tomorrow and flushing the radiator. Hopefully that fixes the heat issues. I’m not sure if it has to do with the not starting, but it possibly could. Someone was telling me that a temperature sensor can cause issues with the fuel injection? If that doesn’t work, it may just be time for that LS swap...

Mr.T

At higher temps the fuel is less dense and that needs to be compensated for. There's temperature compensation in the PCM that should keep the fuel trims close to zero, but I've proven to myself that it's not enough and have increased the compensation via tuning the PCM. There's no fuel temperature sensor, so it must be based on an algorithm using other PCM information like engine temp, the battery temperature sensor, intake air temp, etc. Fuel trims only go so far, then they'll set appropriate codes and the engine might run wonky. Bottom line is that I think abnormally high engine and underhood temps will mess with the fuel calibration.

Last month one of my pick-ups had a similar overheating incident, turned out that the center of the thermostat literally broke away from the outer part. Maybe it would be a good idea to pull the thermostat while you're at it, do a visual, and heat it up in a pan of water on the stove with a thermometer and check the opening - 195F start opening, ~210F full open if I remember right.

KBRANDT0813

It definitely has to do with heat under the hood. I use a thermal overload 100A breaker as a disconnect for my ARB compressor and a few other accessories and it was tripping and not letting me reset it. Even when there was obviously no shorts because there are fuses between the breaker and the devices. Also another weird thing is that it is throwing no codes. I even took it to a dealer to have them check for codes and nothing. I’m replacing the temp sensor today and flushing the radiator and seeing if that does it.

KBRANDT0813

So as I was replacing the temperature sensor, as I started loosening it I noticed antifreeze coming out from the side. After further investigation, I noticed that the actual intake manifold around the sensor in two spots was cracked. So as a temporary fix, I put some JB weld over the cracks and replaced the sensor. The temperature gauge is now accurate and the cooling system seems to be functioning. I am going to replace the intake manifold now that I believe that was the culprit, in addition to most likely a faulty sensor. As for the “vapor lock” I don’t know how to test that, but I am going to take it to a dealer and have them perform a full tune up on the motor, as well as this fuel injection system flush. If that still doesn’t fix it, it may be time for an LS...