Engine Stalls When gets to Operating Temperature

Photojeeper

Photojeeper

New Member
Joined
Jul 10, 2021
Messages
6
Location
Escondido,CA
1990 YJ with Mopar Fuel Injection. When the engine gets to operating temperature the starts hesitating, when I press on the throttle t gets worse then stalls. I’ve replaced the fuel pressure regulator and fuel filter. Also insulated the fuel lines to prevent vapor locking which was a problem. The fuel pump is in the gas tank. Thinking it now could be a fuel pump having an issue. Will test the pressure. Any other thoughts?
 
Well...since nobody else has chimed in...Yep, fuel pressure is a good place to start. Wish I could tell you what it should be on your jeep. If im not mistaken your 90 has the factory TBI, not the 91 up style port injection which may be a bit harder to work with. Seems odd you had vapor lock issues unless its been altered to create problems.
 
It is a 1990 with the Mopar kit which is an Muiltport EFI. Looking at other threads and other forums, I am ruling out the fuel pump. Since if it was bad it would be cutting out and the Jeep would run out of fuel. If I let it cool it runs fine until OT. So I will be replacing the coolant temp sensor at the tstat housing and the MAP, MAT sensors. If if goes away after that then that was the problem. If it continues I will replace to O2 sensor. Will let you know.
The vapor lock issues were when I had the external fuel pump and the kit does not come with the heat shields that the other 4.0 OEM engines comes with. Being from SoCal I do a lot of desert trips, even in the Spring and Fall it can get toasty. Then I got a in tank fuel pump but kept all of the insulation on the fuel lines. Never can be too cool. ;-)
 
The acronyms and abbreviations keep us frequent posters from repeatedly typing the same thing over and over.
Kind of a short hand that most folk figure out pretty quickly...
 
Well the culprit is/was the O2 sensor, although no codes were in the ECM and the CEL did not go on as it should have. Got a new Bosch O2 sensor. One thing I am going to change is the thermostat. I believe that when the shop got a new on for the head they went by the book. The 4.2 uses a 185 degree, but i have Mopar EFI system which requires a 195 degree thermostat. I did run the engine and when it got to the temperature that it would start stalling it didn't. I gave it a short test drive and it was fine. I will do more testing after the thermostat is installed.
 
Photojeeper said:
Well the culprit is/was the O2 sensor, although no codes were in the ECM and the CEL did not go on as it should have. Got a new Bosch O2 sensor. One thing I am going to change is the thermostat. I believe that when the shop got a new on for the head they went by the book. The 4.2 uses a 185 degree, but i have Mopar EFI system which requires a 195 degree thermostat. I did run the engine and when it got to the temperature that it would start stalling it didn't. I gave it a short test drive and it was fine. I will do more testing after the thermostat is installed.
Click to expand...
Not that it really matters but a 90 4.2 should have used a 195° as well. But good you got it figured out, hopefully no more issues. The 195 will definitely ensure you run warm enough for the computer’s liking.
 
By mistake, I got a 180 for mine from the yoyo's at O'Rielly and it threw codes left and right.
ECM was no happy about it.
I'm slowly learning myself that these FI motors like it Hot, but that's not a bad thing, unless you don't have the cooling capacity.
As far as engine life is concerned most wear occurs when they're cold.
.
 
jeepjoe43 said:
By mistake, I got a 180 for mine from the yoyo's at O'Rielly and it threw codes left and right.
ECM was no happy about it.
I'm slowly learning myself that these FI motors like it Hot, but that's not a bad thing, unless you don't have the cooling capacity.
As far as engine life is concerned most wear occurs when they're cold.
.
Click to expand...
Yup. I've relatively learned to leave the cooling system alone and let it do its thing. I am pretty much 100% stock in that regard. Works fine and always stays cool. both of my trucks have preferred the same 210 also. I think it's a thing that started at the end of the carb years in vehicles and really became a guarantee with FI.
 
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