Stalling, overheating and poor fuel economy

jimstj

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Guys, I am having a problem with my TJ I was hoping you might be able to help me with. I know this is long, but I wanted to provide all the info as I think the pattern has multiple factors.

2002 TJ 4.0L 5 spd 4.56 gears 63k original miles with 33’s

updated to remove commentary, I know this is long. so if you read the outline, you don't need to read what follows:


Symptoms
  • Stalling on the highway
    • Sudden loss of power then stall. Highway only and only after driving for at least an hour
    • Discovered that if I very lightly feather gas, it would limp along
    • If I stopped and let it cool for about 30 mim I could carry on. Might or might not stall again
    • Does seem to be correlated to the overheating problem below
  • Decreased fuel economy and acceleration
    • Normal fuel consumption was about 15.8-16.2 mpg (combined hwy / city)
    • About this same time I had installed a Gobi rack and on the longer trips was using a cargo carrier. So was not sure how bad the fuel economy actually was. But then realized just city driving was really bad
    • On this trip up north, with no cargo carrier, I was getting about 12-13mpg.
  • Overheating
    • Started to overheat when the AC on. Did not do this in the past
    • Sudden “check gauges” light and in the red zone. AC off, windows down, heater on full, and back to normal operating temp in about 2 min.
    • Outside temp was about 102 – 105 degrees, also up in elevation (5k-8k)
Blue Driver Data
  • When limping, O2 sensor voltage nearly zero
Repair
  • Had the crank sensor replaced
  • Performance drastically improved and was in elevations up to 8k feet
  • After 100 miles fuel consumption was 16.1 mpg!
  • Used AC without overheating but there were reduced air temps so overheating problem not confirmed as fixed
Next Day
  • Heading back home and it stalls again on hwy
  • Had the AC on but noticed temp climbing (no check gauges light) so turned AC off but still stalled about 15 min later
  • Just needed to let it idle on the shoulder for about 15 minutes
  • Stalled again before getting home
  • Once home, I filled up the tank to check fuel consumption – 11.4 MPG! Ugh
Next Steps
  • Wondering if I had a faulty crank sensor sensor. Was not MOPAR which is my preference for such parts, but this was a NAPA part. I know if they are going to go bad, they usually do soon after installing
  • Fuel pump only has 40 miles on it.
  • Did not seem like the cam sensor, but wondering
  • Bad ECU?



Thanks for hanging in there with this long post. Any help would be appreciated.

Jim
 
Last edited:
Maybe I made this post too long.

To make it easier to consume does anyone have an idea on what can cause intermittent stalling on the highway other than a crank sensor?
 
Any OBDII codes? You indicated that you had brought it to a garage for the sensor replacement. Was that a result of your request, or did they inspect and then recommend that?

I recommend having the cooling system inspected and tested. Once that's working correctly, report back if you still have the problem.
 
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Search for a Mopar Crank Position Sensor and DO NOT use an aftermarket one. While they may work for a while; they will not last or perform the way a Mopar CPS does.
Check the three PCM (ECU) 32 pin wiring harness connectors/pins for oxidation; ensure the connectors are firmly snapped into the PCM receptacles.
 
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If your A/C system is not working properly, the condenser can dump a lot of energy that the radiator absorbs (especially with the high ambient temps you saw), causing the overheating, and the overheating could be causing the stalling and limping. How well is your A/C cooling? If it's struggling to cool the TJ, I'd look at that.
 
Install the DEI thermal kit for the injectors and fuel rail. Replace the OEM manifold heat shield , this helps with Vapor lock. Twin Cats cook the engine compartment and causes significant heat stress on the sensors and PCM. Consider installing an electric fan replacing the stock radiator fan; operates on a thermostat (or manual override switch). I actually wrapped the entire intake air tube and AT sensor in thermal wrap to prevent engine heat loading from screwing with the intake air sensor. Also if your air box is ORM be sure that the air duct is pointing UP and toward the right side of the engine compartment as possible to pull in the coolest (relative term) air and not downward pointing toward the engine block. This helps believe me. Also if your jeep “bonks” (dies with no codes) after it runs on the hottest summer days (with the AC on) replace the shut down relay as those get thermally sensitive over time. Back when my 05 TJ was stumbling and cutting out (only in the summer) I kept a can of cool spray and a infrared thermometer gun with me. I would pull over and quickly measure the hot spots and selectively shoot cold spray on components just to confirm if it made any difference. That’s how I located the shut down relay and the AT sensor causing 50% of the heat issues. I also wrapped the fuel line in thermal insulation from the fuel rail down past the CATs completely under the floorboard to help keep the fuel cool as possible Remember fuel under pressure decreases its boiling point. So when you stop for gas or a quick trip to Walmart’s you’ll get loos of power and stumbling scenarios. Do everything you can to cool the engine compartment, shield the fuel line, fuel rail, injectors, air intake and AT sensor, and manifold.

Also look into keeping your radiator and fluids in top shape and ensure the coolant reservoir is topped off; you’ll be amazed at the cooling efficiency lost by running with a slightly low coolant level.

If you have the original OPDA throw it away before it damages your cam shaft. There is a known MOPAR recall / defect around this. Install the OPDA made by crowne as it has oiling grooves (oil slinger) to life the upper cam disk. The OEM OPDA will simultaneously wear out the cam shaft lobe as it wears out the OEM OPDA leaving you with a major repair bill. I changed mine 5 years ago when I first found out about the recall; I had the cam shaft evaluated by Jeep and they gave it a pass. Apparently I caught in early (70k miles) before any wearing damage could occur. I found this recall when I was trying to solve CP / CKS errors; I replaced the OEM OPDA with the crowne model just as a suspicion but it did not fix the CP / CKS errors for that I had to isolate the CP sensor’s 5VDC source and ground (wired in 12v/5V dc regulator).
Hope this helps. Holler if you need any more details.
Zippyman1962
 
Any OBDII codes? You indicated that you had brought it to a garage for the sensor replacement. Was that a result of your request, or did they inspect and then recommend that?

I recommend having the cooling system inspected and tested. Once that's working correctly, report back if you still have the problem.

No codes. At one time I was throwing a Throttle position sensor code and have sense replace it with a Mopar sensor.

I had purchased the sensor just before heading out and had it with me. I didn't want to replace it in the parking lot, so I found a garage. We did discuss the symptoms and he agreed this would be a good place to start.

Is your thought that the overheating could be making the sensor fail?

thanks for your input.
 
If your A/C system is not working properly, the condenser can dump a lot of energy that the radiator absorbs (especially with the high ambient temps you saw), causing the overheating, and the overheating could be causing the stalling and limping. How well is your A/C cooling? If it's struggling to cool the TJ, I'd look at that.

Hey Sab, the AC works really well. Outside of the fairly common problem of the air blowing out different vents from a vacuum leak, which I have not yet looked at.
 
No codes. At one time I was throwing a Throttle position sensor code and have sense replace it with a Mopar sensor.
Just to make sure: have you checked it recently with a reader? Some codes are stored without the CEL on.
Is your thought that the overheating could be making the sensor fail?
Since it was overheating, I thought that addressing that would be a good place to start. The 4.0 is a tough engine, but I wouldn't tempt fate by overheating it anymore than necessary.
Stalling could be caused by many things.
  • Last spark plug change?
  • Last O2 sensor replacement? It's a bit early for your mileage, but check those codes again and make sure.
 
Just to make sure: have you checked it recently with a reader? Some codes are stored without the CEL on.

Since it was overheating, I thought that addressing that would be a good place to start. The 4.0 is a tough engine, but I wouldn't tempt fate by overheating it anymore than necessary.
Stalling could be caused by many things.
  • Last spark plug change?
  • Last O2 sensor replacement? It's a bit early for your mileage, but check those codes again and make sure.

I have at least 40k miles on the spark plugs. guess it's time to change. Do you feel it should be a mopar only?

I don't have any codes at all at this time.

Even with the low miles I was thinking of installing an oversized radiator. But after researching I found that the best alternative is the OEM radiator. I have purchased one and it is still in the box... lol But not really sure I need it at this time. If I am not leaking any coolant, what is the measure / criteria to replace it?
 
Install the DEI thermal kit for the injectors and fuel rail. Replace the OEM manifold heat shield , this helps with Vapor lock. Twin Cats cook the engine compartment and causes significant heat stress on the sensors and PCM.
Zippy, I like your approach. I did a bunch of things to my silverado to keep it cool and it make towing much more pleasurable. I ordered the heat shields as well as one for the fuel line.

I actually wrapped the entire intake air tube and AT sensor in thermal wrap to prevent engine heat loading from screwing with the intake air sensor. Also if your air box is ORM be sure that the air duct is pointing UP and toward the right side of the engine compartment as possible to pull in the coolest (relative term) air and not downward pointing toward the engine block.

What thermal wrap did you use? and did you just zip tie it to the sensor?

Check air duct pointing up

Also if your jeep “bonks” (dies with no codes) after it runs on the hottest summer days (with the AC on) replace the shut down relay as those get thermally sensitive over time. Back when my 05 TJ was stumbling and cutting out (only in the summer) I kept a can of cool spray and a infrared thermometer gun with me. I would pull over and quickly measure the hot spots and selectively shoot cold spray on components just to confirm if it made any difference. That’s how I located the shut down relay and the AT sensor causing 50% of the heat issues.
Is there a way to test the ASD relay? when you say cold spray, do you mean the sports stuff for injuries?
 
Search for a Mopar Crank Position Sensor and DO NOT use an aftermarket one. While they may work for a while; they will not last or perform the way a Mopar CPS does.
Check the three PCM (ECU) 32 pin wiring harness connectors/pins for oxidation; ensure the connectors are firmly snapped into the PCM receptacles.

Yea, that is usually my approach also. I just picked up a Mopar CPS and will install it. I am just worried that if this overheating is causing the sensor to fail I don't want to replace it now just to have if soon fail.

what is the likelihood that the overheating is causing the sensor to fail rather than an inefficiently running engine due to the bad sensors is causing the overheating?
 
I have at least 40k miles on the spark plugs. guess it's time to change. Do you feel it should be a mopar only?
Thread 'What are the best spark plugs for my Jeep Wrangler TJ 4.0?'
https://wranglertjforum.com/threads/what-are-the-best-spark-plugs-for-my-jeep-wrangler-tj-4-0.4196/
I don't have any codes at all at this time.

Even with the low miles I was thinking of installing an oversized radiator. But after researching I found that the best alternative is the OEM radiator. I have purchased one and it is still in the box... lol But not really sure I need it at this time. If I am not leaking any coolant, what is the measure / criteria to replace it?
Start with the basics. Pressure test the system and cap. You borrow the tester for free at your FLAPS.
 
Does this mean you are currently running a non oem radiator?

Not sure. I am the third owner and this jeep has only about 60k miles. I think there is supposed to be a Mopar embossed under the radiator. I will check.