Hey all, been around plenty, just never had much to contribute. Today marks the end of a very long story, but I thought the information might help someone else so here it goes.

I bought my truck about 4 months ago. 2003 4.0L XLT 4x4. The truck had about 107K miles on it and was very stock. I noticed right away that I was not getting the gas mileage that I was expecting, but figured it was one of the maintenance items that I would be doing any way, so no real harm.

Over the next couple of months I performed the following items:

Oil change(s)
K&N air intake
Spark plugs and wires
Fuel filter
Tranny and diff fluid change
PCV elbow replace
Cleaned MAF sensor
New tires and alignment

During those couple of months I racked up several thousand miles and never saw more than 16 mpg. I also noticed that the exhaust smelled of sulfur (rotten eggs) even after the car had been driven at highway speeds for over an hour. I checked often and never had any trouble codes. or a CEL at all. The engine seemed to run fine and started actually quite well. I would say most people would have never noticed any poor performance. I would notice the smallest hiccup every now and then and a slightly rough idle.

I was also having transmission problems too, I thought. Going into 4th and 5th was slightly rough, more like a shudder. I had a transmission shop look at it and they told me it was the torque converter locking up. I had plans to look into the TCC solenoid.

Today I was driving and was paying very close attention to the temp gauge on the dash, trying to see if maybe the fan clutch was causing my poor MPG. What I noticed was upon deceleration and hard acceleration the temp gauge would go down to about 1/4 sweep, about halfway between normal and absolute cold. Once I would go back to cruising the temp would stabilize just below half sweep (normal temp).

I then thought the fan clutch wasn't releasing after lockup. To confirm this I parked and revved the engine around 2,000 rpm to see what happened to the temp. Sure enough it would drop similar to before. But then I would let the engine idle, and the temp stayed low. Even after several minutes it was still lower than normal. The car had been on for almost an hour and couldn't maintain normal temp when idling.

At this point I knew I had a problem with the cooling system, and wanted to start with the cheapest part, the thermostat. Once I had the housing off and the old thermostat out I found the source of my problems. The thermostat had come apart and the piece that had broke off was clogging the neck into the water pump.

After fixing this the engine is totally different. I guess the plugged water pump was creating quite a drag on the engine. The mid range power and throttle response is totally different. Also, since the engine was never getting properly up to temp, I was always running in the richer warm-up fuel mapping. This also explained the sulfur smell from the catalytic converters burning excess fuel and the poor performance.

The part was a Motorcraft piece and could very well of been original. Thankfully there was a small hole in the offending piece of metal and I didn't blow the engine to pieces. The transmission shifts very smoothly now (why I can't seem to figure out) and the mileage is much better.

Sorry this ended up so long, but there it is, my saga explained. I don't know if this is a one time occurance or a chronic problem. Either way, there it is.

 

dude raligh, didnt know you were on here. congrats on the fix man. wish i could find the mph hemorrhage i have

 

same here, i'm going to check into my thermostat this weekend as well as my pvc elbow.. something has got to give. although my scanguage says i stay at a consistant 190 ish after warmup.

 

Did you have any problems with your heat due to the thermostat?

 

what should normal temp be on the 4.0? my ultraguage shows 176-180 consistantly.

 

I live in a warmer climate and "gut" the thermostat and modify to an approx 5/8" hole, works great just like the aftermarket restrictor plate.

 

x2 lol

 

x2 mine never seems to get up to temp in the winter time.

usually around 172 on chilly days.