just a thought... check your intake manifold bolts to ensure they are still torqued correctly.
Check all of the vacuum ports that you may have added and make sure everything is still sealing right.

 

did you figure out what going on with your truck ??

 

OK, the shop couldn't fix it. But they did give me a clue.

The scanner said that the engine was running at 254*. The truck was backfiring thru the TB. The tech said that kind of temp is WAY too hot and will lean out fuel trims big time. The pain is that the coolant temp gauge isn't going above 200* so why is the PCM seeing the engine as hotter?

The overheating theory is supported by the fact that the engine ran fine all the way home on a cold truck, but pinged a bit as I was pulling up to the house when the engine got hot. So that leaves me with the following:

Bad coolant temp sensor - possible, but shouldn't the dash gauge be boiling? The nice guy at NAPA that lent me the fuel pressure gauge gave me a free new one to slap in there.

Bad T-stat - possible but unlikely as the radiator gets hot. I'll replace it anyway to be sure. I'll boil this one as well to test it with a thermometer.

Bad IAT sensor - it's possible the sensor is bad, OR the wire crimps I used came loose and is making an intermittent connection, causing a false reading on the air charge temp. I will unwrap everything and solder and shrink wrap the connections tomorrow.

Is there anything else I should look for? The tech also suggested that I pull the fuel rail and double check all the injectors for screen clogging. He seems to think that might be an issue as well. He suggested using a 12 volt source to open the injectors and backflush them with carb cleaner. Would a few small batteries be enough to do that? I also have small Ultrasonic jewelry bath I could use as well.

 

a good 12 volt source is a battery charger, or a regular car battery. it would take too much screwing around with 6 volt or 1.5 volt batteries.

i'm confused by the 254* reading on the scanner at the same time your dash gauge was normal. you've got just one temp sensor, right ? does the scanner read the coolant temp, or the IAT temp ?

an injector would explain a lean condition. if you happen to have an extra set from the junkyard, you might swap them out.

 

IDK what's going on with the dash gauge VS. PCM. I'm wondering if it takes both into account and the IAT connection is bad. I just hope I didn't blow a head gasket.

I don't have extra injectors lying around, and they're darn expensive from the FLAPS. it would be over $400 for all 8. Gonna try to clean them first.

 

Did you replace the coolant temp sensor?

 

put an ohm meter on the temp sensor... at operating temp it should read ~700 ohms

from the book...

F temp Min Max
176 1170-1340
194 860-970
212 640-720
230 480-540

if the sensor checks out ok and is not intermitantly open (found one on my audi that was like that) check the wiring back to the pcm. with the battery disconnected, (for your peace of mind only as it doesnt really matter) pull the connector and short the wire to ground if it is a single wire or short both if it's a 2 wire sensor and measure back at the pcm. (I forget if it's a single or dual wire sensor).

the sensor on the TT that was bad worked fine up to a temperature range, then opened up. cooled the motor down and then started working again. had power loss issues because of it. the motor was most likely pining and the timing was being pulled back.

 

Well, I tore into today. Here's what all I did:

Changed the temp sensor and t stat. The temp sensor had a little of that white filler on the outside. I think the seal broke and that might have been causing the issue.

Cleaned the injectors, ohm'ed them to make sure they spec'd out. The FSM stated they should be 12ohms +/- 1.2 ohms. All were right around 11.2. I used a little 9 volt battery to make sure they would open and backflushed them with carb cleaner. I figured the 9V would open slowly and not push enough amps to short it accidentally. It actually worked well. Then I ran each thru the ultrasonic bath for 30mins. One seemed to really dump a bunch of black stuff.

Soldered the IAT sensor connections

Disabled the MSD ignition for now.

Checked the torque on the manifold bolts and a few were loose. Pulled them one at a time and added some locktite. Torqued them an extra 6 in/lbs so the gasket would seal.

Deleted the catch can as there was no appreciable oil in the can.

Set the e-fans to go on a little bit earlier.




I did have ONE "oh crap" moment. There were a couple of bubbles of oil that rose to the top of coolant funnel when I was filling it. I have used the pan I caught the coolant in for tranny fluid before, so it might be a little residue, but I'll be keeping a very close eye on that.

 

Is the temp gauge and the scanner now showing the same value?

 

I don't have a scanner, but I'm going to call the Dealership today to make an appointment to have my fuel sync checked and reset to 0. I'll pick at them about the temp when they have it hooked up too.