Coolant Temperature Fluctuating & Poor Fuel Mileage

[eand28]

Background: 2005 LLY bought it a few years ago with 150K miles, truck now has 171k.

Mods:
LBZ intake elbow
5" exhaust
DSP5 tuning
Built Trans & goerend DK converter

When I bought the truck it was winter time and on the drive back from AZ with it, the truck never would warm up fully. After I got it home, I put new GM 180° and 185° thermostats in the truck which fixed the running cool issue.

I've noticed recently driving the tuck on cooler days (below 60° F), the truck will run about 180-185° unless I get on it, then the temp will rise to about 190°-192° and cool back down once the load decreases. On hot days the truck will consistently run between 190° and 205° running empty or towing. I do not have any issues with the truck using coolant or the heat going in and out. Upper coolant hose stays pliable after driving the truck hard and letting it sit overnight.

I put a lift on the truck with 20x12's and 33" trail grapplers 2 years ago, and my fuel mileage has been 11-14 mpg regardless of how I drive the truck or what level I have the dsp5 set at. Tuning was done by a reputable tuner, so I do not think that is the issue as the truck got 13-18 mpg stock height with the factory pyo's and 245 michelins. I am not sure if the converter is contributing more to my fuel mileage woes now that there's more rolling mass. The truck is a toy at this point so while I am not that concerned about the fuel mileage, it does seem odd it dropped off so much.

The balance rates seem reasonable based on the calculated fuel rate and the truck holds rail pressure.

Am I at the beginning stages of the truck needing head gaskets and/or injectors/CP3 or do I just need to do thermostats again and I have some other issue that is hurting my mileage?

 

[darkness]

The lift, heavier and bigger tires are hurting you mileage. Did your temps start rising after the lift and tires? Have you cleaned your cooling stack? And also how old is your fan clutch?

 

[eand28]

The lift, heavier and bigger tires are hurting you mileage. Did your temps start rising after the lift and tires? Have you cleaned your cooling stack? And also how old is your fan clutch?

Temp fluctuated like that before I lifted the truck. I've never cleaned the cooling stack. Fan clutch it original to my knowledge.

On my drive home today the truck was running 180-185 unless I got in the throttle or was going up a hill then it would climb to 190 and then cool back down to 180-185

 

[darkness]

Those temps aren’t out of line, but you should address those 2 things.

 

[2004LB7]

What thermostats did you use? Brand and source?

 

[Chevy1925]

There is nothing wrong with anything you stated. Crack temp on t stats is 180 and 185, full open is 15* above that.

Lift, wide heavy wheels and larger tires are really going to hurt mpg.

 

[eand28]

What thermostats did you use? Brand and source?

Ac delco through rock auto

Part number 97241129 and 131131

 

[NC-smokinlmm]

I’ve seen an lly get poor fuel economy when the ground to the FICM was compromised and corroded. He regrounded the case and picked most of the mileage back up. I figured the injectors weren’t getting full amperage and dribbling fuel causing poor atomization. This truck has 750,000 on 7 of its original injectors. It’s a trooper…

 

[JoshH]

I’ve seen an lly get poor fuel economy when the ground to the FICM was compromised and corroded. He regrounded the case and picked most of the mileage back up. I figured the injectors weren’t getting full amperage and dribbling fuel causing poor atomization. This truck has 750,000 on 7 of its original injectors. It’s a trooper…

Can you explain a little more? The FICM is rubber mounted from the factory, so did the ground wire in the harness have an issue that he fixed by adding a ground to the case?

 

[NC-smokinlmm]

He fixed the ground to the FICM. I’m not super familiar with them so if my terminology was off my bad. The lily is the only duramax I’ve never owned.

 

[2004LB7]

There is two grounds. One on each side. Or, one in each connector. Both are internally connected inside the FICM. It can run on one ground. Same for the 12V power

 

[Mikey52]

On another forum I remember a person fixing his problem by grounding the case.

 

[2004LB7]

On another forum I remember a person fixing his problem by grounding the case.

This is more common on the TCM that can burn it's internal connection to the connector pin. Grounding the case fixes the grounding. But the case on the FICM isn't grounded to the board from my recollection

Edit. Looked at the board again. The top of the board as a strip that contacts the top cover. Then the screws that hold the cover on will contact the bottom base. So there is case grounding. But doesn't look like it's the most reliable. Would be better to just fix the harness ground

 

[Mikey52]

And it might have been the gpcm, it's been a while since I was on there. I quit using that forum when they got rid of Tapatalk support.