PSA - Check your coolant bleeder screws!

[coffeegoat]

Just like the title - check your coolant bleeder valves, specifically on the 6G74 engine but I assume the issue is similar on the 6G75 as well. I bled the system after my water pump replacement, but several months later was still seeing depressed gas mileage and a little down on power. I replaced the temperature sensors for an intermittent P0125 error (which helped but didn't completely resolve it) but was still seeing low engine temps.

One cool day I opened the hood and noticed the radiator hoses were slightly deflated, inferring air in the system somewhere so I swapped out the radiator cap, again, no dice. Finally, I rechecked the bleeder screws and low and behold I had an air bubble! Perhaps it had been hiding in the rear climate system or somewhere else.

The air bubble is right over the temperature sensor so you don't get a correct reading to the ECU, the temp gauge still looks correct and the thermostat is lower so the engine still operates correctly, but since the ECU doesn't see the right temperature I think it believes the engine hasn't reached correct operating temperature so it keeps the engine running rich, which wastes gas and power. Fixing the air bubble gave me an immediate mileage increase of ~4 MPG and seemed to also increase power (measured via butt dyno).

Anyways, it takes all of 60 seconds to check - so go check your coolant bleeder valve.

 

[evomaki]

Good reminder. Will do.

 

[Monty85]

Good note. I will check this. I was getting 13 ish mpg after installing 33" tires. Now for no reason its about 11-12mpg. I did flush coolant and spent lots of time bleeding it. I dont have weird temp reading But now you gave me little hope for extra 1-2 mpg

 

[The Viper]

I assume I did this when I changed my timing belt and h20 pump, but dang I can't remember. Isn't there 2 bleeder screws up on top of the engine area?

What do you do, run the engine, and then just loosen the screws till coolant flows out, then tighten?

 

[coffeegoat]

Yep there are two bleeder screws near the thermostat, mine had little warnings about not opening them while hot.

I did mine in two steps

1) When the engine is cold open them both up entirely and add coolant to the radiator until they overflow then re-tighten down the . This is just to catch big bubbles.

2) When the engine is hot, carefully crack the both bleeder ports until fluid dribbles out then tighten back down. Once the engine cools off you'll want to add any makeup coolant to the radiator.

Easy peasy, in retrospect I'm not sure if I did it correctly when I replaced the water pump, but either way things are better now.