Coolant loss stops

'98 w/3800, approx. 120k mi.
I was losing an entire overflow tank of coolant per week
for about 3 weeks in a row and gas mileage dropped.
Of course I thought my upper plenum was bad. Then it stopped completely and decent mileage has resumed.
What is happening?
I have been reading of the intake problems and solutions on this forum for about 5 years now and trust
the opinions of the moderators.

Thanks,
Rod

Have you popped the radiator cap off to make sure its full?

If you have 120,000 miles that upper intake is a ticking time bomb... Do not ignore this or you'll end up with a 3800 Series II paperweight... The upper intake and the lower gaskets are a serious problem on these engines..

'98 w/3800, approx. 120k mi.
I was losing an entire overflow tank of coolant per week
for about 3 weeks in a row and gas mileage dropped.
Of course I thought my upper plenum was bad. Then it stopped completely and decent mileage has resumed.
What is happening?


You may have a different leak at water pump? Has it been replaced? Check carefully using a mirror to look inside pulled where the weep hole may leak leaving a brown trail of dried DexCool or sealer.

Check the o-rings where the elbow goes from the upper manifold into the the idler pulley braket and the coolant goes through there to the heater hoses.
Checking for leaks after a long drive in warmer temps is when the pressure in the system may make a leak more noticeable.

Check around the perimeter of the upper intake manifold looking for the telltale brown trails.

If your car had DexCool drained out early or the DexCool has been drained every 2-3 years and refilled with fresh and the system has not had contamination with regular coolant added to the tank to cause globbing of the DexCool, you may not have a leaking upper intake.

Just keep checking and being sure the radiator itself is full and the reservoir tank is up to the cetain mark and then recheck at same conditions outside with car totally cold after overnight and parked level. See if it's losing coolant.

Have the spark plugs and wires been changed in the last 40-50K miles?
Did you change gas stations or brands? Extra cold weather and higher alcohol content really hurts mileage.
Idling to let car warm up in cold weather is especially bad at lowering mileage. If you can, start driving gently to let the car warm up after just a short period of runtime after starting it.

I bought the car 5 years ago with about 60k on it.
And I've never put a pump on it. I've always kept
only Dex-Cool in it.
I'm careful not to let it idle to much.
I'm pretty consistent with the gasoline brand.
The car runs extremely well. That is what puzzles
me about the coolant. When it was losing coolant
I was checking the radiator as well and would add
very little to it.