Fluctuating temperature readout 97 Bravada 4.2

[Ben D]

Bought a 1997 Bravada w/ 4.3 V-6 and on the way home, it overheated. Ultimately I flushed it, changed thermostat, and ran on water w/ water pump lubricant. Temperature fluctuated some at start but ultimately ran at reading of 150 - 170 after a few hundred miles. Small fluctuations. Changed water pump after leak and now massive temp fluctuations. Very rapid changes. Going up hill or around a corner can bring temp from 200 down to 150 almost instantly.
Can there be air pockets somewhere?

[drdd]

change your rad cap to a Stant brand cap. ACDelco caps are faulty and will allow air into your system.

After you've flushed, changed cap and changed stat ... I'd pressure test it.

could be the lower intake manifold gasket. Failure is a very common problem.

http://www.automotiveforums.com/vbul...d.php?t=456944

http://www.automotiveforums.com/vbul...=469926&page=1

Originally Posted by Ben D

Bought a 1997 Bravada w/ 4.3 V-6 and on the way home, it overheated. Ultimately I flushed it, changed thermostat, and ran on water w/ water pump lubricant. Temperature fluctuated some at start but ultimately ran at reading of 150 - 170 after a few hundred miles. Small fluctuations. Changed water pump after leak and now massive temp fluctuations. Very rapid changes. Going up hill or around a corner can bring temp from 200 down to 150 almost instantly.
Can there be air pockets somewhere?

[Ben D]

Thanks. I'd think that I would see sign of that but I'll try.

[drdd]

signs of lower intake gasket leaking are: 1) physical leak that you can see pouring out onto your driveway 2) mysterious missing coolant in your plastic overflow container ... might mistake it for a head gasket

random spiking temperature could also simply mean you need to flush it again ...

I had to flush the hell out of my 97 for about 30 minutes ... had the same spiking symptoms ...

Originally Posted by Ben D

Thanks. I'd think that I would see sign of that but I'll try.

[drdd]

when your engine is cold (in the morning after a long nite's sleep) ... is the coolant "up to the brim" when you remove the rad cap ?

it should be ...

Originally Posted by drdd

signs of lower intake gasket leaking are: 1) physical leak that you can see pouring out onto your driveway 2) mysterious missing coolant in your plastic overflow container ... might mistake it for a head gasket

random spiking temperature could also simply mean you need to flush it again ...

I had to flush the hell out of my 97 for about 30 minutes ... had the same spiking symptoms ...

[Ben D]

Thanks to all for the help. Yes, the coolant was at the top of the radiator. It appears that the radiator cap was the cure. It is still spiking but far less extreme. I'm guessing that there is still air to be removed. It would start happening as soon as the engine reached about 150 deg. on the gauge. Before the cap change the temp would go from (read) 150 to over 200 in ten to thirty seconds, it varied, and drop just as fast when it felt like it. Not sure exactly what was being seen by temp. sensor as I can't see things heating or cooling that fast and heat transfer shouldn't let something local change and be read that fast. Anyway, the cap seems to have been the cure and I'll let you know if anything changes.

[drdd]

keep and eye on it ... it shouldn't be spiking at all ... try parking the truck on ramps to get the air out ...

keep an eye on it ...

on a 97, I'd be just waiting for the lower intake gasket to go.

if still spiking, you might try a Prestone reverse flush kit and flush the hell out of it some more ... there still might be small obstructions of gunk that get stuck in the heater core ... Presonte also sells an acid flush which is great ...

small cracks in the rad itself on the upper neck are common too.

Originally Posted by Ben D

Thanks to all for the help. Yes, the coolant was at the top of the radiator. It appears that the radiator cap was the cure. It is still spiking but far less extreme. I'm guessing that there is still air to be removed. It would start happening as soon as the engine reached about 150 deg. on the gauge. Before the cap change the temp would go from (read) 150 to over 200 in ten to thirty seconds, it varied, and drop just as fast when it felt like it. Not sure exactly what was being seen by temp. sensor as I can't see things heating or cooling that fast and heat transfer shouldn't let something local change and be read that fast. Anyway, the cap seems to have been the cure and I'll let you know if anything changes.

[ericn1300]

My 4.3 used to shoot up to about 225 on warm up, then I could watch as the termostat opened and the temp would drop to 170 or less in seconds. Turned out to be an air block under the termostat insulating it and delaying opening.

My dad's farm mechanic showed me a trick to solve the problem. He drilled a 3/16th hole in the thermostat flange to allow the air thru before the thermostat opened. Fixed the problem and doesn't seem to affect warm up much at all, if any.

[Chris Stewart]

Wouldn't you just love to have half the tricks of a good farm mechanic? Talk about a knowledge base.

[ericn1300]

Originally Posted by Chris Stewart

Wouldn't you just love to have half the tricks of a good farm mechanic? Talk about a knowledge base.

yeah, when i was kid, he used to keep my go-cart running too