1987 suburban tbi 350 overheating.

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shotrod69

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‘87 Suburban with TBI 350.

As I drive up the road the temp gauge will climb to ~220-230, then fall back down to thermostat temp. It will do this repeatedly.

I had changed the antifreeze and put a 195 thermostat back in it for winter. (About 2 months ago)

I had hoped that it was a case of bad news parts, so I swapped the thermostat out with a 180. The logic being that if there was another issue that the 180 would let me have more time then the 195.

I tried the combustion gas fluid test on the over flow tank. I’ll be trying it on the radiator like I should have either tomorrow or Wednesday.

I’m just hoping to find out if it’s something besides a cracked head or head gasket. I’d rather not have that expensive repair.
 
Vbb199

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When's the last time the water pump has been changed
 
fast 99

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Are you saying motor ran higher than 180 with a 180 stat? new condition? Is a temp gun available to confirm temp? Does the fan clutch work? push water out or boil over?

Factory gauges do fail.

Cheap thermostats are notoriously bad, was a high quality part purchased?

I know lots of questions but need a little more info to diagnose.

Water pumps impellers can rust away but it's down the list. Usually coolant is very rusty for that to be considered.
 
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fast 99 said:
Are you saying motor ran higher than 180 with a 180 stat? new condition? Is a temp gun available to confirm temp? Does the fan clutch work? push water out or boil over?

Factory gauges do fail.

Cheap thermostats are notoriously bad, was a high quality part purchased?

I know lots of questions but need a little more info to diagnose.

Water pumps impellers can rust away but it's down the list. Usually coolant is very rusty for that to be considered.
Click to expand...
I’m assuming the gauge is fine since it sweeps no issue.

The 195 that was in it and the 180 that’s in it now are both Napa brand. It’s all I could get.

Sensor for gauge is in head.
195 Tstat would run engine at 210 gauge rock steady before issue developed.
180 Tstat runs ~190, unsure since gauge is non-linear.
I don’t have a temp gun that’s reliably accurate enough for coolant temp.
Fan clutch seems to work, belts are tight.
Haven’t noticed coolant boiling out, after I get to work/home the coolant level in overflow tank is about the same.

There’s a brown coating on the inside of the rad, but that’s been there for 50k miles.
Was also there before I changed the water pump.
There’s coolant that’s in it haze been in for ~10-12k it’s not brown tinted.
 
Vbb199

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When the engine is off, and cold, does the clutch fan spin with some resistance or freely
 
fast 99

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What you describe actually sounds normal. What occurred that made you suspect overheating?

NAPA has 2 lines of stats. Never had any luck with the economy line.

Cylinder head temp will be 10-20 degrees hotter than stat. It is also normal temp to migrate up and down a little until engine heat stabilizes.

HF has inexpensive heat temp heat guns. Might consider a purchase.

If cooling system was drained will take a couple heat cycles for all the air to bleed out.
 
Vbb199

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Sounds like most of the bases are covered for it to run halfway decently.

You could park the nose up on a incline and take the rad cap off and let it run and see if it "burps" any air out
 
Vbb199

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I suppose it could also have stop leak in the system, which will cause issues.
 
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fast 99 said:
What you describe actually sounds normal. What occurred that made you suspect overheating?

NAPA has 2 lines of stats. Never had any luck with the economy line.

Cylinder head temp will be 10-20 degrees hotter than stat. It is also normal temp to migrate up and down a little until engine heat stabilizes.

HF has inexpensive heat temp heat guns. Might consider a purchase.

If cooling system was drained will take a couple heat cycles for all the air to bleed out.
Click to expand...
The suspicion came from it going up to 220-230 then back down about 20 seconds later. After I flushed and reinstalled a thermostat for the winter it would get up to 210(middle of gauge) and not go any high. Roughly 5k miles without issues.

This whole thing has worried me since it wasn’t doing this before.
 
fast 99

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Think you just need to varify the temperature. Those factory indicators [not gauges] and/or old wiring could be faulty.
 
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Vbb199 said:
I suppose it could also have stop leak in the system, which will cause issues.
Click to expand...
If it did, it was before I got it. And the coolant has been drained countless times and flushed 3 times
 
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shotrod69

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fast 99 said:
Think you just need to varify the temperature. Those factory indicators [not gauges] and/or old wiring could be faulty.
Click to expand...
Hard to do that since it only does it while driving.

You’re more then welcome to hang out the window while I drive.
 
fast 99

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shotrod69 said:
Hard to do that since it only does it while driving.

You’re more then welcome to hang out the window while I drive.
Click to expand...
Take hood off and sit on motor. lol

Just get a heat gun and compare to temp gauge. They cost around $20.
 
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