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1995 GMC C1500 Temperature fluctations

Tiny
CAREEFER
  • MEMBER
  • 1995 GMC C1500
Engine Cooling problem
1995 GMC C1500 V8 Two Wheel Drive Automatic 119k miles

I've ran into an odd situation I could use some help on. 1995 GMC C1500 5.7 with 119k miles. Newish water pump (5-7k miles), new radiator (replaced single core with dual core), new 195 deg. Thermostat and Hayden severe duty fan clutch.

All is fine as the truck warms up. As soon as the thermostat opens @ approx 195, the temp drops to ~150 deg. After a few minutes starts to climb again until it hits the 195, then drops to 150 and the cycle repeats itself. Doesn't seem to matter if I'm on the road for 1/2 hour or 4 hours, the temp won't hold at 195 and constantly bounces off the thermostat. I've had 3 different thermostats in the vehicle, 1 regular Stant 195 deg, a Stant Super Stat 180 deg and the current Super Stat at 195.

It seems to me that this would wreak havoc on the fuel injection system and I'm hoping is partially to blame for recent plummet in fuel economy.

Any insight would be greatly appreciated.

CAReefer
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Tuesday, April 15th, 2008 AT 8:59 AM

5 Replies

Tiny
MERLIN2021
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Might be the larger radiator! Too much cooled water re-entering the block. Do you still have the old one? It might be worth the time to test the old one out.
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Tuesday, April 15th, 2008 AT 2:23 PM
Tiny
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Hey Merlin, thanks for the reply. I do still have the old one, but it leaks pretty bad between the plastic tank and core thus the replacement. I'm trying to think back but it seems that the truck was doing the same prior to the radiator change, just not to this extreme.

Shouldn't the thermostat be holding the block at temp or at least closer than the near 50 degree swing I'm seeing now? Am I wrong in assuming that they operate incrementally and not as an "on/off" or "open/closed" type of valve.

Thanks again

p.S. I haven't checked into it yet, but can the plastic and aluminum radiators be repaired at the seams?
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Wednesday, April 16th, 2008 AT 12:22 AM
Tiny
MERLIN2021
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Take it to a radiator shop, sometimes they can put a new tank on. The tstat will open as the tem increases once normal temp is reached, so if it's 195 it opens at 195.
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Wednesday, April 16th, 2008 AT 4:14 AM
Tiny
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[quote="merlin2021"]Might be the larger radiator! Too much cooled water re-entering the block.[Quote]

I do appreciate your input Merlin, but.

The more I thought about this I don't buy it. Larger simply equals more efficient. Replace it with the same size as OEM with a BeCool aluminum or equivalent, would I still have this problem? Seems to me the Tstat should be holding the block at temp once it reaches 195.

Doesn't the Tstat (after the initial warm up/open cycle) open incrementally?

Additionally, the 2 row was an option for this same pickup configuration with the "heavy duty cooling" package.
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Wednesday, April 16th, 2008 AT 11:12 PM
Tiny
MERLIN2021
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Only other thing I can think of, air in the system.
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Thursday, April 17th, 2008 AT 2:55 AM

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