350 TBI overheating

[field57]

I have a 94 5.7 that is overheating when accelerating moderately hard, or under a load etc. Heats almost instantly. It cools down back to normal almost as soon as I pull over and open the hood. Gauge goes up very quickly. I replaced the thermostat with no luck. After I parked the truck and it sat for 48 hours, the upper rad hose was still hard and there was some pressure when removing the rad cap. The expansion tank and inside the rad look clean, and the oil on the dipstick looks normal. Can a rad cap cause this type of issue? I’m leaning towards a head gasket issue, but figured by 48 hours the pressure would release from the cooling system back to the blown head gasket issue?

 

[Supercharged111]

Sounds more like a water pump or radiator issue from here. Head gasket overheats don't recover in my experience. How clean is the radiator?

 

[alpinecrick]

Sounds more like a water pump or radiator issue from here. Head gasket overheats don't recover in my experience. How clean is the radiator?

This would be the first three things I would look at, too.

DexCool was a relatively new thing when the GMT400's were new, and I believe it was not uncommon for well meaning owners--and even mechanic shops--to dump green coolant into the cooing system. That creates sludge, and is bad ju-ju.

A radiator flush kit from the parts store helped one of my warmish running G1500 cargo van run cooler with no other work on it.

I have flushed all my trucks, and was surprised at the black gunk that came out even on cooling systems that I thought were clean.

 

[Schurkey]

How does the radiator stay pressurized after a two-day cool down? Clearly, it's not pressurized because it's HOT. This leads me to believe it's pressurized due to a tiny head gasket leak or casting crack that blows combustion pressure into the water jacket, but is so small that water-jacket pressure won't leak back into the combustion chamber.

Don't forget to verify the fan clutch.

Be sure to inspect the radiator air fins. They rot out due to road spray from the vehicle ahead. When the air fins corrode, the radiator can't remove BTUs from the coolant even if the water tubes are not corroded.

 

[Markmartin01]

Had a similar issue with a new water pump. Turns out the reverse pump for the non AC motor was in the wrong box. Exchanged it and got the correct rotation for the AC truck and it's cool as a cucumber.

 

[field57]

Ya the pressure thing has me stumped. I agree it seems like a combustion pressure is getting in, but not getting out. I’m going to verify the temperature with an aftermarket gauge first to make sure it’s not a sender/gauge problem. Then I will probably replace the water pump and have the rad checked out at the rad shop. After that, I guess it’s head gasket time. Thanks

 

[field57]

I did a compression test today. Dry cylinders were 150 exactly on every cylinder except number 8 was about 125-130. it overheats under 3,000rpm in lark after about a minute. Then if it idles it will start cooling down again and when it does it will suck the upper rad hose flat for a bit. Replaced the rad cap and no changes...

 

[PlayingWithTBI]

Dry cylinders were 150 exactly on every cylinder except number 8 was about 125-130

Spray some oil in to that cylinder and try it again - if compression goes up it's your rings, if not, you may have a blown head gasket. I had 2 cylinders (1 on each side) that were only 5PSI lower than the rest and both head gaskets were blown causing overheating. I was losing 1/2 Gal of coolant a week too.

 

[Schurkey]

I did a compression test today. Dry cylinders were 150 exactly on every cylinder except number 8 was about 125-130.

Means nothing by itself. Still within spec.

it overheats under 3,000rpm in lark after about a minute. Then if it idles it will start cooling down again

How do you know it's overheating? Temp gauge? Or by blowing coolant out of the radiator? Is the upper hose pressurized?

and when it does it will suck the upper rad hose flat for a bit. Replaced the rad cap and no changes...

It isn't possible for the upper rad hose to suck flat if the rad cap is working properly, UNLESS the overflow hose is plugged/restricted. Any vacuum in the cooling system is intended to be relieved by inflow of air (or coolant from the overflow tank, if equipped) through the overflow hose.

So you need to define "for a bit". One second? Ten seconds? Two minutes?

 

[unruhjonny]

If it's acceptable for a newbie to chime in, I'll speak up.

I think the following statement is of paramount importance:

After I parked the truck and it sat for 48 hours, the upper rad hose was still hard and there was some pressure when removing the rad cap.

To me that says there's an issue with the cooling system itself.
It would seem logical to me, that even IF there was a head gasket issue, the pressurized coolant would over a span of 48 hours relieve pressure by pushing coolant into the crankcase.
My money is on a plugged cooling system.
Have you checked for those "stop rad leak" pellets in the end tanks? (I seem to recall those were notorious for causing cooling system issues down the road.)