Overheating problem

[dwg30]

I bought this truck awhile back and have this issue since I have got it. Runs fine but gets hot if I run it down the highway (65-70mph). City or country road driving it runs normal temps and will idle for days and not overheat. What happens when running down the highway is it will push coolant into the overflow but never draw it back into the radiator. It will eventually fill up and run out the cap. I have replaced the radiator, water pump, and t-stat (even removed it), with no change. Going down the highway it goes from normal temp and gradually starts climbing. If I slow to 55mph or stop and let it idle for a few minutes it will come back to normal. The only thing I have not changed is the radiator cap. I have read that a symptom of a bad cap is it will not draw fluid back from overflow into the radiator. Will it also cause it to run hot down the highway??


Sorry forgot...its an 86 K20 with a 350 4-speed.

[Lt.Del]

I'd replace the cap b4 i would've replaced anything else.

How do your radiator fins look?  None bashed in or bent, clogging the airflow?
Nothing blocking the radiator for the free flowing of air?

Do you have a catalytic converter?  If so, it may be clogged not creating a free-flowing exhaust, which causes the engine to get hot.   Do you have that valve on the right hand exhaust manifold?  Sometimes they stick closed, again, creating a back pressure of the exhaust at high speeds.

[Irish_Alley]

like Sgt said go with the cap first. go with a stock cap not one of higher pressure it just causes the coolent to get hotter b4 it boils and flows out

[Prong]

How's the fan clutch? Make sure there's no black goo around the hub and it shouldn't spin too freely by hand when cold (about 1/4 to 1 full revolution is all--less is better).

[Lt.Del]

I wouldn't think it is the fan, however, those are much overlooked, especially w/ AC problems. They dont last as long as people think, however, for it to get hot when going on highway speeds, it is not the fan.  The fan doesn't need to activate when doing 65mph.

[dwg30]

The fan seems to be working properly. It is moving a lot of air at an idle and really should not matter at highway speeds. The exhaust is free flowing as well, headers with dual flowmasters. The radiator is practically brand new. Fins straight and clean and seems to flow freely internally.

I was running a 160 t-stat before it was removed. Getting on the highway with it sitting on 160 I can run about 10 miles before it gets to 210. It just slowly starts climbing until I slow down or stop. I have never had a cap go bad on me and don't know if the cap would cause it to do this? 

[VileZambonie]

Run it with the radiator cap off and see if as it warms up starts bubbling.

[dwg30]

I have looked at it while at operating temp and could not see any bubbles. You meaning like a bad head gasket? I have about ran out of options. It has to be either the radiator cap or internally restricted. I will pick up a new cap today and try that.

Has anyone had a cap go bad and cause a overheating problem??

[Blazin]

Has the intake ever been off it. Some of the older intake gaskets came with a knock out spot for the water passages. Someone might have replaced the gaskets, and not knocked them out??

[VileZambonie]

Replace the radiator cap and see what happens.  210 isn't overheating so try it and let us know.

[dwg30]

Thanks for the replies. I have not had a chance to get by the parts store to get a cap yet, been real busy. Hopefully tomorrow.

I know 210 is not overheating. That was just an example. It has gotten over that but I try to slow down or stop to keep it from overheating. With a 160 or no t-stat it shouldn't even come close to 210 going down the highway.

Blazin i am not sure if it had been off before I got it. I bought the truck about a year ago and it has been doing it since I got it. I only drive it maybe 150 miles a month. When it has been cooler outside it does not happen, but since it has gotten back above 70 outside I have started seeing the problem again. I guess if the cap does not fix it I will have to start tearing into it.

[406 Q-ship]

Your truck have a fan shroud, and if it does how far does the fan blade sit inside the shroud?  Is the fan run really close the radiator core?  There can be a weird air thing going on where the air is being pushed forward through the radiator, at idle the fan pushes air forward through the core colloing it just fine, at lower speed the forward vehicle speed is high enought to counter act the weird air direction but when you get to higher speeds the engine RPM is high enough to not allow sufficant air to flow through the rad. core.  I know this sounds bizzare but I have seen it happen and fixed it by repositioning the fan on a car. 

[Irish_Alley]

the air passing through the rad should be enough to keep it cool at high speeds its when it slows down or idles is when it should over heater if the fan is in-op or the antifreeze is low thats why electric fans are usually only on at low/idle speeds. the fan last time i checked pulled air. and yes you shroud should be about the same size as the fan but bigger.

[Russ130]

Normal operating temp should be over 210 that's why there is a radiator cap to increase the boiling point so it won't boil over. If it is running at say 240 or above that is pushing it and 280 is critical. Anything under 220 I wouldn't worry about. Boiling over into the overfill and not pulling it back in is a sign of a bubble in the system which will need to be burped. Since there are no bleed off screws you will have to do this the hard way. What mixture of coolant are you using? Straight water runs the coolest but should not be run because you will have no rust and scale protection, though you can buy additives. I'd run a 70/30 solution in the warmer months and 50/50 - 60/40 in the winter. The lower number being the coolant and the higher being water.

[dwg30]

I understand what is over heating/safe operating temps. The temp would get well over 240 if I let it. There is definatly a problem somewhere with it getting that hot without a t-stat. 

The fan is several inches from the raditor and the shroud is in place. The mixture was around 50/50 but has been diluted some by adding water that has been pushed out the overflow.

How would I go about trying to burp an air pocket out of the system?