Best of Deals Car Reviews Repair Shops Cars A-Z Radio Show

Can't go over 45 mph?

Been doing a lot of repair’s on our 1994 Chevy blazer. It’s a 4.3 v6. We finally got her running and now it seems like it can’t take high speeds for very long without over heating and eventually smoking some. Had to bypass heater core, it’s clogged. That helped with temp. Also replaced radiator and cap. Replaced both heads, took thermostat out. That helped with temp too. It has a new water pump, new exhaust manifolds, has good spark plugs, oil changed and oil filter. Kind of at a loss. Please help!

A plugged heater core will not cause the engine to overheat.

On some engines removing the thermostat will cause overheating.

If 45 mph is the top speed and the engine isn’t surging when accelerating it is likely that the exhaust is restricted and the catalytic converter is the usual cause for that problem.

What year-what engine?

1 Like

Did it overheat before you replaced the heads? Are the heads new or used and are you sure they are correct for the engine. Did you check that all the water passages lined up between the block ,heads and gaskets?
Are yiy sure you have the right rotation water pump? Small block Chevys have been made in a lot of variations and just because something bolts up doesn’t mean it is compatible.

Aside from ther plugged exhaust already mentioned, late timing can also cause overheating,

I have a vauge recollection of some Blazers and Suburbans having double wall exhaust pipes that can delaminate and clog internally.

I’m not sure on a lot of those questions. Our neighbor has done all the
work on it, he’s a veteran mechanic, been doing it his whole life so I
trust to some extent that he would have done the best he could. He helped
us pick out the heads. The over heating issue is kind of strange in the way
of when and how much. Before the heads were replaced it was running okay
then one day the truck got real hot and wasn’t running well, that’s when we
found out the heads were cracked. Must have been weak already, I we hadn’t
done much driving with it before they cracked. It had been sitting for
almost 3 years. It doesn’t have a catalytic converter and the overheating
issue lessened after taking the thermostat out. If we keep the truck under
45 mph it drives fine. But it seems like when we get on the highway it
starts to get hot then doesn’t cool properly and when we come to a stop it
starts smoking real bad.

And the heads were used and had been smoothed over so the gaskets would
fit. He actually used a weird blue glue instead of gaskets. Said it was a
cheap alternative. Before the heads cracked we got new exhaust manifolds
and alternator I think. It ran okay but then the heads cracked. It’s been a
complicated mess and sucked a bunch of money out of our pockets. More than
its worth for sure but we don’t have money for a new vehicle…

Cylinder heads don’t crack after 25 years because they are weak, they crack from severe overheating. If you overheat your engine again you may crack your replacement heads.

Any mechanic should be able to correct this, have you explained to your neighbor that this truck runs hot on the highway?

These classic vehicles are quite simple, this should be easy for your neighbor/mechanic to diagnose if you leave the truck with him.

Sorry but this does not sound good! In my opinion there is no glue in the world that is an alternative to a head gasket. And just may be the source of your remaining problem. Also has the radiator be thoroughly checked out.

1 Like

Chevy blazer with the 4.3 v6?

Sorry, I wasn’t fully aware of what it was. He used the glue to secure the
gaskets, not in stead of lol and the radiator is new! How do we check it?

I’ll take a stab at this even though my crystal ball told me nothing about the make, model, year, and engine of this mystery vehicle.

It had cracked heads from a previous overheating episode and sat for three years.

I’ll bet that the tensioner for the serpentine belt is rusted in place and is not keeping tension on the belt. This allows the belt to turn the water pump, but it is slipping.
It will look fine and work at idle, but as you drive at higher speeds the belt slips.

I would pull the belt and see if the tensioner moves free through it’s entire range.

You keep overheating this and you’ll need a new engine.


Thanks, I really appreciate this, all of you. I will relay this info to my

In spite of the fact that the heads were “smoothed over” . . . I would have checked them with a straightedge

Any sign of oil in the coolant, or vice versa?

a block tester would quickly confirm if combustion gases are present in the cooling system . . . and would point to a blown head gasket, warped heads, cracked block, etc.

Since this neighbor is a “veteran mechanic” he should know the term “block tester”

Can I assume the heads are some kind of aluminum aftermarket, high performance heads?

No sign of oil in the coolant. The heads came from another chevy. Nothing

You may want to hook up a backpressure gauge and take a reading

Your mechanic will quickly know if the catalytic converter is plugged up, which can affect your truck’s ability to reach higher speeds

Are you certain the used heads are from the correct type of GM 4.3 V6 . . . there were a few different generations. And it wouldn’t be surprising if heads from a 2003 model, for example, don’t work properly on your truck

I found a few 94 motors with 80k miles for $450. Of course that’s like closing the barn door after cows are gone.

It doesn’t have a catalytic converter. I will try the back pressure Gage.

no offense, but I have a hard time swallowing that

unless . . .

somebody removed the catalytic converter and installed a straight pipe

somebody hollowed out the converter

it’s a gray market vehicle