Calling all shade-tree mechanics!

I have a '94 GMC Sonoma pickup with almost 90K miles on it. It’s been overheating when I drive on long, fast drives, like on the highway. Once I go back to city driving it cools back down. It doesn’t seem to be the fan clutch, I’ve replaced the hoses. Last year I had the radiator flushed and filled and the level of coolant is fine. Last year I also replaced the thermostat. (I had overheating problems last year, but it was the fan clutch)

Anybody care to diagnose the problem?


  1. Don’t assume the new clutch is NOT the problem.

  2. is there any debris between the AC ccondenser and the radiator?

  3. Define overheating. There are a LOT of motor heat behaviors that aren’t considered overheating.

  4. Buy a bottle of watter wetter and see if that affects your temps any. It works, is an excellent product, and is pretty much a quart of distilled water, some coloring, and a detergent. (But the RIGHT amount of detergent to not cause problems.)

4a - I’ve seen watter wetter work in many situations where a marginal cooling system needed a little help.

4b - but it’s not necessary if the system is working to OEM standards, you really oughta find out why it’s overheating.

How well is the water circulating. When you crank it up before it gets hot or at least before it’d be dangerous to remove the radiator cap. Is the water circulating at a good rate. Maybe the waterpump itself is going out? Of course the water wouldn’t be circulating at all if the thermostat were stuck shut.

BTW it is a good idea to replace the radiator cap once in a while. Especially if it has overheated a few times. They hold pressure at specific rates so get the correct one. I recently went through this. The guy had put a new radiator cap on his car. (It was a real pretty chrome job) but wasn’t the right pressure cap. He spent a couple hundred bucks guessing trying to cool the motor down. I walked over to the sucker and put the original cap back on and he went… :smack: ya never know, sometimes it’s little things.
good luck

I don’t think it’s the fan clutch because the last time the fan clutch went out the fan would spin without resistance. There’s resistance now.

It doesn’t just happen when the A/C is running. But I’ll check it out.

When I drive on the highway the temp inches up to the red zone and the “Check gauges” light will come on. It starts going up within 4 interstate miles.

I hadn’t heard of this product - I’ll give it a try. Thanks!


t-keela - I’ll try another radiator cap. I looked at the existing one and it looked okay, but I’ll pick up a new one. How can I tell if the water is circulating well? (My step-father is doing the work, but he’s a good shade-tree mechanic)


Once the motor is warmed up and you’re sure the thermostat is open, place your hand on the radiator hoses, the one downstream of the thermostat should be hot…

I’ll second the radiator cap advice…if the t-stat is sticking, you should have a meltdown situation on your hands…

Also, you might verify that the temp sender is working correctly. They fail in wierd and mysterious ways.

sould there be a part of the shroud that’s loose? (So that at highway speeds, it obstructs the air path thru the radiator, but doesn’t at slower speeds.)

Start it up cold and leave the radiator cap off for a few minutes. You should see the water circulating.

Also - with the radiator cap back on and the engine running and warmed up - check all the hoses again. Look for any that look collapsed; that can be an indicator of a circulation problem as well.

Still the original radiator?

This is kind of a long shot but are there any kind of loose metal pieces in the grill that could shift and be restricting airflow at highway speeds but retract back to normal position at lower speed city driving thereby allowing normal airflow through the radiator again?

The low pressure hose is supposed to have a spring running thru it to prevent is from collapsing under load…Is it possible is spring is missing?

A motor running hot can be damned tricky sometimes. My brother’s Dodge Dakota went through some shit recently because the dumbass who put a new battery in for him, set the battery on the cak and when he picked the battery up he turned it just enough to break the seal and on the way home of course the truck got too hot…small head gasket leak. Pain in the ass to repair, so I told him to get a quart of liquid glass and seal it up until he could afford to tear it down. I drove an Olds cutlass like that with a cracked head for over a year. It ran good and never got hot. I put new heads on it later but my last year in college I drove that damned car every day.
You can’t SEE that the radiator cap is good or not usually. It’s kinda like a thermostat. You can boil it to see if it’ll open or not. But like the radiator cap they are cheap and easy to replace. Odds are if the truck is 10 years old and has gotten hot a few times the cap is weak and at high speeds it’ll have a tendency to get hot. If the cap doesn’t hold the pressure it’ll open up and let the water bypass. If it is stuck and won’t let the pressure off as the water gets hot…that’s even worse. You’ve got a pressure cooker that get hot even quicker.

Is it circulating…once the thermostat opens up you can see the water jetting through the core in the radiator. If the pump isn’t working it’ll circulate some but only because the temperature variance in the water is forcing it to. Your dad’ll know what I’m talking about. A water pump can be a pain in the ass to change on some vehicles but usually it’s not too bad.

I assume this truck had a belted fan and not an electric one?

Update on car problem - Well, I tried the Water Wetter and the car still overheated, although more slowly. My step-father removed the thermostat and it still overheated, although not as much (it never got into the red zone). We checked the air-flow and there was nothing loose that could impede cooling. The next thing he’s going to try is replacing the water pump.

Any other suggestions?


Put a bullet innit. :smiley:

I had a volvo that overheated on the highway. Replacement radiator was the solution. Perhaps some of the channels in your radiator are blocked with crud.

It sounds like the radiator to me as well. Flushing the radiator may not completely clean it, and overheating at high speeds is usually caused by the radiator or thermostat.

Overheating problems can be frustrating to diagnose, but are fairly simple.
for the truck to run at the correct temp you must have three things Air flow, water flow, and no engine problems. If you don’t have enough of each under a particular running condition the truck will overheat.
Truck overheats only at low speed: look for a bad fan clutch, problems with the fan blades (missing or trimmed), slipping fan belt, bad thermal switch for electric fan
Truck overheats only at freeway speed: look for sand, dust, dirt, dog hair etc. blocking airflow to rad. We used to have to remove rads from cars and hold them over the floor about 12" up and drop them flat on the floor. You would be amazed at how much dirt is dislodged sometimes. Also aftermarket grilles, license plates in the wrong place, missing or mis-adjusted air guides.
Truck overheats all the time: probably not an airflow problem but look for plugged fins in rad.(see freeway speed)
NOTE: a fan clutch will generally not cause overheating when driving at speeds above 20 mph or so. I only know of one case where a fan problem contributed to O/heat at speed in all the years I have been doing this. Due to an accident the fan blades on this car were about 1" too short. Car would O/heat going up a hill.

Water flow
Truck overheats only at idle: low coolant, air pocket, incorrect coolant/ mixture colasping lower hose,
Truck overheats only at freeway speeds: Bad T/stat, bad water pump (corooded impeller) restriction inside engine, poor flow through rad, incorrect coolant/mixture
Truck overheats all the time: Low coolant, incorrect mixture, bad water pump, poor flow through rad due to restriction, poor flow through engine due to restriction

engines problems
Overly lean conditions (overheats at idle)
Ignition timing retarded (more often O/heats at speed)
Blown head gasket (maybe not fully blown, but leaking)

Truck only O/heats when low on coolant (no visible leaks)
Incorrect coolant/mixture
Slightly blowen head gasket or internal crack in engine
Bad pressure cap

Some things to keep in mind:
Fan clutches are to help the airflow at low speed, at high speed they disengage to allow for better gas mileage/quieter running. So a bad fan clutch gives low speed O/heating, and normal on freeway.
Bad pressure caps do not cause O/heating. Putting coolant under pressure does not make the engine run cooler, it allows the coolant to get hotter before it boils. When coolant boils it will boil out of the system, and when the coolant gets low, then the system will overheat. A bad pressure cap by itself will not cause O/heating.
There are a number of different coolants being used by car makers nowadays. While writting this I called a buddy of mine and asked how many, he told me that at last count he was up to 13 different types (not brands but types) of coolant. Mixing the wrong one together can cause serious issues. My suggestion is use what the car maker put in the car from the factory, the engineers that designed the cooling system know more about it than you do. Bottom line is that for a newer car (say 1990 up), a bottle of Prestone may not be a very good idea.
Getting back to the OP if this truck were to be brought into me for a diagnosis here is what I would do:

  1. Check for the presence of hydrocarbons in rad. If hydrocarbons are present, head gasket is blown, or head is cracked. With the right equipment a shop can do this test in just a few minutes.
  2. Pull rad and have it sent to rad shop for cleaning and flow check. (yeah I know it was done last year, but it is this year NOW)
  3. While rad is out, take a high pressure hose, and working from the engine compartment side shoot water through the A/C condenser. When done shine a light from one side and verify that the light can be seen all the way through all parts of the condenser
  4. Replace T-stat (cheap!)
  5. Remove water pump and inspect impeller. I have seen technicians go round and round on O/heats and when the finally pull the water pump the impeller is rusted away to almost nothing. If you are a belt and suspenders kind of guy you might want to go ahead and replace it while you are there.
  6. Fill with factory coolant mixed to factory spec.

You couldn’t determine if the water pump was working or not?
Have you checked the oil dipstick to see if there’s any water in the oil? The oil will look muddy. That’d be due to a blown head gasket or a cracked head.

You don’t have access to a diagnostic machine do you?