overheating at idle

[95 Tahoe]

I recently rebuilt my 95 Tahoe with a 350 TBI due to knocking. I've had it back together for a couple of months now. Since the weather has warmed up it has started running a bit warm at idle. If I rev the engine to 2000 rpm the temp comes back down. Temp gauge is bumping 225, not sure how much I trust it but it's warmer than before. Before the rebuild it sat around 210. It has a new water pump and a 185F thermostat. The fan clutch was replaced about 6 months ago and seems fine. I think my radiator might be fouled and the sludge hardened up while sitting for a month while rebuilding the engine. Next step is a coolant flush. Any thoughts?

 

[GoToGuy]

Check cap seals, gasket, pressure test, is it holding pressure? New thermostat? If not did you check before reinstalling, boiling pan O water temp check? Have you ever done a good flush of cooling system ? Remove upper fan shroud, how many twigs, feathers, grass, small dead things are packed into condenser and radiator. Back blow crap out of fins.( I had a dead pigeon in mine once) That's what I'd start with.
Good luck.

 

[Schurkey]

If I rev the engine to 2000 rpm the temp comes back down.

Rule of thumb--which is not true in all cases--is that overheating at low speed is lack of airflow, overheating at high speed is lack of coolant flow.

As already said--look for debris in the airflow path including folded-over fins on the radiator and condenser.

There's two dozen other potential causes of overheating, including but not limited to lack of spark advance, lean air/fuel ratio, missing cooling-air ductwork--fan shrouds, air dams, etc., dragging brakes that make the engine work harder, water-jacket blockages, and so forth.

Temp gauge is bumping 225, not sure how much I trust it

Worth verifying the temp gauge with a thermometer in the radiator, an infra-red temp gun, or a second temp gauge and sending unit.

An infra-red temp gun can also find cold-spots (plugged coolant tubes) on the radiator, if the plugging is bad enough, and if the condenser isn't in the way.

I think my radiator might be fouled and the sludge hardened up while sitting for a month while rebuilding the engine. Next step is a coolant flush. Any thoughts?

How old is that radiator? If it's original, it's likely overdue for replacement based on age and miles. If you have a local radiator shop, you could ask them to clean it, and see what happens. Radiator shops are getting scarce--the Communist Chinese along with the EPA are running them out of business. One offers low-dollar "new" replacements, and the other cries and whines about the waste they generate.

 

[Carlaisle]

I would install a mechanical gauge in the head temporarily to confirm if it is actually running hot and, if so, how hot. If you remove the upper shroud and fan you can easily use an IR temp gun to check for any obstructions. My primary suspects would be the water pump and thermostat, in that order. I would swap the old parts back in - including the old fan clutch - and see what happens to your problem. Just because those parts are new does not mean they were manufactured correctly or are even the correct parts for your vehicle.

 

[95 Tahoe]

I did some troubleshooting today. I think I found my problem. I am barely getting a 10 degree drop across the radiator. I think it's fouled. It would make sense because it worked fine before the rebuild and sat dry for weeks with years of crud in it. Fan will not stall with a piece of cardboard so the clutch is more than likely good.