# D21 sudden overheat?



## jeremy107 (May 29, 2013)

Hello all, I've been having a hard time fixing a overheating problem on my 94 HB D21 4 cycl. Truck has 91k on it and been taken care if pretty well from what I can tell. Got it last week for a gas saver to work all week. 

Well all of a sudden it over heats on the way to work interstate driving? High rpms. If I'm at a low rpm or in the neighborhood it doesn't overheat, but right when I get on the interstate it spikes? Haven't replaced anything yet and the ECU is not throwing any codes 55 for no malfunctions. Any help would be great!! Heater works fine also AC works fine. Just high speed rpms causes a overheat? I'm confused!


----------



## smj999smj (Jan 1, 2006)

If it's find at idle and low speeds, that rules out the fan clutch. Make sure the the coolant level is full, make sure there is no debris in front of the AC condensor or between the radiator and condensor. Make sure the condensor fins are in good shape and not bent over, restricting airflow. If that's okay, you could have a bad thermostat, restricted coolant flow through the radiator or a bad head gasket. I usually take a Lisle "spill-free" funnel and attach it to the radiator neck and fill it about a 1/3 full with coolant. Then I run the vehicle at at constant, high RPM (2000-3000) and watch for air bubbles coming up through the coolant system. Assuming there has been no recent coolant system work and the engine is not air-bound, this is typically a sign of combustion gases working their way through the coolant (indicating a failed head gasket).


----------



## jeremy107 (May 29, 2013)

This morning before work I took some water out to see if coolant was low, took prob half a water bottle 8oz maybe 12 oz and it was full..I started it and went inside to finish getting ready for roughly 10 mins came back out and it was maybe a 1/8 or 1/4 up the gauge. (Which is where it sat running or driving for the first week when I got the truck) Took it a few miles down the road at 200-2500 rpms to see if it overheated before I'd jump on the interstate to drive 30 miles. But yes it continues to creep up the gauge so I took it back home and took the CTS V to work. 

Looking for things to do this afternoon. Thermostat,rad. cap, and flush coolant out and fill with new maybe? What's signs of bad radiator or water pump? Confusing it gets hot at high rpms but low rpms neighborhood speeds 45 and below it doesn't. Wouldn't it just get hot in general if it was a head gasket?


----------



## cajunlte (Sep 22, 2012)

I am in the same situation. Son's truck started overheating about 2 months ago and I have since changed the water pump, head gasket, fan clutch and removed the thermo without resolving the issue. 

smj999smj, Is there an easy way to check for a clogged radiator before I replace it with one from NAPA? Could a mechanical non-cooling system issue cause the system to not keep up with the temp spike?


----------



## ajriding (May 10, 2010)

cajunlte, did you resolve the issue? sounds like the radiator was the last possibility?


----------



## cajunlte (Sep 22, 2012)

I suspect the radiator. I replaced everything with NAPA parts and gave it a good flush using the plug on the block and she now runs cool. Almost fully done with the thing now after replacing the MAF sensor. I ended up having to replace the exhaust manifold because I couldn't get the EGR tube removed and now have the O2 sensor left. I suspect the lack of having an O2 sensor is causing a slight loss of power around 2500 RMPS


----------

