2002 Running Too Cold

2008wagon · December 13, 2009

Hi everybody,

This is my first post, I don't have a truck anymore and I'm posting for a friend of mine whose truck I am working on.

It's a 2002 4x4 Z71 with a 5.3L and auto trans. What it does is this: most days it's fine, but sometimes the engine temp will drop, you can watch the gauge going down while you're out at highway speed (this never happens at idle, ever) and when the gauge gets really down there the engine will start missing, almost like it's cutting the ignition or something. Then after 10-20 seconds it'll pull out of it, the gauge will go back up, and it'll run fine.

I have a scan tool, so I monitored the actual engine temp. At idle it sits at a rock solid 185 degrees which is good. But out on the highway this thing will drop all the way to as low as 155 degrees, and then is when it starts missing.

It has a brand new GM thermostat, it does not leak or use any coolant, and it throws good heat inside. I noticed that the fan is particularly noisy on this truck, like it is engaged all the time. I spun it with my hand and it is VERY stiff. Do you think this fan could be causing the low temp concern?? It almost has to be but I've never seen a vehicle do this before due to a clutch fan.

Please tell me, anyone, what you think.

Green_Monster · December 13, 2009

Engine coolant temp sensor? Should be on the drivers side of the engine, if I remember correctly.

Try this:

Coolant temperature sensor

minibike132 · December 13, 2009

^^^^^^^^^I would have to agree with the ECT. If the computer starts changing the fuel mixture and timing for a colder engine when it is actually hot it might explain the missfires when this happens.

You might have two issues because the fan clutch shouldn't be rock solid and 185 is way on the low side. You should be running somewhere between 200-220.

2008wagon · December 13, 2009

^^^^^^^^^I would have to agree with the ECT. If the computer starts changing the fuel mixture and timing for a colder engine when it is actually hot it might explain the missfires when this happens.

You might have two issues because the fan clutch shouldn't be rock solid and 185 is way on the low side. You should be running somewhere between 200-220.

It's an O.E. 186 degree F thermostat from the dealer that was just put in. That being the case would you still say it should run 200??

And I feel the same about the ECT theory. But a coolant sensor that far out of whack would SURELY give a code wouldn't it? There is only one code in this thing, and it's for the EVAP sytem. And this code was there he says long before this truck ever started exhibiting this symptom.

And the ECT doesn't feed the gauge does it? Isn't there a separate coolant temp sender on these trucks, or not??

minibike132 · December 14, 2009

^^^^^^^^^I would have to agree with the ECT. If the computer starts changing the fuel mixture and timing for a colder engine when it is actually hot it might explain the missfires when this happens.

You might have two issues because the fan clutch shouldn't be rock solid and 185 is way on the low side. You should be running somewhere between 200-220.

It's an O.E. 186 degree F thermostat from the dealer that was just put in. That being the case would you still say it should run 200??

Yes, after the t-stat opens the rate of temp climb will be slower but you should still be in the 210 range before the fan kicks in fully.

And I feel the same about the ECT theory. But a coolant sensor that far out of whack would SURELY give a code wouldn't it? No not necessarily, actually if it were going to set any code it should be the one for engine not reaching operating temp. If it went dead short or open it would set a code immediatly but if it is just swinging the pcm would just think it is actually running cooler. There is only one code in this thing, and it's for the EVAP sytem. And this code was there he says long before this truck ever started exhibiting this symptom.

And the ECT doesn't feed the gauge does it? The gauge gets it's signal over class 2 data line from the pcm, it's the same sensor in this vehicle. Older ones used 2.

Isn't there a separate coolant temp sender on these trucks, or not??

I'm kind of up in the air on which direction to go here. I think I would get out the infared thermometer and shoot it at the t-stat housing to see if the sensor is accurate. Of course your issue is more going down the road so that is kind of difficult unless you could pull over and shoot the thing before the temp recovers. If it is actually low go after the fan, if it is reading incorrectly go after the sensor or sensor circuit. I still think you might end up doing both.

2008wagon · December 17, 2009

^^^^^^^^^I would have to agree with the ECT. If the computer starts changing the fuel mixture and timing for a colder engine when it is actually hot it might explain the missfires when this happens.

You might have two issues because the fan clutch shouldn't be rock solid and 185 is way on the low side. You should be running somewhere between 200-220.

It's an O.E. 186 degree F thermostat from the dealer that was just put in. That being the case would you still say it should run 200??

Yes, after the t-stat opens the rate of temp climb will be slower but you should still be in the 210 range before the fan kicks in fully.

And I feel the same about the ECT theory. But a coolant sensor that far out of whack would SURELY give a code wouldn't it? No not necessarily, actually if it were going to set any code it should be the one for engine not reaching operating temp. If it went dead short or open it would set a code immediatly but if it is just swinging the pcm would just think it is actually running cooler. There is only one code in this thing, and it's for the EVAP sytem. And this code was there he says long before this truck ever started exhibiting this symptom.

And the ECT doesn't feed the gauge does it? The gauge gets it's signal over class 2 data line from the pcm, it's the same sensor in this vehicle. Older ones used 2.

Isn't there a separate coolant temp sender on these trucks, or not??

I'm kind of up in the air on which direction to go here. I think I would get out the infared thermometer and shoot it at the t-stat housing to see if the sensor is accurate. Of course your issue is more going down the road so that is kind of difficult unless you could pull over and shoot the thing before the temp recovers. If it is actually low go after the fan, if it is reading incorrectly go after the sensor or sensor circuit. I still think you might end up doing both.

Well it's fixed. Once I really got in there, it was obvious in 2 minutes. The coolant sensor was broken in half, and stuck back together in a half-assed way. And that makes sense because this guy said the truck started doing this immediately after he had it to the dealer to get a tune-up. This sensor in only about an inch or so from a plug, and I guarantee they broke it and tried to hide it by just sticking it back together. I changed the sensor, gauge is now up where it should be, truck is running great.

Thanks for everyone's input.