Help please - Burban temp used to run too hot - now too cold!?!

[partybear]

Hi all.

I have a 1996 Chevy Suburban K2500 with the 7.4 ltr.
124,000K no previous engine probs. - regularly maintained.

Last week as I was driving home from work (highway) I noticed the temp gauge of the burban approaching 250!! I was close to home so I turned up my cab heat all the way - Which was blowing cold air.

I pulled it into my garage. I found the radiator to be 3/4 full, which may explain why the cab air was blowing cold (air pocket by core).
The first thing I did was replace he thermostat after flushing out the system (yes it has dexcool) and I put fresh dexcool (I will flush it out for the green in the springtime). That seemed to solve the problem
I ran it in the garage and to/from work and it never got above 210 degrees (cab air warm as can be).

Now here is the dilema - Tonight as i am driving home from work (30 minute drive.. all highway), the temp Guage never got above 130 degrees and the cab air is only luke warm.

Could it be that the thermostat is bad already and is left open? I'm not even sure a bad thermostat could be left open. Even though it is about 25 degrees outside (lovely Cleveland weather), I can't imagine the engine staying that cool. I would assume a bad temp sensor but it did work this morning and the cab air blowing only luke warm would back up the engine running cool. Could it be a combination of thermostat and temp sensor? Oh heck I'll shut up now and hopefully get some good advice.

Thanks in advance and Cheers!

- Jim
Signed - Clueless in Cleveland.

P.S. not related but i thought I would ask. Near the radiator I noticed a gounded power cord and male plug coming from below the engine. Could this be an electric core heater?

[bad68chev]

The temp sensor has no effect on engine running hot or cold, just tells the computer the temperature of the engine. The electrical cord is probably a block heater, you plug the vehicle in at night when its real cold outside and it keeps the engine warm so it will start easier. I would check your t-stat and also make sure you don't still have an air pocket in the system.

[gregory914]

Also check to see if you have any fluid in your oil, and if you have any leaks around the water pump. My 97 5.7 had such a small leak it would take months for the fluid to be low because the fluid would evaporate and not leave a puddle.

And yes that is an engine block heater that replaces one of the freeze plugs and sometimes needs to have the connections cleaned. I know this because I also live in Cleveland and had that thing trip my breakers cuase salt is conductive and will short out your outlet. You only have to plug it in for 2 to 3 hours before you're going to drive and you can hear the fluid boiling like a coffee pot. I only use mine when it's below zero!

[partybear]

Thanks for the pointers. I thought that cord was for a core heater, but wasn't sure- doubt if I'll ever use it. Thx

As for the heating issue:
I earlier mentioned a bad temp sensor because I thought it may be giving me a false reading (engine running hotter than the sensor) in combo with an air pocket.

The oil is is normal, no discoloring, no smell and normal consistency.
The exhuast has no coolant or "sweet" smell so I'm hoping I'm good there. I ran the engine when it was cold, and I ran it without the rad cap and I noticed the fluid rushing through the rad when cold. So something could be up with the stat. I'll replace that again this weekend. I'll also look at the water pump seal, I did smell a very slight hint of coolant when I lifted the hood. so that could be it. Thx.

QUESTION: I also noticed the engine fan (behind the rad) was on instantly when I started the engine. Is there a relay or a clutch for this? Shouldn't it running be delayed until it reaches a certain temp?

Thanks Guys!



Jim

[bad68chev]

The radiator fan (if electric) will come on any time the ac compressor is on as well, This includes when system is set on Defrost (Compressor comes on to help take moisture out of the air). If the ac compressor is not on and your fan is staying on all the time you may have a wiring problem (relay,short, etc). hope this helps.

[Larry Derouin]

Sounds like your thermostat failed "SAFE" in the open position. Which is a hell of a lot better than failing shut. Change out the thermostat again (sometimes the cheap things are such a pain in the ass.)

Change out all of your hoses in the engine compartment and block off the rear hoses if it's not needed. If needed then flush out the rear and run new hoses all the way back. Also take an put a mechanical guage in the right hand head (you'll see a pipe plug between the #4 & #6 cyls. Mount the gauge in the engine compartment and when you get a spurious reading, pop the hood and double check against the mechanical. Do this with a 3 gauge set and mount on the firewall or one of the side walls. That will give you a set of separate gauges to go by when running the engine and working on it. (Keeps the grease out of the cab too.)

Larry

[J-Ri]

Just wondering, why would you put dexcool back in? That stuff is crap.