Overheatimg Mystery

MonroeBobo · June 11, 2010

Hi, this is my first post. I have 1995 Yukon4x4 5.7liter gas I purchased 5 years ago. I have put 15k on it since I purchased it. It has been a great running truck. The temp gauge has always read around 180 give or take a 2 or three degrees under all conditions. I do not loose any coolant and the truck seems to run great other than the overheating issue. The coolant appeared clean and theres no sign of rust anywhere in the system.

Last fall while driving on the beach in sand it began to run a little warmer than norm(195 or so). I didnt pay it much attention. During the winter all seemed fine. When spring came, I noticed it was running 200 or so. Being proactive and knowing I pull a camper (3600lb), I put a water pump on. It didnt make any difference. I put a radiator on it and it made no changed at all.

Went to the beach in early may (no camper). While driving in the sand the temp woud climb to 220 sometimes 230. It usually would take less than 5 minutes to get that warm. I also believe if I had continued to drive the tempeture would have continued to climb. I would stop or get out of the sand before it got any hotter and it would slowly return to 205 or so.. On the way home from the beach it ran 205 to 210 it was also a hot day on the trip home.

After getting home I put a thermostat 195 Failsafe on and it made no changed at all. I removed the transmission line from the radiator and ran an extra trans cooler as a test to see if the trans fluid could be getting hot. That made no changed at all. I purchased a sunpro temp gauge and installed it and it read the same temp as the factory gauge. I had the catalytic converter check to see if it was clogging and it came up as good as new. I put a new 15lb stant radiator cap. I burped the system several times. Iput in new hoses.and belt. I let the thermo cycle with the cap off and never seen any bubbles and to see if it appears to be flowing good and it appears to flow fine. The fan clutch was checked by me and the mechanics. When cold it will turn only as far as you turn it with your hand. It sounds like a jet under the hood for the frist minute or two after starting calms down and when it begins to run hot it is the same jet sound.

Being frustrated I took it to a local shop I trust and they keep it fir a week and couldnt pin it on any paticular thing. They said it could be the heads leaking(no water in the oil or oil in the water}.I ask would that fix it and they said it MIGHT fix it. I got the truck back and decided to take it to a radiator shop who has been in business since I was a kid (i am 44yrs old). They cannot find the problem. They did a compression test on the heads and said they are fine. They took the radiator apart to ensure something wasnt clogging it up and it was fine.

Some other symptoms: While driving if I give it wide open throttle it will rise 10 degrees or so and then fall back to 205 or so within a minute or two. The normal temp seems to change with the tempeture outside. in otherwords. 80 degrees outside 200 to 205. 70 degrees outside 190 to 195 and so on. Any time under a load it begins to climb to 230 or more. I did let it get to 250 once.

I am at complete loss. I have done so many things to it I may have forgot to list them all. Although I am not a mechanic, I have done nearly all my maintainance on all my autos since I got my first car. I dont know what else to do. I dont know what else to check. Posting on a few forums is my last attempt. Any input or thought will be greatly appreciated.

Thanks Monroe

SLESierra · June 11, 2010

Sounds like it needs the system chemically boiled out you have a restriction somewhere

redvett · June 11, 2010

Since you have no indications of overheating i would recommend you take a scanner and read the correct engine temperature. The Gauge sender and the CTS are two separate systems the CTS being more accurate. I would be suspect of the IP Gauge.

compulynx · June 11, 2010

Beach, sand, saltwater....Just a shot, but have you closely checked the connectors on the sender and gauges? I am betting that the gauges are not accurate. The only other issue I could think of is a timing problem.

Oh, one other thing. Check the bottom hose to be sure the spring inside is not rusted out. If so, it could be collapsing under heavier RPM's.If yours does not have a spring, try a new hose.

C

MonroeBobo · June 11, 2010

I installed a sunpro after market mechanical temp sending unit and gauge. The new gauge ran the same temp as the factory. Is there another reason that I shouldnt trust the sunpro gauge. I put new hoses and belts on there. The spring in the bottom hose appeared fine. I also wonder about the Timing. My only problem is that the truck Idles and accelerates so well. There does,nt seem to be any miss at anytime. Is it possible for the timing be off and me not be aware of it. Again I did all this work myself. I had the two shops help diagnose it after I ran out of ideas. They didnt come up with anything. They did double check the new radiator to ensure it was fine on the inside. I plan to change the plugs and wires and check the timing next wednesday on my day off. I want to put a fan clutch on there because its the only thing I havent replaced. But i hate just throwing parts at it.

I wonder if the CTS you are referring to isnt off causing the truck to run lean or something. But not lean enough to really be noticable. Im really trying to get ideas for my next day off work so maybe I can solve this thing.

Thanks Monroe

LeoS41280 · June 11, 2010

Has the intake manifold been off?

MonroeBobo · June 11, 2010

I flushed it out myself with the prestone flush form the parts store. While I got a very few really fine particles it for the most part was very clean looking. I wonder if somehow part of an old hose or the like could have broke loose and partially clogged a passage. It does seem to have good flow with the radiator cap off.

No havnt had the intake off for any reason. It appears its never been off. then again maybe it was long before I got it.

Thanks Monroe

redvett · June 12, 2010

If you havent done so i recommend you use a scanner and read the data the CTS should be 195f to 210f also look at LTFT & STFT. Also check the the initial timing it should be zero with EST wire unpluged and engine at operating temp.

I installed a sunpro after market mechanical temp sending unit and gauge. The new gauge ran the sa me temp as the factory. Is there another reason that I shouldnt trust the sunpro gauge. I put new hoses and belts on there. The spring in the bottom hose appeared fine. I also wonder about the Timing. My only problem is that the truck Idles and accelerates so well. There does,nt seem to be any miss at anytime. Is it possible for the timing be off and me not be aware of it. Again I did all this work myself. I had the two shops help diagnose it after I ran out of ideas. They didnt come up with anything. They did double check the new radiator to ensure it was fine on the inside. I plan to change the plugs and wires and check the timing next wednesday on my day off. I want to put a fan clutch on there because its the only thing I havent replaced. But i hate just throwing parts at it.
I wonder if the CTS you are referring to isnt off causing the truck to run lean or something. But not lean enough to really be noticable. Im really trying to get ideas for my next day off work so maybe I can solve this thing.

Thanks Monroe

MonroeBobo · June 12, 2010

I have the OBD I diagnostic system on my truck. I believe it will only blink codes. I have never use a scanner before. Can I get more info than the blinking Check engine light will give me. By the way,the check engine light has never come on. also what is LTFT & STFT.

Thanks Monroe

When I figure this out I will post the fix.

98 Z71 SLT · June 13, 2010

Fan clutch

Should NOT sound like a jet.

That was the clue.

Also, would not hurt to check ignition timing. Refer to your underhood emissions label.

MonroeBobo · June 13, 2010

Wednesday is my next day off and I am trying to gat ideas and get ready to work on it again. I will change the fan clutch and check the timing also. I seems a lot of people in other forums thinks its the fan clutch. Sure would nice if thats it.

Thanks Monroe

redvett · June 13, 2010

A scanner not a code reader will display all th info coolant temp,RPM's,IAC counts,KS counts, MAP,TCC, transmission data and much more. LTFT is long term fuel trim and STFT is short term fuel trim. Your CTS is much more accurate than the Gauge sensor and is digital readout.. Like i said before check the timing and check the CTS against the Gauge.

I have the OBD I diagnostic system on my truck. I believe it will only blink codes. I have never use a scanner before. Can I get more info than the blinking Check engine light will give me. By the way,the check engine light has never come on. also what is LTFT & STFT.
Thanks Monroe

When I figure this out I will post the fix.

MonroeBobo · June 13, 2010

A scanner not a code reader will display all th info coolant temp,RPM's,IAC counts,KS counts, MAP,TCC, transmission data and much more. LTFT is long term fuel trim and STFT is short term fuel trim. Your CTS is much more accurate than the Gauge sensor and is digital readout.. Like i said before check the timing and check the CTS against the Gauge.

I have the OBD I diagnostic system on my truck. I believe it will only blink codes. I have never use a scanner before. Can I get more info than the blinking Check engine light will give me. By the way,the check engine light has never come on. also what is LTFT & STFT.
Thanks Monroe

When I figure this out I will post the fix.

I plan to get a scanner. Are they vehicle specific or are they universal. If you have a recommendation thats not to terribly expensive on a scanner I would appreciate it. The scanner ought to be cool to learn from. Kind off looking forward to it. Seems like a good all around diagnostic tool.

Thanks Monroe

P.S. LOL I am also learning why all that crap appears automatically where I need to type my next post in the reply box.