1999 gmc 2500 5.7L, 60k miles chassis. Reman engine 2k miles ago. Was running fine but got 10-12.5mpg with slight hesitation off idle. Ran 170F indicated. Had read faulty temp sensors can run rich w/o CEL so replaced temp sending unit and coolant temp switch with complete tune up. Shop cleared old stored codes and it ran alot better. But instead of always 170F I got ~185-195 idle with heat on, ~210+ idle no heat, ~220+ idle with a/c on, ~210-220 driving A/C. I put in another new 195F tstat, sender and switch. No change. Burped system 4 times w/ heaters cranked. Radiator tested fine at shop. No noise or leaks from pump. Oil is clean/clear, coolant is green/clear.Vacuum and fuel press test fine. Shroud on. Heater works. Installed heavy duty fan clutch. Now 200F no a/c and 210 w a/c. Laser readings after 30min idle and 10 min drive at 55mph. HEAT ON - tstat house 175-180, up hose 165, top right rad 160, mid left rad 135, low hose 125-130, pass head near 6 and 8 195-205, pass head rear 195-200, drive head rear 185-190, temp send unit 185-195, dash reads 210. A/C ON - tstat housing 199-202, up hose 186, top right rad 185, mid left rad 165-170, low hose 165, pass head near 6 and 8 210-213, pass head rear 206, driver head rear 193, driver head near 5 and 7 207, temp send unit 215-218, dash reads 220-225. then 55mph for 10 min at 215-220 w heat on. no boil over. any ideas?

 

My 1st idea - it's not an F-body, let alone a 3rd gen F-body - how would we know? This is a 3rd gen F-Body forum, not a 2500 forum.

My 2nd idea - if it's anything like a 3rd gen F-Body - then it's NOT overheating until the guage reads over 240. Every car on this site runs 220 all day long - these cars were designed to run that hot.

 

your first idea - our beasts share the same powertrain dna. slightly different mutations but, nevertheless, very closely related. obviously there will be lots of information and expertise is transferable from cars to trucks and camaro's from silverado's. i'd bet alot of camaro owners have had or still have a GM truck in there driveway and would be happy to share the experience.

your second idea - good info. i had heard that alot of temp related troubles that come about after sensor swaps occur because the parts house gave out a z28 sensor instead of a vortec sensor because, for some reason, the part number may be the same. the result is that the needle sweeps to a nonsensical number indicating a false oveheat condition. your info seems to confirm that possibility. thanks.

Anybody else have any ideas or experiences they'd like to share? any help greatly appreciated. thanks!

 

Well im assumeing you fixed the poor fuel economy/hesitation issues? If you have it doesn't sound like you have a overheating problem as normally SBCs or the emissions era on run about 220. Maby the AC at idle on 220+ (depending how far north of 220 it goes) but beond that ide say you were running too cool befor for your application and now are normal.

 

it runs noticeably better. i don't think i gained 100hp or anything but it is more responsive and seems happier with the wires, sensros, etc. have not driven it much because of the overheat issues. would really be happy if i could get temps down around 195 or so. engine is reman with 3/36 warranty. if the heat tabs melt off my warranty is void.

 

For starters, make sure the timing is correct.

Also, I have a 91 2500 GMC w/ a 350. The thermostat is stuck open in it and it runs around 160 I do believe in the summer. I would check to see if your rad is dirty or possibly have a lot a of corrosion on the inside of it.

 

you need to hook it up to a computer and see what the comuter is showing for the temp and not be so hung up on the gauge which sounds like its not reading correctly...better yet hook up a manual gauge that way there is no question. and as far as it being a reman engine and worrying about the heat..dont worry those companys always find a way to screw you outa it...trust me i worked for autozone and advance...

 

Radiator tested fine at who's shop? Any violence in the overflow tank after driving with the 215+ temps?

 

No violence in the overflow. Zero boil over. Never has.

Timing, once dizzy is pointing to #1 at TDC, is handled by computer. That part is a little different than the TBI injected engine. I'm sure the timing is fine, all old codes cleared out and no new ones so far.

Am going to investigate a mechanical gauge, as a diagnostic tool, tomorrow. Will install it in place of the temp sending unit and see what it shows. If it gives the same readings as the guage at idle with and without a/c, what then??

anyone know if those actron scanners at the parts houses read temperature off the computer? am i stuck paying a shop another 70-90 diagnostic fee?

radiator is good to go. The pros at our city radiator shop powerflushed it in May and retested it for me last week. Zero obstruction.

How cold should coolant be flowing back to enigne in the lower hose be to keep enigne at 195? Or, is there something about my longblock (all of a sudden) that suddenly causes coolant to absorb more heat than system can handle? could 3 new thermostats all stick? is my pump not flowing enough water through the block? and the $10,000 question, why after running ~180F for weeks and only recently switching cap, rotor, wires and two sensors is this happening?

 

this morning went back down to the shop, after a 20 miles 65mph drive without heater on, and had fella plug in a scanner for sensor data. read 199 while gauge was indicating 220-225. didn't increase when i turned a/c on either. fella says that catastrophic engine failures typically include some sort of serious overheat either as the cause or as one of the byproducts of failure. he went on to say that when this happens the gauge can exceed its range and "stretch". as a result it reads incorrectly. the failure may have resulted in damage to my original sensor causing it to read low, really low, because the gauge read cold before the new sensor and now it reads 25F too high.

Make sense or sound like B.S. ?

Anyone know of a way to re-calibrate the gauge?

Am I stuck buying new gauge cluster?

Could I swap in one with a tach or is my app wired differently?

With either new cluster, will my odometer get screwed up?

 

put in a 4 core aluminum radiator and a 165 thermo.....im willing to bet you wont have any more over heating problems....

 

4 core and a 165..... not bad!!

bigblock rad with a 195 may be better for my purposes.

more info:
after my visit with the scantool i felt pretty good. not out of the woods yet but at least the sun was shining, right? wrong. bought mech temp gauge and installed it in place of factory unit. here's what i got....
idle no heat or a/c: 195-200
idle heat: 190-195
idle a/c: 210-215
idle a/c in drive with wheels chocked and ebrake on: 225-227 and holds. then you blip throttle a few seconds and it falls to 220 before slow climb back.
no boil over
stays 195-200 while moving.
traffic is going to be a ****. unacceptable.
i still think something is wrong with my systems heat disapation capacity. it'll cool down if you bring engine to 1200-1500 rpm and get fan sucking more air. someone else replaced the radiator before i bought vehicle. you think the radiator i have is for a 4.3 or 5.0? could that be why it's border line?
you think some small e-fans on either side of tranny cooler would help at low speed/idle?
you think a 7.4l rad would be a better idea?

 

Well if your cooling system didnt have the ability to dissipate enough heat then the temperatures would slowly rise beyond the goal temp (unfortunetly not by just a few degrees either if given enough time). As for radiators and stuff factory equipment should keep your car no questions asked. If its not than you have some other underlying problem that should be corrected first. A classic example of what i mean is i remember talking with someone about their overheating camaro people were sayin camaros factory system sucks (which it kinda does but anyways) sure buy a huge aluminum radiator dual fans bla bla bla too bad that's not going to fix the real problem, a blow head gasket. So again first befor you start to try and compensate for other possible problems by adding a large radiator and fans make sure everything else is OK first. Now the way i see it is first we have to establish the basics. for starters, what is the temps normally on another truck of your type? Being that im not to familiar with them ide say your temps dont sound out of the norm but comparing with other owners would confirm or deny this fact. You asked if may be someone installed the wrong radiator if that would do it? May be again being from a camaro site i wouldn't know off hand whats interchangeable on your truck although you can always look it up on a parts website like autozone or napa and compare part numbers between applications. Lastly assuming you've compared with other owners and confirmed your truck is running hotter than it should then we can start digging deeper into things (ensuring everything is working and the parts installed are correct ect). If its concluded that there is no problem and you would like it to run normally around 200 or so then again mods can be made to make it run cooler. However i cannot stress enough this should be done only after its been confirmed that there are no other underlying problems. Although it all comes back to the initial problem of what is the normal operating temp for your truck? Until we know that theres really not a whole lot we can do ya know what i mean?

 

[quote=

How cold should coolant be flowing back to enigne in the lower hose be to keep enigne at 195? Or, is there something about my longblock (all of a sudden) that suddenly causes coolant to absorb more heat than system can handle? could 3 new thermostats all stick? is my pump not flowing enough water through the block? and the $10,000 question, why after running ~180F for weeks and only recently switching cap, rotor, wires and two sensors is this happening?[/quote]

Really there's 2 fundamental issues that cause overheating.

1 the problem is just the opposite of what you mentioned in most cases if the cooling system cannot absorb enough head energy then you overheat. See it takes X amount of heat energy under a certain amount of pressure to change the temperature Y degrees. The greater the heat capacity the more energy it takes to change the temps. Low heat capacity caused by things like improper coolant mixture, low pressure, ect can lower your coolants heat capacity and allow less heat energy to cause a greater increase in temperature.

2. More heat energy is entered into the system than can be dissipated. This is caused by 2 fundamental issues. First, the motors generating more heat. This could be caused by ignition timing, running lean, blown head gasket ect. The second is you cannot dissipate the heat. Could be caused by a things like corrotion in the radiator acting like an insulator, fans not working, fan shrowds not installed, faulty thermostants, ect.

lastly you ask why it was running 180 before? This can be caused by a multitude of things. However this question could be looked at two ways why is it so hot now or why was it so cool before? I think as stated earlier this is too cool or at least i should say by factory spec your running cooler than it should. Course we can sit here all day debating potential causes of why but first we need to figure out whats normal and do some troubleshooting. Otherwise this problem (if it is one) will turn into a very expensive project if were going to just throw parts at it based on guesses and speculation.

 

ok here. a 1999 tbi engine is going to run between 195 and 220. period. it is designed to run at that temp. you said you replaced a bad sensor or sensors and that is when the temp went higher. i have not seen one of them engines run that cool with out cooling system mods. you said that you have a 195 thermostat then it is going to run about 10* warmer than the thermostat that you put in it. you can try a 180 thermostat but i believe that you are not having a cooling issue at all. also the hesitation could have been directly related if not caused by the temp sensors. overfueling due to incorrect reeadings to the computer.

 

i agree with one92rs.
bigblue you are chasing a non exsistant issue. your temps are perfectlly normal. my 1500 runs at 210 all day long. with the a/c it rises to 220 from heat added by the condensor in front of the radiator. this is normal. your truck is fine, and the heat plugs will stay in tack. dont waste anymore money paying a shop to chase down your fears.

 

Its acting normal for stock cooling system. Running that temp wont hurt any thing.
But if you must cool it down for some reason about the only way you will get it to is if you stick in a 180 or 190 degree thermostat.

Yeah the shop will work on your truck all day long to fix a non existant problems so long as you pay them.

 

ok first off, those temps are normal, Im an ex GM tech (laid off due to economy) you should be running about 210-225, my car runs 223 in the summer months and 230 in the winter months, you are NOT over heating till you hit 260, you get to 260, you have problems, anywhere up to 235 is normal operating temperature. dont worry about it unless your guage gets to the top and your the amber warning light lights up and your DIC displays "coolant hot, Idle engine"