Hummer h2 limo occassionally overheats

[pteam]

One of our hummer h2 limos occassionally overheats, only occassionally. Bought it used with like 40k miles from another company. It ran fine all winter no over heating.

Now today it gets to 250 degrees we shut it down. Same thing last summer, maybe once a month it would get close to overheating. Today a/c in the front was kinda warm but cold in the back. Does this have anything to do with it? Also there isn't a coolant leak that is noticable as it parks in the same spot everyday inside. What do you think could be causing this? What can be done to solve this or help while we figure it out? We've had it to the dealer last year and they couldn't duplicate the problem nor figure it out.

[tomp]

H2 radiators are not efficient enough for the H2 from the factory. It is ample when everything is brand new but with the slightest amount of debris in the front grill slots, the efficiency goes to crap.

At a minimum, clean out the AC coil in the front by lightly spraying cold water from the rear of the radiator and then from the front. If that helps even the slightest bit, then you are on the right track. If not, try replacing the thermostat.

Finally, I'd have to question what you tranny temps are at prior to the engine overheating? It is possible the your tranny is heating up first due to it being a limo and then causing the antifreeze to climb with it. This is due to the ATF fluid running through the radiator on passenger side. I would suggest a very large efficient aftermarket tranny cooler to replace the cheap OEM unit. Since it's a limo, I would imagine the tranny is working overtime.

I would finally replace ATF and engine oil due to the excessive temps

[pteam]

Thank you good suggestions. I will add these to the lists.

The thing is we have 5 h2 , 4 are limos, and none of the others have problems. All have the stock radiators and tranny coolers and this is the only one to have a problem. So its something out of the ordinary.

We have replaced the thermostat like 5 times. Last time we put a fail safe in it, so if it does overheat (remember its a limo and it does alot of idling at wedding and when 25 people are getting out) the thermostat will stay OPEN. Its not the thermostat.

Would it be ok to completely take out the thermostat for the summer only and put one back in, in the winter? Would this help?

What we plan to do is complete coolant flush, then install extra aux fan in front of the radiator, and 2 aux fans behind the radiator blowing towards the engine.

We put a new radiator and fan on the h2 last year, cant remember if we did the water pump. Could the water pump cause it? Last year we thought it was a leaky head gasket and we ran some sealant in there, and it didnt overheat since then because it didnt overheat all winter, then bam it did it again today.

Thanks for your help guys! Trying to get as many ideas to check as possible.

[SummitUp]

Originally Posted by pteam

Would it be ok to completely take out the thermostat for the summer only and put one back in, in the winter? Would this help?

The primary function of the thermostat is to allow the engine to warm up quickly from a cold start. But it also performs an important function of controlling the actual flow rate of the coolant through the radiator (by creating a restriction). For optimum cooling the coolant needs to pass through the radiator at an optimum rate to allow it time to release or "give up" it's heat. I've seen situations where removing the thermostat actually makes the situation worse because the coolant moves through the radiator so fast it doesn't have time to exchange or release all the heat it could if it were moving through the radiator at a slower rate.

Originally Posted by pteam

Last year we thought it was a leaky head gasket and we ran some sealant in there.

Adding radiator stop leak in an overheating situation is definitely contra indicated. Besides stop leak being nothing more than a temporary band aid for a leak somewhere in the cooling system, it can tend to clog up the coolant galleries running through the radiator and diminish the cooling efficiency even further. I would definitely flush the system and get a <tranny cooler> making sure to bypass the ATF lines going to and from the radiator so at least the heat from the tranny won't be dumped into the radiator.

edit: I should have asked: What is the temperature reading on the tranny temp gauge when your overheating. Also, (a very remote possibility), you may not be overheating at all, but rather you could have a bad temperature sensor sending bad info to the temp gauge on your dash. I've seen it happen...

[pteam]

Heres what I have planned so far, tell me what you think:

1. 160 degree "fail safe" thermostate for hummer h2
2. full coolant flush, going to wash out the inside hoses with a hose
3. single aux fan I believe its 9 inches or 7 inches for in front of radiator on h2
4. double aux fan for behind the radiator to blow into the engine for engine cooling
5. Water Wetter, used to add into coolant of engine to help run coolant cooler, buy 2 bottles

[pdsq99f4]

Make sure your fan clutch isn't just freewheeling in both directions. This could cause it to not pull enough air at idle. Just because you don't see coolant on the ground doesn't mean it's not leaking. Look around the oil pan to make sure the water pump gaskets aren't leaking down the front of the motor and the coolant just drying to the oil pan.