Overheating in a GM 3.5 DOHC V6
 

I'm a bit stumped...weird overheating problem. No leaks, radiator not blocked. Electric fans working properly. No indication of blown head gasket. Intermittent overheating. Figured it was defective thermost so I pulled it...auto parts store sold me the wrong one so I reassembled without the thermostat since they were closed. Strill doing the same thing. Waterpump maybe? That's all I can think of. Unless the water is flowing too fast without the restriction of the thermostat.

Dueller, I'd still think bad thermostat. It isn't unknown for thermostats to be bad from the factory. Get another one and put it in a pot of boiling water, watch what it does. If it works [the wax pellet melts and the sealing plate moves] then pull it out and install it.

Two points from 928-land that have some pertinence:

1. The 928 CANNOT run without its thermostat. Without a thermostat, a 928 will overheat in a hurry! That is because the t-stat on a 928 not only opens a passage to the radiator, it also blocks a bypass passage. I don't know if the gm 3.5 is like this, but it is something to consider.

2. Some replacement 928 water pumps have plastic impellers. This is because the earlier metal impellers were known to occasionally shift and grind into the soft aluminum block. The fix? Plastic. The problem? The plastic impeller would occasionally come loose from the metal shaft and the car would mysteriously overheat. I wonder if that can happen to this V6? Unfortunately, the only way to find out if this is the case is to pull the water pump.

Best of luck-

N

I had this problem on my Toyota Tundra. Turned out that the tranny cooler had blown inside the radiator mixing the tranny fluid with my coolant. From the outside everything looked fine, but the decreased cooling ability due to contamination was the culprit.

I would think you would have noticed this while doing the thermostat but...

what car is it in?

the DOHC 60* v6 is a fav swap in fieros in both 3.4 and 3.5 versions
but air pockets can hurt cooling you need to burp the system to get the air out
how varies by car

there are some bad plastic WP impellers in other GM 60*v6 motors
watch for flow thru the removed cap

posiable the stat was stuck open NOT closed and removing it recreates the same condission of
to much flow to fast for propper cooling

OK..update. Replaced thermostat...still overheats. Repaced water pump, serpentine belt, tensioner...still over heats. It will sit and idle and maintain cool temp. Get in it and drive a few miles and the temp will climb. Cooling system seems to build up huge pressure.

Radiator cap?

Or the dreaded blown head gasket?

Ok...replaced radiator cap even tho the old one seemed to test OK. It will sit and idle at proper temp. But if you drive it 3-4 miles ay 1600-1800 rpms it will stradily begin to get hot and then peg temp gauge. As the temp is going up you can turn on the heater and it will blow hot. However at some point it will stop blowing hot as the temp climbs,


You can then stop and let it idle and the temp will come back down. However if you sit in neutral and hold the rpms steady at 3000 it takes a while before the temp begins to climb.

I'm stumped.

Are you sure the radiator isn't blocked? You might look at Len's post as well, the trans cooler thing...check your trans fluid, it'll turn a gummy pink if it mixes with coolant...NOT good for the trans.

No..doesn't appear to be a radiator/trans cooler problem...no signs of cross contamination.

The upper radiator hose will get Really hot while the lower is relatively cool.

Mechanic is guessing a cracked head as opposed to a blown head gasket. Thinks when it gets warmed up, the crack opens up and blows compression into the cooling system....hmmmmmm

sounds thermostat...but that too has already been covered. Suggestion: Find a good shop...

I'm not familiar with chev v6's, but the symptoms you describe sure sound like a headgasket or cracked head.

Works fine until the cooling system is overpressurized, then flow is impeded (no flow to heater core, lower hose cool, etc) and then overheats.

Exact scenario in my Samurai this spring, was just the gasket... head wasn't even warped. Same thing in my friend's Honda this summer.

Does it overheat if you leave the rad cap off?

Nope:(

So if you leave the rad cap off, does the lower hose get hot?

If so, I'd wager my favorite set of ratcheting wrenches that it's either the head or the gasket. It's overheating because the cooling system is overpressurized and won't flow properly.

I don't think you mentioned what kind of car it is, or what your plan is for it. A friend of mine had a bad HG on his 2000 Jetta, but he didn't want to dump any more $$ into it. It would only overheat in traffic or idling, and would work fine with the rad cap off. I tightened all the head bolts 1/4 turn and put in a can of Bars Leaks. Stopped overheating and held up for 8 months. Might be worth a shot if the car's not a "keeper"

there's a kit to check the radiator that will tell you if your head gasket is blown or you have a cracked block/head. don't remember the name but it's a container type thing held in radiator fill with some chemical that changes color indicating engine gases in the water. might be cheaper checking to make sure before paying for gasket or head change. I've seen it recently at a parts house. I've seen it used many times and it works fine.

Sorry to say, but I had the same problem on a GM V6 and went through the same stuff you did. Turned out to be a cracked head.

Again...did you CHECK the thermostat by boiling it? I've found two [2, count them....] bad thermostats by boiling them. Always boil your new thermostat!

What one component can BLOCK coolant flow the most?

Best of luck!

N

Check the timing. And the temperature of the cat.

Already been mentioned but ...my truck (Chevy V8 ) developed an 'air lock' after I replaced a radiator hose. Took me two tries, (draining complete system) to get it to clear.

Bypass the heater core to make sure it is not blocked. You said at some point the heater stops blowing hot.
Also, Are you sure your temp. gauge is working correctly?