GM 3400 V6 thermostat replacement - how'd you get the rear bolt out?!
 

I'm getting some pretty erratic temeperature readings and no heat
whatsoever on my 97 Trans Sport Montana, so I think it's thermostat
time...

If you're not familiar, the thermostat housing on the 3400 is directly
below the throttle body and right smack next to the exhaust crossover
for the front manifold, leaving maybe 1/4" of clearance between the
side of the crossover and the head of the rear-er of the two bolts that
attach the housing to the block. It looks my best bet would be to get
at it from the rear with an open-ended wrench, but I still don't see
that bolt coming out with that pipe in the way.

I don't think I'll have much trouble with the front bolt, but
does anyone with 3100/3400 experience have any suggesstions for freeing
the rear bolt that don't involve removing the exhaust crossover?

Thanks a lot!
-Bryan

Bryan wrote:
>
> I'm getting some pretty erratic temeperature readings and no heat
> whatsoever on my 97 Trans Sport Montana, so I think it's thermostat
> time...
>
> If you're not familiar, the thermostat housing on the 3400 is directly
> below the throttle body and right smack next to the exhaust crossover
> for the front manifold, leaving maybe 1/4" of clearance between the
> side of the crossover and the head of the rear-er of the two bolts that
> attach the housing to the block. It looks my best bet would be to get
> at it from the rear with an open-ended wrench, but I still don't see
> that bolt coming out with that pipe in the way.
>
> I don't think I'll have much trouble with the front bolt, but
> does anyone with 3100/3400 experience have any suggesstions for freeing
> the rear bolt that don't involve removing the exhaust crossover?
>
> Thanks a lot!
> -Bryan

No heat to the heater is a no coolant problem, not a thermostat problem.
When did you last change the leaking intake gaskets?

I did the lower intake gasket at 99k miles. Currently has 103k.

More info: At a cold start after 10 mins or so of idling, temp gauge
climbs like normal to the point where it normally sits (195ish F), but
usually just keeps rising steadily. Bringing the RPMs up to 2k will
sometimes make it come back down to normal, but it eventually climbs
back up and wants to stay there. This made me think the thermostat
wasn't staying open when hot, and that the increase in RPMs got enough
coolant flowing through the thermostat to force it to open a little bit
to circulate enough coolant to bring the temps back down. It also has
a new water pump.

"Bryan" wrote


> I don't think I'll have much trouble with the front bolt, but
> does anyone with 3100/3400 experience have any suggesstions for freeing
> the rear bolt that don't involve removing the exhaust crossover?

You don't actually have to completely remove the rear bolt. The
housing has a slotted hole for that rear bolt....so you can loosen
the rear bolt until it's almost all the way out, and then you can wiggle
the housing out.

By the way, the rear bolt will come all the way out without interfering
with the exhaust crossover.

Ian

> You don't actually have to completely remove the rear bolt. The
> housing has a slotted hole for that rear bolt

Excellent; thanks for the help!

"Bryan" > wrote in message
oups.com...
> More info: At a cold start after 10 mins or so of idling, temp gauge
> climbs like normal to the point where it normally sits (195ish F), but
> usually just keeps rising steadily. Bringing the RPMs up to 2k will
> sometimes make it come back down to normal, but it eventually climbs
> back up and wants to stay there. This made me think the thermostat
> wasn't staying open when hot, and that the increase in RPMs got enough
> coolant flowing through the thermostat to force it to open a little bit
> to circulate enough coolant to bring the temps back down. It also has
> a new water pump.
>

My first thought is that perhaps there is still some air trapped in your
cooling system. If you had the gasket job done only 4k miles ago, I'd have
whoever did that job look at it - I doubt it's a coincidence.

doug

> My first thought is that perhaps there is still some air trapped in
> your cooling system

Thanks for the suggestion; I actually thought of that, too... Earlier
today I opened up both air bleed screws (#1 on the thermostat housing
and #2 above the water pump) with the engine running and let them run
until steady coolant came out...shouldn't be any air in there now. I
did notice, however, that the coolant coming out of the bleeder on the
water pump was hot, while the coolant coming out of the thermostat
housing was cool. The upper radiator hose was pressurized, but never
got hot, either.

I replaced the intake gasket with the help of a friend who spends 8
hours a day under the hood (of Fords, albeit), and afterward drove from
Iowa to New Orleans and back, among other journeys, with no signs of
trouble in the temp department, and no signs of coolant leakage into
the crankcase as noticed prior to replacing the gasket.

I also replaced the water pump about 600 miles ago after a small leak
developed that ended up slinging coolant all over the underside of the
hood. No overheating or in the few weeks following that, however, so
I'm pretty sure this problem is new.

Hopefully I'll have some time this weekend to take a crack at it.

"Bryan" > wrote in message
oups.com...
>> My first thought is that perhaps there is still some air trapped in
>> your cooling system
>
> Thanks for the suggestion; I actually thought of that, too... Earlier
> today I opened up both air bleed screws (#1 on the thermostat housing
> and #2 above the water pump) with the engine running and let them run
> until steady coolant came out...shouldn't be any air in there now. I
> did notice, however, that the coolant coming out of the bleeder on the
> water pump was hot, while the coolant coming out of the thermostat
> housing was cool. The upper radiator hose was pressurized, but never
> got hot, either.
>
> I replaced the intake gasket with the help of a friend who spends 8
> hours a day under the hood (of Fords, albeit), and afterward drove from
> Iowa to New Orleans and back, among other journeys, with no signs of
> trouble in the temp department, and no signs of coolant leakage into
> the crankcase as noticed prior to replacing the gasket.
>
> I also replaced the water pump about 600 miles ago after a small leak
> developed that ended up slinging coolant all over the underside of the
> hood. No overheating or in the few weeks following that, however, so
> I'm pretty sure this problem is new.
>
> Hopefully I'll have some time this weekend to take a crack at it.
>

Sounds like you've done a very thorough job of troubleshooting this. Let's
hope that it is the thermostat - and it will be an easy job!

Doug