My 1997 Pontiac Bonneville SSE has about 140,000 miles on it. (Bought
used with about 41,000 on it.) We did not have an overheat problem
until the past few months. Two sequential flushes of the cooling
system followed by replacement of the thermostat did not help. It
does OK until one gets into stop-and-go OR until one has to go up a
long incline on the Interstate. Use of the AC does not seem to have a
dramatic impact. What DID have an impact was doing an oil change,
putting on a Bosch filter, noticing that the oil pressure did not seem
up to snuff, yanking off the Bosch filter and putting on a Mobil 1
filter. Oil pressure up now, much better, still shows tendency to run
a little hot at times, but not fearfully hot as before Mobil 1 filter.

There does not _seem_ to be anything wrong with the waterpump . . .
but . . .???

Seems to run about as well as it ever did but gas mileage is awful and has been awful foer a long time -- before overheat symptoms.
(was 28-32, is 18-22 -- wish I could do that!)

Tranny does not always want to behave properly; seems to be too eager
to shift into overdrive, then wants to "burble" (I do not know how
else to describe the sensation.) when it ought to be downshifting.
When it is not upshifting too soon and/or "burbling," it runs just
fine, the engine seems to have plenty of power and the tranny will
sometimes behave perfectly. Shifts are very smooth, almost
undetectable when the darn thing is running "right.".

I would like to make the car last another 18 months. I have
restricted the wife to local travel only.

My main concern is the intermittent overtemperature behavior. Any
thoughts on the tranny issue (maybe a computer issue?) will be warmly
appreciated as well.

Thanks in advance.
BBQToo
--- END INITIAL POST ---

97 Bonnies had some trans concerns with valve bodies and soleniods. Its al internal and not a D-I-Y job.

As for your overheat problem maybe it is not the coolant systems fault! Maybe it has something to do with your mileage concern. If an engine is running too lean it will burn hotter causing the coolant to be hotter. I wold get the car to a shop have them scan the computer and see if anything is abnormal.

Thanks, Flatrater, I appreciate the info. I suspected tyhat there would be no DIY solution. --- BBQToo

Also you may have a transmission cooler in your radiator. Many vehicles cool the tranny using coolant by routing tranny lines into a coil in the radiator. If this is the case it could that your transmission fluid is getting overly hot and warming your coolant.

Thanks, TorinoGT69. Sounds like a reasonable point. Most, if not all, cars with automatic transmissions have coolers in their radiators.

If that is the source of the issue, then the question becomes, what is making the transmission so hot?

Thanks. --- BBQToo

Check the torque converter. I had a '97 Bonneville SSE, didn't want to shift into gear. The torque converter was bad, it was getting so hot that when they pulled it off, it was purple. That might have something to do with it, might not...just thought I'd offer. :smile:

Will take it to the shop with some suggesdtions for the mechanic (he is my neighbor, and is a reasonable person; he actually will listen to the customer.

Many thanks for the advice.

--- BBQToo

I have a 95 and have posted a thread about a temp issue not quite the same as yours - more along the lines of the fans and what controls them, etc. I do know that on mine the tranny has it's own little radiator that sites in from the the engine radiator but I think that it uses the engine coolant running through it - it's not actually inside the main radiator.