Last week, my Bonneville was acting strangly. I was driving at highway speeds, noticed the engine lacking on power. When I rolled up to a stop, then started to go again, I had heavy pinging (detonation?) No abnormal gauges or lights. Puffs of blue smoke were coming from the exhaust. I finished my day and drove home, when I got to within a couple of miles from home, it occured to me that it seemed like the symptoms were overheating so I turned off the AC and cranked the heater on full blast. (it was already 105 deg that day) The engine started to act more normal after that.
Removed the thermostat, it was stuck closed (boiling water did not open it)
Replaced thermostat with a new one, changed the oil, ran some Prestone flush thru it. Changed the sparkplugs (old ones were kinda dark pink and one was showing a little blister) Ran some Seafoam thru the intake (watch that smoke!)

All seemed well for a few days of highway driving.

Yesterday, I smelled a faint odor or coolant while driving, I stopped, checked it, it took about a half gallon (after I let it cool) Then while coming home yesterday evening, I stopped from highway speeds at another stop sign, and when I went to take off, same puff of smoke came from the exhaust and the heavy pinging was back!. Turned off the AC, turned ON the heater and finished driving home with no more problems.

Coolant is still full. No trace of anything in the oil. Fan is coming on when it should, plenty of airflow thru the radiator. I plan to do a compression check here in a bit. What else should I be alert for? blown head gasket? intake gasket?
This car sees heavy highway use (200-250) miles per day if that makes any difference and the weather has been very hot (triple digits every day).

try a coolant flush, maybe the radiator has a clogged passage or 2 and isnt cooling properly...

is your temperature gauge working?

Temp gauge seems to be working, it isnt showing to run any hotter than it usually does, I ran some Prestone Flush thru it already, left it in there for a days driving and flushed it out that evening.

It has the base cluster, which is only a temp gauge with numbers that start at 100 and then the high side at 260, always runs about 1/3 of the way off the cold side.

Coolant will never show in the oil until it'* too late. It can also be ingested into the engine. Change your oil to be safe. What does the temp gauge read? Did your idot light ever come on?

Back, compression check was 170 on all cylinders.

Don, 118k on entire car, only repairs have been normal maintence and timing chain replacement. (bought from original owner and they had ALL I mean ALL reciepts.)

However, here is something interesting, when I pulled the plugs, I noticed that the 2 transaxle side plugs were kinda nasty and wet again. HMMM. I studied this for some time wondering what was getting in there. Didnt feel or smell like coolant. Looking up at the TB, I noticed the vacumn line for the transaxle modulater fed in right there. Yanked the vacumn line off the modulator and TB and it was full of trans fluid.

So maybe its not an overheat problem anymore and the pinging and smoke is from ingesting trans fluid? Maybe a new modulator will have it back to health again?

These plugs have less than 1500 miles on them LOL. Reckon they will clean up? I have more Seafoam, can I just use that again too? Waiting on my wife to get home from work to get another modulator, It doesnt look too hard to change. I hope AZ or O reilleys has one.

I appreciate all the fast help!

edited to add, I only use AC Delco, tried Bosch once in my 88 Royale, never again. Only got 10k or so out of them and it wasnt a pleasant 10k

Not trying to hijack here, but FWIW, I run Bosch Platinum +4'* in the GT. It does run a bit hotter than I would like (probably only because its like 100* outside and I have an UD water pump pulley), but not a big deal. But performance wise it runs awesome and I get pretty good gas mileage as well.

Shawn