My 1978 Pontiac Bonneville Brougham started running 225 degrees while on a long, mountainous trip so I was running the heater to keep things cool but it was heating up my transmission fluid causing it to overflow out of the vent a week ago. I added more fluid and everything was good. I replaced the fan clutch and still had it running between 210 and 225. Purchased a new thermostat and gasket and when I went to remove the the radiator hose which was a Dayco universal flex hose from the radiator there was part of the inside liner of the hose was restricting the coolant flow. I have installed a Dayco molded upper hose and still have an overheating issue. I’ve flushed out the cooling system with plain water and baking soda/water mix and still hitting 210 degrees. What am I missing? Also even removed the radiator and flushed it out prior to this posting

Hello,

  What color is your transmission fluid? I'm concerned that radiator fluid is getting into the transmission fluid through the radiator's transmission fluid coolant tank.  

  You could also have a head gasket leak.  Take a look at a You Tube video for something called "Block Check Fluid". This will test for the presence of combustion gasses in the airspace above the radiator fluid level.  There should be none.

https://www.youtube.com/results?search_query=Block+Check+fluid 

  You might also put some fluorescent radiator dye into the radiator fluid and use the kit (the goggles and light pen) to check for the dye at the end of the tail pipe.  EBay has a kit for this and the Block Check tool.

https://www.ebay.com/sch/i.html?_from=R40&_trksid=p2334524.m570.l1313&_nkw=radiator+dye+kit&_sacat=0&_odkw=radiator+dye&_osacat=0

  It is also possible that the fins of the radiator are plugged with dirt and dead bugs after so many years.  Air can't get through and there is no good way to clean that debris out.  Its time for a new radiator.

Have you dropped the fluid pan on the TH350 to see if the clutch pack friction disk material is laying in the pan?

Rick

 

B52bombardier1

 

      The transmission fluid is dark red and has the burnt smell to it. Now that you mention it I should get the coolant head gasket leak tester tool to see if I have a head gasket leak. The original radiator was replaced probably about 10 years ago and it’s only a 2 row radiator. I’m looking into putting a 4 row radiator in to provide better cooling of the engine and transmission along with an auxiliary transmission oil cooler in the system. There’s  been a burned exhaust valve on cylinder # 8 since 1981 but it’s never really bothered the performance at all. I’ll have to drop the transmission pan and see what it looks like. Curiously thought if I end up rebuilding the transmission is there a way to flush the torque converter?

5 hours ago, 1978 Bonneville said:

B52bombardier1

 

      The transmission fluid is dark red and has the burnt smell to it. Now that you mention it I should get the coolant head gasket leak tester tool to see if I have a head gasket leak. The original radiator was replaced probably about 10 years ago and it’s only a 2 row radiator. I’m looking into putting a 4 row radiator in to provide better cooling of the engine and transmission along with an auxiliary transmission oil cooler in the system. There’s  been a burned exhaust valve on cylinder # 8 since 1981 but it’s never really bothered the performance at all. I’ll have to drop the transmission pan and see what it looks like. Curiously thought if I end up rebuilding the transmission is there a way to flush the torque converter?

Yes you can flush the torque converter, the best way is take it to a transmission shop. A good shop will be able to flush it. That said you have several issues to resolve. A Turbo 350 is a good stout tranny, but only when built right. Of all the GM automatics the 350 needs to be as clean as a new dinner plate you would eat off of! What you describe is a 350 either gone or on its way out either way you can’t save it! I could advise further on a 350 build, but first you need to resolve the heat issue, obviously. 
Your overheating is more than likely a head issue based on what you’ve said. If the exhaust valve was burnt in “81” it’s way worse now, more than likely part of your problem! Depending on where it was burnt it could now be impacting the intake valve. What did/does the coolant look like! Does it have an iridescent look to it? 

Somewhat the theory on radiators these days is to go with a two row that has 1.25 inch wide fluid tubes.  These are nearly the same overall thickness of a four row but allows the passage of cooling air through the fins more efficiently - it cools quicker.  This works for me in my LS swapped 70 Chevy El Camino with the same 1.25 tube style of rad from a company called "Cold Case".

  Its 93 degrees F here in Louisiana today with about the same percentage of humidity and my radiator had zero trouble with overheating.   Mercifully, the air conditioner was working great and the meat thermometer needle in the center vent said "38 F".

Rick  

Last Indian,

     No there wasn’t an iridescent look to the coolant. I have seen some deposits in the full neck of the radiator cap. The engine didn’t start reaching 225 until I made a trip thru the mountains of mid Pennsylvania to which when I had to replace the original radiator hoses 5 or 6 years or so ago the upper molded hose wasn’t readily available so I settled for the Dayco universal flex hose which some of the inner liner came loose near the coolant inlet and was impeding coolant flow. I ordered and replaced it with the Dayco molded upper hose which I hoped would’ve remedied the problem but no luck. Before I rebuild the transmission I’m gonna pull the heads and have the seats reground and either have the valves ground or replace all of them