Where do you live? Out here in Phoenix, that car just gets hot sometimes, and there isn't much you can do about it. I have the same problem if i've been driving the car most of the day... around 3:15 or so the temp starts creeping up. Not a good answer by any means,

Maybe your coolant system is slightly air bound and may take some time to vent out air. What is the coolant recovery tank doing? Does coolant recovery tank level increase with hot engine, then decrease when engine cools down? Does the outlet hose from the engine (thermostat outlet) stiffen up when this overheating occurs? What is the condition of the rad cap? Bottom line, you're not getting heat transfer through the radiator. You may have a blockage somewhere in the coolant system also. I would recommend back flushing as well as forward flush again.

I've been fighting an insufficient cooling gremlin for some time on my 96 SE. AC does seem to be a serious contributor to the overheating problem for me as well. So far I have tried everything you have, including a backflush of the radiator and repairing the LIM gaskets which showed quite a lot of gunk buildup. I am currently running a cooling system cleaner which I will be draining out tomorrow and replacing with regular coolant. Since running the cleaner, my temps have stayed lower. Today I also Simple Greened the radiator fins, let that sit, and hosed that all off. The water that came off was almost black it was so filthy. I also have some banged up fins on the lower part of the radiator, but nothing anymore severe than many cars I've seen.

Long story short, I think these cooling systems were slightly underbuilt, and normal wear and tear has a huge negative effect on them. All this work I've done seems to be bringing my temps back under control. I drove it today with the outside air temp at 114 and the temp gauge never pushed over 200-210. Considering 2 weeks ago it would border on the red ON THE HIGHWAY, I would call that an improvement. I did drop in a 180 t-stat as well, but that won't do much for you if the car can't cool off in the first place.

I took out my radiator of the car & cleaned the fins out with a garden hose. It'* amazing how much sand gets stuck in the "vee" of the fins that you can't flush out while in the car. I just laid the radiator flat on the ground and sprayed from all directions.

It helped my cooling out a bunch.

It'* a 92 SSE with a 2 row radiator.

I didn't get a chance to do much with it this weekend, but I'm going to go with a 180 t stat, and clean out the rad fins, even though they look not too bad. It'* just so odd that it only does it when it'* over 95 degrees or so, but if it'* only 90 outside, no problems at all.

It'* not so odd when you realize that the problem is that your rad. is cooling at a barely acceptable capacity. 95* just puts it over the top.

Has anyone tried installing a larger radiator in their Bonneville? Or maybe an electric water pump that could push more coolant, especially at idle?

I'll measure it tonight, and post an update tomorrow.

jr's3800 is doing so right now. His failed and he received a larger Cadillac rad instead of the bonneville one he ordered.

jr's3800 Radiator Update