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I'm trying to diagnose a '97 bonneville NA overheating problem. The car is using coolant slowly. During normal driving the gauge hovers between 220 & 240. When the heater gets turned on, it will start to overheat.

It's at 180k and I don't know the previous maintenance history, but suspect that the LIM gaskets need to be changed anyway. Is the overheating while the heater is on a different issue? I don't understand how it could overheat only when the heater is on to begin with.

Note that the exhaust is not white yet and coolant is not getting into the oil.

MattStrike wrote: I don't understand how it could overheat only when the heater is on to begin with.

Note that the exhaust is not white yet and coolant is not getting into the oil.

This. Normally they tell you to turn on the heater when you're car is overheating to stop it.

I don't know. Maybe a stuck thermostat but with that mileage I think I would just change the intake, just for good measure.

Think I'd start with a good radiator flush and new thermo. If it still has Death Cool get it out ASAP!
Like already stated.....if the LIM gasket hasn't been done yet. Do it before something serious happens.

Like it has already been mentioned replace the plenium gasket. i dont know what it is called exactly but there is a small metal pipe that is butted up next to the gasket, and when the pipe gets hot it will mest the gasket. I wish i took a pick of the mod i did. I took the origal pipe and cut it to make it flush with the engine (it gets put into the engine like a 1/2 an inch befor sticking out.) then we welded a pipe that fit snug into the original one making it so the part that sticks out does not rub up against the gasket.

The UIM itself should be replaced, and the LIM gaskets. You should use a non-AC Delco replacement, either APN or Dorman. Both have the reduced diameter EGR stovepipe and sleeved insert now to the best of my knowledge.

The coolant flush and thermostat are no-brainers at that point. Radiator function should be checked as well, as sludge buildup may be choking it.

Is the series 3 aluminum UIM and/or LIM a bolt on swap? The only reason to do it that way would be to save money by getting the parts at a junkyard.

The S3 LIM is exactly the same, the UIM will need a nipple tapped in to the neck for the vac lines the opening on the side will need to be tapped and plugged and you will need to modify a TB adapter for the TB to fit. Its not hard to do. Almost for got there is a little tab on the back side that needs to be trimmed off because of the way the alternator is mounted.

For the price you'd pay for a S3 LIM plus the parts to make it work, you could get the Sleeved UIM for $110 with everything but the LIM gaskets.

The "overheating when heater on" is really and overheating when A/C compressor engages during the heat/defrost or defrost only settings. New parts on the way... The defrost settings have a red bar, making the driver think it was the heater setting. Makes more sense now.