In 2003 I bought a '97 Bonnie without doing much research. Afterward, I heard about the plastic IM and potential issues with the gaskets, and Googling around convinced me that my motor was going to explode soon, probably the next time I drove it. Haha.

Anyway, I went to my local mechanic, and he hedged on it a bit, but said it wouldn't hurt to replace the stuff, as the car had 90,000 miles on it. He'* very reasonable price-wise, so I decided to take no risks, and have him do it. After that, I drove the car another 100,000 miles with no major engine trouble. So all was good, I guess, but I have to wonder if I was being overly cautious.

Fast forward to my "new" 2002 Bonnie, which I got a week or two ago. It'* got 120,000 miles, and I have no service records. It was well cared for and looks clean inside and out. My question is, how much should I worry about this problem on a 2002? Is it different than a 1997 in significant ways? While I have done a small amount of research, it'* still not clear to me what the actual defect is. Is it the IM itself? A specific gasket? Both? Did GM improve the situation between 1997 and 2002?

I don't want to make any hasty decisions this time around....maybe I should just watch for leaks externally and check the oil frequently? Spending a few hundred bucks to significantly decrease my chances of engine trouble in the next 50,000 miles or so seems like an OK idea to me, but I guess I shouldn't spend money now that I would be fine spending later, or never, if I was safe simply monitoring the situation. I don't drive my cars hard at all (my wife says I drive like an old man); I put on maybe 12-15K miles per year, mostly between home and work (~15 miles one way). Occasional trips of a couple hundred miles, no plans to depend on it for a long road trip at this point.

Like I said, I have done a bit of research, but it'* been spotty, and a perspective from owners of this motor would be very helpful, and very much appreciated.

the UIM and LIM gaskets are still an issue with them. if you look at where the LIM meets the head and see aluminum color between the two you LIMs have been replaced with the updated aluminum gaskets

Taking care of it was a good idea even if just for the piece of mind.

There'* a huge myth running around about how your engine is going to explode in to tiny pieces if you don't get the DEX cool out of the engine and then replace the LIM and UIM gaskets.

Now for the truth.

True: The plastic LIM gaskets fail in as little as 10K miles, but can last 100K with no real problems, by fail I simply mean they will seep a little but not make your engine explode.

Not true, UIM gaskets generally do not fail.

True: The UIM does fail. Some super smart guy decided it was a good idea to rout the hot EGR gases threw a plastic UIM and do this about a inch away from the coolant passages. The heat cracks the plastic between the coolant and the EGR stove pipe allowing coolant to enter the throttle body. This then runs right in to the LIM and fills the cylinders causing a hydrolock. You either get a no start, or a bent connecting rod.

False: removing the DEX cool will not fix the problem.

True: Aluminum LIM gaskets will fix the problem regardless of the coolant you are using.

UIM fixes include a aluminum UIM, blocking off the coolant passages (Not really needed), or disabling the EGR.

Missed a few questions.

Your 2002, is this a supercharged engine or a NA engine? If NA the best thing you can do is remove the throttle body and look at the area where the EGR tube is. If you see any dampness or coolant, it'* time to replace the UIM. Here'* a write up I did on LIM gaskets, https://www.gmforum.com/specifications-general-information-132/lim-gaskets-myth-fact-288248/ Please feel free to ask any questions and we will be here to help.

Thanks for the useful replies, you guys. I was just gonna step outside and see if I could identify the gasket material by looking, but I guess it'* decided to rain REALLY hard here right now. So I should be able to assess whether I have any water leak problems later today.

It'* going in to my mechanic tomorrow anyway for an oil change and to replace the OP sending unit, and have him generally look it over, so I will have more info soon.

I presume my motor is not the supercharged one....I don't actually even know what trim this car is. It just says "Bonneville," no additional letters on the nameplate. I suppose there'* something about the VIN that will tell me for sure?

you more than likely have the K motor and your car is an SE model

Stock LIM gaskets are black, the aluminum gaskets are a light green.

Are there any badges on your front doors? If it doesn't say anything but has the Bonneville script on the trunk it'* most likely and SE

I watched my coolant tank, expecting it to drop at any moment. It took 200K miles, but posted this morning about LIM replacement last nite. My LIM failed in the same spots as the pics in Danthurs link.

Dont worry, just look at the coolant tank at each fillup.

Well, it'* done, actually. Took it in last week after confirming visually that the gaskets were the stock ones. I also stuck my finger inside the rim of the oil cap and got some orange-y stuff on it. My mechanic pressurized the radiator and said it lost pressure a little too fast, and there were no visible external coolant leaks, so he went ahead and did the replacement.