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LIM nylon gaskets

10-09-2011, 11:25 AM
For those of you who still have those nylon LIM gaskets with Dexcool and think you're ok until you begin losing coolant, think again. I replaced my wife's LIM gaskets with the aluminum framed ones. The car wasn't using any measurable amount of coolant, so I thought it was still fine, but I replaced them anyway as insurance.

1) failed coolant port passage in LIM.
2) gasket track seemingly in good condition (gasket removed). No evidence of cracking/degradation.
3) gasket track of failed coolant port passage.
Note that nylon appears to be pulled away. The only force I can see that would do this is contracting coolant as the engine cools when shut down. Note that the nylon appears to tear away along a predictable pathway near the edge of the track. While Dexcool seems to play an important role in the degradation of the LIM gasket, it appears as if the gasket is inherently weak at this point.

Even though the damage on the left side of the coolant passage gasket frame appears minimal, it was in fact torn right through. I have not observed any partial failures yet in my gaskets suggesting that the tears may happen suddenly without warning. However, the silicon gasket material often holds for some time. As bad as my nylon frames were, the were not leaking any significant coolant. In fact, I didn't think I had a problem at all; I was just replacing the gaskets for preventative maintenance.

The bottom line is this: if you still have nylon gaskets in a Series II motor, change them to aluminum framed gaskets even if you're not losing coolant yet. They could fail drastically at any moment.

10-11-2011, 04:24 PM
Thank you for the post back. Out of all the times people have done this job and had it leak again, I think it's failed again on about any type of gasket.

10-11-2011, 10:23 PM
Thank you for the post back. Out of all the times people have done this job and had it leak again, I think it's failed again on about any type of gasket.

I can see the job failing with aluminum framed gaskets if:
1) LIM bolt threads are not cleaned properly. This wasn't an easy job. The instructions on on popular pontiac tech forum were not sufficient to clean the threads. I had to use a thread chaser, then degrease. Only then would you be able torque them properly. When I dismantled my LIM, a few bolts felt like they were barely finger tight while others required more force to remove. I believe the job was done before but the bolts were not cleaned properly, nor was any fresh thread locker used.

2) Aluminum can be pitted by Dexcool if it becomes too acidic from mixing with air. I could see the aluminum frame pitting if the coolant fails to protect the aluminum, and if it pits by the silicon gasket it could then leak under the gasket. However, I wouldn't expect it to break apart completely like nylon.

3) Unclean gasket surfaces or scores along the surface. I found the valley gasket surface had a deep scratch on mine. That would cause an oil leak if anything. I spent way more time cleaning gasket surfaces than anyone I'm sure.

I'm sure that some shops may be quite lax with items 1 and 3. Item 2 would be a problem caused by the owner not maintaining the cooling system.

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