Three years ago I replaced the following items:

LIM gaskets (aluminum framed)
LIM (GM): The DexCool had eaten away the aluminum.
UIM gasket (APN kit)
UIM (after market unit from APN with increased clearance for EGR stovepipe)

I now have coolant consumption and I can see it is leaking out between the UIM and the LIM, at the throttle body end. My question is this:

Is it possible that I simply need a new gasket for the UIM and/or throttle body? Or is it likely that the LIM gaskets are no longer sealing as well? Should I orderer another APN kit, or can I source the gaskets elsewhere? Has anyone had an APN UIM fail ?

i have a leak between the uim and LIM on 00 SC bonnie. it was due to not using enough RTV when installing gaskets along the troth the LIM sits in. a new set of gaskets is about the only way your going to get it to stop. also how old is your coolant? i have never seen the aluminum ones fail (other than from installer error)

The way I read this is your leaking between teh LIM and the UIM. If so, the LIM gaskets are likely fine. It'* the O rings between the LIM and UIM that brings coolant up to the throttle body. Pull the UIM only and replace those gaskets. Make sure it'* torqued right, and recheck after a week or so.

I replaced the radiator and all the coolant about a month ago, using the Prestone "mix with anything". This weekend I will do a tear down, planning to replace the UIM and TB gaskets. I'll pick up a new set of LIM gaskets as well to have on hand.

It was cold out this morning when I started her (30s) and I had white exhaust, which made me think I had coolant in the combustion chamber (it went away and I had clear exhaust eventually). She does burn oil (166k) so maybe the coolant is mixing with the oil and burning that way. I'm pretty sure I would know if I had blown a head gasket (I still have plenty of acceleration). I can't find evidence of coolant on my dipstick on in the oil filler hole. When I replaced the radiator, my coolant was scuzzy - I assumed it was because I had a small crack in the radiator side tank, and the system was getting contaminated that way.

At this point I'm hoping that the UIM is the problem, and that coolant and oil are mixing a only a little, and that the regular oil burning (either rings or valve seals) is how the coolant is getting burned as well. What fun!

You have a plastic UIM right? Pull the throttle body and look inside at the EGR hole. The UIM tends to crack there and leak coolant in to the intake at that point.

I have seen the aluminum gaskets with up to 50K miles on them and they don't appear to ever degrade or have issues. I'd think the issue is the UIM to LIM gasket. Afterall..it'* only sealing the 16psi of coolant pressure with 89in lbs of torque on it'* bolts.

I will get a picture of the degrading around the EGR stove pipe for you, I have mine torn apart right now because of a ton of problems I was having and this ended up being part of it. There is also a small block underneath the throttle body that closes off the coolant passage or something along these lines. Mine started leaking last year and I had to just pull it off and I think there was a gasket there that went out and I got a new one and RTV'd it really good and it stopped the leak. I am assuming this isn;t your problem since you said it looked like it was coming from between the UIM and LIM but this is right there too. If it is the problem you wouldn't have to teat up the intakes.

I just got finished installing a new UIM from APN on my 3.8 and I remember reading that a new TB gasket is available from GM that is softer and seals somewhat better than the other stock gaskets. Mine had just been changed(gaskets) 9K miles ago and then the UIM cracked at the EGR stove pipe. The TB gasket I pulled off was all screwed up because at the bottom where the water goes through there are three opening and the wall on one was broke and the gasket had 2 protrusions that matched the inner 2 walls but you could see where it had been leaking. I purchased a 1/4" torgue wrench from Harbor freight sales and it worked great. If it is leaking between manifolds it sounds like the oring like seals on the bottom of the UIM could be bad or the TB gasket as well. Good luck

What is the thought of using a FelPro gasket as compared to the new GM gasket? Which one would you select and why?

Well I pulled the UIM yesterday and the APN gasket I put on 3 years ago failed. The coolant O-rings had indeed come apart. In fact they weren't even there! They completely dissolved into jelly and plugged themselves in the throttle body. I also had oil pooling in the LIM valleys (near cylinders 2 & 5 of course) so I took that as an indication that the LIM gaskets weren't working, since the bolts must be wicking oil up there. I replaced the LIM gaskets and hopefully things seal this time. Took me 9 hours from start to finish. I hope I don't have to do this again. I forgot why coolant has to circulate through the TB ... is it really necessary? I seem to remember a common mod for TPI engines in F-bodies was to bypass this.