Must reading for all L-36 owners (and especially those who have squeaked out 140K without poppin' somethin!):
Re overhaul kits, here is some background on repair and replacement methods that retain factory coolant flows through the UIM and throttle body. Some of us here on BC built on the concept developed by Ken Spragg who first marketed an overhaul kit for L36 UIMs. The KenCo kit comprised a sleeve to epoxy into the (often perforated) EGR passage and a reduced diameter stovepipe both of mild steel. It used to sell for $80, and more recently for $70 or less. Ken'* idea was great - provide a heat shield in the most vulnerable area of the plastic UIM, the EGR bore, and reduce the diameter of the EGR stovepipe from .750 to .500 providing a .125 insulating gap between the hot pipe and the heat shield. Properly done, this is a very good fix. However, it takes skill and patience to center the repair sleeve in the EGR bore and to keep it properly aligned.
Everybody copied Ken'* concept - or, parts of it. GM increased the gap between the stovepipe and the plastic from practically nothing in 95-98 to about .065 in the 99+ models. They accomplished this by changing the EGR bore diameter in the LIM from .750 to .625, and by changing the stovepipe from a straight piece of .750 stainless to a straight piece of .625 stainless. That extra gap gave the plastic a longer expected life, but still did not prevent eventual deterioration of the plastic from heat. GM did not add a heat shield like Ken did or we would not still be dealing with the problem of perforated UIMs.
We liked Ken'* kit, but not the $80 price tag, or the use of mild steel, so, with some help from Bob Dillon, who recommended off-the-shelf 7/8" tubing for a sleeve instead of more expensive custom sizes, a low-cost stainless steel repair kit was eventually developed. For 95-98 engines, this included a .625 reduced diameter pipe and a thin-wall heat shield that together provide an insulating gap of about .090" and which was made available to BC do-it-yourselfers for $15. So, with $5 in JB weld and $15 in parts, you could repair your own UIM. For 99+ , only the sleeve and JB were needed -$10!
The problem is that perforation around the EGR is not the only problem with the plastic UIMs. They also can warp out of plane on the lower sealing surfaces around the EGR and coolant bores, and the sealing surface for the throttle body gasket often begins to gap open over time. So, a new UIM is often a good choice.
Most guys don't want to wait for mail-order parts, and just buy the widely marketed Dorman UIM replacement kit. Recently, Dorman has provided a reduced diameter (.625 x .510) aluminum stovepipe that will fit right into the LIM bore of 99+ L36s. This bettered GM'* '99 improvement by further increasing the gap between the hot stovepipe and the plastic to about .123. But still no heat shield.
For 95-98 guys who want to use the Dorman with a heat shield, a ring can be pressed onto the Dorman pipe to adapt it for the .750 LIM bores, and a .750 x .710 piece of stainless can be driven into the LIM bore to shield the plastic and provide a .100 gap for $10 in parts. 99+ guys with the smaller LIM bores need to apply some hi temp copper silicone to hold the shield in place.
The KenCo kit (unless they have changed it recently) stovepipe has a .750 shoulder to fit into the LIM of 95 - 98 L36s. The KenCo pipe could easily be turned to fit into a .625 99+ LIM bore and so achieve a .125 gap with a heat shield.
The new APN upper is really the best off-the-shelf fix we have seen for the UIM. It comes with a reduced diameter .500 pipe with a .750 shoulder, and a factory- sleeved EGR bore. The shoulder of the pipe will need to be turned to .625 for 99+. The APN upper can be purchased mail order for about $125 including shipping which makes the APN less expensive than the unsleeved Dorman. A little patience waiting for the mailman not only saves you money, but gets you a better design.
If you are still thinking of repairing your own UIM, remember to calculate the value of the new UIM gasket that both APN and Dorman provide. When you add that to the cost of a KenCo kit, it is pretty hard to justify compared to a new APN upper. If you have skill and time, and are inclined to do so, and your old upper is not too badly warped, you can save $100 and still get a very good fix by sleeving it yourself. If you want to repair your own in stainless and need any help or any of the parts described above , PM me.