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PostPosted: Wed May 28, 2008 7:28 pm 
Retired Admin/Techinfo Admin
Retired Admin/Techinfo Admin

Joined: Mon May 19, 2008 2:08 pm
Posts: 26004
Location: Space Coast, or at least it used to be
Year and Trim: 03 SLE, 95 SE, 95 TS SE
Original Author Jr's 3800 & Bill Buttermore, with the help of many members to be added later

Upper intake manifold failures, and lower intake manifold gasket failures - what we all need to know and understand

Upper Intake Manifold (UIM)

For the years we have been here on Pontiac Bonneville Club we have seen many 3800 Series II composite upper intakes fail. Sometimes the result was simply the replacement of the upper intake, other times the result was total and complete engine damage. A few actually reported that the engine either seized up or just threw a rod through the block due to spun bearings. Most of the time the cause was the engine ingesting coolant and during the combustion process, that excess moisture makes it into the oil. Once the oil has been contaminated with coolant, the oil can and will become acidic and will destroy the bearings in the bottom end of the engine in a hurry.

For the first few years of the 3800 Series II L36 we did not have these problems as the engine was too new at the time. But at about the 4-5 year mark we started seeing a lot of these intakes rupture, and in most cases the problem was corrected before any real damage occurred.

I myself was a victim of this upper intake rupture and had to have my car towed to the dealership as my car was under extended warranty. It was repaired by the dealership and I had no further problems. But after 6 years I decided to replace the intake just as a precaution. The intake again was degraded to the point it was ready to rupture, and I was glad I had decided to repair the issue. But it’s not a cure. There may be a cure for those interested, but we will come back to that a little later.

There has been talk that GM changed the upper intake in the later model years. They in fact did not make any changes to the upper intake, it’s the same one that was used in 95 with the first 3800 Series II engines. The only thing that changed is that Rochester had become part of the Delphi automotive group. So the newer units 98+ if memory serves, will say Delphi via a little sticker near the PCV valve location.

The most common mode of failure for the Rochester/Delphi upper intake is degradation around the EGR port; the upper intake actually touches the EGR Stovepipe. Over time, the heat from the EGR Stovepipe slowly degrades the plastic to the point where it’s almost ash; the problem here is that coolant flows through this area to the throttle body. When enough of the plastic has degraded and the cooling system is under pressure, the upper intake can leak or rupture resulting in coolant being pumped into the intake, in turn being ingested into the cylinders, resulting in a very whitish smoke being emitted from the exhaust. It is also possible for the engine to become hydrolocked. This usually happens when the motor is turned off and the cooling system is under pressure. And at the time, if you were to check the coolant overflow tank or the radiator, chances are that you will find one or both of these to be very low.

In the later model years (99+), GM went to a smaller diameter (5/8”) EGR pipe that creates a 0.065” air gap between the hot pipe and the plastic EGR bore hole in the upper intake. This helps, but does not cure the problem - as we have heard of and seen a few of the newer cars with failed manifolds. But so far, they seem to have a lower failure rate. Only time will tell how the 00-05 3800 Models fare. By early 2006, GM dealerships were seeing a few of the 00-02 models come in for an upper intake replacement due to failure of the unit. So it’s starting to appear that the 2000+ L36 owners need to be watchful as well.

Reading the Date On the Rochester / Delphi Upper intake

If you have a GM upper intake, whether it be a Rochester or Delphi unit, you can figure out when the upper intake was made. This may be helpful in determining if the upper intake was ever replaced.

On the right side of the intake or throttle body side, you will find a date stamp with an arrow pointing to the Year of manufacture for the intake.

In the center of the intake you will find a Day of the month stamp. This tells us what day the upper was manufactured.

On the left or drivebelt end of the intake you will have a stamp for the Month of manufacture.

Lower Intake Manifold (LIM) gaskets

The lower intake manifold gaskets in 3800 engines deteriorate over time creating internal coolant leaks that can cause the same type of damage as a ruptured upper intake. While UIM failures are limited to the normally aspirated (non-supercharged) L-36 engines built from 95 – 05, the plastic-frame LIM gasket design was used in both supercharged and normally aspirated engines at least from 92 – 05. The original equipment plastic frame gaskets carrying silicone rubber sealing beads break down and begin to leak as shown below:

Depending on where the break occurs, coolant may be sucked into the combustion chambers, leak into the crankcase, wick up the LIM bolts, or otherwise leak to the outside of the engine, sometimes pooling in depressions on the LIM casting. Sometimes the seal fails between an intake port and the crankcase causing poor idling and a vacuum in the crankcase that can damage the engine oil seals. Many believe that orange Dex-cool (organic acid) coolant, used by GM since 1996, reacts chemically with the plastic frame gaskets causing them to break down much more quickly than was observed when the older green (phosphated) coolants were used. Because the lower intake manifold gaskets are easily accessible when changing the upper intake manifold, it is strongly recommended that LIM gaskets be replaced when the UIM is removed.

Keeping the Engine Cooler to Avoid Heat-related Failures of the Gaskets and Upper Intake Manifold

Some of the members on this site have tried to reduce the overall operating temperatures of the engine and trans as they all relate to heat as the engine is concerned. As others have, I installed a 180F Thermostat in both of my cars as well as a Transmission Fluid Cooler to help aid in keeping overall temps lower. Whether or not this will actually help the gaskets or upper intake last longer is up to the owner. I have had decent luck with reducing the operating temps of the engine and trans. I have seen gaskets come off 3800 II's that looked 10xs worse than what I removed from my car.

You can go to a Lower Temperature Thermostat such as a 180F with no real trouble. But depending on the climate you are in, we may recommend that you stick with a 195F or at the least if you do want a 180F you use it during the summer and switch back to the 195F during the cooler months.

As for the transmission cooler, again it falls to your discretion as to whether or not you would like to run a transmission fluid cooler. If you decide that you want to run a cooler, take into account the climate you are in. In a cold climate you may want to run a Thermostatically Controlled Cooler so the trans will not get too cold. Now, if you live in a climate that’s the reverse of that, such as myself (Florida), then you would be fine to run a general trans cooler to cool the trans all of the time regardless of the outside temperature.

I feel that the above items in one way or another relate to the overall operating temperatures of your car.

Fixing the Problem

Generally when the Upper Intake ruptures, it will be replaced. We also highly recommend that you replace the Lower Intake Gaskets and the Coolant Bypass Elbow(s). Doing the Upper Intake alone and not replacing the other items will turn into more trouble down the road. So for that reason, spend the $40-$60 on the Elbow and Gaskets for the lower intake and save yourself the headache down the road.

You can get these gaskets from GM as well as the Elbow. You should also be able to go to the auto parts store and get Fel-Pro gaskets as well as an Elbow. These are very common items. My parts store had the items right down to the upper intake on the shelf. If you cannot get the GM aluminum gaskets for the lower intake manifold, at least make sure that you get 2nd generation gaskets - they will have metal washers around the bolt hole locations to prevent the gasket from being overtorqued.

We also recommend that you use a reduced diameter stovepipe to create an insulating air gap to reduce the heat stress on the plastic EGR bore. This is especially important for the 95 to 98 engines where the hot stovepipe typically touches the plastic of the EGR bore. When installing the stovepipe, make sure it is seated firmly in the Lower intake manifold. When installing the Upper intake, check to see that the stovepipe is centered in the EGR bore. If not, the alignment of the stovepipe can usually be adjusted a bit by tapping gently with a rubber mallet.

About a year ago, we became aware of a new kit that appears to offer the best off-the-shelf fix for the upper intake manifold. The combination of a metal heat shield to protect the plastic upper and a reduced diameter stovepipe is the best method we can currently recommend to resolve this problem without altering the cooling system. Using either of the two reduced diameter pipes supplied with the kit, an insulating gap of .125” is created between the hot pipe and the heat shield. The kit is marketed mail-order by Automotive Parts Network; it includes a UIM gasket, a throttle body gasket, and other parts, and is priced under $100. ... 17806.html

Here is the APN UIM showing the heat shield in the EGR bore and the reduced diameter stovepipe as it appears when assembled. Image

Dorman markets a replacement UIM kit that includes reduced diameter stovepipes that will directly replace the larger stock pipes. When one of the smaller pipes supplied with the kit is installed in the LIM, the insulating gap between the hot pipe and the plastic EGR bore is increased to .123”. The Dorman upper is a good replacement for the original equipment part, and includes a UIM gasket, throttle body gasket, and other parts, but the EGR passage is not protected with a heat shield. The Dorman kit is available widely and sells for about $150.

Regardless of the method chosen for repair, it is very important that the cooling system be purged of air when it is refilled. The design of the L-36 engine makes this usually easy task more difficult to accomplish. A procedure is described in Techinfo that will help you ensure that the critical areas around the hot stovepipe are filled with coolant and not air.

Other Fixes or Methods of Repair

Before you attempt to repair a failed upper intake, carefully examine the plastic surface where the throttle body attaches, and the area around the EGR bore where the coolant passes between the LIM and the UIM. These areas are prone to warpage, especially if the throttle body bolts have been overtorqued, or the engine has overheated. A little warpage is typical and can be tolerated, but if the surface is badly warped, the throttle body or coolant passages may not seal properly. Do not re-use an upper manifold if you detect severe warpage in these areas.

Ken-Co sells a repair kit for failed upper intakes. The kit includes a machined 0.5” diameter mild steel EGR pipe that replaces the stock 0.750” pipe in 95-98 Series II engines, and a mild steel sleeve that is epoxied into the EGR bore of the plastic upper. When completed, an air gap of .125” is created between the hot EGR pipe and the metal sleeve. The sleeve protects the plastic upper intake by reflecting and dissipating heat that is radiated across the gap. Club members have reported good results with this kit, but fitting the sleeve can be difficult, and the cost ($70 including postage) seems high for many.

Replacement Parts

Here is a link to APN’s sleeved upper: ... uct=229679

Here is a picture of the two reduced diameter stovepipes, UIM gasket, and vacuum fitting that are included in the APN kit.

This is Dorman’s Replacement Upper Intake (Dorman / Motormite) PN # 615-180.

The Dorman kit comes complete with a new upper intake manifold gasket (shown below), throttle body gasket, a new PCV valve, vacuum fitting, and injector o-rings.

Recently manufactured Dorman units also include two reduced diameter (0.510”) aluminum stovepipes that fit into the ¾” bore of 95-98 lower intake manifolds and the 5/8” bore of 99-05 lower intake manifolds. The Dorman pipe provides an air gap of 0.123” between the hot pipe and the plastic.

GM Lower Intake Manifold Gaskets (Fel-Pro Similar) Note steel reinforcement rings to prevent overtorque damage to the plastic frame.

GM Aluminum Frame Lower Intake Manifold Gaskets. We hope that this newest design will provide the best LIM gasket fix yet. As of March, 2006, these are available at the dealer for $59

Other Part Numbers

GM Second Design Plastic Frame Lower Intake Manifold Gasket set: PN# 89017400
GM Aluminum Frame Lower Intake Manifold Gasket set: GM PN# 89017816
GM Coolant Bypass Elbow: PN# 24503423
Dorman “help” replacement cooling elbows with o-rings (include two elbows for 99-05 models)
Fel-Pro Lower Intake Manifold Gasket set: # MS95809-1 (2nd design with pins for end seals)

We feel that its very important that this is read by every 3800 series II Vin K owner, regardless of what platform the engine is in.

I want to thank Mkaake, Bill Buttermore, Bob Dillon, Boosty, Bill Wren, Arbelac, Showshocka2k, and everyone that had made this upper intake knowledge possible. Thanks everyone. This continues to be a work in progress so if there are any part numbers you want added or any more info please PM me, Jr's3800... Thanks again

Last edited by Jrs3800 on Thu Dec 10, 2009 7:28 pm, edited 2 times in total.

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