caddydaddy

The intake manifold gasket on my Moms '90 L98 is leaking coolant. I'm not sure if it's leaking into the engine or not. But when she drove it home from having the dealer inspect it, the engine made a growling noise, so hopefully the bearings aren't being torn up by the coolant. The dealer quoted $500 to R&R the intake gasket, so I plan to do it myself.
Anyways, the engine only has 45k miles on it, but I'd like some tips on replacing the intake manifold gasket. I'm a Cadillac man, so the last Chevy I worked on was a '95 Astro 4.3 V6! The pass side valve cover gasket leaks as well, but I'm sure I can figure that one out!
Anything else that should be done while I got the intake torn apart?

Thanks!

65Z01

Get a FelPro intake gasket set along with their valve cover gaskets and a tube of Black RTV or the Right Stuff and some Permatex #2 for the intake bolt threads.

I always chase the intake bolts and the bolt holes in the heads to assure a good torque reading. Then coat the bolt threads with some Permatex #2.

The intake base job is labor intensive though not difficult to do. Be sure to roughen and dimple the front & rear engine lands and the mating surfaces under the intake base to assure a leak free installation.

You might also throw in a new T-stat and air filter while it's open.

If there is over 1/2 tank of fuel be prepared with some small heater hose and two clamps so you can loop back the fuel feed & return lines, as these must come off the fuel rail.

When installing the plenum hand start ALL the bolts to assure you don't strip any threads in the plenum. You will likely need to use a rubber mallett to "persuade" it on removal as it tends to stick to the runner gaskets over time.

After you bolt up the intake base let it stand for >24hrs to allow curing of the RTV on the front & rear engine lands. Then retorque the bolts and install the rest of the intake.

caddydaddy

OK, thanks! Do you have any idea where the leaks typically occur? I'm not too familiar with how the coolant flows through the intake and heads of this engine. I just hope it didn't get into the oil!

bogus

A couple of other thoughts on the intake...

Use some copper RTV spray on the intake gaskets.

The gaskets themselves will shift, it's not a bad idea to tape them down with some masking tape, just to hold them in place while the intake sets into place.

I use Copper RTV on the end seals. It's designed to deal with oil and stay soft. The black RTV will harden over time. I don't know about "The Right Stuff," but I have heard good things about it.

65Z01

The coolant flows mainly through the front of the heads & manifold; the intake gaskets will have a steam hole for the rear coolant passage.

I apply a thin coat of black RTV to each side of the intake gasket around the water passages. This tends to hold the gasket in place as well as provide (I hope) some added protection against leaks.

Typically these intakes leak oil across the rear land. If it is leaking coolant that's not visible on the top of the intake base it's likely into the lifter valley (possibly into the #1 or #4 intake port). Drain the oil and check it out; change the oil after the R&R in any case.

Morley

The coolant leaks I've seen in the TPI's happened at the back corner at cylinder #8 and at the front corner by #1. The one over #8 was caused by a casting imperfection in that area, it had pits in the metal that the gasket couldn't seal against. To fix this I cleaned the pitted area with a wire brush and brake parts cleaner. then I filled the pitted area with JB Weld and filed smooth once cured. That repair lasted 18 years, and is still there (Manifold was changed a couple months ago.)

Trog

Are you sure its the intake & not the head gasket. Mine was the rear of the head gasket at 78K miles...

caddydaddy

The dealer said it was the intake.
We took the intake off, what a project that was! Those torx screws didn't like to budge!
There looked like some bad corrosion around the rear coolant blocking plate on the rear of the passenger side head. I'm kinda guessing that's where it was leaking into the engine.
The valve covers had quite a bit of light brown goo from the moisture, but there wasn't too much under the intake.
So hopefully, if all goes well tomorrow, we'll have it back together and running!

Thanks!

cv67

Once you get the base torqued down, let it sit for the day or 24 hrs if you have the patience and retorque it before throwing the rest of it together. OR, slap it all together like I did, and start it the next morning. (It started leaking again.. )

caddydaddy

LOL, ok, I'll try that! That's too bad I have to let it sit for a length of time! I'm in NY from CT to fix the car, and have to leave tomorrow! Hopefully we can get it all done by tomorrow!

TheCorvetteKid

Don't mean to hi-jack the thread, but how thin a coat or RTV do you put?

I ran a 'bead' (maybe 1/4" round) around all of the water passages the last two times I did my intake gasekts. In fact, I can see the sealant pushed out at either ends of the intake manifold, and there was even a little inside the bolt holes when I first put the intake down (I cleaned out whatever RTV there was before threading in the bolts, of course).

My intake is leaking oil along the back rail again, so before I tackle this job for a third time , I'd like to know exacly how much to put on the intake gaskets (pictures would definitely help here!!!). I have a feeling that I'm putting too much sealant around the water passages, which may be keeping the intake from sitting properly and causing this recurring oil leak problem.

65Z01

The RTV sealant takes roughly 24hrs to cure; starting the engine inside that length of time can blow out the RTV sealant causing another leak.

Good luck.

JOHN89

Place a 1/4" bead on the engine and let it skin over. Then place a 1/4" bead on the intake itself on the front and rear only. Dont let that skin over. Install it the intake immediately. This will ensure no oil leaks. As far as coolant leaks I would recommend a softer type intake gasket than the felpro blues. They are too hard. Every time I use them I pull oil into the intake ports under high vacuum conditions.
Mr. Gasket Ultra makes a softer gasket that when combined with copper RTV will eliminate leaks. They will be a bitch to get off though if you change them in the future. Can you say razor blade?

Good luck!
John