Long time reader first time poster lol. I have a 2000 blazer 2wd 4.3. It is losing coolant from somewhere. Nothing showing under the hood or on the ground. New heater core as the other had failed, new hoses, (even the bypass). The oil looks like oil with no mixing, and no steam from the tail pipe. It's picked up a sporadic "hiccup" at red lights which I'm attributing to needing a tune up. It hasn't had one in quite a while. It has over 300k. It's my understanding that it has the infamous plastic intake gaskets unless the previous owner had them done. It had about 120k when I bought it 13 years ago. My question is, does coolant flow through the plenum and if so I'm guessing I probably should do the lower as well. Is there any other spots i should look into before popping the top? I'm very familiar with the old school small block, 350 ect. Would doing intake gaskets on the 4 3 would be pretty much the same?

 

The coolant passages in the cylinder heads are sealed to the lower intake manifold using the intake manifold to cylinder head gasket. Only at the front does the water pass through the intake. At the rear, the intake caps off the cylinder head passages.

I've heard of leaks from loose hose clamps and pinholes causing some loss that were very hard to find because with the engine hot the coolant would spray on to a hot engine part and immediately evaporate. This could apply also to a small internal leak of the LIM gasket allowing coolant into the lifter valley.

A way to test for leaks that will get around the hot engine problem is to use a coolant system pressurization tool. You can often rent these at an autoparts store. You attach a pump with a built-in pressure gauge to where the radiator cap goes and pressurize the system with the engine cold. Then you look for water leaks. Be sure to check the oil afterwards too.

I used this technique to find the leaks in my radiator.

 

Thanks for the info. I'm definitely gonna grab the pressure tester. I actually ruined a 3.8L with one not too long ago so I'm a little hesitant lol. The 3.8 had the egr stove pipe issue and it would hold pressure, until I started it. Then I basically pumped coolant into a cylinder and it was done after that. Would a failed plenum gasket cause coolant loss. I understand that the lower intake and heads have mating passages but can tell by looking at replacement plenums online.

 

I've done the radiator pressure test on my 4.3 so you will be fine. Jut keep the pressure to about 15 psi, the rating of the radiator cap.

I've included some pics from when I replaced my LIM gaskets so you can see the parts. There are good threads on how to do this repair if you need to do it. When I did it, I took the opportunity to upgrade to the later version of the injectors.


With LIM removed. Coolant flows across the the LIM at the front.


Underside of LIM. The rear of the LIM is to the left and its front end is on the right.



LIM in place.



Plenum and throttle body installed.


Underside of plenum. The throttle body opening is at the top which is the front end.

 

Awesome, thanks for the visuals. I actually did see some coolant dripping last night finally. Looks like the radiator sprung a leak.

 

"Only at the front does the water pass through the intake"

Yes, BUT on my 95 4.3Z, the PS rear intake manifold bolt has COOLANT behind it. It leaked coolant when I removed that bolt.

in your first and second pictures, see that LR bolt is not INTO the water jacket, but right BESIDE it...

In your 4th picture down, left rear IM bolt is wrong, theres no flat washer. That bolt held the rear engine lift bracket and the bracket, which acted as a flat washer, is not there. Note the other IM bolts have heads with integral washers.

The other mystery leak in the 4.3 is the specially molded heater hose from the IM to the driver side nipple on the heater core, on my 95 S10, the nipples that hose connects to are two different diameters, so a 5/8" plain hose section leaked at the manifold. If the wrong hose is used, the joint leaks near the manifold and the water runs down onto the transmission where you cant see it.

Also, this is a GM 350 engine cut down, theyve had chronic problems with the IM leaking when the manifold bolts work loose.