Does the intake plenum on my unmodified '94 'vette LT1 engine have coolant running through it? I've been trying to chase down a coolant leak and one of the things the FSM states as a possible cause is the plenum -- "Cracked Plenum and/or leaking gaskets". I'd really rather not pull it off if I can avoid it.

A little more info for those who are wondering why I'm asking this question.

• I have a significant coolant leak but it's not external -- can't see any coolant on the engine or on the ground. I do smell it in the exhaust.
• The cooling system flunked a pressure test.
• I did a compression check on all eight cylinders and they passed with flying colors. As an aside, now I REALLY HATE to work on #6 and #8 cylinders.
• There is no coolant in the oil
• I just finished doing a visual and a dye penetrant test on the throttle body after removing the coolant cover. It's not cracked.
• I put all new hoses on the engine a year ago.

Any help would be greatly appreciated as the car is now banished to the garage while I work on it, and I'm missing some of the best top-down weather we've had all year.

Thanks,

Qack

 

Sounds like a head gasket problem. Get a block tester kit from your local parts store and run the test.

 

Could it be the heater core? Do you smell coolant inside the car?

 

Oops. I should have mentioned that the heater is currently bypassed, so it's not that.


It failed last year - I ended up with the passenger footwell full of antifreeze one fine day. Since I live in a warm climate, I just bypassed it. Replacing it is on my to-do list, but with very low priority.

 

I ran a compression test. What does a block tester kit do?

 

We ended up doing the same thing with the brand new heater core in my dad's '76

I'm not that far away from you so I know how rarely we use the heaters

I think the head test will indicate if there are any cracks in the heads that are allowing coolant to get into the combustion chamber.

I don't know enough to make any further suggestions. I think it's odd that you can smell it in the exhaust but it passes a compression test. Does it generate white smoke when running?

I know that on our L98 engine there are coolant lines that run coolant through the throttle body and it seems that a lot of people bypass this.

 

There is no coolant running through your intake.Check for external coolant leaks with pressure tester.Then do a hydrocarbon test to see if combustion gases are in coolant.Does your coolant level stay the same or is it going/leaking somewhere?Was the car overheated recently?Was the car worked on recently before this happened?Coolant does go through the throttle body & maybe its getting ingested there somehow?Are you sure your smelling coolant at the tailpipe?Any white smoke coming from exhaust like what was previously asked?

 

block test should help clarify. Imean loos of coolant and smelt at tailpipe indicates coolant through the exhaust port. Of course keep fingers crossed that thisi s not the case

 

A block tester is a plastic tube the fits where the radiator cap goes. You pour blue fluid in the tube. Run the engine. Air from the cooling system will pass through the fluid as the engine warms up. If it turns green or yellow, you have exhaust fumes in the cooling system. That means the head gaskets are leaking or a head is cracked.

 

Thanks for the info Paul. I guess an old dog can learn new tricks! I had never heard about these before, but it looks like a great tool, and using one fits neatly into my current plan.

Since I can't find anything, I'm currently putting everything back together (have I mentioned I hate #6 and #8 spark plugs?). I'm then going to run the engine - carefully - and check for leaks, smells or whatever else I can find. After I've run it until everything is warmed up - assuming I don't find a problem or am obviously risking the engine if I continue to run it -- I'll test the coolant with one of these kits.

Wish me luck.

 

Update: I buttoned everything back up and did a block test. I'm getting exhaust in the coolant from the left side of the engine. The right is just fine. I'm leaving for two weeks, but after I return I'll be taking off the left head as that is where the leak is. Not sure what I'll find, but hopefully it's just the head gasket.

Speaking of which, I want to order the parts now so they are here when I return. Since this is going to remain a stock LT1 for as long as I own it, which I plan to be for a long, long time, any advice on head, exhaust and intake gaskets? Are the Fel Pro line good replacements for the originals? I will also be replacing the head bolts as that seems to be the consensus.

Paul, thanks for recommending the block test kit. Neat tool and they loan it out at Autozone so it didn't cost me anything!

 

How did you narrow it down to one side("getting coolant from the left side")?Did the spark plugs or left side combustion chambers have coolant on/in them?The block test-thats what I previously meant."We" always called it a "chemical head gasket tester".There is a very slim possiblity that the block could be cracked as well.I'm not trying to scare you,just letting you know.Good time to do some power upgrades(ported heads & cam & to take pics of the job as you go-if doing yourself.Could help some of us

 

I've never seen a liquid dye block test kit for rent there. Probably since the dye is something that gets wasted with each use. I've only seen it for sale but not part of their free rental program. Maybe it's regional.

You should do both sides and not just one. It doesn't pay to half *** it. You probably can't buy just one head gasket anyway and it isn't that much more work to just unbolt the other head which would already be plainly accessible at that point.

With a cooling system pressure tester and plugs removed you can see which cylinders fill with coolant under pressure.

 

Absolutely do both heads. Also ounce you have them off, take them to a machine shop and have them pressure checked and resurfaced if they need it. Its cheap insurance

 

Thanks, Paul. I've been thinking seriously about doing that, and you're right, it is cheap insurance. I'd be foolish not to do it. I also think I should take the intake plenum to that same machine shop.

With that said, I've trying to resist that dreaded and ever-present boogeyman called project creep. Hey, as long as I'm going this far, I should ... <insert your own list of favorite tweaks, upgrades, modifications and "just in case" items>

have a good one.

 

I'm ready to button the beast back up, and here are the lessons to date. It's too freakin' hot to take pictures, but they wouldn't do much good anyway.

1. I had a problem removing the "coolant air bleed pipe" from the back of the head.

I'm glad I have small hands, because reaching the bolt is difficult. I managed to get a socket on it, but rounded it off trying to remove it. I did some research and discovered that the bolt is aluminum. Oops. I had used a twelve-point socket on it. Never use a twelve-point socket on an aluminum bolt - it's not strong enough. Unfortunately, all my six-point sockets are elsewhere right now, so I went to Sears and purchased another set. I cleaned up the bolt a little with a very small file, and successfully and easily removed it with the six-point socket.

2. The "coolant air bleed pipe" itself has been discontinued by GM but can be purchased elsewhere. However, the steel/rubber washers are still stocked by Chevy as they are used elsewhere on cars. The description for these washers is "Seal-Eng Cool Al" and the part number is 1018689. You'll need two per head.

3. It pays to have a terrific parts department - mine is at Sir Walter Chevrolet in Raleigh, NC. I've been using them for this car for three years and they have always been really helpful, going well above the call of duty. I think they've adopted this old codger because they like his toy and his passion for it.

4. I now have real respect for "southern" cars. I grew up in Wisconsin and lived much of my adult life in Connecticut. Based on past experience, I came to believe that removing the three bolts holding the exhaust system to the exhaust manifold was a thankless all-day job for a shade-tree mechanic, and removing the bolts holding the exhaust manifold on was another full day of frustrating work. It took me about ten minutes for each! There was no evidence that any of this had been done before, and the car is sweet 16.

5. When I finally removed the head, I discovered that the head gasket looked fine. However, at the back end of #7 cylinder, the gasket appeared not to have been sealing. Also, #7 piston was clean as a whistle, as was it's combustion chamber, unlike the other three cylinders that had the typical "aged" look. I took the head to a machine shop. They cleaned it, pressure-tested it and vacuum tested it. It passed with flying colors. It was, however, slightly warped. They only had to remove 0.004" to flatten it.

6. Since I'm giving credit where credit is due, I had a really great experience with the machine shop. I used T-Hoff in Raleigh, NC. They clearly know what they are doing. They wouldn't be my choice if I were "building" the engine, but for this job they were perfect.

Since everything else looked good, really good -- I had believed this engine was well cared for when I purchased the car and I know I have taken very good care of it -- I decided I would not do a top-end rebuild. In my estimation, the engine does not need it. I put on about 5,000 miles a year and it's mostly cruising with the top down.

I'm off in search of sealing compound and thread locker, then it's into the garage to start putting the beast back together so I can do some of that cruising! I'll let you know the results when I've finished.