Engine loosing coolant, but not showing signs of blown coolant seals? Read this.
 

So after doing research, and initially being under the impression my 12a had blown coolant seals I was grateful to find out this was likely not the case. The car did not demonstrate any of the usual signs for blown coolant seals. Either way my car was still loosing coolant rapidly. The coolant was puddling up where the intake meets the block as shown below.
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Turns out, Mazda routed coolant through the intake manifold. On a 32 year old car those O-Rings are bound to be on their last legs, right...?
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They're practically gone! The O-Rings blew, taking the intake manifold gasket with it. I purchased two new O-Rings and the Intake Manifold Gasket from Racing Beat for $25 shipped! I'll post back in a few days once I finish putting the intake back together. If this doesn't solve my coolant issue I don't know what will.

I originally found this fix on another RX-7 forum (click here) but I decided to post it here on RX-7 Club for redundancy, and to have pictorial evidence of the problem available to all.

Nice to remind those who get in a panic when coolant starts to disappear. Thanks for the heads-up!

Stu Aull
80GS
alaska

I'm trying to figure out what the dark gray sludge is in there on top of the o rings....

It's a mixture of coolant and dirt. Oh and dead gasket paper.

Yeah, needless to say I'll be doing a full coolant flush after this job!

You should pop some 20mm frost plugs in the while you have the manifold off, prevent the problem from ever coming back....

Note that the frost plugs don't always seal completely, at least mine didn't. I just redid this
as my frost plugs leaked and I had no orings in place (assumed the plugs would not leak).

On mine the plugs leaked slightly allowing some coolant to collect in the intake manifold. This
was found after removing the intake. I ended up with a similar leak because I never put in the
orings thinking the frost plugs would seal it off for good. My bad.

I put new frost plugs in, new orings and a new gasket and now no more leaks. It may bleed
some fluid past the frost plugs into the intake, but that doesn't bother me much. Once it fills
up its not circulating much so it won't effect intake temps that much.

Should also be noted that the intake/block junction point is where any liquids will collect over time; spilled gas from floods, carb cleaner, engine cleaners, road spray/salty water, you name it.

All sits on top of that gasket forever, unless it's intentionally cleaned out, brewed by engine heat over years, difficult to reach with all the stock gear in place... Pretty nasty mix of gunk. Who knows what it does to the gasket and o-rings over time?

Hmm, I hadn't though of putting frost plugs in there. How does it affect performance? I've heard throttle response is better.

It keeps coolant in the engine when pull the intake manifold. But then the ACV port still allows heated exhaust pulses in with no coolant to keep things cool, leading to a hot manifold. No one ever mentions this except for me.

I like to use quicksteel in the ACV port. Now the only way the manifold gets hot is from the headers. A heat shield helps. Or wrapped headers.

Then the venturi effect, while small, cools the manifold down enough to make more power. But you gotta take the above steps if you want a cool manifold.

Do you have any pictures of this Jeff? I'm very interested in this now. I'm doing a TII now so I need to run coolant through my intake manifold to cool the turbo, but this seems like an awesome trick for an N/A engine build.

Thanks for the reminder man, good to show people some good maintenance techniques!

You should probably fill in your ACV ports, then, to get a cooler manifold as well, on your T2. What do you think? I filled them on a 20B and a couple of 6 ports.

Anyway, enjoy.
https://www.rx7club.com/attachments/...browncar14.jpg

Yep, I did this as well this last time too!. Forgot to mention it.

All I have to do is fill that square port in the center iron with Quicksteel and I'm good to go? That would be awesome since I have no use for that system anymore! This one's getting a full emissions removal as well so none of that stuff is useful to me.

Yep, just fill it! But it contains an oily carbon mixture that nothing will stick to so I like to clean it down to bare metal as much as I can with a dremel stone. Then vacuum out what I can. It pulls air up through the exhaust port sleeves so it cleans up pretty well (metal, carbon and grinding stone particles). Then the quicksteel can go in.

I use 1/4 to 1/3 the length of the quicksteel slug as it come out of the plastic container. Cut it through with a razor blade and peel off the shrink wrap. Now smash it in your hands until the two colors mix and it feels warm. Then push it into the hole. I like to use a razor blade to spread or push it out evenly into the corners. Then lick a finger (ew gross) and smooth the surface down so it looks something like the picture I posted.

There is a little technique but you'll get it if you've ever used anything similar before.