So I finished cleaning up several coolant leaks in the engine bay of the FD that I just bought. Previous owner suspected a blown coolant seal, and I bought the car knowing this. However, for < $20 I wanted to verify myself. Conditions of the car are as follows:

Car had been sitting for a few months when I bought it. Erratic compression front and back when I first started cranking the engine. After a little bit, even bounces all around front and back. Car fires up readily hot or cold, no white smoke, no sweet exhaust smell. No more coolant leaks that I can see/hear/smell, and the line from the AST (aftermarket) to reservoir is not kinked or pinched. Coolant that I could see was on the pretty diluted side. Oil is low but not tripping the buzzer. Oil pressure is also "low" but I hear bad things about the sender.

First thing I tried after fixing all the leaks was the controversial "champagne test." With the car stone cold, fill cap off, and a clear (full) bottle of water upside down in the neck, the car idled with no bubbles. However, once the thermostat opened, the bottle overflowed and I saw mass bubbles, and shut down quickly. I'm not 100% sure the system was burped at this point, but I was under the impression that even with the thermostat shut, I should see bubbles if there was a leak. Thoughts?

Next, I tried to see what conditions were causing the reservoir to overflow. All warming of the car was done by just idling. Started with an over full reservoir and full system (coolant would not return to the system previously b/c of the leaks). Warmed the car up until the fans kicked on, reservoir was overflowing obviously. Next I let the car cool back down. All (and I mean alllllll, bone dry all) of the coolant previously in the reservoir was successfully sucked back up by the system. Then I warmed the car back up until the fans kicked on. At this point the gauge was in the middle, and the reservoir was at the L mark on the stick, and the car was pretty much at a steady state as far as I could tell. As a final test, I raised the RPMs to ~2000 and held for ~15 sec. This caused the reservoir to fill fully and overflow. Sigh.

I figured I'd ask to see what you guys thought since FD's aren't my specialty. Any thoughts (or condolences, heh) are appreciated.

 

I'm was in the same place as you a while back, these things are definitely a bit tricky to diagnose. From what I can tell, the champagne test isn't the most reliable method of determining the problem (I tried it too). When I asked the same question, everyone recommended that I do a coolant system pressure test, and it without a doubt, it showed that I had a coolant leak somewhere. After pressurizing the system, you can check around for external leaks. In my case, I was able to pressurize the system and found no external leaks. After that, I pulled the ECU fuse and cranked the engine a few times then checked the plugs, which ended up being wet with coolant (the pressure test had pushed coolant into the combustion chamber).

I'd recommend a pressure test, it doesn't leave you guessing.

Justin

 

Pull the fuse and crank it like ^ said. But to me it sounds like it's blown. If you fill it up with coolant..and all the sudden your low. Prob means you've got a bad coolant seal.

 

Yea, that's what my feeling was. I've got a pressure tester & import adapter waiting for me to pick up after work. I'll give it a go and report some results.

As far as the actual test goes, I seem to recall there being a need to block off one of the AST -> reservoir hose? FSM doesn't say anything about it... Will just a bolt in the reservoir side of the line do it or is there more?

 

First pressure test the cap for the filler neck and make sure it has no leaks. If all is well, put the pressure tester on the AST, and either pinch off the overflow line, or do like I did and just pull the overflow hose and put a vacuum cap on there.

 

^^All good advice. One giveaway on the champagne test is that with the water pump housing cap off and you start the car the coolant bursts out as the car fires if there is a blown combustion side coolant seal. The coolant then continues to slowly overflow out of the neck as pressure builds in the system and combustion gasses enter (also causing bubbles).

As mentioned above the system pressure test really is a better way to go about testing for leaks. If the pressure drops off with no visible external coolant leaks then odds are you have a bad seal.

You should also be consuming coolant as well. BUT, with no white smoke I would really be searching for an external leak while the system is pressurized.

Good Luck.

 

Alright, picked up a pressure tester and "import adapter" and did some fiddling. I'm not sure if it's broken parts, broken tool (doubt it) or operator error, but this thing isn't sealing correctly.
The previous owner had installed an aftermarket aluminum AST. I believe it is this one from RP.



These were all the adapters in the kit


And both adapters had holes in them, I assume for testing the pressure release type caps.


Regardless, I hooked up the water pump housing cap (small one, no clip) to the end of the red adapter, plugged the hole with my thumb, and pressurized to 20psi. The cap held with no issues.

Then I hooked up the AST cap as follows

This was the only way the AST cap and pressure tester could be combined using the parts in the kit. I'm not sure if this is correct. For whatever reason, the cap would not hold 5psi for more than a second. It was like pumping it to open air. The leak sound was definitely coming from the AST cap side of the assembly, and almost sounded as if it was coming from the very center under the cap. If the cap and tester were working, shouldn't the cap hold pressure until .9bar and then let some bypass?

Additionally, I was unable to get the pressure tester to seal to the RP AST. It looks like the tester fitting is the same size as the cap (the silver adapter in the picture appears also to be symmetrical) so I figured they would be the same. I thought the air may have been just pushing air to the reservoir, but it was the same even with the hose kinked shut. The sound also indicated a leak between the top sealing surface of the AST and the tester. Agh

In the slim case that it is the AST cap failing, could it be the reason I'm seeing a ton of coolant overflow? Once the thermostat has opened and the coolant has expanded, could the pressue of the system be allowing far too much to bypass the bottom seal of the AST cap and end up overflowing the reservoir?

Thanks for the help

EDIT: For all of these tests, I made sure the rubber on the caps and tester were wet.

 

Fwiw I had a problem not too different than yourself, except for the compression tests. Kept overflowing and I would then overheat. Turned out to be a bad ast pressure cap. Got a new one at advance auto for 7 bucks and never had another problem.

 

Buy a hydrocarbon tester for $40ish from napa and call it a day. If the coolant is contaminated you know it's not line or a hose.


I did have trouble with coolant burning out at shutdown. But a new radiator and ast has fixed it~more likely the ast.

 

Updates:

Since the cap gave me funky readings last night, I decided to go pick up a new one first thing. Threw it on the tester, pumped up a few psi, and the new one held fine. Guess the old AST cap was bad for sure.

Decided to clean the AST sealing surface gunk using some 1000 grit paper, and put the new cap on. Fired the FD up, warmed it up and no coolant. YAY!!!! Revved to 3k and held, no coolant overflow, YAY!!!! Revved again and held, coolant overflow, NOOOOOOOOOOOOO!!!

So I'm going to see if I can get the AST to hold pressure from the tester now that it's cleaned up.

On a related note, this oil pressure sender BS is making me nervous. I topped off the oil with some 20w-50 to make sure the pump had enough to push through the system. On startup and while cold, the gauge would read ~30ish at idle and spike sharply to ~60ish once I revved and held past 3k. However, once hot, the gauge reads low (<30) or 0, and doesn't respond to RPM in the same manner it did while cold (much lower, slower, smaller spike when revved). I know RX-7 oil pressure issues are uncommon in general, but could a failing oil system put additional burden on the coolant system, resulting in overheated coolant and overfow?

 

Final updates:

I got the AST and pressure tester to finally seal correctly. After kinking the AST hose, I pumped the system up to 15psi. A leak around the coolant level sensor in the tstat housing was the only external leak I could see or hear. The system was very very slowly losing pressure and I guessed at very least it was through the sensor leak. After a 7hr soak, the gauge was at 0psi.

Removed both plugs, inspected, replaced them, and cranked with the EGI fuse pulled. BOTH plugs were wet with what smelled like coolant. Awesome.....

Thanks everyone for your help.

 

You could always try cleaning the electrical connection on the oil pressure sender and see if that fixes the problem. Or just Call Ray Crow at Malloy Mazda and pick a new one up. They are not that expensive...

Glad to know it was just your coolant cap causing issues.

 

if ur leaking through the sensor, then seal that up first. any leaks, and it isn't going to be at the pressure it needs to be which will cause overheating of the coolant. And go rent a liquid block tester from autozone. That will tell u if u have coolant in the combustion chamber.

 

Another trick that worked for me was after pressurizing with the pressure tester, to crank the engine without a fuse and two plugs pulled. Have a plastic dish stuffed in there below the plug holes. I caught green coolant.

David

 

If it's not smoking and it's starting easily you probably don't have a coolant problem.

A bad cap will cause the reservoir to over flow. I'd buy a new AST cap.

Even a mild coolant seal leak will cause smoke on start up combined with running on one rotor or ruff idle after the car has been sitting for a week or more. On a daily driving basis you may see a little initial smoke and the start is normal but once it sits you'll have the smoke, ruff idle or one rotor idle and or hard start.

If your car has been sitting for 2 weeks and starts up with no smoke you have no worries about coolant seals.

 

I replaced the AST cap and still had the reservoir overflow.

After performing the pressure test, when I pulled the plugs (front AND rear leadings) they were soaked enough that I could shake the liquid out of them and watch/hear it hit the ground. Is there any other way for coolant to get in there?

I'll try the dish and see what comes out.

 

Hook up the pressure tester to your AST. Pump it up to 17psi and let it sit for a few hours (I think overnight is even better).
Start the car... if it stumbles and sounds like it's running on one rotor, you've got a blown coolant seal. You'll probably also see white smoke coming out of your tailpipe.