'94, 89 K miles, stock engine, original owner here. Never a problem previously, but just recently pulling the coolant reservoir dipstick is showing greasy dirt on the stick above the coolant level. Level is on the "F" and dirt begins about 1/2 inch above that. The coolant itself is clean.

The manual shows a hose from the AST to the top of the coolant reservoir. The AST seems to be clean, though. Any idea where this crud is originating?

 

Its common. Just make sure to do flushes at regular intervals.

 

Mahjik,

Thanks, but how do I flush the air? Is that the AST connection bringing in the junk? Or something else?

 

https://www.rx7club.com/3rd-generation-specific-1993-2002-16/normal-see-dirt-particles-stick-67949/

 

Thanks again, Mahjik. I don't think that link answers my question, though, since my coolant is not "scummy" or visually dirty. I am planning on flushing the system just on general principles, but I really think that dirt above the coolant level is coming from somewhere other than the coolant itself. I'll report back after I find the culprit.

 

Its most likely also coming from the vent hole in the neck of the overfill tank. Either way, its a common thing on these cars.

 

Ah, that makes sense. Maybe I will try & design a standoff "sleeve" to shield the vent hole against dirt. Or something... haven't seen it up close yet.

 

^ Just flush the overflow reservoir; I guarantee it's never been done.

Remove the hose from the AST neck that goes to the overflow tank. Stuff a garden hose in the overflow tank and run generous amounts of water through it, being certain to let water gush out everywhere Leave the AST hose disconnected until all the water drains from the tank and then connect the hose and top the tank back up.

 

DamonB,

You are right... the coolant has been drained and refilled per Mazda's recommendations, but neither the system nor the reservoir has ever been flushed.

I understand, and I like the idea, but how is the tap water going to drain out of the reservoir after being flushed?

 

The hose from the overflow to the AST is connected at the bottom of the overflow. If you disconnect this hose (and point it toward the ground) the overflow can drain its contents through it.

Put the garden hose through the overflow's fill cap and the tank can empty through the AST hose.

 

DamonB,

The illustration in the Cooling section of my 1994 workshop manual (page E-2) shows the hose from the AST connected to the top of the coolant reservoir, just at the bottom of the filler neck. Of course, it could be drawn incorrectly... are you sure about that?

 

Regarding flushing the rotary, which I have not done before...

It looks as if external (hose) water pressure is necessary as opposed to using the engine's water pump, since so much stuff has to be removed to pull the thermostat. Would you experts recommend reverse flushing, i.e. water into the thermostat housing and out the top of radiator, or vice versa?

 

After considerable searching for the right coolant, I settled on Prestone Extended Life Antifreeze/Coolant. This is a comparatively new product, colored yellow (neither the green nor orange types). Prestone says on the label that it has the 5 year/150K mile life of the GM "Dex-Cool" but is compatible with ALL types of Antifreeze.

I pulled the RF fender liner and removed the coolant reservoir. The rubber collar over the join between the filler tube and the main reservoir was broken, so that seal was gone. The vent grommet in the upper surface of the reservoir, and all the area around it, was covered with sludgy dirt. I think that might have been caused by coolant splashing out the vent hole and lying on the top surface of the tank, collecting dirt over time. The interior of the tank had fairly clean coolant in it, but there was a lot of precipitated "gunk" on the bottom of the tank. The long tube on the AST-connected hose was fairly dirty on the outside but clean on the inside. I made a replacement for the broken collar using an old crankcase breather rubber tube.

I found that draining the radiator collected only 4.0 quarts of coolant. Since the system capacity is 9.3 quarts, that turned my "flush" into a frustratingly long process. I had a can of Prestone "Super Flush" and 6 gallons of distilled water. The first refill was the flush stuff and distilled water to top off. Ran the engine, drained (4 quarts of) that, then repeated refill/run/drain with just distilled water three more times. After that, last fill was 3.25 quarts of the Antifreeze and .75 quarts H2O. Altogether I used 5 gallons of water, and I figured that amounted to the cooling system having been 89% flushed, with 11% of the original mix still in there. Since the coolant reservoir was now clean, over 90% total should be "new" mix. If I did it right, mix should be close to 35% ethylene glycol and 65% distilled water.

I had read in a post here that there is a coolant drain plug on the driver's side of the "block," but I could not find such a plug. Nor does my 1994 factory workshop manual show one in the illustrations of the engine. If anyone knows how to find it, I'd appreciate a clue!

 

Look for a bolt on the bottom drivers side center iron.

 

Mahjik,

Thanks. Would that be on the iron piece between the spark plugs? I looked there... saw nothing. Is it really low, like on the actual bottom of the engine, or is it still on the "side?"

 

It's on the side but low. Immediately above the oil pan.

 

Thanks, DamonB. I guess it is behind cable harnesses or something... every time I've been under there to change plugs I've looked for it... haven't seen it yet. Guess I have to move something or other.

 

Here it is.

 

I'll see if I can snap a picture from my engine this evening for you.

 

I think it's possibly combustion gasses contaminating the coolant, AKA, a failing coolant seal.

 

I'm an original owner too and have noticed such a black residue for years. (although I don't think mine is as bad nor as thick as what you describe.) It's just little grains of rubber coming from the inside of the hoses. It's normal and nothing to worry about unless you feel your hoses are getting weak.

 

.

 

DamonB and Mahjik,

Thank you both for the pics! I swear I have looked at that exact location and have not seen that plug; I will have to get under there AGAIN, I guess. Sorry to be a bother!