I have been having hot start issues for the past few months. It behaves like some of the videos I have seen here that show symptoms of low compression. It is an '04 MT with 75k+ miles. I don't baby it and routinely hit 5k RPM during normal driving and 8K occasionally. I understand they should be run hard.

I am thinking of paying for a compression check and know the procedure has to be done for rotary engines and there should be three numbers for each rotor.

There is only two plugs for each rotor, so why are there three numbers for each?

Is there a specific name for the compression test on rotary engines, or do you just have to quiz the shop to see if they know what they are talking about?

So, if the numbers come back bad, I know the first suspect is the apex seals.

I also read here that it could be the coolant seals. I do have a milky film on the dipstick, but only during the winter. In the summer it cooks the moisture out. I have read this milky condition is a known issue so I never though much of it. I checked my coolant level yesterday and did add about 6 oz. I still need to check for bubbles in the tank the way another member had while it is running.

Where are these coolant seals and does replacement require a full rebuild?

How slippery is this slope if I have bad apex seals? Six months? 1 year before failure?

Can a partial rebuild be done?

I know prices will vary by region( I am in NW Florida), but about how much for a engine rebuilt if I can find a competent shop?

Thanks for any help.

 

There are two plug holes per rotor, but each rotor face sweeps across each plug hole, so each plug hole will still be measuring the same 3 faces.

It is exceedingly rare that a shop has a rotary compression tester if they aren't a Mazda dealership. The exceptions are if it is a rotary specific shop, which tend to have a tester on hand.


There are many failure methods, and a compression test will only tell you if compression failure is present. A compression test will not tell you if a coolant seal has failed. Compression failure could be side, corner, or apex seals though, not just apex seals.

Milky substance on dipstick is entirely normal across the board, still no reason to concern yourself with it. If you want to validate that it isn't a coolant seal failure, you can pull an oil sample and have it sent off to Blackstone Labs for testing. They can tell the difference between water from condensation and coolant.


Any parts that need replacing inside of a rotary engine always require a full teardown of the engine, so a full rebuild. Reusing any existing seals is generally a bad idea.

How much longer you have depends on how bad the compression is. If you are high 6s, maybe 10-20k left. If you are in the 5s, you probably don't have more than a couple thousand. If you are in the 4s, any day could be the last for the engine.

No, a partial rebuild can't be done, in that you always need to replace all the seals. However you can sometimes re-use other components such as rotors, irons, and the e-shaft. It's rare that you can re-use the housings.

Your three closest options are Rotary Resurrection in TN, the best low cost rebuilds available. Pettit farther south in FL, which do really high quality rebuilds/complete-builds, but you pay for it (think $5k+), or Mazmart near Atlanta. Mazmart doesn't do rebuilds any more, however for $3,500 + your old engine, you can have one of their reman engines. It's a known price ahead of time rather than a flexible rebuild price, and generally competitive to rebuild costs. The price is pretty stable and they have good customer service, so you won't get shafted by a bad engine like you might if you bought a reman through the dealer. A bad engine is still possible through them, but you won't be left high and dry if that were to happen. Mazmart takes care of their customers.

 

Well it is due for an oil change, so I will send off a sample. I will try to find a shop to do a compression check and maybe even take it to the dealer to get it done if I can not find one that has done them before.

So on a compression check I should have four sets of three readings? Total of 12 numbers?

I don't know if it matters, but it does not leak any oil at all.

Thanks a lot for the in depth reply.

 

You should have 2 sets of 3 numbers. The test is only done with 1 spark plug hole per rotor. Using the other plug hole too doesn't do anything else meaningful, since it's still the same rotor faces it's seeing.

Plus at least 1, possible 2 RPM readings. Some dealers report the RPM for each rotor, which usually just means that the 2nd rotor that they test will have an RPM that is 1-3rpm slower than the first rotor that they tested, due to battery drain from the first test. 1 RPM number is perfectly fine, since the two RPMs will always be very close together.

See this post in this thread that has attached pics of 3 of my compression tests and their scores, so you can see how you might get them from a dealer. https://www.rx8club.com/series-i-tec...7/#post4408673

 

Mazda designed the engine with 2 spark plugs per rotor (and as many as 3 in some race motors) because of the length and shape of the combustion chamber.

Hot start issues are almost always a sign of low compression. Ypu really need to do a compression test with a rotary-specific tester. Any reputable shop that specializes in rotaries should have one. I use the Mazda tester. Its hard to tell how much time the motor has but the problem will get progressively worse. A proper rebuild replaces all the engine seals and coolant seals.

The milky residue is caused by moisture in the oil. Mazda released an updated oil filler neck that tends to reduce the problem.

 

Thanks for the help.