Mildren Automotive


Mazda RX8 poor starting ability

Sadly, an increasingly common fault the RX8 suffers from is difficulty starting when hot and cold start flooding. Just to be clear this IS NOT a normal characteristic of the car. A healthy RX8, or any rotary engine for that matter, should start like any other car when hot or cold.

Two things may be the cause of hot start problems.
1. an engine suffering from low compression.
2. a slow starter motor

The only real way to determine which of these is affecting your car is by having a rotary compression test. It is important to have this test carried out by a rotary specialist, as the testing needs to be done with rotary engine specific compression testing equipment. Using conventional piston engine test equipment will produce inaccurate measurements.

Left; Genuine Mazda Rotary Compression Tester.


Right; Mazda Rx8 04′ manual recently in workshop for testing.


A rotary compression test will tell you quite a few things if you know what you are looking at. When you recieve your results you should have at least seven numbers – six  of these tell you the compression reading from each face of both rotors and the 7th number will tell you the exact speed that your starter motor is cranking. According to Mazda, your results should read between 6.9kgf and 8.5kgf at 250rpm.

Cranking speed and compression are directly related.  I’ve found you want more than 240rpm cranking speed. Just to be clear, in the same way that a slow starter motor won’t start a healthy engine, a very fast starter motor (i.e. 280 + rpm) will start an unhealthy engine and so can be used to buy time with an unhealthy engine, before the inevitable rebuild.

DPP_1683 DPP_1684

Which brings us on to fixes. If, after you’ve had a compression test carried out, you find that your starter motor is slow but your engine is healthy, simply fit a faster starter motor and away you go. Unfortunately however in 75% + of cases it is low compression that is the problem, not helped by a particularly slow starter motor on older Mazda Rx8’s. In this instance you have two options:

  1. You can either fit a fast starter motor to mask the main issue but this will simply be a temporary bandage fix.
  2. You can opt to have your engine stripped down and rebuilt with new parts giving it a new lease of life, before it breaks, possibly causing a higher rebuild cost.

If your Rx8 has starting issues, hot or cold, or you are concerned your engine is not performing correctly, and would like help finding the most cost effective solution that is right for you, contact Greg Mildren.

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