General Engine Troubleshooting

Last updated: January 4, 1999
>(1)  What are the possible causes of the low compression in an 
>engine (please list a few) or it's just merely an indication of 

Bad Apex Seals cause low compression on a rotary engine. The only way to test is to actually use a compression tester. I don't have the specs on what good compression is supposed to be, as my shop manual is out in the garage, but a decent mechanic should be able to look this up in his manual.

The ECU can not cause low compression, period. Compression testing is simply a measure of how much the combustion chamber compresses the air/fuel mixture as the rotor rotates.

Your first mechanic mentioned that you would have to replace the engine to cure low compression. Not true. Rotaries can be rebuilt easily. It might just need new apex seals. Note: the rebuild might cost a lot, depending on what needs to be done (e.g.- the turbos may need to be replaced also, if they happen to be bad, so a whole new engine may end up being cheaper). Make sure the mechanic actually does a real compression check. No need to rebuild the engine if it is not bad.

If you do need to get the engine rebuilt, you may want to go with a rebuilt engine from one of the RX-7 tuners around, such as Mazdatrix, Pettit, PFS (Peter Farrell), or Hayes. They should all be able to ship you an already rebuilt engine (with or without turbos). They may or may not need your old engine as a core swap. See my site, and go to Upgrades --> Vendors & Parts --> Bay Area RX-7 Club Vendor List to locate these places.

>(2) What is a possible symptom of a low compression (difficulty 
>in cranking up the engine etc.)? What is the possible 
>causes of the engine stalled during deceleration?

Rough idle. Do a compression check.

Also, other things might look like low compression on the tester, but may be other problems. See my site, and go to How-to --> Engine (Other) --> Apex Seal Troubleshooting.

Possibly ground problems may cause the engine to die during deceleration. Could also be bad plug wires.

Check the vacuum lines that control the turbos. These are a source of problems on a lot of the cars. See my site, and go to How-to --> Turbos --> Vacuum Hose Replacement. There are some other pages under Turbos that may be of benefit, such as Boost Testing and Troubleshooting.

>(3) Does the "black smoke" at the exhaust indicate possible faulty 
>engine or faulty ECU or could be both? If it is unsure, which do you 
>think could have the higher possibility or should change first?

Black smoke is too rich of a mixture. Could be caused by a bad ECU, but also by a lot of other things. In a fuel injected engine, it could be a bad O2 sensor, bad/dirty injectors, etc. You said your mechanic replaced the plugs, which was a good guess, but he should have also done the plug wires too. They tend to go bad in a turbo rotary due to the heat. You may also want to check the air cleaner to see if it is dirty and blocking air from getting to the engine. The ignition coils also may be a good thing to check, but not as likely.

My guess would be a bad O2 sensor. See my site, and go to Upgrades --> Engine Control Electronics --> O2 Sensor Replacement for more info. It is unlikely that the entire ECU would fail.

>(4) I didn't actually notice the turbocharger giving out white smoke in 
>the past, however, is there any possibilty that a faulty turbo-charger 
>could give out white smokes (the second mecahnic mentioned that 
>he noticed the white smoke only after driving the car around for a 
>while)? What's the first indication of a failed or faulty turbo charger? 
>What could be the possible impact of a failed or faulty turbo-charger 
>on other parts of the engine? How often do we need to change the 

White smoke in the rotary is due to bad O-rings (similar to having a bad head gasket in a piston car, this will let water/coolant into the combustion chamber). Water in the combustion chamber turns to steam, and is white. See my site, and go to How-to --> Engine (Other) --> O-ring Failure Information.

You don't need to change the turbos unless they break (although if you overhaul the engine, it might npt be a bad idea to have them rebuilt.

I would get the black smoke problem fixed first, and see if the white smoke goes away. Maybe it is just changing to a lighter shade of black once the engine gets warmed up.

>(5) Is there any posibility that the car is "partially flooded" due to faulty ECU?

More like running really rich, but other causes than the ECU (see above).

>(6) What's the normal RPM level of a normal and acceptable 3rd gen. RX7 during idling?

Around 750 RPM, plus or minus.

>(7) Does RX7 uses any starter unit? Any possiblity of the starter giving problem 
>that causes the engine  having difficulty to start/crank up? I do notice that it's 
>easier to crank up the engine during hotter afternoon than colder early 
>morning. Anything to do with the spark plug etc.?

My guess on the starting problem would be due to a bad ground. This is a well-known problem on the 3rd gen RX-7. See my site, and go to How-to --> Electrical & Ignition System --> Electrical System Troubleshooting. There is a link to the 3,000 RPM Hesitation problem which describes the grounding problem.

Also, there is an excellant section there on Starting Problems written by David Lane, and it should help you figure this out.

In general, I would take the following approach to getting your car running right:

  1. Do a real compression test. If your mechanic can't or doesn't have the proper tools, take it to a Mazda dealer, who should have the equipment. If this tests out OK, go on. If it doesn't, you may want to have the engine rebuilt before proceding on. It may or may not be worth getting fixed, so you don't want to spend money on the rest of this if it is not worth fixing.
  2. Do the grounding fix.
  3. Follow the starting problem methodology described by David.
  4. Perform the boost test.
  5. Check/replace the vacuum lines that control the turbos.

From: Scot Kight Date: January 3, 1999

Normal idle is 750. If you have an idle of 1000-1200 and have a stock engine, there is an intake leak (probably) If the idle hunts around 700-800 you have an airpump problem.

Anything over that, and its more serious.

Inital idle while cold will start at 3k (car in neutral) or 2k (car in gear) then slowly work its way down to 750rpm.

To elaborate some on the cold idle Scot refers to, the car is designed to rev up to 3,000 RPM when started in neutral. This is supposed to warm up the pre-cat faster for emmissions reasons. Since revving an engine up to 3,000 RPM when cold and the oil may have drained off of everything is not the best idea in the world, Mazda built in a feature that will disable this when the car is in gear (and clutch depressed). Mine doesn't go up to 2K when the car is in gear except when it is cold out. It will rev up a little but almost immediately drop down to normal idle. --Steve

[ Mail me ] [ To RX-7 Files home page ] [ To my home page ] [ Copyright Notice ]