Wrong plugs + carbon = no compression???
 

I recently bought a 90 GXL. When I brought it home it ran REALLY rough, wouldn't idle unless you kept giving it gas. The next day I found a loose vac hose and when I re-attached it, it idled great(at times rich), a bit rough under 1k but it didn't shut down. I dropped it off at the Mazda dealer to be checked out. They did a diagnostic and found the front rotor was fine but got no compression reading from the back rotor(used a digital comp tester). Someone one the forum suggested it might be due to carbon buildup..... Well while cleaing out and detailing my car(what else can I do with a near perfect interior and a low power engine....) I found a set a spark plugs in the glove compartment. The boxes read that there were 2 7's and 2 9's. However in the boxes were the plugs that were replaced. They were all 9EQ's. I think whoever did the 2nd to last plug replacement installed the wrong plugs. Could this lead to the carbon buildup and hence the bad compression reading?? Thanks!

Nope wouldn't change a thing. How many miles are on the engine.. might start looking foward to a rebuild.

^ditto

Yeah, if a dealer did a proper compression test and got a bad or zero reading on one rotor, there's absolutely no way around that, it needs a rebuild or replacement.

Running all 9's won't cause an engine to blow, or build excess carbon. The numbers represent heat range, and some of the more modded turbo guys run all 9's for better resistance to detonation (so goes the theory, anyway). Running all 9s on a nonturbo would have no real effect, good or bad. I'd say stick with stockers, though.

It's got 63k miles. I know, it's not much at all. Everything else about the car is great, just no power....

Thanks for the advice and the plug info Kevin.