The cylinder #4 spark plug has been looking pretty fouled on the Miata.

Cylinder #4 looking fouled isn’t exactly a new problem, but I wanted to run some tests to see if I could understand what might be the cause; here’s the results.

Compression Test

I just got a new compression tester because the hose on my old kit started to leak. Engine was warm and I cranked each cylinder 8 times before reading the peak pressure (like I always do).

Leakdown Test

Leakdown between 10-20% is normal. Anything over 20% is usually cause for concern.

This video shows the majority of the leakage is into the crankcase, indicating that the cylinder rings are what’s worn. This isn’t surprising for a motor thats got 230,000 miles on it, the last 30,000 of which haven’t been easy miles :)

Conclusion

Based on these results, the fouling doesn’t appear to be due to compression-related reasons. I guess this means it’s either something with the ignition or the fuel system on cylinder 4. I’ve seen the fouling occur before and after I swapped to coil-on-plug ignition. It’s also been a problem that spans between spark plugs changes, so I think that’s enough to rule out the ignition system. I think the next steps are to finish connecting the 4th EGT temperature sensor so I can see how well that cyclinder is burning relative to the rest. I might also run an injector flow test to see if that cylinder is getting more fuel than the rest.

Update (4/23/2022): I ran the injector flow test. Results show that cylinder #4 and #1 spray similarly to each other, and are less that #2 and #3. I don’t think it’s a fuel problem.

Update (5/30/2022): I finished installing the 4th EGT sensor. Results show that cylinder #4 is running similarly to the others. Still not sure why the spark plug on #4 looks so fouled….

-Dan