mkingham
mkingham New Reader
6/8/22 6:54 p.m.

You may have seen some posts from me, working on a miata that has since become my sisters. My dad, pkingham, has been posting on this forum forever. Since my sister claimed the miata I bought a 2005 Mazda RX-8 Shinka with the manual transmission. It will not start hot or even warm, and seems to be significantly down on power. I bought it with bad water seals, and I rebuilt the engine, and since then it will reliably cold start, and run. If you start it and turn it off within 30 seconds, it will restart. It was unable to idle until it broke in some, and now it idles at about 1000 RPM. 

 

Troubleshooting:

I tested compression and it is reasonable on all 6 flats. I have replaced the ignition coils, plug wires, and spark plugs. I pulled and tested the fuel pump which seems to work at a variety of voltages, and did not change throughout a 20 min test. I cleaned the MAF. I upgraded the starter to a better starter from the gen 2 cars. 

 

I’m looking for any ideas for what else to look at or any resources that can help me figure this out.

Pete. (l33t FS)
Pete. (l33t FS) MegaDork
6/8/22 6:57 p.m.

what IS the compression?  Hot and cold.

What clearances did you set the side and apex seals to?  Usually an inability to run well on a fresh engine means the apex seals were too long.

 

Did you have the side housings resurfaced or at least verified that they are not warped near the exhaust ports?  Rotor housings, are they all warped or is the chrome flat?

mkingham
mkingham New Reader
6/8/22 7:15 p.m.

In reply to Pete. (l33t FS) :

I don't think that I tested compression cold, but hot it was between 100 and 120 depending on the flat, using this compression tester. I don't remember what number I gapped the side seals to but none of them were too long, and most of them were a few thousandths to short. I checked the side housings, and they were within spec. I had the rotor housing professionally resurfaced, so presumably the chrome is flat although I did not validate. 

Pete. (l33t FS)
Pete. (l33t FS) MegaDork
6/8/22 7:19 p.m.

Hmm.

Not having any direct MSP experience, but the difference between .002" side seal end clearance and .008" is extremely noticeable to me.  Poor low end and midrange power.

 

Compression seems good but I would want to check it hot and cold to see if there is a discrepancy.  And the coils are always suspect smiley New doesn't mean good.

mkingham
mkingham New Reader
6/8/22 7:21 p.m.

In reply to Pete. (l33t FS) :

I don't thing they are that far off , would that noticeably affect hot start performance?

Pete. (l33t FS)
Pete. (l33t FS) MegaDork
6/8/22 7:23 p.m.

In reply to mkingham :

It shouldn't, but RX-8s seem to be really sensitive to compression compared to peripheral exhaust engines.

toconn
toconn New Reader
6/8/22 11:20 p.m.

I had this problem rebuilding my RX-8 engine. The side seals were too long. The car started fine but would start to struggle as the temps increased. It turns out the side seals expand under heat & load and put pressure on the corner seals which then squeezedthe apex seals. When the apex seals push down on their stamped spring, they dont spring back because the corner springs are pinching them and that's where the compression loss happens. Could be that you're not seeing it on a compression test because you dont have combustion forces slamming the rotor around as hard.

fidelity101
fidelity101 UberDork
6/9/22 11:23 a.m.

compression test then I would go down ignition route if compression is okay. 

Our Preferred Partners
Pe2SXhwQ9XFu9gID5NygNnDJ8667nfDosDKhOCsZealzXdAu6I8VkMdqe7QOSOtU