Hi, I’ve been chasing this issue for a few weeks now and cannot for the life of me figure out what the problem is. I have a boosted 1.8 swap in my 90 Miata. MS ecu setup. I have this issue where my car runs great, will get into boost just fine for about 10 minutes of driving then it will slowly go lean for a while and then AFR plummets into the 20-22 range under no boost. Weird part is when you turn the key off for about 20 seconds and turn it back on the car runs great for another few minutes then back to real lean. Checked the FPR and it does not hold proper fuel pressure it’ll start at 45 range with vaccum then slowly drops to the 30 range and under any kind of throttle it drops to 0-20 range. I’m not real good on explaining things but my boyfriend has the exact same set up in his car and everything works perfect. Bypassed the fuel filter, same issue. New fuel pump with new sock, new o rings, new upgraded injectors, new braided fuel lines up front. Tried a different brand FPR same issue. Checked voltage at the pump and it’s fine even when car spikes way lean and bogs out. Studs in fuel sender unit to bypass stock sending unit connector. Fuel relay is routed through new fuse box we added for the MS. Practically everything engine related running through the MS. I’m at a loss

 

You should post your megasquirt tune along with a datalog of the issue occurring.

 

Is the tank clean inside? I had a bike that would do the exact same thing you describe. It ended up having rust in the tank, after a few minutes of riding it'd pick up enough rust to clog the filter sock and lean the bike out to the point it wouldn't run. Turn it off, all the rust falls down to the bottom of the tank and it's good to ride again for a few minutes.

 

Tank doesn’t seem too bad inside, put one fuel pump in and ran it for a day or two and the sock is still clean. Switched out pumps because I thought the first one might’ve been a knock off since I got it free of marketplace but even brand new fuel pump didn’t change anything. Last option is the rubber hoses, thinking they maybe collapsing in on themselves but I don’t have high hopes

 

I'm still waiting on the tune/log.

 

I think we finally figured it out this time. The tank didn’t look too bad but we tried every other option besides an oem sock. Cleaned the sock out and drove it, sock was gross as hell after 10 minute drive. Ended up sticking hand down in the tank and there was just piles of rust in the bottom clogging the sock. Drained all the gas of. Spent about 3 hrs wire wheeling and shop vacuuming the inside of the tank. Cleaned the sock and put brand new gas in and it seems to have completly fixed the fuel starvation. For now we will see

 

I'm pretty sure the rust will come back at some point. I'd recommend planning to pull the tank for a proper fix. I used a POR15 fuel tank repair kit on my 82 RX7 fuel tank to great success but it was a lot of work. The POR15 kit is about $120 USD. I don't know how much a new tank is, but I know miata parts are still a lot easier to find than FB rx7 parts. If available, I'd rather buy a new tank even if it was 3x the cost of the POR15 repair kit.

 

Yep, it solved my problem for about a half hr of driving then immediately clogged the sock up. If you swerve really hard it unclogs for a little then will clog right back up. New gas tank on Wednesday hopefully that’ll be the end of it