Lean under boost
 

I have a 1.8l engine we just converted to MS. I don't think this is a MS issue but..... With car at operating temps, AFR are great under 14psi. Once the car is driven for 30-45 minutes, it wants to get real lean under boost. Has a new Walbro 255. The lean issue went away when I filled the tank with gas. It was low. However, with the tank level just under full, the lean issue returned. I thought it was the pump getting hot. I replaced the pump with a used good pump from an RX7. Still same problem, after a while the lean under boost comes back. I am trouble shooting but l;ooking for some hints from those familiar with Miata's. My thoughts
1) put fuel pump on it's own relay with power directly from battery
2) Alternator problem - low voltage output at high rpm and hot
I don't want to go with an inline fuel pump although the car had an auxiliary fuel pump on old Link setup for some reason?????

Thanks for any help.

Bobby

adjust MAT correction

Anything you would offer as a reference? Change the correction to 50%?

Does this explain why the problem went away when I filled the car with "cold" gas?

set the whole curve to 100%, tune the car, then adjust for colder/hotter as needed.
that's assuming you have IAT heatsoak.
we've discussed injector heat soak before too, usually also battled with MAT correction table as well as ASE since it usually occurs most on hot-restarts.
you gotta post logs demonstrating all of this behavior though, otherwise we're just guessing/assuming

Awesome. Thanks for the response. I appreciate your time.I will get some logs this evening.

I am still having the lean issues. 2 times now I have had the lean issues and put gas in the car and the lean issue goes away. I can also park the hot car and once it cools the lean issue goes away. The logs 16.36 and 16.37 are with the lean issue. I put gas in the car and the issue went away. See logs 16.48.

Bad fuel pump?

It still could be MAT correction.

Don't forget to shield the bottom of the fuel tank from the hot exhaust.

I replaced the fuel pump. I also pinched off the fuel return line while having a lean under boost event to see if it was low fuel pressure. I just wired the fuel pump through a relay with power directly from the battery on a 12 gauge wire. Your "shield the bottom of the fuel tank from the hot exhaust" comment is interesting. I do not believe any shielding has been done. What symptoms do you see with an unshielded fuel tank? When I put fuel in the tank (should be cool fuel from the gas station since it is stored underground), my lean issues go away.

The logs really don't indicate much difference in MAT.

But tell me more about shielding the fuel tank.

Thanks,
Bobby

Exhaust pipe runs right under the fuel tank. On the stock exhaust there is shielding between the fuel tank and the exhaust pipe. If you have redone the exhaust you should add some shielding.

Thanks