Fuel Pump and Battery Voltage Drop on Warm Restarts
 

I swapped the Ford 2.3T in my 91 Miata, so the installation should be about the same as the typical 5.0 swap. I have been chasing a problem where the voltage to the fuel pump drops after a warm restart causing high AFRs. I then decided to use the fuel pump wire on my universal microsquirt 2.3T harness as a trigger for a relay so I get battery power to the fuel pump, but the same thing happens; good voltage on a cold start and throughout driving, but the voltage drops once I start it up again after not running for several minutes. I have ruled battery and the alternator out and I am now at a loss. Is it possible that my battery cables are to thin to the starter? I just can not seem to make any other connection here to the correlation with warm temperature and a voltage drop. I did upgrade the battery cables to 4GA and ran the negative battery cable directly to the starter bolt, but this did not solve the issue. I have also upgraded to a 130 amp altinator. Does anyone have any experience with this problem and has corrected it?

How much voltage drop?
How long does the "high AFR" reading last?
Do you let the O2 sensor warm up before the warm start? The sensor reading weird may be due to sensor temperature.
I'd want amperage measurements of the fuel pump as well.
Does the fuel pump draw more amps during the warm start scenario?
If so, bad fuel pump. This would be an abnormal fuel pump death. Normally when they are dying, they will pull big amps when hot and the pump fails. You let it cool down a couple of hours and the pump starts working again...

Your application of a completely different power supply eliminated most of the common issues for voltage drop.
During starter engagement the entire system is operating at a serious voltage drop (1+V total drop) but immediately after the starter stops turning the alternator will increase system voltage 1+ volts.
So you have 11.2 during starter engagement but 13.5+ after it's running on a system that will show 12.5-12.6 (fully charged battery) at rest.

Your post makes it sound like this issue lasts longer than 5 seconds.
How long does this issue last?
Besides the AFR gauge readings does it run differently?
Do you have any other indications you actually have an issue besides the AFR gauge?

One final thought, the fuel pump is cooled by the fuel in the tank.
For test purposes I'd want at least 1/2 tank of gas so the pump was always submerged.

First of all, thank you for your well-thought out and informative reply! This issue has been prevalent since I first started the car about a year ago. The voltage drops anywhere between ~.30 V to ~1.5 V depending on long the car runs before I restart it. Yes, I do let the O2 warm up before starting the car. I have not tested amp draw at the fuel pump. The voltage drop issue lasts much longer than five seconds, sometimes throughout the entire duration of driving the car, again depending on how long I drive the car before I restart it after it is at operating temperature. The main indication that something is wrong other than the AFR gauge is that it idle hunts and drives drives poorly after a warm restart. I am pretty sure I have had a full tank while this problem was persisting. The thought of a bad fuel pump escaped me because it is a brand new Walbro 255, but I should know that new does not mean it could be defective. The fact that I switched over to E85 had something to do with destroying the fuel pump is not the case because I had the voltage drop when the car ran on premium fuel as well. I will test the amp draw at the fuel pump with a full tank of fuel and report back either Friday or Saturday. Thank you for your insight.

I checked amperage draw at the fuel pump on cold start and after the voltage dropped after a warm restart; the amperage is consistently 5.7 amps.

How was it that you eliminated an alternator issue? Low voltage after the engine is running sounds like an alternator. Be aware that a higher amperage alternator frequently requires a higher RPM to regulate.

Is the alternator internally regulated, or regulated externally by the ECU?

If you don't need a 130A alternator, it probably isn't a good change.

I have personally experienced high output alternator issues due to heat soak.