I have a 92 truck with 22RE on a 82 straight axle frame so I am using an external fuel pump. Usually runs fine, but occasionally dies as if running out of fuel and won't restart but a day or so later it starts and runs fine after sputtering a bit at first. I suspect the circuit which runs through the AFM may be the issue and wonder how best to test this? Would tapping into the wiring and adding a switch to bypass the AFM be the logical test, or is there a better way?

 

I'm not an expert on the AFM's but my understanding is the fuel pump won't run without receiving a signal from the AFM. AFM is crucial to the EFI system as it tells it how much air the engine is consuming as well as controlling a bit of the air fuel ratio. You might need to pop the cap off of it and adjust it because it may have come out of adjustment and may be causing it to run too lean/rich

 

Thanks for your reply. The AFM has two functions, one of which is to close the circuit to allow the fuel pump to run only when some air is flowing. This acts as a safety to shut off the pump when the engine stops in a wreck (many vehicles use a oil pressure switch to do this). I am not aware that you can just "pop the cap off to adjust it". Mine seems to be a sealed unit so I would like to know how to open it up.

 

Do you have a carbureted engine or EFI? The '92 EFI needs a fuel pump in the tank. As rustfarmer points out, part of the function of the VAF is keep the pump on when you release the key, as long as the engine is sucking air. It's the easiest thing on your whole truck to test. Here's (pretty much) everything you'd want to know about the VAF-COR circuit: https://www.yotatech.com/forums/f116...-quits-312747/

VAF come out of adjustment? If you had to ever adjust it, why did Toyota seal it up tighter than Tutankhamun's tomb? Your O2 sensor is the final arbiter of mixture, so if the VAF was "out of adjustment" the ECM would detect the mis-match and compensate for it by means of "Learned Fuel Trim." Yeah, you can hack open the VAF, and there is a factory adjustment in there, but all you're doing is fighting with the ECM's Learned Fuel Trim. You could adjust it so far that the ECM can no longer compensate (and get a too-rich or too-lean Code), but you really can't improve matters that route.

DON'T "add a switch." It doesn't help, and you might be tempted to leave it there rather than fix the problem. As rustfarmer pointed out, the VAF-COR circuit is a safety mechanism, so I don't think you want to bypass it.