I'm having a problem here, I have a 90 F150, 5.0 with Mass Air Conversion, AOD, MSD 6A ignition box, shorty summit headers, Comp Cams 351W cam. The conversion was completed about 4 months ago and everything ran great. I now have what seems to be a fuel starvation problem that started in the rear tank. It happened on a hot day and it just acted like I ran the tank dry but it was 3/4 full. I switched to the front tank and it ran great. The next day no problems from rear tank. about 3 days later I was towing and it happened again, I switched to the front tank ran fine for about 5 miles and then the front tank started acting up. Again it was a hot day, load, uphill. It left me stranded and I had to leave it at a local shop and continue on to where I needed to go. So far they are stumped. I have new gas caps that are venting properly, new fuel filter. injectors have been cleaned, they say both fuel pumps test out at proper PSI specs. I pulled codes on my computer stating AC on during test(it wasn't), Canister Purge circut failure (but this has been on there since the conversion, and I contacted the conversion harness manufacturer, no idea) and neutral start circut failure. The shop said they pulled up other codes but I haven't recieved that info from them yet. I am getting ready to get it back from them and start tearing into it myself since they are getting no where. They have never seen a mass air conversion and think it is an injector incompatibility issue. Whatever. This conversion was done with the PMAS conversion harness, an A9P computer with the proper mass air sensor by the letter of the instructions and I drove it for about 4 months and from NC to CO. It ran great with no codes other that the CANP circut failure.
I think it is fuel delivery. Do you guys have an idea what this may be? How do I test for the bad tank switch?

 

Your last question shows that you're thinking just what I was thinking as I read this. The tank selector valve does sometimes fail and cause interesting problems. I've had them draw fuel from one tank and return to the other, or just fail to switch at all.

The valve has 6 ports: Supply and return for each tank, and the line to and return from the engine. The earlier ones were basically a sealed unit but later ones are servicable:

http://stangler.com/motorsports/arti...alve/index.htm

I don't know of a procedure for actually testing it, I think it's more of one of those remove and replace types of deals. Perhaps the link above will be of some help.

 

Thanks for that link, When I get it back I am going to look at the valve first. Do you know the failure rate on these valves?

 

No idea, but it's somewhat common from what I've heard.

 

I'm going to replace the valve today or tomorrow, depending on parts availibility. If it works I will post my symptoms and solution. If it doesn't I'll post the fix I do find.

 

Well my 90 does not use a fuel tank selector valve or or a fuel reservoir. It uses the Fuel Delivery Module (FDM) instead. The fdm does everything the old system does but in a self contained unit in the tank. So it looks like I have a bad FDM in the rear tank. Time to drop it a and replace it I guess. It seems I have a fuel return problem that is causing the FDM to basically choke out causing an overfilling problem. I'll do this and see what happens.

 

Sounds like fun...I hate dropping tanks!

Let us know how it works out.