CRANKS FINE BUT WON'T RESTART AFTER SITTING 60 MINUTES OR OVERNIGHT

09/00 Workhorse P42
VIN #5B4KP42R813327341
Chevy 5.7L / 336,789 miles

I have a customer in Savannah, GA with a Workhorse stepvan fleet; I live in Charleston, SC and go there every couple weeks to maintain it. They have a stepvan (info above) that is kicking my butt and would really appreciate your insight. The original complaint two weeks ago was that it was turned off at a route location then would crank over good but wouldn't re-start after 30 minutes or so. It was towed to a local garage and they replaced the fuel pump; it was picked up the next day and driven back but then wouldn't re-start at Unifirst so I was asked to come check it.

I first scanned for diagnostic codes even though the check engine light wasn't displayed and there weren't any; I then checked for ignition spark at the plugs and had good fire. I installed a fuel pressure gauge and checked the pressure key-on at 32 psi at the engine when the service manual says it should be 55-60 psi; when I turned the key off the pressure dropped approximately 6 psi in 10 minutes. I then cranked the engine over and sprayed carb cleaner in the air intake, the engine started and ran and the pressure was 44-48 psi with the needle fluctuating. I inspected the replaced fuel pump and saw nothing unusual but replaced it with a OEM pump and the pressure was 54 key-on and the engine started and ran normally; I also replaced the fuel filter and it definitely had blow-through restriction, the original garage didn't replace it. I road tested the truck and it had very poor acceleration so I removed all the spark plugs since I had never replaced them and found all of the center electrodes worn out and #5 plug fuel-fouled. I performed a cylinder compression test and all cylinders were within 10 psi of 125 psi so I installed new plugs/wires/distributor cap and rotor; I also replaced the oil pressure switch because it was seeping oil. I then road-tested it and the performance was much better, basically what it should be.

By this time it was late and I had to get back to Charleston so I told the location the truck was good. They called me three days later and said the truck ran fine for two days but that morning had done the same thing on the route, cranking but not starting after 30-60 minutes at a customer location. I had originally ordered the fuel pressure regulator located under the intake manifold because the fuel pressure gauge needle was still rapidly fluctuating even with the new pump installed and I thought the regulator may be causing that. I went there this past Saturday with another vehicle tech friend and we rechecked everything I had previously done then checked for the continuing issue. We removed the manifold and replaced the regulator with a dealer/OEM one and checked everything with the manifold off, all looked good. We checked the key-on fuel pressure and it was now at 48 psi but the engine started and the gauge fluctuation was gone. I shut it off and we worked on other trucks for an hour or so then checked the fuel pressure; we had left the gauge connected and the pressure was down to about 42 psi. I attempted to start the engine; the pressure went up to 48 with key-on but had no-start; I found that after 4-5 start attempts and being patient that I could get the engine to start. We disconnected the steel fuel supply line at the engine and fuel filter then used compressed air to blow through the lines; found no restriction but some grayish, dirty fuel came out so I removed the filter and manually blew through it; it had very slight restriction but also the same dirty fuel in it. I then disconnected the line from the filter back to the pump, did the same thing and got the same result; no restriction but grayish fuel. I pulled the pump, inspected inside the tank and saw the fuel was clear all the way to the bottom of the tank with no visual indication of trash or other foreign substance. After reading some more I saw an indication that the MAF sensor may cause a similar condition so I checked the sensor wiring/OK and swapped the sensor with a known good sensor off the same make/model/year stepvan and then waited about thirty minutes before trying to restart, still got the same result with the same pump pressure readings. Regardless, every time it doesn't start I can prime with carb spray and the engine starts immediately and runs good.

I apologize for the long explanation but wanted you to know all the details. I'm assuming the problem is something obvious that I'm missing; that's why I took another experienced tech with me Saturday. I'm going down there again this Saturday and hope you'll be able to give me some positive leads to follow, thanks.