Good Troubleshooting Puzzle
 

Hello friends,

The past couple weeks I've been working on getting my '92 GMC Vandura 2500 running. I parked it out behind my house a little over a year ago and hadn't driven it in that time. It was running fine prior to parking it. I put a fresh battery in it and it fired right up...and ran for twenty seconds and then died. I started it up again and it ran again for only twenty seconds. I was able to drive it twenty seconds at a time over to my shop to start troubleshooting. Here's what I've done so far:

•Checked fuel pressure. It's a 350 TBI and pressure is 13psi. The injectors are firing just fine. As a cautionary measure I rebuilt the TBI and replaced the fuel filter.

•Checked vacuum. The engine produces 22" vacuum at idle and doesn't lose any vacuum when it dies. I also checked every vacuum line coming off the manifold and every one of them was in good condition and not leaking. I also checked the brake booster vacuum line and it held vacuum just fine.

•Checked spark. I put a timing light on the #1 spark plug wire and it fired consistently until the engine died. I also tested the pickup coil on the distributor and got a couple readings out-of-spec so I replaced the distributor with a new one that included a new pickup coil and ICM. The new pickup coil on the distributor tested within spec.

•Checked EGR. I took the EGR valve off and tested it to see if it held vacuum. It did. I also blocked off the EGR passage underneath the valve to see if that affected the engine dying. It did not.

•Checked exhaust. I suspected that a mouse may have built a nest up inside the exhaust system which was causing excessive back pressure. I ran a plumbing snake up the tailpipe and didn't encounter anything before I hit the muffler. I also put my hand in front of the tailpipe and felt the exhaust coming out while it was running which seems to indicate that there's no serious blockage.

I'm at an impasse. I don't know what else to check. There's no Security light on the dash and since it's a 1992 model I doubt it has a Passlock security system that would disable the injectors. What else should I check? I'd like to get this thing running again so I can take it on a few remaining summer trips. Any help is appreciated!

Today I pulled the #5 and #7 spark plugs to check for any abnormalities. They looked absolutely perfect. My next theory is that maybe there's some kind of problem that comes up when it goes into closed loop mode. Perhaps there's a problem with the oxygen sensor or coolant temperature sensor.