Van backfired, barely made it 2 miles home, scanned for codes, none found, none pending. #3 and #4 cylinders showed 90 & 150 psi compression and leaked compressed air thru both intake and exhaust ports, valve train off. Other 4 cylinders had 210 psi compression (heads machined 2 yrs ago after overheat). Replaced heads with reman heads that had reground valves, new springs and valve seals.
Used new head bolts and replaced lifters this time; used original push rods as they weren't bent.

Inserted fuel pump relay, turned key several times to feed the fuel rail. Fired it up and van backfired as if one cyliinder was firing with the intake valve open, idled up to 800rpm then died. Same thing several times. No codes. Didn't have this issue when I had the heads machined and replaced the head gaskets 2 years ago. Scanned for codes, none found.
Suspected aftermarket coil was crossfiring. Replaced with new aftermarket coil; same symptoms. Still no codes.

Pulled valve covers and fuel pump relay and had a friend turn it over with the starter. All valves were opening and closing. Backfired once thru the throttle body, probably leftover fuel in a cylinder.

Now I'm thinking there's two possible causes. Either the valve timing is off or the PCM is triggering the coil at the wrong time.
Since there's no timing marks, I can think of no way to check valve timing, except maybe sticking a long screwdriver or the like into a cylinder and turning the crank by hand til it's at TDC. I've talked to 3-4 Chrysler mechanics with 10-15 yrs dealership experience and none have ever replaced a timing chain in a 3.8 L motor, so I'm thinking PCM, since there were never cam or crank sensor codes.

Question: Is it necessary for #1 cylinder to be at TDC when the rocker arm rail is torqued down? I had a friend watch for both #1 cylinder push rods to both be at their lowest point while I manually turned the crank before I torqued the rocker rail. Didn't stop to think it could be TDC before the intake stroke. Could the valve timing being 180* off when I torqued the rocker rail cause this issue?

Don't want to buy a PCM if it's not necessary...

Any suggestions are appreciated, thanks.

 

UPDATE: Checked the timing chain play - turning the crank pulley by hand clockwise til one piston at TDC (long screwdriver through a spark plug hole), then reversing until the piston moved shows only 7 degrees of slop at the crank pulley. Since I've always heard the point at which a timing chain might jump is 10-12 degrees, should I assume the valve timing is still correct?

Thinking outside the box, another possible culprit came to mind: the injector wiring that I insulated 4 years ago with shrink wrap on each wire in the harness. But theoretically, fuel delivery should not have any effect on the relationship between a cylinder firing and the intake valve being open. Replaced the MAP sensor on a hunch, now it looks like I might need a new PCM. Reassembling valve train today, if it doesn't rain.

Anybody have any experience with refurbished PCMs being flashed with the VIN number?

Thanks.

 

UPDATE:
Problem SOLVED!

After replacing the coil (warranty) - for potential crossfiring as a source of the backfiring - without improvement, I started thinking about lean mix being the cause.
Because there's no schrader valve on the fuel rail, I made up a hose assembly to put between the fuel rail and the fuel line. Oddly, I got zero pressure, though I could hear the pump run.
Replaced the fuel pump in the tank (also warranty), and got the same result with the new pump, no fuel pressure. Blew out the fuel lines and they were clear, except for one. Compressed air would flow from the engine compartment to the fuel pump fitting, but not the other direction. Disconnecting all the segments revealed the fuel filter mounted on the top of the tank was plugged, with charcoal colored gas dripping out of the line when pressurized.
Replaced the filter and the van runs smoothly now!

Now I have to replace the charcoal vapor canister to keep the new filter from plugging up, but I thought I'd see if there was any way to rehab the one on the van with 237,000 miles on it.

Dodge wants nearly $300 for a new one, and eBay has one for $103, but as long as this one's off the van, I thought "why not rehab it?"

Anyone know of a way to replace the guts in a vapor canister?

Thanks.