Hello,

My 83 volvo 240 is at a friend's shop now after it turns over, but refuses to start. I've just replaced the wiring harness, checked for fuel and spark (ok), checked the fuse box, timing is ok, and am running out of ideas for the car.

My friend thinks we should change the ecu, b/c after the harness was removed the car backfires a little bit vs before it didn't. On ebay, there's two types: 0280000503 or 0280000510. The engine is the original b23f, lh2.0 system. Which one should I get? Thanks for the help.

Solomon

Just a suggestion since it happened to me once. Check the fuel pump relay. It's located inside on the firewall behind the dash/carpet on the pass. side.
Ben

I am not sure the ECU is the problem, most 503 (or 510) ECUs fail in the "failure to adjust mixture" mode. Not usually a failure to start. I guess it could be but before spending too much I would borrow one from a running car to prove it is what you need. Are you sure you did not reverse the idle air valve harness plug with the similar one for the, I forget, throttle switch? Or leave an air hose loose or disconnected in the harness swap.

Changing the harness introduces a lot of possibilites. Is it 100% certain the firing order is correct? Backfiring means there is fuel in the exhaust. Understandable if it has been cranking for a while without starting. However, to ignite the fuel in the exhaust a spark has to occur while an exhaust valve is open.