1988 740 Weird Timing

Larrys740 · Post by **Larrys740** » 22 Oct 2007, 22:03

Have been troubleshooting my 1988 740 8 valve fuel injected wagon for about a year. Mostly just hard starting, but last couple months running rough at idle and REALLY hard starting. Have replaced: fuel pump and filter, ignition module, distributor, cap, rotor, plugs and plug wires. Coil tests good. New radio interference relay, fuel pump relay, fresh air hose. Cleaned throttle body, reseated all connectors even at the computer, all wires test good to everything. Will swap air mass sensor soon with my 1989 just to check everything. Good compression, fuel pressure (and new pressure regulator). Vacuum is steady at manifold even with the rough idle. Also cleaned injectors. New oxygen sensor. It WILL run smooth very occasionally.

OK, now the weird part.

After installing the new distributor, I hooked up my timing light, and no kidding, watched the timing mark make a complete rotation slowly around the balancer. It looks like a one-piece balancer. 360 degrees in about 20 seconds, while the engine was running! Figured the computer could not tell the engine position, but the hall sensor checked fine, and the wires tested black=ground, red= +12volts, and blue=+5volts. There was a good coil spark when shorting the blue wire to ground, and the fuel pump activated for a moment. No flywheel sensor on this model.

Have tried a lot, sometimes twice, but the car is still idling rough, won't start at all half the time, and misses a lot.

Anybody think of anything that fits these symptoms and results?

waynej · Post by **waynej** » 23 Oct 2007, 09:08

Is your timing light the kind where you can dial in a certain amount of advance? If it is try it on another vehicle. I had one that went crazy like that.

Post by **sseir39** » 23 Oct 2007, 10:30

The vibration damper or crank pulley can wear out allowing the outer portion to rotate. The rubber part of the sandwich (between the inner portion and outer portion of the pulley) is what goes out and can cause strange timing marks.

Larrys740 · Post by **Larrys740** » 23 Oct 2007, 11:12

Thanks, guys! But the timing light is a plain-jane lamp with no adjustments, and the balancer is a one-piece unit. I still believe that the computer is not sensing the engine position. It did start today with some difficulty, and ran pretty smooth except at idle. May take a video of the timing mark advancing around the wheel. Have already heard the Volvo mechanic say "That can't happen!".