dry start rpm jumping

[318lotis]

dilemma, m42 turns over but not starting, when it does start, rpm jumping up and down 800 to 2000 at idle. Now no starting at all. I cannot hear fuel pump prime. would jumping rpm be sign of bad fuel pump?

[DesktopDave]

I'd bet either a big vac leak, fuel pressure issues or the crank sensor.  Wandering idle RPM usually indicates a vac leak.

I'd do the crank sensor first, it's easiest.  Check & clean it...might be losing signal and the DME is cutting the fuel pump as a safety.  Pins 1 & 2 on the sensor pigtail should be about 680 ohms...IIRC resistance on pins 1 & 3 and 2 & 3 should be very high, >10k ohms.  If the sensor is in spec, check the clearance between the face of the sensor and the trigger wheel.  Should be very close, like credit card thickness or less.  Clean the oil & rust off the sensor face & see if that helps your problem.

Vac leak are usually on the idle control valve line or the main intake boot, but can be anywhere unfortunately.

After you've checked those out, maybe think about testing the pump pressure, checking for a loose connector at the pump, loose fuse, flaky relay, etc.

[318lotis]

thanx for help Dave,  i determined fuel pump was bad, placed new fuel pump, i can hear new pump running while trying to start. but i cant hear it prime. strange, but still no start, even though original pump was bad. Is there away to by pass electrical? could be short in wiring harness. thanx, mark