Hey guys, long time lurker and I have always been able to find my answers by searching but this time I am stumped.


1994 6.0 xjs. Flat and backfires like the timing is off. Checked all sensors, all wiring, rephased disty to rotor. checked just about everything. Both sides are hot, idles mint but no power. stumbles with the odd backfire under accel, just flat. weird

 

It is fuel or spark (statement of the obvious). Test for fuel pressure and flow, test for throttle position sensor functioning correctly, test for fuel injectors functioning correctly.


For spark, have you changed the plugs and gapped them accurately, fitted new HT leads, checked ignition wiring looms very carefully, test amplifiers which can breakdown under load/fit new ones anyway, cleaned/replaced dizzy cap?
Greg

 

I'm sure all you guys get tired of me talking so much about the crank "sensors", but here we go again. These "sensors" are my first stab at poor running Marelli cars. I can give the whole sordid story again, but trust me, no need. Check the gap or the sensor to flywheel, and timing fingers on damper first. They need to be about .020 from the flywheel and damper timing fingers. The "sensors" send voltage to the Marelli ECU. When one or both "sensors" go bad, they send a different voltage than the Marelli ECU is programmed to operate under, and hence, poor or no running. I have said this over and over, and will continue to say that they are small magnetos and go bad over time, and as all magnets do, lose magnetic strength with heat.

 

Agree that these sensors (variable reluctance device) can result in non working ignition and hence engine if they're faulty. Sometimes because auto electronics are viewed as magic they can be blamed when not at fault. (My sensors were replaced in attempt to fix non running engine, it turned out that they were OK and a relay was faulty)

My repair manual lists correct sensor gap as 0.018 to 0.042in on 5.3 engine (88 to 93), however for 6.0l engine with Marelli it's in metric as 0.2mm to 0.8mm (which would be 0.008" to 0.032in

Whilst incorrect gap or failing magnetic strength will have an effect they produce an alternating voltage signal, the magnitude of the signal influenced by speed of rotation (I.e engine rpm), however the circuit triggers when signal crosses zero.

They are a mature technology and one advantage is they can operate at fairly high temperatures (up to 300℃)
https://en.m.wikipedia.org/wiki/Vari...uctance_sensor


Photo of repair operation manual showing alternating voltage signal from crank position sensors

 

I 100% agree with this. I also know that one of the reasons aircraft magnetos are changed every 2000 hours is because of potential failure after a known number of cycles. On our Marelli cars, the rear "sensor" produces around .6 volts AC at cranking speed (300 rpm), and can go as high as 80 volts AC at full throttle. The front "sensor" is much less, as it is triggered by the three fingers on the damper. I say, and will continue to say the these things should be part of a routine service change. They perish. Or, take your chances. Btw, its not just Marelli cars that have these. My last repair of this nature was on a 97 XJR with roughly 85,000 miles. Immediately fixed the problem.