Misfire?

[discobrit]

1994, Disco I, 3.9, 80K miles, manual transmission

Under moderate load exhibits an intermittent "misfire", rough running/loss of power.

I have read all the messages I could find on this subject and believe it may be the VSS, because:
-Idle is steady and controlled even when switching the aircon on and off (not the stepper motor/air bypass)
-Diagnostic display shows no faults (Oxygen sensors OK?)
-Engine is very clean inside - no sludge (valves not sticking)
-Plugs look good and leads are fairly new
-Timing is OK
-Distributor looks good (fairly new cap)
-New fuel filter
-Good temperatures and not losing water
-No water in oil
-Injectors were very clean ~1year ago

I noticed the idle speed sometimes stays pretty high (over 1Krpm) until reaching a complete stop. Don't know if this is normal.

-Is there any way to test the VSS?
-Any other suggestions? (Ignition amplifier? Ignition coil?)
-What's the best way to check fuel pressure? What should it be?
-Will a diagnostic tester detect faults that are not shown on the on-board display?
-Where's the best place to buy a VSS?

All help is greatly appreciated.

Happy holiday!

 

[Doug W]

Speed sensor is worth checking. I found out an interesting detail the other day. I have an enhanced ECU on board, and it started to short the fuel pump two weeks ago. Out for repair/replacement, and put the original back in from pre-engine swap. I suddenly had old misfire symptoms from pre-engine swap, exactly the same. So the ECU itself was causing misfire above 3400 rpm. Worth finding a truck with a matching ecu and testing....

 

[discobrit]

Seems like I fixed the problem, fingers crossed!

I changed the VSS - after taking it apart to try and fix it. $90 is a rip-off for something that must cost about $5 to make - there's a magnet, a reed relay and a 120ohm resistor (looks like 120ohm) in a plastic housing.

I also cleaned the stepper motor, checked the fuel pressure (was good), checked and tightened all the vacuum hoses, took the distributor apart for cleaning and lubrication of the advance mechanism, checked and cleaned all the sensor connections, checked all the ignition leads (found one bad plug lead). - So I'm not sure what the cause of the original problem was.

It's interesting to note that while I was waiting for the new VSS, I tried running with the old one disconnected - it ran great (but the speedo didn't work of course).

 

[Andrew Homan]

Sounds like a bad plug wire. get an ohm meter and check them including the coil wire. Its a cheap fix and even cheaper to check. had almost the same problem and found the #1 wire was open. replaced and all is well.