Land Rover Repair: Rough Engine Timing:, engine management components, throttle position sensor

Question
Hi John,

Recently, my 1996 Discovery 4.0 (GEMS) 125K on the clock, has been acting like the timing is off. I was on the highway doing about 60 mph, when the front end started a slight vibration and stays present at all RPM changes (idle or driving).

When I got home a few miles later, the engine idles fine at about 700+/- RPM, but with a slight shimmy to it. Inside the cab of the truck, you can tell the engine isn't idling properly due to  mechanical vibration. It feels like the timing is off. No popping or backfiring though.

I put new Bosch Platinum plugs (4 point) and Magnecor 8MM wires on plus new O2 sensors, but the problem still exsists.

The idle is constant and correct RPM and the shimmy or engine vibration doesn't really go away with throttle increase.

Any suggestions?

Dan

Answer
Hi Dan,

it could be injector related which is why you are having backfires.  Have the injector harness tested.

Rough engine running problems can happen when ignition points, electronic modules, pickup coils, rotors, and distributor caps are not properly installed or damaged.

Also, vacuum tubing and wiring associated with timing devices connected to the distributor and other engine management components can also be disturbed or damaged and cause the engine to run roughly.

Recheck the wiring to make sure this is correct.  I've heard that existing engine firing troubles are only magnified when new wires and plugs are installed.  Just in case, check the distributor, rotor, coil wire and coil.
 
I've rarely heard of a LR needing timing adjustment, not to say that it can't happen.  With so many sensors involved, I'd suspect the Throttle position sensor, the fuel temperature sensor, the engine temperature sensor, the air intake valve, vacuum leaks, the MAF sensor.  All these can cause your firing to be off.

Best of luck,

JohnMc