Hi guys:
I have been chasing a misfire for awhile. I have received advice from this forumn and others.
To recap:

1996 discovery 100,000kms
misfire occurs at highway speeds when going up hills (steep grade or not) it seems to have something to to with the length or duration or quality of engine load.

I drive the same route and encounter the same hill 1/2 mile constant grade (not steep). At the bottom of the hill I am traveling 90kms and 1\2 way up the hill the rpm's rise enough that the kick down gear engages. 3rd gear does it's thing and shifts back into 4th (D) speed is about 80 km and misfire begins the vehicle shakes violently and loses power no noticeable effect with the gas pedal,manual down shift is required to keep going. Once we are on the flats vehicle is capable of 160kms easily.

So some advice indicated that a valve could be sticking I drove up the hill with a vacumn gauge attached and noted no fluctuation. this would indicate no valve is sticking open.

fuel pressure and volume have been verified as within oem specifications.

coil packs/pugs/wires have been replaced and primary and secondary ignition systems can be considered functional.

Reluctor ring on flywheel giving info to crank posistion sensor was found to have some bent teeth. these were made straight.

I am looking for more clues any thoughts?

thanks

john

 

VSS.
Vehicle speed sensor, when it starts to fail it cuts spark to the engine.
The VSS is what sends the speed signal to the speedometer and it is also the vehicle governor, it is set to limit vehicle speed (here in the US) to 110mph, it keeps youi from going faster than that buy shutting down cylinders one at a time by killing the spark.
Replace it.
www.discoweb.org has a good write up in their tech section on how to do this.
Let us know how you make out.

 

I agree, pretty common issue.

 

Thanks for the reply. I will do a VSS this week and post with the results
john