Land Rover Repair: 98 Discovery Engine Mis-Fire, vehicle speed sensor, crankcase oil

Question
QUESTION: I have been having problems with the LR misfiring from time to time over the past year or so.  It will start out of the blue cutting out for a second then coming back and it will do this for a while then not do it again for a month or so then do it some more.  Some times it will kick on the check engine light and I get a "engine misfire" printout.  

I was advised that this might be the VSS.  So as a test the last time it happend I shifted to a lower gear and slowed down.  I was told the VSS only operated in high gear.  When I shifted down it still was cutting out and back in.  So does this eliminate the VSS?  I removed the TPS and IAC and cleaned them and this appeared to help for a while.  The IAC had some black build up on the cone.  

I am not much of a mechanic so any help would be huge.



ANSWER: The vehicle speed sensor is not going to give you a misfire code.  This year and model suffered badly from sticking inlet and exhaust valve, which cause random misfires.  I would give it an oil change add a good quality detergent with the oil to free up rings and sticky valves.  I would also perform a chemical upper cylinder de-coat and add a fuel additive to the gas tank.  Finally, I would suggest new plugs, an air filter and set the base idle.

---------- FOLLOW-UP ----------

QUESTION: I replaced the spark plugs & wires about a year ago.  I bought some SeaFoam and put in the crankcase oil and put a can in the tank with 10 gallons of new gas.  Is this what you had in mind?  I was not sure what you thought would be the best detergent or chemical coat.  Thanks for the help.

JP Stidham

ANSWER: I have never used this particular product, but it does seem to be the proper kind of additive.

---------- FOLLOW-UP ----------

QUESTION: Ok.  I had the codes read again and here are the codes.

P-0307 -- Cylinder seven mis-fire
p-1316 -- Manufacturer Control Ignition system or Mis-fire

I have replaced the spark plug on cylinder #7 and have had the codes cleared.  Do these codes prompt any equipment that should be replaced do you think?

Thanks so much for your help.  I think I might be getting on top of these repairs.  I would kill for a diagnostic tool for this beast.

Thanks again.

Answer
These are both misfire codes.  I would check for a spark on cylinder 7 and check compression on all cylinders and compare to cylinder 7.  Also check for bent teeth on the reluctor on the flywheel.

Other than this, give the additives a few days to work.