Land Rover Repair: Running rough,smoke, on cold start......., throttle position sensor, vacuum leak

Question
John: 1998 Disco ,V8....118,000 mi.,ran hot last fall, had a full topend job done, has run very nicely since, 5,000 mi. ago, recently when started in the morning, it idles rough, and light gray smoke, after is warms up or run a bit, it is very quiet and smooth? Nothing shows up on the computer? Any ideas? Thanks!
Clark Gordin     Brandon,MS USA

Answer
Hi Clark,

Rough idle with smoke could indicate ignition timing problems.  It can also point to a vacuum leak, valve clearance problems, injector problems.  If you don't have any backfires you can usually rule out the timing chain. If you suspect a lack of power it could reinforce my theory that it is ignition related (possibly injector timing).

The smoke symptom makes me lean towards ignition (pardon the pun).  The fact that it clears up after your Disco has warmed up, rules out headgasket failure.  If it was your headgasket, you'd see the smoke all the time and your oil would start to cream over.

I'd have a mechanic run some tests on your ignition system and injectors.  Could be a simple timing problem that the ECU adjusts each time the engine is started.  

Best of luck,

JohnMc
Quebec Canada

NB:  a fellow 98 Disco owner told me about a similiar problem.  1 year ago he had top-end work done and recently he too was experiencing rough idle problems.  It is possible that the TPS (Throttle Position Sensor) is failing.  He was able to take his out and perform a thorough cleaning.  He feels that the overheating may have shortened the lifespan of one of the sensors.  Also check the Idle stepper motor.  These two units are heat sensitive.

John