Land Rover Repair: Land Rover Idle problem, throttle position sensor, air fuel mixture

Question
John, do you think it could be the Idle control valve?


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Followup To
Question -
I have a 1995 Land Rover Disco and have recently had an issue with it idling will stopped.  The tach jumps between 1000 and 1500, then will stall.
Answer -
Hi Grant,

I had a similar problem with my '95 Disco.  Problem was with the TPS (Throttle Position Sensor).  Dealer repaired it for ~$500. This is the most likely cause.

Normal rev at idle should be around 750rpm.

Could also be a vacuum problem...possibly a leak in one  of the vacuum lines.

Another (remote) cause could be timing chain slipping or worn.  I think you'd see a power loss problem if it was the timing chain.

If you have a diesel engine, could be wax forming in the injectors. Otherwise, fuel injectors could be fouled or the air/fuel mixture is too lean.

How is your air filter?  When was the last time you had a tune up performed?  Spark plugs and wires could cause idle problems.  

So, these are the possible causes to your rough idle.  I am leaning (pardon the pun) towards the TPS fault. I went through the entire list above before having the TPS changed.  Learn from my mistake.  Talk to your dealer or local LR mechanic to see if they will concur with this.

Best of luck,

JohnMc  

Answer
Hi Grant,

You mean the idle air valve.  One owner has mentioned that his was sticking and he cured it by using some commercial carb cleaner.

It rare but you never know.  Same with the idle adjustment linkage.  Again, so rare that it is usually ruled out.  I have yet to see these two components fault out but ... you never know.  

Many years ago, my dealer  screwed me for close to $3000 in tracking down a rough idle.  The final visit (after I threatened litigation) the dealership decided to replace the TPS and reprogram the ECU.  Lo and behold... the problem went away!  Cost: ~$500  

Now if they adjusted the idle linkage (manual says not to touch this) or cleaned the idle air valve (not even mentioned in manual service book) I have no idea.

Most causes of rough idles are related to sensor failures, fuel delivery problems (injectors, plugs, wires, coil..), timing problems, or serious valve problems such as a sticking exhaust valve.  This is common on Disco II engines but not on our '95 Disco with the 3.9 engine.

I hope this helps.

JohnMc