3.2 throttle issue since 1995

[911hillclimber]

My 73T had a 3.2 engine installed in 1995, 50K miles old.

It is now around 130K miles in and untouched apart from the alternator about 10 years back and annual servicing.
Plug and cap replaced 2 years ago.

The engine runs really well, bit of blue smoke on start-up after dormant for a few weeks etc, nothing to worry about I think.

From cold to hot it runs and starts perfectly, lovely engine.

However...

Ever since I had it installed and wired up (by Bob Watson) the throttle action has been irritating, but, only taking the engine from 800 tick over to 'just' getting going.
If i really squeeze the throttle pedal super carefully the engine will come off tickover and rev as you would want, but if you drive the car normally on the road there is this sudden jerk and all is then well.

I feel this is when the throttle lever on the engine throttle body just comes off a micro switch on the side of the body. The switch position cannot be adjusted and is depressed (closed?) when at tick over by the throttle arm.

So:
What does this switch do and how do you set the throttle action to be as smooth as a normal car?

It is a real irritation when in traffic!

[911hillclimber]

Posted again in the short bonnet section.

[RobFrost]

The switch can be adjusted. If i remember rightly there's either a screw that adjusts when it is engaged, or you loosen a screw and rotate the switch.

It puts the DMU into zero throttle mode. In that mode, the idle control valve is activated once revs drop below 1100 revs so as to catch the engine before it stalls, and both the idle control valve and an algorithm that advances and retards the ignition, are activated, to maintain idle at 800 revs on earlier cars and 880 on cars with the later cpu.

If you want you can pull the lead off and try driving without it. Sometimes if my idle goes awry I prefer it without the switch.

Causes of poor idle are usually a sticking idle control valve or a vacuum leak. To check for a vacuum leak pull the oil cap off. The engine will respond if there's good vacuum and won't if there's a leak.

It sounds to me like your car might already be at a few % throttle open before your switch opens, so when the car comes out of idle mode, it jumps suddenly. Alternatively the switch could be sticking so it pops out of idle mode suddenly half a second after you start depressing the accelerator. You can test if it's sticking by opening the throttle within the engine bay, because you can hear the switch click open. You can also check it opens and closes at closed throttle and not a few % before by listening to when it clicks.



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[911hillclimber]

That is truely excellent Rob, thank you v much.

All that makes sense to me even!

The tick over is rock steady at 800 indicated, never falters, never uneven hot or cold.
The switch click when you move the throttle lever on the stationary engine 'just' so slightly.

Hoping to spend a bit of time on the car's brakes tomorrow so will also have a good look at the what you say on the engine.

Taking the oil filler cap off when engine running gives a 50 or so rpm drop, back on, 800 is restored almost within a second.