3.2 engine: awkward throttle response from tick over

[911hillclimber]

I have a 1985 3.2 in my 73T, bog standard except for an 'ssi' exhaust.
The engine has never been apart and is about 120K miles old and runs fantastically.

I fitted it about 18 years or more ago when it was 60k miles old and it has always been really sensitive to pulling away just with the revs above tick over and makes trickling along in traffic a bit 'kangaroo'.
It is just where the micro switch on the throttle body is released, ie the metal control arm with the throttle rod attached just lifts off the small switch.
When off this switch the engine goes perfectly, it starts like a old 911 etc.
It has always done this since I bought it but would like to smooth it out, unless 'they are all like that sir'

Any comments or advice please?

[smallspeed]

I had an old Mercedes e-class (280CE I think, might have been the 230CE) which had a similar thing - it would only do it some times, but it would have a bit of a lumpy idle and would be a bit lurchy pulling away.. was auto so a little less obvious. The other thing it would do very occasionally is stall if you came very slowly to a stop. I buggered about with everything and never fixed it, then I was chatting to some 100yr old guy on the MB parts desk in Leicester who laughed at me, went out back, returned with a tiny little spring and swapped it with the one on my car.. Problem gone!

Not sure if its the same system, but it is a similar age so might be.. On the MB there's two throttle return springs - one large one to close the throttle, and a small tension spring about 3-4mm diameter and about 10mm long which then pulls the "linkage plate" down onto the switch closing it. It effectively takes any slack out of the throttle linkage that might develop over time to make sure when you're off the throttle the switch is operated

This spring on my car looked fine, but it had lost almost all of its tension (it had bugger all to start with really - its about 1/2 as powerful as the spring in a biro) and wasn't doing the job anymore, so sometimes the switch wasn't closed - on the MB the switch effectively sets idle condition, so at zero throttle opening and with the switch open the engine runs but lumpy. When you slowly come to a stop the revs dip a little and it sometimes stalls, etc., etc. If you snap the throttle shut or let off the pedal quickly then it has enough force from the main spring to close the switch, hence it didn't do it all the time.. I guess its like going "closed loop" on a modern car

Anyway - long reply which can probably be boiled down to the following two questions...

Is the throttle linkage actually activating the switch? and is the switch actually working?

[Nige]

Idle Control Valve maybe?

[911hillclimber]

All springs present and correct.
The issue is one I've driven round for 20 years, is absolutely repeatable but just makes the driving smootholy at just, ie 900rpm, in traffic awkward.
I keep trying to sanitise this great car, but having never driven another 3.2 stock engine I have no bench mark.
As you open the throttle spindle less than 1 degree the levers just come off the micro switch, it is all that close, the switch I refer to is the Idle Switch..
At any speed over 900 or so it is turbine smooth to the rev limiter.
Tick over is Singer sewing machine quality, ultra stable, ultra repeatable.

[nj1052]

Sounds like it could be an issue with the switch itself not operating cleanly. Worth seeing if you can borrow one for a swap over.

[911hillclimber]

When the throttle lever come off the switch (it depresses the switch when at idle) there is a nice crisp click, exactly like it was in 1997 when it went into the car.
Not sure exactly what it does.
There is no mention of it's function in the Haynes Manual, it just appears on an illustration of the engine system.

[nj1052]

I guess you could put a meter on it and see if it switches cleanly with no "bounce"

[911hillclimber]

Will give that a try.
It is a very small switch much like I've used on electrical control circuits in the distant past, light currents only.

[Nige]

You sure the switch you are on about is not WOT (Wide Open Throttle) so unless you are planning burnouts from standstill I don't think it's your problem.

Sure it's not clutch judder?

[Nige]

This might help setting it up rather than guessing.


You can also download service manuals which has more of the above in from my Dropbox account
https://www.dropbox.com/s/33gi4678e0psm ... s.zip?dl=0

[911hillclimber]

Thanks!
I presume that connector goes to the micro switch, but an easy test.
I'll have a go after Silverstone this weekend, don't want to cock anything up!

[Harry]

Hi,

Open up your Air Flow Meter and have a look at the tracks the contact runs up and down. The tracks wear and you can move the contact to a new portion of the track to fix it.

The symptoms of this issue are similar to what you are describing.

Google AFM track repair and you will plenty of info about it on Pelican etc

Cheers,
Harry.

[911hillclimber]

Old thread Harry!
Thank you for your response though.
I've done nothing to the car since the post I must admit as it runs so very well off tick over.
The engine has done this since I bought it @ 60k miles in '96. It now has around 130k on it and runs a dream except for this.