Throttle Pot adjustment

[MisterTee]

Good evening everyone

My engine was rebuilt approx 500 miles ago and runs pretty rough at tickover - sometimes hunting (revs rising and lowering) for about 10 seconds before settling down.

I’ve also noticed that when slowing down and dipping the clutch the revs drop so low that the car almost cuts out.

I have borrowed a star tester previously (thanks again Ray) and one of the error codes mentions that the throttle stop is set too low.

Any ideas how I adjust this, or do I need a new TPS?

Thanks in advance.

Andy

[v8s4me]

If you loosen the screws can you rotate the TP a few degrees in either direction?

I had similar symptoms when my engine was rebuilt and it turned out to be a split 'O' ring on an injector.

[mentall]

MisterTee said:

I have borrowed a star tester previously (thanks again Ray) and one of the error codes mentions that the throttle stop is set too low.

Any ideas how I adjust this, or do I need a new TPS?

Hi Andy

I've had these symptoms before, and I expect to get them again when I re-commission the car after (so far) 12 months lay-up. Here's my take, but I know there are other opinions:

What code did you see that indicated throttle-stop setting? As far as I can recall there is no EEC-IV 2-digit code that mentions the throttle-stop, though I don't know what further information the Star Tester adds.

And I believe that this is because the 2.9 EEC-IV controlled system has NO throttle-stop in the usual sense. Tickover is wholly controlled by the ECU through the ISCV, and a closed throttle condition means just that: a closed throttle.

There is however a stop screw controlling the throttle spindle. But its purpose is NOT to adjust the throttle opening! It's to prevent the (very strong) return springs from causing the throttle plates to seat so firmly in their bores that they stick when the mechanism begins to open them, giving a jerky start to air admission, and jerky initial operation to the TPS. I have found this to happen on my car, if the screw is backed-off.

So: I back-off the adjustment, allowing the plates to close. Then I tighten the screw until it is just contacting the cam on the spindle, acting as a back-stop but not holding the throttle open in the slightest. And I check by gently pulling on the linkage that there is no sticking at the closed position.

Of course, as you have identified there are lots of other potential faults in the system that can cause similar problems. But I have found that smooth snag-free operation of the whole setup, from the throttle pedal all the way to the TPS, is essential.

It's easy enough to check for a faulty TPS with a multimeter: no jumps or dropouts as the spindle is slowly turned. Best done on the car, turning the spindle with the linkage. Or even better, by monitoring the voltage on ECU pin 47. You can even do this with the engine running!

Good luck, and keep us up to date!

John

[lancepar]

I found this site useful when adjusting the TPS, but it may not apply to a "S"

http://www.v8engines.com/carbs-2.htm#setting-up

[MisterTee]

Great, thanks for the replies.

I’ve give them a go when I next get anywhere near the car and will let you know how I get on.

Andy