Interesting K Series Idle Issue

[revilla]

Had a funny one last night. Drove the Caterham to work and back. All felt OK, although I did notice it felt a little bit "ragged" on the lift-off. Didn't think much more of it, not had many opportunities to drive it lately so just put it down to my feet being used to the tintop.

When I got back my wife needed something delivering so I thought take the Caterham. As I pulled up a roundabout in the next village about 4 miles away, it decided to idle at 2000rpm. Clearly wasn't entirely happy so I went home, took the tintop and decided to have a look at it this morning. Didn't fancy breaking down on the M69 in the dark.

I was expecting a vacuum leak, a split vac hose or cap but all looked good. Plugged in the OBDII scanner and nothing looked really wrong, but I did notice the MAP at hot idle was 39kPa which was a tiny bit higher than normal for my car (different engines will differ quite a bit depending on cams, timing etc. but I knew mine was usually about 33kPa from experience).

So I looked at the throttle stop screw. Seemed to be a long way in, so I adjusted it until the idle settled properly after blipping the throttle and recalibrated the TPS. When it is set correctly, the revs should drop smartly back to the idle speed set (825rpm in my case) without dipping, and usually with just the slightest hint of a "hover" around 1500rpm as the ECU takes over on the IACV.

I had to wind it out 3 full turns! Quarter of a turn is usually enough to be significant.

I took a photograph of the screw in the position I had set it, then took the car out for a test drive (it was back to its happy self again):

When I got back I thought I'd put a bit of locking paint on the screw to make sure it stayed where I put it, and after only 10-15 minutes driving I found it like this:

It had wound itself in by a full 90° in less than a quarter of an hour under engine vibration. It didn't feel particularly loose, but the locking paint which had been applied had flaked clean off the screw and it was able to wind itself in (downwards under gravity) with the engine vibrations when the throttle was open.

I scraped off the remains of the paint spot from the stop with a blade, cleaned the stop and screw with acetone to remove and grease, dried it off then put a blob of paint on it again.

Hopefully it will stay put this time.

MAP back to 33kPa, idle speed back where it should be, TPS correctly calibrated at 3.92%, idle return normal, all of the idle transition "shuntiness" gone.

Just thought I'd write it up in case anyone else had similar issues.

[revilla]

PS: This is a VVC with aluminium plenum and aluminium throttle body. To sure if the screw would be so free to move in a plastic TB or not.

[Wrightpayne]

In the words of OMC How Bizzare!

[elie boone]

So a locking nut should do the trick.

[revilla]

Yes, Elie ... I decided to put a lock nut on it.

The existing screw was a bit too short (M5 * 0.8 * 16mm) and to ensure consistent behaviour as it was rotated it, it had a rounded ball end on it. So I bought a slightly longer (20mm) screw, slipped some nuts on it so I could grip it in a drill, spun it in a drill (anti-clockwise so it didn't unwind from the nuts) and worked the end with a file, then wet and dry, to make a nice ball end of my own. Only took five minutes:

That then fits nicely:

A word of warning: I decided to make the same change to a spare engine I have (I like to keep them identical!). I know Rover intended the throttle screw to be "set for life", but they didn't expect me to be changing exhaust cams and cam timing either, which does tend to mess with the idle and requires this to to be tweaked for best results.

On the spare engine, the screw was locked in place with a high-strength red threadlocker. The screw head snapped! I managed to get the stump out by applying heat (with  tiny jet-flame lighter, straight onto the screw itself) but the threadlocker was tough stuff, as soon as it cooled it locked again so I had to keep heating, half a turn, heating, half a turn...

Once out I ran an M5 tap through the hole to clean the threadlocker out.

If anyone does want to take this screw out and finds it doesn't want to come easily, I'd apply heat BEFORE the head snaps so you can spin out out with a screwdriver!

[Wrightpayne]

I tried to adjusted mine after fitting a 52mm TB as the car occasionally stalls coming up to a junction and its rock solid! I'll try some heat! 

[revilla]

Yes that's what you get if it needs opening up a bit. As you coast to a junction, the throttle is closed and the ECU is fooled into think it's idling but too fast, so it tries to compensate by closing the IACV. As soon as you dip the clutch it dies.