[urq] suddenly Lean - Now I'm Rich!

[SpotatAshleys]

Afternoon all.

Forget the begging letters, I haven't won the lottery, but I may have solved the "suddenly lean" problem (big sigh of relief all round)
Kick off you shoes and relax - this is an epic.

Having ruled out all the obvious (and cheap to rectify) causes I was faced with replacing the metering head (and still may be - but read on) but facing the unpalatable thought of forking out £300+ on something that might be the cause I decided to start again from the beginning and check everything one more time.

Starting with the metering head I removed the vent pipe from the waste gate that enters the top of the igloo and replaced it with a long pipe in to the car and attached a vacuum/pressure gauge to the other end. I then took a test run because I could not believe the metering flap could slam shut when the throttle was closed with such force that it would stick. I didn't think it could be a powerful enough change of pressure. What I found was that during most situations the environment in the roof of the igloo (i.e. downstream of the metering flap) is one of slight and constant vacuum. However during a gear change under boost this changes, initially there is a momentary increase in vacuum and then a sudden and equally brief sudden surge in pressure - certainly enough to slam the flap shut with great force.

I then removed the igloo and made absolutely sure there was no way in which the flap could stick on its stops. No evidence that it had or could stick.

Sure of this I then moved to the pipes from the manifold to the diversion solenoid on the bulkhead and the thermotime switch and WUR.

All pipes were confirmed as being in good condition, all clamped and secure. The WOT switch was tested and worked fine, coming in to play at about 50% throttle (I feel this may be a bit too early - any one have any thoughts/ facts regarding this?). Anyway the switch works and also activates the solenoid. The solenoid works as it should because I  disconnected the pipes and carried out pressure tests to confirm and also checked all the pipes were connected on the correct order.

The thermotime switch was removed and tested in a bowl of water with a thermometer and stopwatch. All OK.

This left the WUR. I had previously checked the continuity of the bimetal spring heater circuit and feed of power to this. All was fine. Then in an idle moment I decided to dismantle two WURs that I had from KV engines to see how they work - they have a different part number from the WR engine WUR (actually same number but WR has suffix C) so I wondered "how different can they be?". If I am right, the only difference is the orientation of the electrical connector - on the KV it is aligned so that the wires approach from the bulkhead direction whereas on the WR the wires approach from the front - to put a KV WUR on a WR will have the wires foul on the dip stick tube.
So I dismantled a KV WUR after first ensuring that the diaphragm was intact and it held vacuum/pressure. I reversed the orientation of the connector, re-assembled and ensured that it still held vacuum/pressure. Checked continuity - all OK.

The WUR that I removed from the car did not hold vacuum or pressure so I think I may have found my culprit

As I understand it the WUR works as follows:

At cold, the valve within it allows free flow of fuel in the control pressure circuit, as the bimetal spring warms, it and the coils springs close the valve, increasing control pressure and leaning the mixture. So as the engine warms up so the mixture leans off to a normal running condition. Under this normal running condition the pipe from the manifold, routed by the inactive solenoid applies vacuum to the diaphragm in the WUR maintaining the spring pressure against the valve and thus maintaining high control pressure.
Under WOT condition when extra fuel is needed in a rush, the WOT switch triggers the solenoid which purges the vacuum, replacing it with either ambient pressure or positive pressure causing the diaphragm to move away from the spring, allowing the spring pressure on the valve to decrease, opening the valve and reducing the control pressure, thus increasing fuel delivery. - Am I right in my understanding of it's operation?

Anyway I then swapped WUR on the car for my checked KV WUR. I re-set basic mixture and took it for a test run. Then and since I have not had a "lean" incident.
As Inspector Clouseau would say: "Kato my little yellow friend, I think the case is solv-ed" 
 
So my question now is - if I have indeed cured my problem by replacing a WUR that had a faulty diaphragm why was it not rich all the time rather than suddenly going lean? If the diaphragm was broken, there would be no pressure on the spring and so control pressure would low all the time - or have I just answered my own question? If I had set it up with a correct basic mixture with a broken WUR, when I ask for more juice at WOT  there would be nothing more to give during a quick shift but in a slow shift or if I just kept my foot in without shifting the metering head would simply "catch up" deliver the necessary fuel to meet demand?

If it recurs now then the only possible explanation can be the metering head either being worn or some muck is causing it to stick when slammed shut sometimes. But for the moment I think I have solved it.

I guess if I had access to fuel pressure metering gauges etc I could have diagnosed this a long time ago - I seem to remember reading somewhere that problems with K-Jetronic are usually down to one of three things: Fuel Pressure, Fuel Pressure or Fuel Pressure. 


Anyway, as a foot note, since I effected this "cure" a few days ago, I have not been able to drive after visiting A&E over a foreign object in my eye. I bet that by the time I can drive again some new problem will have occurred.
The joys of quattro ownership......

All the best to you all...many thanks for all the help you gave on this one. I think Ado gets the prize for pointing the finger at the WUR. Let's hope this is the last of it.

Keith

'87 WR
'63 Daimler V8

Louis-Alain - is this the effort you were looking for? I think you'll agree I haven't been idle - stupid may be but not idle.
Thanks again for the help.