Date: Thu, 12 Jul 2007 18:34:19 GMT
From: Paul Halliday <>
Subject: Lucas C900 LPT - Stabilising the idle and fuelling

Hi guys! My Mrs' 1992 C900 LPT has been running swimmingly until recently. It popped a manifold plug out, which has now been replaced, as have the other two, along with all the vacuum pipes and the failed dump valve. Before I changed the vacuum pipes over, I attached a pressure gauge and was getting about 12 InHG at warm idle. I replaced the vacuum hoses and it rose to 14 InHG. The dump valve was found to be leaking, so this has also been swapped over and all the air leaks are now gone. I can spray carb cleaner over all the potention areas and no change in engine pitch ... no leaks. Yay! However ... When first firing up the car, I get a sluggish start and billows of smoke pouring out of the back of the car, along with a strong petrol smell. Out of interest, I checked the resistance across pins 1 and 6 (Lucas AMM) and found the reading to be quite high - 501 Ohms, to be precise. This should be between 331 and 341 Ohms, from what I have read. It could well have been setup higher than normal to compensate for an obvious air leak and now things are fixed there, it's over-fuelling. Is that plausable? I wonder if this is causing a massive over-fuelling? Interestingly, left in park (it's an automatic ... I don't know if that makes a difference), I rev the car at, say 2500 RPM for 30 seconds, or so and let off the throttle, the engine vacuum is great, at just over 18 InHG (spot on!), the idle good and the exhaust clear. Leave it for a minute, or so ... the idle goes lumpy, the vacuum creeps to 15 InHG and the exhuast starts to smoke again with a heavy fuel smell. I wonder if holding the revs causes the lambda to stabilise the fuelling and all is well ... until idle, again :roll: I have read that the earlier Lucas systems, say 1990, needed to be adjusted, but the later ones, say, 1992 onwards did not because the lambda sorts it out. Is this hooey? Should I adjust the AMM? If so, how? I have a blue plastic dot that would appear to obscure a hex socket ... am I in the right area? If I shouldn't (need to) adjust the AMM ... where should I be looking for the problem? Is there a kind of reset for the system? Should I pull the battery negative for an hour, or so? I'm a little stuck, as you can imagine. This is far more complicated than my 8V tractor and I'm very much "green", as far as T16s are concerned. I should say it threw a 'Check Engine Light' this evening, but after two more warm starts it has not, so I was unable to see what the engine thought was the problem. Please help ... Paul 1989 900 Turbo S

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