Site News - 11/22 Member of the Year Voting
Date: Wed, 18 Jul 2007 17:44:40 GMT
From: Paul Halliday <>
Subject: Re: Lucas C900 LPT - Stabilising the idle and fuelling

in article, Paul Halliday at wrote on 12/07/2007 19:34: > 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 plausible? While this is plausible, it was not the cause of the problem. FYI, only the Lucas fuelled cars in 1990 needed to be within that range as they did not have a lambda sensor. Lucas fuelled cars from 1991-1994 did not need to be adjusted at the AMM, since the lambda took control of the fuelling. > 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: This at least showed that the vacuum leaks have been sorted. As I said above ... There is still something amiss causing a leak. > 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? See above ... and yes, disconnecting the ECU will clear all stored fault codes. > 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. Okay, so I pulled the fault codes by finding the yellow/white wire in the engine bay by the cabin air intake and attaching a switch to it and grounding the wire. The fault codes can then be retrieved by using the ignition barrel key and the switch. The full procedure is here: I found "No fault found", essentially, so pulled the spark plug out of cylinder 1 to see if was black (to check out my over-fuelling theory above). I discovered that it was wet! What kind of wet? Coolant wet :( That would explain where all the water was going :) Yes ... The headgasket has failed. No big deal, really - I've dropped the car off at Aeromotive for them to weave their magic and expect everything will be nice and tight when I get the car back. For giggles, here's some videos of the smoke and the steaming turbo/exhaust: <> <> <> Also, here's a picture of the turbo/exhaust showing water coming through the combustion chambers and into the exhaust and a wet spark plug. <> <> Cheers all, Paul 1989 900 Turbo S

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