Date: Sun, 04 Nov 2007 02:32:56 -0800 From: PJGH <pjgh93nospamlemail.com> Subject: Poor vacuum and rough idling on 1992 C900 LPT (Lucas CU)
Hi guys - I really need some insight into an ongoing problem with our 1992 900 LPT convertible. We have a low vacuum reading and a rough idle which is really really bugging me. The car is an LPT, so Lucas fuelled. Here are the facts. From cold, the vacuum gauge shows about 10 inches. When warm, shows about 12 inches. The idle is a little low and a problem occurs when drawing to a stop ... the idle drops quite alarmingly and the car almost stalls. After a second, or so, the idle comes up again. The car is an automatic, BTW. When sitting in neutral, if the car is shifted to drive or reverse, the same behaviour occurs - the idle drops low, nearly stalls, then recovers. At rest, I can watch the vacuum gauge which climbs to 6-8 inches when doing this. Interestingly, if I apply some electrics like the power roof, or put the windows up when they're already us, the vacuum climbs to 6-8 inches and the idle starts to fall. Okay, so I suspect a vacuum leak. Not so ... I have been over everything. I have even disconnected all the vacuum lines except for the FPR and dump valve and attached a different, known good gauge and the same low vacuum is present. I have been over and over the vacuum ports and engine bay with carb cleaner trying to sniff out a leak, but nothing. All the vacuum lines, even the cabin air vent lines have been replaced with 3mm silicon. The timing is good and the fuel pressure is good. I have a 3 bar FPR, BTW. Compression is fine. In fact, the headgasket was replaced within the last couple of months. When it failed, number 1 cylinder was filling with water. The AMM is set to 336 ohms, but apparently on the Lucas cars (post-1990 ... the ones with lambda and a cat) that doesn't affect anything anyway since the ECU sets the mixture by reading the lambda anyway. Backpressure? I did the "hold 2500 RPM and see if the vacuum falls" test. It didn't. It sat at about 18 inches, which yes ... is low, but did not fall. Out of interest I do hear a "jingling" noise when lightly revving the car, or driving off. What is that? A dropped baffle, or a failed cat? Any ideas? Voltage? 14.2 at the battery, 13.2 in the cabin ... tested on the cigar lighter. Is this something to be concerned about? Finally, when the car is first started, there is a lot of white smoke. It could be condensation, but for the fact that it utterly reeks of petrol. I think the car is running mega-rich and the low vacuum and idle is as a result, although I have not seen a rich mixture push the vacuum that low before. Once warmed up, the smoke is not present, but the low vacuum and poor idling persists ... that, and when any electrical load is applied, the vacuum moves to 6-8 inches and the idle drops. I know my alternator is well grounded, but have not checked the brushes. Does this sound like that kind of an issue? The car is an automatic - are these symptoms related to something going wrong with the auto box? Perhaps mis-aligned? What does "adjusting the bands" mean? Sometimes I have to push the stick hard forwards to get the key to turn in the ignition. Gear changing is okay, smooth enough and all the gear selections are the correct gear. The CEL is not lit and there are no stored fault codes. I do have to wonder about the idle control valve ... what happens if this gets physically stuck closed? The ECU will still see it electronically working (so no CEL), but what about a mechanical failure? Could this behaviour be related? Could the AMM be at fault without throwing a CEL? It's a mystery and a head-scratcher :( Any thoughts?
