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Discussion Starter #1
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?
 

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Orca said:
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.
You'd expect that - the alternator'll be putting extra load on the motor.

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.
I think it affects the mixture at idle on lambda/cat cars. On my non-cat Lucas car, the AMM was set to about 500, with the tamper proof seal intact. Setting it to 335 brought the CO to about 6% against a target of 1-1.6%...

<EDIT: Just checked, and it's about 150ohm>

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?
Any idea where physically it's coming from? Front/back/top/bottom of engine bay, further back in the car?

I do have to wonder about the idle control valve ... what happens if this gets physically stuck closed?
When I got mine, the idle would sit _very_ high or low (as in anything from 2000rpm to dying) for a few seconds then reset itself to normal. I took the IAC off and gave it a thorough blast through with brake cleaner - which initially ran out utterly black, then cleared. When I refitted it, the idle was smooth once the residue of the cleaner had burnt off.

Given that yours is a cat car, I'd be more tempted to use the right stuff - carb cleaner - but the brake cleaner was sat there handy... <grin>
 

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Discussion Starter #3
The jingling is about in line with the front doors. I do suspect the cat, since there is no mid-silencer. I thought I had read that there was a significance to the jingling sound in a cat, but I can't find anything now.

I'm just off to clean out the AIC and throttle plate with carb cleaner. I'll refresh the air filter while I'm at it.

Any more thoughts, guys?
 

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Hello Orca,

I would start with Alternator , Poor output or bad earths can manifest themselves in all sorts of ways , The drop in Vacumn may simply be the symptom of the Ecu not getting correct voltage and or bad earths . This could in turn be affecting fuel mixture . I know this is not precise , But I think its a good start . Rule the Alternator out before you go on .
 

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Discussion Starter #5
I've cleaned the AIC valve out and the throttle. I have checked the resistances of the AIC valve, the TPS and the ballast resistor - all fine. I re-checked the voltage in the cabin and it is the same as at the battery. When a big load is applied, the voltage drops from 14.2 to 13.8 - stop the load and it returns to 14.2 in the cabin. That looks good to me.

I have checked that there is little resistance between the earth at the plugs of all the injection components to the -ve terminal on the battery and none came back as anything to worry about ... meaning all earths are good. My next check is the wires from the fuse box to each of the components, but I'm not hopeful that I'll find anything there. The ECU is getting good voltage - thanks AussieBob ... that was a quick on to check.

Out of interest, should I be able to suck/blow through the dump valve vacuum hose without resistance? I would have thought not - just to check, I removed the vac line from the manifold, plugged up both ends and even disconnected the dump valve and plugged the hole at the throttle ... no change ... I thought I was onto something there :( I still don't think I should be able to suck/blow down the vac hose without resistance, though.

I double-checked the vac capsule on the distributor and the engine pitch changes when the pipe is sucked. That's another one to rule out.

I did find the jingling - one of the exhaust clamps has completely perished and some of the metal was jingling. Removed ... jingling stopped :lol:

So ... I think we can rule out bad grounds, we have good voltage, there are no ECU faults, but the problem persists. Any more thoughts?
 

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Orca said:
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...
See what the plugs look like.
 

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Discussion Starter #8
Checking the timing (again) is going to be next weekend now ... I will post the results as well as do another compression check.

Jim - I should have time to look at the plugs tomorrow. If the are not black and fouled, then we are to assume the fuelling is okay; just the timing is off. I can't think how else the vacuum could be so low. I re-iterate - I am absolutely certain that there are no vacuum leaks. If they are black and fouled, replace them?

Also, given that the job of the AIC valve is to keep the idle set properly, particularly when engine load changes (according to the Lucas manual), should I suspect a duff AIC valve? When people have duff AIC valves, what symptoms do they see?

I'd still like someone else with a recirculating dump valve to try sucking through the vac hose - it should have some resistance, shouldn't it? In fact, with the dump valve off, it should hold pressure when the vacuum port is sucked and a finger placed over the end, right?

After my work this afternoon, the car did drive generally better and I was seeing more like 12-14 inches of vacuum at warm idle in neutral and park. I doesn't drive bad, at all ... just not quite right, what with the near stalling when coming to rest, or when changing into a drive gear from neutral. It's my wife's car and she's quite happy driving it like it is ... I just want it right ;)
 

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Orca said:
I'd still like someone else with a recirculating dump valve to try sucking through the vac hose - it should have some resistance, shouldn't it? In fact, with the dump valve off, it should hold pressure when the vacuum port is sucked and a finger placed over the end, right?
Depends on which end of the hose you're sucking on. If you disconnect the hose from the dump valve and suck through the hose, you'll get no resistance 'cos any amount of air can flow through the manifold with the engine off. If you disconnect the hose from the manifold and leave it on the dump valve, then you should get resistance (after a bit). I've had the recirculating valve go bad. I could hear the hissing with the engine running though.
 

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Discussion Starter #11
cdaly said:
Depends on which end of the hose you're sucking on. If you disconnect the hose from the dump valve and suck through the hose, you'll get no resistance 'cos any amount of air can flow through the manifold with the engine off. If you disconnect the hose from the manifold and leave it on the dump valve, then you should get resistance (after a bit). I've had the recirculating valve go bad. I could hear the hissing with the engine running though.
I was removing the vac line from the intake manifold and sucking to actuate the dump valve, as the normal engine vacuum would. There was no resistance at all ... even with a prolonged suck :lol:

At that point, I removed the large dump valve pipe from the manifold and plugged up the manifold. The engine change briefly, but reverted to 12 inches of idle vacuum thereafter. It occured to me this morning that I did not plug up the dangling hose, so unmetered air could well have been drawn into the intake where the dump valve recirculated. Duh ;oops: I will re-visit that test.

I am looking most seriously at this dump valve and the AIC valve as the potential problems. I will negate the dump valve by removing it and plugging the re-circ hole (it's an LPT, so not strictly necessary ;) ) and see if I can persuade someone near me with a Lucas fuelled car to let me try their AIC valve.
 

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Discussion Starter #12
I pulled the plugs today and as far as I'm concerned - they look normal.
Just for fun, I removed the dump valve and plugged everything up - no change, so despite having what appears to be a duff dump valve, it is not affecting this low vacuum.

Yesterday, with the heating on quite high, the car actually stalled when coming to rest in gear and at least once when the fan cut in while sitting at idle. Later on in the day, it didn't do that ... atmospheric pressure?
 

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Discussion Starter #13
It wouldn't fire up at all the other day, so it's back to the mechanics. A quick look on Friday afternoon confirmed the ignition amplifier to be a dud - that's been replaced and the car now fires up. My mechanic is going to undertake an in depth look at the car to ascertain the cause of the poor starting, driveability and hopefully will find the cause of the poor engine vacuum - it ain't a vacuum leak, BTW.
 

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Does the vacuum drop and then the engine speed, or does the engine speed drop and then the vacuum? Or, in other words, is the low vacuum as a result of low engine speed, or is the low engine speed as a result of low vacuum? (I know very little about the turbos, but the above might help.)

Is it the brake booster? ... When moving the gear lever, do you also move the brake pedal at the same time. (Never even drove an automatic, so again this is just an idea!!)
 

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I too was wondering about the brake booster, you say it happens when you're coming to a stop. Have you checked the brake booster out? Perhaps its failed and you're leaking vacuum though there.
 

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Discussion Starter #16 (Edited)
Thanks for the replies Guys - the car has ABS, so no brake booster. That was one of my initial thoughts before I understood how the ABS system interacted (I'm more used to old-fashioned engineering on my 8V).

To re-cap - I have 12 inHg at idle with the car in park. Put into a drive gear and that drops to 10 inHg. Turn the steering wheel at idle and that drops to 4-8 InHg depending on how quickly I turn it. If I put on loads of electric (defroster, headlights, heating, etc) it also drops to, say, 8 InHg.

While driving and coming to rest, the idle drops - depending on the weather and what electrics I have on, the car can stall, otherwise the idle drops rights down, the vacuum drops right down and the car shudders and then recovers.

Since fitting a new battery and a new AIC valve (sorry ... News Flash! I have replaced the battery and fitted a brand new and very costly AIC valve ... didn't fill you all in on that detail above), the idle problem is a lot better and I'm sure would be cured if only I could get to the bottom of this low vacuum.
 

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Orca said:
Thanks for the replies Guys - the car has ABS, so no brake booster. That was one of my initial thoughts before I understood how the ABS system interacted (I'm more used to old-fashioned engineering on my 8V).

To re-cap - I have 12 inHg at idle with the car in park. Put into a drive gear and that drops to 10 inHg. Turn the steering wheel at idle and that drops to 4-8 InHg depending on how quickly I turn it. If I put on loads of electric (defroster, headlights, heating, etc) it also drops to, say, 8 InHg.

While driving and coming to rest, the idle drops - depending on the weather and what electrics I have on, the car can stall, otherwise the idle drops rights down, the vacuum drops right down and the car shudders and then recovers.

Since fitting a new battery and a new AIC valve, the idle problem is a lot better and I'm sure would be cured if only I could get to the bottom of this low vacuum.
Hmm, sounds like its not responding to load. Putting it in gear loads it a bit, turning the steering wheel loads it a bit, alt loads it, I'm wondering if you AIC valve still isn't working quite right, clean the connections and grounds maybe? Are you sure you're throttle plate is closing all the way?

I think the vacuum is a symptom not a fault.
 

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Discussion Starter #18
Matt88S said:
Hmm, sounds like its not responding to load. Putting it in gear loads it a bit, turning the steering wheel loads it a bit, alt loads it, I'm wondering if you AIC valve still isn't working quite right, clean the connections and grounds maybe? Are you sure you're throttle plate is closing all the way?

I think the vacuum is a symptom not a fault.
I had a look when I cleaned the throttle body out with carb cleaner and it looked okay. I did mean to re-visit with a feeler gauge. I'll be in touch with my mechanics tomorrow, so I'll mention it as a possibility. I really hope they get to the bottom of it - it's gone way beyond me now.

Interestingly, I was reading this morning that slipping when changing gears on an automatic is linked to a badly set throttle cable. I've no idea if the slight slip I experience is actually a "slip" ... I've not driven another automatic to compare. The sensation is a kind of "thunk" between gears, but I gather that's normal for the old Borg Warner boxes. I wonder ... might be onto something here.

Should the throttle be totally closed on a T16 (Lucas powered), or slightly open (like a third of a turn on the throttle stop screw)?
 

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At idle it should be totally closed, otherwise there will be air going in that the AIC isn't handling, will throw it off.

As for shifting on an auto, you're always going to have a bit of a "thump or thunk" upon gear shift, you've got hydraulic things being slammed about at fairly high psi, if its very pronounced or if it's accompanied by a jump in rpm or if its slow to change gears/not wanting to up or downshift and has a rather thunkathunk to it, then things may be heading south.


Setting the TV and bands is always something I do after aquiring one though, its fairly simple, and it really effects performance and longlivity.

Is your TPS adjusted properly? Perhaps it doesn't know its in idle map and isn't adjusting accordingly to load. You might check and see if its clicking into the idle position.
 

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Try this...

I had the same symptoms (rough idle, rich running, occasional stalling at stops) and found that the vacuum hose attached to the distributor vacuum advance was attached at its other end to the intake manifold behind the throttle butterfly. It is important that this hose be attached to the nipple right at the butterfly or else you will have timing at about 30 degrees at idle. I've seen a bunch of 900s that have this vacuum hose attached to the wrong nipple, causing this problem.

Paul Bob
 
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