3.9 playing silly buggers while hot

Check ALL your vacuum hoses for splits - replace any that look iffy. You done the right thing by cleaning the stepper motor, did mine awhile back as I had a rough idle too and it worked a treat! You might have an HT lead breaking down causing the rough running when its warmed up, wait for dark, start the engine, pop the hood and look for any sparks arcing from the leads and if so you've discovered summat else to fix. Is your v8 one of the later 3.9s with cats in the exhaust?

DeadTom said:

how does one check that the advance is working? ive had a gander and the hose, dizzy, rotor and everything looks in good condition, but not sure exactly how to test the vac advance?

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Generally, take off dizzy cap and the plastic flash guard. Disconnect the vac pipe from the intake end then lookin at the dizzy suck hard on the end of the pipe. If vac advance is working you should see the base plate in the dizzy move.

If the diaphragm in the vac advance has perished you'll be able to keep sucking air through it.

It would also be a good idea to grab hold of the rotor arm and see if you can wiggle it from side to side. If the rotor arm feels solid it would indicate the mechanical advance in the dizzy has seized in which case you'll need to pop the dizzy out, disassemble it, clean and lubricate the mechanical advance then put it all back together but make sure you don't damage or lose the little springs.

aah thanks for that, even a dunce like me should be able to follow that =)
nope, no cats on mine, its one of the earlier 3.9's i guess.
just got one of my plugs out and its got a buildup of something that looks like rust all over the electrode and cathode. should i be worried?

apologies for all the rookie questions =)

well, dizzy is in very good condition, but the vac advance is broken. you can suck air through the hose and hear it coming through from the vac advance unit.
would this cause the rough idle?
any idea where i can get a new one? fleabay?
also presumably i can still drive it, and it will just be the same as before?

yes
possibly

I reckon you want to get your local garage to put gas analyzer probe over coolant header tank-sounds like HGF and tipped heads.

DeadTom said:

tried that already, its definately not binding on the block, i think its just being stubborn.
also, what should the plugs look like in a healthy engine?
the one i managed to get out ok looks like the top one in this pic:

though mine is not quite as covered in carbon.
according to the website i stole that pic from, if it looks like that then it could be any of the following:

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Looks a bit rich, mine was too till I had it set up right, did you have your afm set up to 030?

DeadTom said:

well, dizzy is in very good condition, but the vac advance is broken. you can suck air through the hose and hear it coming through from the vac advance unit.
would this cause the rough idle?

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Should not affect idle at all.

DeadTom said:

well, dizzy is in very good condition, but the vac advance is broken. you can suck air through the hose and hear it coming through from the vac advance unit.
would this cause the rough idle?
any idea where i can get a new one? fleabay?
also presumably i can still drive it, and it will just be the same as before?

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Duff vac advance might cause some fluctuation but normally causes stalling when coming off the gas to stop. You should still be able to get the engine to idle.

You can get a vac unit from a variety of places - try Paddocks, Famous Four, Dingocroft etc.

Yes you should still be able to drive it but be ready to have the engine stall from time to time.

classic kev said:

Duff vac advance might cause some fluctuation but normally causes stalling when coming off the gas to stop.

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Can you nominate a source for your information.
A vacuum advance operating off Manifold vacuum has an impact when going off idle. That is, it switches off the advance when accelerating. But if the unit was not working it would have an impact on idle but not going off idle.
But, as with most motors, the rover V8 operates off of ported vacuum (not manifold vacuum) and has no impact on idle or coming off idle. To get power out of a motor you need a lower advancement of the timing. That is, no vacuum advance. It is only at cruise with light to moderate acceleration that vacuum advance has a benefit and when it actually operates.
Outside reduced fuel economy and the engine no being quite as smooth at cruise, most people would not notice a vacuum advance unit not working.

I have played around with running the advancement off of manifold vacuum before on a Rover V8 to get smoother idle, but you would not notice the difference anywhere else.

p76rangie said:

Can you nominate a source for your information.

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Simple physical observation. I've seen this a few times here and there.

Also, (no doubt you're going to correct me) running with the vac pipe disconnected causes a vacuum leak which is pretty much what happens when the vac advance unit packs up. This does have a tendancy to upset hot wire injection although I would agree the effect is only slight.

Perhaps if you re-read what I said - I think you'll find the word "might". I'm sure the rest of your technical analysis is correct but of course you need to account for the mechanical advance as well - does this not advance ignition timing as engine speed increases and the effect of inlet vacuum (ported or otherwise) drops off.

The long and short of it is this - there are many reasons that cause poor idle and rough running on Rover V8's often it is something simple so does it not make sense to start with the easier bits before getting bent out of shape? Who really cares how or why vac advance works, the important thing is it is better if the system works as it was designed and intended.

Cheers.

classic kev said:

Simple physical observation. I've seen this a few times here and there.

Also, (no doubt you're going to correct me) running with the vac pipe disconnected causes a vacuum leak which is pretty much what happens when the vac advance unit packs up. This does have a tendancy to upset hot wire injection although I would agree the effect is only slight.

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thats why ive left the vac hose on there, but blanked it off so it wont be pulling air in through the pipe. i figured it would make it run a bit lean if anything, as the air coming in through the vac hose wouldnt be noticed by the MAF sensor

well after taking off the old vac advance and blanking the hose, taking the dizzy apart and cleaning the rotor arm, cleaning the stepper and removing and cleaning half the plugs, it nows runs a bit better both idling in neutral and in gear, but the idle speed has gone up to ~1k when in neutral, which strikes me as being a bit high

DeadTom said:

well after taking off the old vac advance and blanking the hose, taking the dizzy apart and cleaning the rotor arm, cleaning the stepper and removing and cleaning half the plugs, it nows runs a bit better both idling in neutral and in gear, but the idle speed has gone up to ~1k when in neutral, which strikes me as being a bit high

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O.K. idle should be about 725 +/- 25 rpm. You don't say if you have replaced the vac unit but let's not worry about that just yet.

Have you checked the ignition timing?
Have you checked/set the base idle speed?
Have you cleaned the plenum and ram housing and checked/set the throttle disc?

If that doesn't get it to settle down you may well need to start looking at the sensors. Coolant thermistor is a reasonablt common gremlin, resistance values can be found in Haynes but do the other bits first.

DeadTom said:

well after taking off the old vac advance and blanking the hose, taking the dizzy apart and cleaning the rotor arm, cleaning the stepper and removing and cleaning half the plugs, it nows runs a bit better both idling in neutral and in gear, but the idle speed has gone up to ~1k when in neutral, which strikes me as being a bit high

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Do you notice the ECU adjusting the idle or does it just seem to stay at the one speed all the time. The ECU will only adjust idle if the car is stopped and the TPS indicates that there is no throttle being used.

If people have played around with it in the past to try and change the idle, they may have bent or adjusted the idle striking tab on the throttle body or may have adjusted the base idle setting.

Thats why we could never get mine to idle right , as the tps was never giving the right signal.

Sorted it with a new tps and now its better than ever!

havnt replaced the vac unit yet, but i did put the old one back on just to hold all the bits and pieces inside the dizzy from moving around.
havnt checked the ignition timing or anything yet, gotta get my hands on a strobe first.
now it seems to be that it idles ~1k when its cold, then goes back to its shaky nearly stalling habits and idling at ~700 when warmed up.
think im going to replace the plugs and vac advance, then see how it runs.

the only other thing i can think of is that the fibre washer i used in place of the gasket on the stepper is ~1mm thick, which i guess means the stepper doesnt sit quite as far into the plenum as it would have done with the paper thin gasket. would that cause issues?

p76rangie said:

Do you notice the ECU adjusting the idle or does it just seem to stay at the one speed all the time. The ECU will only adjust idle if the car is stopped and the TPS indicates that there is no throttle being used.

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when i slow to a stop and put it in neutral or park it will tick over at ~1k for a few seconds then drop to 750 where it will stay until i put it in gear and drive off again.
this happens pretty much every time i slow to a stop, so i guess thats the ECU adjusting the idle?

DeadTom said:

when i slow to a stop and put it in neutral or park it will tick over at ~1k for a few seconds then drop to 750 where it will stay until i put it in gear and drive off again.
this happens pretty much every time i slow to a stop, so i guess thats the ECU adjusting the idle?

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So it appears that you do not have an issue with the idle revs.

DeadTom said:

when i slow to a stop and put it in neutral or park it will tick over at ~1k for a few seconds then drop to 750 where it will stay until i put it in gear and drive off again.
this happens pretty much every time i slow to a stop, so i guess thats the ECU adjusting the idle?

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What's wrong with this? Sounds pretty normal to me.

So let's re-cap then. You know you need to replace the vac advance unit so you may get a bit of hesitation on opening the throttle and perhaps stalling coming to a stop but you're not getting the latter so it isn't really an issue.

The idle speed is about right at 750rpm although there is a slight rise in idle when you remove the transmission load from the engine - this is normal. The ECU then does its' thing and winds in the stepper to get the idle speed down so it is back to somewhere between 700rpm and 750rpm. Sounds fine to me.

At face value I'm not sure you have a problem. Fit new vac advance unit, check your ignition timing and hopefully be happy. While you're at it also make sure your ignition leads are properly routed and clipped, check the other connections - coil, ignition amplifier and also make sure you're getting good output from the alternator and the battery is good. Have fun.