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[sjc]

It's possibly the actuator on the rear multi-ram, you could try switching the plenum between the cars.  Just remember, everything you eliminate gets you closer to the fault

Found this thread with some relevant info on how the system works - rear valve open at idle and over 3000rpm, actuator operates to close it between those 2 points.

https://www.vauxhallownersnetwork.co.uk/threads/multi-ram-problem.86442/

[Tick Tock]

Quote "Its not connected right.....in fact its connected very badly....!"

8 hours faffing about and no further forward. Swapping the pipes to the correct position didn't make any difference, but thanks to TB earlier I suspect there should still be vacuum held in the system after turning the engine off.....  which I haven't got and need to investigate further.

I suspect it's something stupidly simple - we'll get there.

[TheBoy]

If no leaks in the vac tubes and elbows, its common for the one way valves fail. You can get inline one way valves if this is the issue, but prove/investigate vac system first...  ...before spending more time swapping stuff

[Tick Tock]

Not going to get a chance to work on the car today.

Next session I'll start from scratch and remove and check all vacuum piping, refit as per maintenance guide, and report back.

[Nick W]

Not going to get a chance to work on the car today.

Next session I'll start from scratch and remove and check all vacuum piping, refit as per maintenance guide, and report back.

As TB said, you should check all the components too.
Which is where the vacuum pump comes in handy.

[Tick Tock]

Armed with a vacuum pump, pieces of pipe and soft plugs.......  Disconnected manifold pipe at plenum and sealed - system holds vacuum for at least 5 minutes. I can confirm the multiram vacuum tank non-return valve is fubared, and that's the same on the other car which runs well. A new inline non return valve will keep vacuum in the multiram pipework (on the shopping list), but I really don't think this is the cause of the rough running at low revs. When I unplug manifold pipe from brake servo, it hisses, so obviously that non-return valve is working.

Operating the multirams by hand when increasing revs makes no difference, so I'm back to square 1.

Fuel? No problem at high revs and the car goes like a scalded cat.

Ignition? Runs sweet on tickover.

Any other suggestions appreciated - and thanks for the assistance so far.

[sjc]

Is it still sluggish under 3000rpm with your foot to the floor?  Wondering if it’s MAF related, pretty sure you can test that by unplugging it - will bring on EML but should still run just not at 100% efficiency. Same with O2 sensors.

[Tick Tock]

That's a good 16 hours in so far....  I'm no further forward and the swear jar is overflowing.

I've learnt a lot about the vacuum system and multirams, but as much as it pains me to say, I'm going to have to start swapping parts from the good car to this one just to try to get to the bottom of the problem.

[cam.in.head]

yes .when you have another identical car part swapping is a good idea. one at a time until the fault swaps too !
some frown on this idea but nothing wrong with it providing you dont end up with 2 faulty cars !
and you keep tabs on what you are swapping and doing it methodically.
fault finding the other(technically correct way ) is fine normally until you have one of these 'strange issues'.
in the end you usually find its something simple or obvious and the signs were there !

[Tick Tock]

Update - as I didn't seem to be getting very far at the weekend. Took the car out for a fast drive to see how it performed at high speed and when I got back it seemed to be running worse than before. This is after all the engine bay pipes have all been connected up as per the diagram.

Removing all the spark plugs, number 2 & 5 are much darker than the other 4. Now suspecting coil pack maybe breaking down, so spent time removing it. I really don't like the idea of trying the one from the good car, as it's a nightmare to get to, so have a new coilpack on its way. No cracks or bulges or any visual indication it's faulty.

When it's all back together with some new plugs, I'll report back.

[sjc]

Yep, sounds like coil 3 in the dispack is on the way out or you have a shorting plug lead to either of those 2 cylinders.

[TheBoy]

Before swapping parts, fix the know problems if you haven't already (ie, the vac), else you could be chasing phantoms for ages and making things worse.

[Tick Tock]

After waiting several days for coil pack to arrive, fitted it this afternoon and reassembled plenum, intake, scuttle, etc, and now it runs just sweet again - job jobbed.

Vacuum non return valve ordered for multiram system, still hasn't arrived yet, but at least engine is responsive once again and under bonnet plumbing is in order. 

[sjc]

Happy days!!!!

[grifter]

After waiting several days for coil pack to arrive, fitted it this afternoon and reassembled plenum, intake, scuttle, etc, and now it runs just sweet again - job jobbed.

Vacuum non return valve ordered for multiram system, still hasn't arrived yet, but at least engine is responsive once again and under bonnet plumbing is in order.

Just out of interest, where is that located in the bay? you said it's fubared but does it give any obvious symptoms of failure other than not holding vac?