You're really really sure there aren't any vacuum leaks anywhere?

I reinstalled the plenum and trumpet thingy with silicone, but thats twice i've had the inlet of and reassembled, so it would be pretty bad going to have managed to screw up the sealant twice.

Theres two takeoffs on the manifold for the gas system, but the hose was melted so i've blanked them off for now. I Might try and hose the inlet manifold with WD40 tomorrow with the engine running and see if that makes any odds.

And again, if it was a vacuum leak, you'd expect it to run fine with the MAF unplugged, right? As the vac leak causes the MAF to read incorrectly, and if the MAF isnt there reporting the incorrect data, the engine will run normally.

If it's GEMS then it won't idle at all with a faulty or unplugged MAF, but will kind of run above about 1200rpm.

Like i said, it ran fine before with the maf unplugged. The old one would crap out when it got hot. Unplugging it allowed the engine to run fine. It idled fine, drove fine, and (unintentionally) completed an MOT emissions test like that, albeit failing because it was rich. I only realised it was unplugged when i got home from the test!

I'm going to reconnect the missing LPG hoses in a bit, and see if it runs properly on LPG. If it does then it would seem to narrow the issue down to fuel delivery. However when trying LPG previously, when it did actually work(ish) on petrol, seemed to show one bank full rich and one bank full lean, which probably indicates an issue with the LPG system or injectors. I've noticed there appears to be no filter at all on the vapour side of the LPG. Might be one on the liquid side, but i cant see anything, so it wouldnt surprise me if the injectors are clogged up.

Mine only has a filter on the inlet side, nothing after the injectors, and the many I've worked on recently were exactly the same, I presume that's a standard install

Mine has no filter at all between the vaporiser and injectors. I dont know if theres one before the vaporiser, certainly nothing in the engine bay.

Right, spent some more time this morning banging my head off a wall. Tried it on LPG, didnt work, no better at all, so maybe its not a fuel pressure issue, or maybe the LPG is just broken as well.

Unplugged the maf, and got it running. Seems to act odd when first started without the MAF, you need to give it lots of throttle and really work hard to keep it alive, and after maybe 5-10 seconds it sorts itself out and starts running. I eventually got it to idle, by super-gently letting the throttle out. Initially it was surging up and down but eventually it settled to a rather unsteady idle. With it idling and me able to actually get out of the cab it became immediately obvious there was a vacuum leak. The hissing was so loud i could hear it from my garage 20 meters away clear as a whistle.

Grabbed a tin of brake cleaner and sprayed it over the number 1 injector as that seemed to be where the noise was coming from and the engine RPM's shot right up then the engine stalled. HMM!

So decided to pull the fuel rail out again (for what feels like about the 17th time) to try to see what was going on.

Discovered this:

https://www.dropbox.com/s/eqiuxgtvn62r6ij/2016-10-09%2011.41.46.jpg?dl=0

If you note the upper circlip groove position, you'll see that the Audi injectors (middle) have the groove a lot further down, and the new injectors at the bottom had both grooves. The lower groove means the injector sits further up into the fuel rail, and thus doesnt full engage with the manifold. When i fitted the new ones, i used the bottom groove, and thus those were also fitted incorrectly. So i swapped the injectors over to use the upper groove and reinstalled them. They fit much better in the manifold now, but having put it all back together and tried it, its still no better, and spraying the injectors with brake cleaner still causes the RPM's to shoot up. The hissing is quieter, but still there.

Its really stressing me out now.

So there's your air leak. You have to be careful fitting the injectors without damaging the O rings that seal them into the manifold. When I put mine together I slobbered them with the red rubber grease used on brake caliper seals. It might be that some of the O rings have twisted so aren't sealing.

If it doesn't run right on petrol, with a multipoint LPG system, it won't run right on LPG either as it is the petrol ECU that is controlling the fuelling.

If you need , I can grab one and give it a measure, I've only once used the original o rings when I removed the fuel rail, had leaks everywhere, change them all every time now. Think there 6mmx2mm

14.5mm on the standard Bosch. 15mm on the GEMS

The new injectors came with new o-rings, so ofcourse i used those, but they were 14.5mm, as obviously thats the standard size of orings used for the past 30 odd years. As usual rover decided it would be fun to do something different for no reason.

So, just took it all apart again, swapped the 15mm gems o-rings onto the new injectors, reassembled. Guess what? Exactly the fucking same.

Paying closer attention to my spraying, i think it might actually be the inlet manifold gasket thats leaking... spraying the joint between the manifold and head seems to make the engine rev up. But thats just insane, why on earth would that start leaking!?

I can see now why people warned me off these cars. I'm several weekends and probably a grand down, and i have a car thats more broken than when i bought it, and the more bits i fix, the more seems to go wrong.

Yep- they can be frustrating little minxes at times, but you did buy it for £360 as a non-runner so you're just about up to market price now.
That's not meant to be a smug p!ss take, by the way, just an attempt to put your troubles into perspective.
These cars were and are complex for their time. Now they're old, complex and tired. Yours and mine are 20 plus years old. Mine's already circled the earth 5 times, Gilbertd's around 12 times.
From my experience of rebuilding cars and bikes twice as old as these, anytime you touch something, something else will be disturbed that has sat there getting crunchy and decomposing since it was built. Anything electrical (cables and connectors) will cause pain, Anything flexible will break. Any seal will probably leak. Any fatigued metal will crack. That's just the way it is.
Your car's not turning out to be a "quick fix", which is frustrating and a bit of a kick in the ego. We've all been there and share your pain. It's time to have a cuppa or something stronger and walk away from it for a bit.
You know your way around a set of spanners, you have the right diagnostics tools, you understand how these things work and you have knowledgeable people around that you can call on, so you're not in a bad place. You've just got to methodically keep on the way that you have- identifying issues and dealing with them, one by one.
Stick with it. It is only a collection of a few thousand parts loosely connected, after all!

yeh i guess your right enough. I just know theres very likely cylinder head or piston ring issues lurking as well, i just want to get it to a stage where it actually runs properly before i start down to road of properly diagnosing that.

I'll need to see whats involved in doing the valley gasket, i guess i'll need to drain all the coolant as well :/

Have you blocked off all four vacuum taps on the manifold?

for the gas injectors? or for something else?

The gas injectors are all still connected up.

hmm passing thought, i should probably check the gas lines are snug and not leaking before ripping the manifold off.

If you're wondering about vacuum leaks I should not start at the gas injectors but at the manifold. Basic wrenching first, start with a fresh valleygasket, no need to drain all your coolant just a bit below pumpheight.

Install the gasket _by the book_that is in the right sequence and the right torque. Install manifold bolts in the right sequence and the right torque. This all seems a bit overdone but it is very important to exclude leaks afterwards, aluminium manifolds for V8 flex and stretch a lot compared to steel installs. When needed plug the LPG nozzles first and check it all in petrol mode and go from there step by step, you might borrow a compression tester to do basic testing cylinder by cylinder just to rule out as many causes as possible.

There are the two breathers, the fuel tank evap connection and the pipe running off to the fuel regulator. If any of those are leaking it'll throw your idle off.

Ferryman: Sure, makes sense. A lot of the hoses on this engine are a bit knackered though, and a loose fitting/perished hose on a gas injector would potentially cause a vacuum leak. I've been running it solely on petrol, as the LPG system needs attention but i want it working right on petrol before adding the additional variations of the gas system.

Gordon: ah right, yes those are definitely connected up. I had to fit a new bit of hose to the FPR and the evap one has some sort of quick-disconnect fitting. The two breathers i've also replaced as the hoses were split.

Well i ignored this for most of the winter. Couldnt be arsed. Finally got a mate round for the weekend and we decided to pull it apart and sort the inlet manifold.

Stripped it down this weekend, removed inlet manifold, replaced the valley gasket, put it back together, fired it up.

Exactly the fecking same. Spraying brake cleaner on/around the injectors on the passenger side caused revs to shoot up. Wont idle at all with the MAF connected. Will only idle with the MAF disconnected once its warmed up a bit and the lambdas come online.

Pulled the thing apart again, stripped the injectors out again, checked everything again and reassembled. No change.

I have a inlet pressure tester i use on my other turbocharged cars, It connects to the intake, and lets you pressurise all the boost pipework etc to test for leaks, so in desperation nailed that on the thottle body and tried to find the leak (why i didnt do this before stripping the manifold off i've no fecking idea!), which discovered air pissing out from underside of the throttle body....

Pulled the plenum off, and stared with disbelief at the cause. My mate called me a knob, i called myself a knob, and stared some more.

Pic:

I removed the heater plate off the bottom of the plenum when i removed it the first time. Completely pointless modification, but the pipes annoyed me and it doesnt really do anything anyway. Unfortunately, i completely missed the fact that three of those four holes are drilled straight thru into the plenum. So theres 3 fecking huge holes in the underside of the manifold.

Total fanny.

I'd also thrown the plate and bolts in the bin, and they're some nonsense imperial thread, so ended up having to drill them out and tap to M8, before threadlocking in some short bolts.

It was late, but put it back together and turned the key. Fired up and dropped straight down to a perfect idle. Not taken it out for a drive yet though, but fingers crossed its sorted.

Plonker..... However, what are the other two holes in the underside of the plenum? One near the top in the pic that appears to be countersunk and one next to the throttle cable plate. I'm fairly sure mine doesn't have them.