ABF lumpy, VW mechanic stumped!

[Classic16v]

Hi everyone,

I'm looking for some help with my Mk2 ABF converted engine. The conversion was done a couple of years ago, and I had absolutely no problems with it until a couple of months ago.  It's running the standard management system, straight out of the P-reg Mk3 it came from, and has a pretty standard cone air filter on it.

This winter it started to run a little rough at low revs/idle, and every now and then on a really cold day would be very lumpy and lacking in power up until about 2000 revs, until it had warmed up a bit.  As this was intermittent, I didn't think much of it.  It doesn't get used a lot in my current job - maybe a couple of times a week - and after coming back from a week away at the beginning of February (having sat in an airport car park in the snow) it started fine, ran very roughly, then died as soon as my foot was off the accelerator.  Again, started fine, revved ok, but died once my foot was off.  I managed to get it home (died a couple of times coming up to junctions where I had to stop!), and then all of a sudden it miraculously started idling again, albeit in a very lumpy manner, like it was mis-firing.  At home I removed the ISV, soaked it in white spirit for a couple of hours and then flushed it through and dried it out. Once it was back on, the car would no longer start!  It was still humming/vibrating with the ignition on.

The garage round the corner from me came and towed it - the mechanic there worked at a main VW dealership for 8 years and used to have Mk2's, as did a mate of his.  He's now had the car for 2 weeks (as I've been away, and not needed it) and has tried lots of different tests (luckily without charging me too much!) but has not got very far.  Below is a list of the things he's tried:

 - new HT leads, ignition amplifier, ignition coil, distributor
 - worn vacuum hose plugs replaced on air intake system
 - tested spark plugs (said they have been flooding, so when it doesn't start from cold he's removed them, lit them to dry them, replaced, and then it starts)
 - checked engine timing and ignition timing
 - wet and dry compression tests (dry test all within tolerance limits, wet test suggestive of slightly decreased compression in cylinder 2)
 - tried running diagnostics (not Vagcom - another system) on 16 pin connector but it only showed very basic info, and no live data. Didn't show any faults
 - also tried adjusting injector pressure (floods itself sometimes on a cold start, and he's had to remove the spark plugs, and dry/heat them before putting them back in to start it)

So we've got to the point where it starts (sometimes...), and while it's running it's still lumpy at idle.  Whilst idling (700-800 revs) it intermittently drops revs sharply, but the engine management seems to catch it around 500 revs and kick it back up to 800. It's misfiring up until about 2000 revs (especially under load), and struggles to rev up through that range, but above 2000 it goes ok (still feels down on power though). Every now and then it pops, like it's backfiring. If the revs are held around 3000 they'll sometimes drop down to about 2500 sharply, and sometimes when engine braking just under 2500 revs it becomes very jerky.  The mechanics final comments are "engine is breathing heavily and suspect worn or mechanical failure."

So any thoughts would be appreciated!  ISV? Crank sensor? My knowledge is limited, so any ideas would be gratefully received.  I appreciate I may need to spend a bit more changing various bits, but I want to do it in a logical fashion, and I'm currently saving for a wedding so I'm a bit limited!

Thanks in advance,

Alex

[Simeon]

*Update 30/04/2014 - page 5 - found crank pulley keyway was stripped, fixed that but still not running right*

Couple of additional points (I fitted this engine) it's running mk2 coil set up and has had, since the conversion just over two years ago, new lambda and ecu temp sensor (non genuine). Engine was on 89k when fitted, prob around 110k now so not a high miler.

Loom was checked and wrapped before fitting, particularly the wiring to the throttle body. Alex has checked the idle switch is operating correctly.

[tshirt2k]

They replaced all those bits without changing th ecu temperature sensor. Cold start problems and wet plugs are the clue. Hence the reason I wouldn't go to a vw specialist.

[Simeon]

Presumably it wasn't changed as it's relatively new, easy to rack up a large bill if you start swapping out everything. Worth a try though, thanks for the suggestion

[Classic16v]

I can see how the temp sensor affects the startup, but can that still explain the lack of power and misfiring when revving up to 2000 revs with a warm engine?

[Chris.]

Yes, It affects fuelling.

[tshirt2k]

I can see how the temp sensor affects the startup, but can that still explain the lack of power and misfiring when revving up to 2000 revs with a warm engine?

If the ecu thinks the coolant is cold then it will I over fuel, as cold engines need more fuel to run. It will be very rich all the time and overly rich mixtures don't produce much power.

Seems a large bill of parts has been racked up already, may as well add a little bit more. 

[Simeon]

If the ecu thinks the coolant is cold then it will I over fuel, as cold engines need more fuel to run. It will be very rich all the time and overly rich mixtures don't produce much power.

But conversely, it shouldn't have any trouble starting when it's cold...

Either way, Alex will no doubt report back on whether it fixes it

[tshirt2k]

If the ecu thinks the coolant is cold then it will I over fuel, as cold engines need more fuel to run. It will be very rich all the time and overly rich mixtures don't produce much power.

But conversely, it shouldn't have any trouble starting when it's cold...

Either way, Alex will no doubt report back on whether it fixes it

Depends what resistance it is when it fails.

[Diamond Hell]

Check crank sensor, too.  Might be in the process of failing.

Always handy to have a second engine to swap these things out from.