Could do with some help from the more experienced guys on here so hopefully some guys will chime in.

 

My brother has my old 1.2 9n and its now developed a misfire it seems (happened a couple of times now) - strangely it only triggers the MIL on start up, idles rough whilst 'on choke' and then when being driven seems to be fine. I have cleared the code before but have forgotten which cylinder the misfire was detected on, ill start tracking it from now on. The plan moving forward is as follows

 

- switch ignition coil from problem cylinder to another, leave it until the light comes back on and see if it has moved.

- Plugs were done 10k ago but if the problem persists on the same cylinder even after swapping the coil, i'll then take a look at the plug in that cylinder.

- If swapping the coil and inspecting the plug doesn't lead to either the misfire moving or suspect looking plug then I'll investigate further.

- If all of the above doesn't lead anywhere I'm thinking personally it needs a compression test, with the idea that it may have a cracked valve or valves (the 1.2's are prone to this) 

 

I also am not ruling out it possibly being an injector problem, i may be able to check the long term fuel trims with VCDS which could potentially point to it being a dodgy injector if its reading lean, however I'm not sure why the misfire would only be detected on start up from cold. Hoping to get someone else's take on the problem or some possible theories as to why its only present on start up. May be looking into this way too deep but i like to get stuff right rather than throw parts at the problem. Hopefully it isn't a cracked valve but I'm fearing the worst.

 

Finally, if there was a cracked valve would that cause it to burn oil? since there has been no obvious change in oil level and no blue smoke.

 

TIA

Swopping the coil pack would be the first thing to try along with a compression test & if you have the kit to do a leak-down test ? if after swapping the coil pack it stays with the same cylinder then swop the injectors

Will get that done once he's home. Hoping it's not a cracked valve.

 

I don't have the kit do do a compression test or a leak down test but will either pay for it to be done if it's a reasonable price or invest in the tools to do it ourselves once we exhaust all other possibilities 

You can get a compression tester for under £12 on ebay http://www.ebay.co.uk/itm/AUTOMOTIVE-PETROL-ENGINE-COMPRESSION-TESTER-KIT-VALVE-TIMING-GAUGE-PRO-CYLINDER-/350906166022?hash=item51b3a32b06:g:Fa4AAOxyA7tSZXqb  Leak-down testers are more expensive and need an air compressor

Cheers nige!

 

so upon scanning the car I found fault codes for misfires on cylinder 1 and cylinder 3 as well as multiple cylinder misfire code. 

 

Ou oh and fuel too low code, that's to do with the fuel sender, unsure why that is there at the moment but it cleared just fine and hasn't come back.

 

i checked the long term fuel trims for idle and they were at -10% ish! Very lean! But for partial (normal driving) they were zero'd.

 

im now thinking is it likely that all injectors are failing intermittently and only at idle - or the coil packs? Seems very unlikely, though I can't completely rule it out I guess. Starting to think there may be a vacuum leak somewhere causing it to run badly at idle, confusing though since you'd think I'd have a system too lean code, unless it won't show since at idle/until the car is warm or off choke as such, the ECU runs open loop lambda, hence signals from those sensors may be ignored? 

 

Vacuum seems likely though would that cause multiple cylinder misfiring. My gut feeling is yes, simply since more air is getting in than the ECU allows for. 

A vacuum leak will cause un-metered air into the system which will make it run weak until the system goes into closed loop and the lambda signals are then used to try and trim the fuelling schedule, but it wouldn't explain why it's only showing on 1 & 3 cylinders?

There was four codes

 

one was cylinder one, the ther cylinder 3 and then another code for multiple cylinder misfires/random.

 

i imagine that means they've all been misfiring.

 

the other code was the low fuel level.

 

must get the carb cleaner out and spray it on the vac lines to see if there's a leak 

Multiple misfires just means more than a single cylinder, so  as you've got codes for 1&3 that'll explain that :)

Swapped coil 1 with coil two and misfire didn't move after a few drive cycles.

 

Yesterday i removed the PCV valve and i was able to blow through both ways, which doesn't seem right? Potentially not restricting enough so getting too much air back into the inlet manifold? need to properly check the pcv i guess. If it was me i'd just buy a new one and hope but my brother is an apprentice so want to avoid throwing stuff at it without solid evidence that its the cause. If the PCV doesn't solve it ill be compression testing it.

The PCV valve should only open when a vacuum of 0.8 BAR is applied, here's the test procedure PCV testing.doc

Thanks again nige. I did find that procedure on elsawin, however i don't have a tool to test it. What i'll do short term to see if it is the PCV causing the problems is remove it, add a filter to the rocker cover to vent to atmosphere and then block the port on the inlet, monitor fuel trims at idle and hope they zero out. If they do then i'll be sending my brother down to vw for a new pcv valve. 

back to this - hate to bump an old thread but my brothers car just chewed an ignition coil (now sorted) 

 

i suspect there is still a bit of a problem with it, going to monitor fuel trims tonight though. Occasionally at idle it seems as if its going to stall, the engine judders and then it almost wakes up and springs back to life. I know this is a common problem with these cars but i'd like to get to the bottom of it - it'll help me and others no doubt. 

 

Anyone got any ideas they can throw in the hat that might cause that dodgy idle. Sometimes its fine and then you can drive off, a few m ins later come to a stop and it'll idle like crap again so very random.

Have you cleaned the TB ? or tried to do the TB alignment with VCDS ?

Yeah it was taken off, cleaned and re aligned not long ago :) 

 

starting to think from the stuff on the web that the timing may be out due to the chain jumping a tooth or two. Will try to confirm with vcds tonight though, hope this isn't the case though.

So I've discovered it likes to misfire its tits off at idle - mainly cylinder one but a few times on cylinder 3. Literally as soon as it gets to idle it'll misfire, runs really rich (10% or so) 

 

not sure if rich because misfire or ire or rich because of something else.. then causing a misfire because it's saturated. 

Swapped injector 1 with injector 2. Same symptoms pretty much as soon as it starts to get over 60 degrees it'll start misfiring on cylinder 1. Me thinks valve damage :( though wouldn't that mean it misfires all the time?

It would normally, you might have a carbon build up on that cylinder/valves that takes time to get hot then as soon as the fuels injected the red hot carbon ignites the mix early causing the misfire ???