My Misfire...

[Shug]

Right, some may know I've had a misfire with the car - ever since a cockup saw it rev to about 7500 from cold at startup last week.

Leads and plugs have been replaced. Definitely running on 3, not 2, so I'd think a coil was impossible. Checked injectors and they are all firing. Cam position sensor seems to be reporting sensible values (but what part of the VVC cam does this read?) Compression check shows decent compression in all cylinders, so no bent valves.

Could part of the VVC mech be fubared and the cam position sensor still see sensible values? Don't suppose one of the mechs could be gone in a way that only affected one cylinder?

Any other learned opinions folks?

[tenkfeet]

Have you checked all the electrical plugs are on from when you were tinkering around before .Tps ok ? Does the emerald try to protect the engine when it see's it going mental ? Is the throttle linkage now longer , does it run on part throttle ? Has the cat decided to fall apart ? Does the head sound rattly? Just guessing but hey.

What you did is the standard way to start a hire car and they keep running .

[woody]

Wasn' me.



Was just about to text and see how you're getting on with it. Did you get the bits of the damaged lead, and the plug it was stuck with, out ok? A few of the neighbours were also concerned I'd got another to break for spares.

Surely if it were the VVC mech. either all four or a pair of cylinders would be affected? Is it not possible that a coil can partly fail (don't know enough about these obv.) Have you traced what cylinder is not firing yet?

[Rich H]

Block and comp test (Wet and dry) first. I know you don't want to know, but....

[Shug]

Block and comp test (Wet and dry) first. I know you don't want to know, but....

Compression check shows decent compression in all cylinders, so no bent valves.

Comp check is fine. My only thought is that part of the eccentric cam system on one end of the VVC mech might have bust, leading to the outside cylinder firing okay, but one of the inners not running (if you know the VVC mech, you might get what I'm on about)

Wood - got the bits of plastic out fine, used a more sophisticated version of the bodge we tried (the nozzle was made in 2 stages and gas-flowed better )

Next task is pull plugs again, as I got it running on three for 20 secs or so and it stank of petrol (no firing flames out of the exhaust, so exh valves are opening at the right time) - so one will be soaking, that'll be the cylinder.

[robin]

Check the spark plugs, obviously, but it's hard to see how you get spark plug failure from over-revving.

Whip the cam cover off and check out whether both inlet cams are turning at the same rate; with the spark plugs out and turning the engine over via the crank pulley bolt you should be able to observe all eight inlet valves doing more-or-less the right thing - I'm sure you know what I mean.

If you were unlucky and made piston-valve contact you *may* have jumped the cam belt around, so I would check both front and back pulley timing marks line up properly at 90 BTDC.

Failing that, if you smash the VVC mech I think that it is possible to break just one cylinder - the VVC cams are independent of one another - each half cam is further split in half, IYSWIM, so yes, you could shag just one cylinder - though it would probably end up still sort of working, in that case.

If it does turn out that you've damaged the VVC mech I would split the head and inspect all the valves and valve guides for damage - it's easy to crack a valve guide.

Cheers,
Robin

[mwmackenzie]

It's not unheard of for one of the VVC cams to snap, Ali has seen this a couple of times and has one in at the moment with a snapped VVC cam. As Robin say the VVC cam is split so essentially it has 3 cams if one of the VVC cams snapped it might give you your symptoms??

2p

Mark

[robin]

If you do end up dismantling, I found this guy has put a lot of effort into taking good quality photographs of each step ...

vvc mech pics

[robin]

No, I think the VVC has 5 cams! One exhaust and four inlet, I think! The #2/#3 inlets are driven through independent shafts that run through the #1/#4 cams.

If you think through what has to happen to make the VVC work (each cam has to slow down during the valve opening period and speed up during the valve closed period, so that average cam speed = 1/2 engine speed), I think it follows that you cannot achieve this with #3/#4 being forced to rotate at the same angular speed throughout the entire rotation.

Cheers,
Robin

[Mikie711]

If it turns out you have snapped a cam, god forbid, give me a shout as I have a couple left over from on of the heads I stripped. In fact I have the whole VVC gubbings if you need any of it

[robin]

Shug, to answer your question about cam position sensor, I think it reads #1 position/duration, I think - possibly #2, certainly not 3/4.

Cheers,
Robin

[Shug]

Thank you Robin. Next task, whip cam cover off and take a looky. Mike - if it does look gubbed, I'll be giving you a call!

PS that link is brilliant. That's the first time the full operation of the mech has clicked in my head. Had a rough idea before, but that's cemented exactly what does what. Class

[Shug]

Phew... fixed...

Had a small brainwave - have had it happen before with the DTHTBs. Basically on these, one pair is the master pair and one the slave, with the slave working off a finger & yoke design to balance with the master set. There is a leaf spring as part of this that, if you somehow shove it, will hang up, leaving the slave pair slightly open with the master pair at idle.

Guess what's happened then?

Funny thing was, it was firing on three - maybe not the same three each revolution, but it wasn't obviously a big enough imbalance that it was going to totally kill the pair of cylinders affected. Thought of it as I was driving to my parents tonight - walked up to the car, tweaked the spring and *click* it snaps back into place. Turn it over and it catches and runs like a goodun.

Thanks f**k for that, as it now needs an MOT, tax and insurance - it would not have been going back on the road for a while if I needed to spend anything more on it!!!

[tenkfeet]

[woody]

Result. Glad it was something simple afterall. Probably wouldn't have been the full throttle start afterall then.