VR6 misfire under load (now sorted, cheers)

[Funkster 0]

Hi All,

 

My 'rado developed a misfire just before christmas. I was out driving in it and it was behaving itself, and then all of a sudden 2 cylinders dropped out. If I let it rev quite high with minimal throttle, I think all 6 are firing, but as soon as I plant the pedal it goes back to running on 4. There are times, I think, when 5 can be felt running but it's light throttle and at some mid-point in RPM before all 6 light up.

 

Research on here has shown that the coil pack is a likely suspect, and mine has a crack next to one of the mounting bolts, but given the price I'd like to know for sure before replacing it. Has anyone local to me (leamington spa, warwickshire) got vag-com? I don't fancy driving it with the misfire, and anyway Stealth (10 miles away) are shut when I'm at work (

 

The reason I'm only posting now is that it took this long to get an HT lead removal tool. Changed the plugs today too see if that made any difference (it didn't) and the old plugs looked pretty good, if a bit tired.

 

Many thanks in advance for any hints/advice/help!

--

Olly

[dr_mat 0]

Sounds like the coilpack, if you've lost 2 cylinders.

There's three coils, so two cylinders go at a time with it.

And I don't think a coil pack fail will show up on VAG-COM, either. (The engine will just notice the extra fuel in the exhaust and try to run it leaner on the remaining cylinders, I don't think it'll be able to directly tell that there's a misfire (or no-fire) on some cylinders. Could be wrong, but...

[Funkster 0]

Indeed, good old wasted spark eh?

 

Tricky to prove for sure though as it's just by feel and I'm more used to 4s and 8s. Maybe someone kind will lend me a known good coil pack for a few hours to try it out :oD

 

Bentley book on the way btw, but from the states - so please expect a few dumb questions before it arrives )

 

Cheers,

--

Olly

[Funkster 0]

Oh, yes as you mentioned it - I know there aren't any coilpack fault codes... I was just wanting to check that it wasn't something else. Would be painful to shell out for the coil pack and then discover that it's a sensor!

 

Thanks,

--

Olly

[dr_mat 0]

I think all your evidence points at the CP, no other combinations of failures would give a double misfire and ONLY a double misfire (on the same cylinders to boot!), that I can think of.

[Funkster 0]

Right you are.

 

Just had a fantastic idea - I shall break out the HT current clamp and my picoscope, and measure the current in each of the HT leads. That should tell me what's up - I've used this trick before with my MGB GTS which has home-made coil amps and it's very revealing.

 

What I need to know though, is which wires are which on the LT side of the coil? Any chance of a wiring diagram ahead of my manual arriving? Beer tokens available if someone can do me a scan of the relevant page!

 

Thanks,

--

Olly

[Funkster 0]

Humph.

 

Results so far are inconclusive. I got the 'scope out and started poking around, and couldn't find any HT output that was consistantly weaker than any of the others. However, I did see quite a lot of variation on each plug between bangs.

 

That said, here's a bang from plug #1:

http://homepage.ntlworld.com/oliver.stephenson/p1.png

 

and another from plug #4:

http://homepage.ntlworld.com/oliver.stephenson/p4.png

 

Plug 4's HT current is only 1/5 of the plug 1 current on this bang, but I'm sure I saw healthy currents in that lead at other times and weak currents in #1 sometimes also.

 

Waggling the HT leads around seemed to make a bit of difference, but that might well have been a placebo effect - me trying to decide that the leads are faulty rather than the coil )

 

Ho hum... I guess I'll get the analogue 'scope on it tomorrow and see if that turns up anything interesting, and maybe try the strobe too. I tried spraying water on the leads, and didn't see owt.

 

Can anyone lend me a set of HT leads? :oD

 

Cheers,

--

Olly

[Roger Blassberg 0]

The coilpack fires both cylinders in the pair simultaneously, one only as a power stroke. It is necessary for both HT leads of a pair to be properly earthed for either of them to function. A faulty HT lead on one of a pair will cause a misfire on the other one of that pair, apparently showing two faulty cylinders where only one is in fact faulty, so maybe you have just one leaky HT lead.

 

Am I teaching Grandmother to suck egs??

 

Best wishes

 

RB

[Funkster 0]

Not at all! All advice is gladly received.

 

Any idea what resistance the HT leads should have? I'll measure them this evening and see if one is wildly different, but if they're all quite similar they could all be performing poorly with only one being bad enough to miss. Someone wise once told me a good range of resistances for plug leads, and I foolishly didn't write it down anywhere :-/

 

Thanks again,

--

Olly

[Kevin Bacon 5]

5 to 6Kohms

 

Just buy the bloomin coilpack :lol:

[Funkster 0]

Just buy the bloomin coilpack :lol:

 

My credit card is wilting just at the very thought of it )

 

Thanks,

--

Olly

[Kevin Bacon 5]

I think German & Swedish sell them for £110 + VAT, so better than the dealer price of £180!!

[Funkster 0]

I used to work right next door to german & swedish. Will that be the lower quality one (is it BERU? I'm sure I read that somewhere on here) or the better OEM one?

 

Muchas gracias,

--

Olly

[Kevin Bacon 5]

It was the Vortex people that were complaining about the Beru ones but as far as I know, nobody on the C forum that's used one has run into problems yet.

 

The original ones are made by TEMIC but IIRC they both have the same Telefunken internals.

 

The worst that can happen is you keep getting new Berus under warranty if they fail within a year!

[Funkster 0]

Cool, thanks for the info.

 

Just checked my HT leads - all can be made to read ~5.7k with some wiggling, but numbers 2 and 4 read 100k or even open circuit (meter goes up to 2M) most of the time. They don't look like the sort of leads that I can take apart and repair, so I guess I'll invest in some new ones before replacing the coil pack - even if it doesn't fix the miss, these leads are definitely past their best.

 

Thanks for the support,

--

Olly

[Kevin Bacon 5]

Sounds like you may have found the cause then! As the tension increases under load, they're giving up and arcing out to the head!

 

I highly recommend the VW red ones, they're not cheap but good for another 10 years. If not, Blue Igniters from C&R are a worthy substitute at £70 and have factory ends on them for a good fit.

[Supercharged 2]

Blue Igniters from C&R are a worthy substitute at £70 and have factory ends on them for a good fit.

 

Lifetime warrantee too I might add...

[Funkster 0]

Where does one get red vw ones, and how much are they? Just had a quick look at the C&R site, and I think 77 quid is about as high as I'm prepared to go for HT leads. I'll give 'em a call tomorrow and see how soon they can get 'em to me.

 

Presumably the 61 quid ones from the VW dealer or equivalent from german & swedish would basically do the job with no worries?

 

Thanks,

--

Olly

[dr_mat 0]

The cheapie set from the dealers are pants - don't waste your cash...

Either buy the two leads you need from the dealer (the quality, original ones are available as separate items), or buy a full OE set of black ones (they're only red when you buy 'em from VW) for about £70.

[Funkster 0]

Sorry, I'm being a bit thick here... where do you get a full set of black OE ones if it's not from the dealer?

 

Thanks,

--

Olly

[Kevin Bacon 5]

VR6 leads are quite critical as they're difficult to seat on the plug properly without a metal tool and the engine literally roasts them aswell, so quality is paramount imo. Hence the OE red ones.

 

The red ones are super tough.

 

The black VW ones to avoid are the Quantum ones or any lead with a "BBT" end on it. Look for Bremi or BERU ends, which both the Blue igniters and red VW ones have.

[dr_mat 0]

To clarify what I said: VW will sell you the OE (red) leads individually, or a black set that are pants and to be avoided. You can also get a full set of the OE BERU leads (only in black), from some other company, possibly C&R but I am having trouble remembering..

[Funkster 0]

Okay, thanks for the clarification. I'll go ahead and order a set of blue igniters as long as the lead time isn't too long.

 

Cheers again,

--

Olly

[Kevin Bacon 5]

Normally next day if C&R have them in stock.....

[Heardy 0]

The leads that i got from Euro car parts are excellent.They are numbered the same as the originals and have Bremi ends. They also fit in the guides too :)