Missing a beat or two

[Roger Blassberg 0]

Just of late the VR6 is starting to miss a beat or two when opening the throttle from low revs/min. under load, in such circumstances as coming out of a roundabout.

 

The plugs are new-ish, (5000 miles at the most). I took them all out and wire-brushed them; nr.1 plug was a bit black, but not too bad.

 

Apart from this, the engine runs very well and smoothly above 2000 revs/min. and gives good mpg., new lambda probe, no obvious leaking sparks from the leads. I'm not certain as to the state of the coil-pack, though. If I spray water at it (a very severe test I know), there is a leak from nr.4 lead cap(centre, top row) upwards. But in the dry there is no apparent sparking.

 

Is this indicative of a coil pack in its last throes of reliability? It looks to be fairly easily removed: is it just a case of unscrewing the four socket-headed screws and unplugging the leads? What are the telltale signs of failure, if any?

 

The other question is this: if, and only if, I fully open the throttle suddenly by operating the quadrant on the throttle body, there is a momentary gulping induction roar and drop in revs, a slight pop in the exhaust and then a recovery to the expected revving up. All hoses seem to be secure, but there is a constant hissing coming from the area around the ISV and what I take to be its silencer (a plastic housing with rounded edges right at the back of the engine and close to the heater pipes as they go to the bulkhead). Is this normal or is there an induction leak somewhere? Normal, more progressive throttle operation is entirely as expected.

 

Best wishes

 

RB

[andy 0]

The hissing is normal, its the ISV sound deadner, air just passing through it.

Coil pack removal is as you say, plus removal of the multiplug on the top of it thats currently hidden by the lead guide/cover.

I'd go with the leaking ht lead personally, you need to eliminate this first, most running faults are ignition system related. Your test, though extreme is highlighting a problem that may not manifest itself when you look under the bonnet normally but may cause a misfire under load as you described.

Something else, not that its that important is the fact that you wire brushed your plugs, afaik thats not recommended these days.

[Roger Blassberg 0]

Thanks for your thoughts, Andy. I'll try swapping nr. 4 lead with nr. 5 (if they will reach) and see if the arcing is transferred with it.

 

Do you think that the plugs have been damaged by the wire-brushing? - I used one with brass wire bristles so not too abrasive.

 

What about this "gulping" when I snap the throttle fully open? I accept that, the laws of physics being what they are, the engine cannot respond instantaneously to any given throttle position and that maybe I am being over-critical of an engine with lots of rotational inertia.

 

Best wishes

 

RB

[andy 0]

No I wouldn't have thought the plugs would be damaged for now, I think its about longevity thats all. I certainly brushed mine off until a couple of years ago when I found out you're not supposed to!

The gulping sounds familiar to me, it sounds like a standard VR6 characteristic, my VR6s have all done it. (If its the same gulping!)

[Trig 0]

As far as the gulping goes take a look at this topic & see if you think it's the same thing or not... Pay close attention to RW1's post. :)

 

http://the-corrado.net/.archive/forum/viewtopic.php?t=27154

[dr_mat 0]

The gulp is pretty normal. You should hear it when you fit an induction kit!

The revs shouldn't drop though. Perhaps your MAF sensor is fubar? Does it get worse if you unplug it?

[Roger Blassberg 0]

I'm confident that the MAF is ok - it's quite new. If I unplug it, the engine stalls from idle. The drop in revs is very slight and transitory, and doesn't threaten to stall.

 

It sounds as though this is a bit of a red herring.

 

Best wishes

 

RB

[Roger Blassberg 0]

Back to the misfiring.

 

I took off the coil pack and had a good look - no cracks or signs of deterioration - so I got a set of replacement HT leads and the problem seems to have been solved.

 

Thanks for everyone's interest and suggestions.

 

Best wishes

 

RB