Misfire under acceleration

[Chim450]

I had a 4.3 engine rebuild earlier this year. I’ve done 600 miles since and have gradually been running it in with a couple of hard accelerations up to 3000 revs or so. Today it has been really lumpy under heavy load especially in 4th gear going from 30-60mph, it coughs and splutters and then seems to clear. It runs absolutely fine otherwise although there is a bit of a petrol smell.
I had the throttle bodies refurbed and the coil pack replaced at the same time as the rebuild. I’m thinking probably HT leads, but think they may have been replaced (not sure though). Has anyone had similar?

[so called]

What you describe sounds just like when my coil pack was on the way out.

[Chim450]

Ok. I think I have narrowed it down a bit. At idle the NS exhaust is puffing and rattling and the exhaust gasses feel cooler compared to the OS. So in my reasoning one or more cylinders are not firing on the rear bank. This all seemed to happen after I was accelerating on an uneven and bumpy surface. Do HT leads ever come adrift? I’m just loathed to remove the bonnet to have a look as I have just had it resprayed. Will it harm the engine at all if not running on all cylinders?

[Malcster]

I've recently fitted a sytec OPT020, and can confirm it's a perfect fit, and works well.

[nawarne]

Chim450 said:

Ok. I think I have narrowed it down a bit. At idle the NS exhaust is puffing and rattling and the exhaust gasses feel cooler compared to the OS. So in my reasoning one or more cylinders are not firing on the rear bank. This all seemed to happen after I was accelerating on an uneven and bumpy surface. Do HT leads ever come adrift? I’m just loathed to remove the bonnet to have a look as I have just had it resprayed. Will it harm the engine at all if not running on all cylinders?

Agree with comment re: coil pack.....don't discount HT lead core failure either.

Ignoring it will ultimately damage the cats/soot the lambdas too.....as well as petrol washing the bores and thus contaminating the oil!

Ultimately, I believe it's sensible to (carefully) lift main bonnet and inspect/replace the coil pack. For about £150 you'd eliminate the most likely cause of a misfire - a world of pain if it continues.


Nick

[Chim450]

nawarne said:

Agree with comment re: coil pack.....don't discount HT lead core failure either.

Ignoring it will ultimately damage the cats/soot the lambdas too.....as well as petrol washing the bores and thus contaminating the oil!

Ultimately, I believe it's sensible to (carefully) lift main bonnet and inspect/replace the coil pack. For about £150 you'd eliminate the most likely cause of a misfire - a world of pain if it continues.


Nick

It’s booked in at X-Works for Friday. Normally I would have a go myself, but just too nervous about damaging the paintwork. I’m going to get a bonnet conversion once the paint has had time to properly harden.

[Chim450]

Well.....after a long day at X Works it’s fixed. Swapped just about everything out (coil pack, ECU), balanced throttle bodies, performed leak down (result was excellent according to Paul), finally changed HT leads (they only had 500 miles on them!). Hey presto, it accelerates like a nutter. Now, to put them miles on and back to Powers for 1K service.

[Chim450]

Well.....after a long day at X Works it’s fixed. Swapped just about everything out (coil pack, ECU), balanced throttle bodies, performed leak down (result was excellent according to Paul), finally changed HT leads (they only had 500 miles on them!). Hey presto, it accelerates like a nutter. Now, to put them miles on and back to Powers for 1K service.