Rover V8 Misfire - advice please!

Rover V8 Misfire - advice please!

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1750Spyder

Original Poster:

4 posts

Yesterday (10:38)
quotequote all
Hi looking for some help / advice on diagnosing the cause of a misfire, Rover P6 3500, missing on Cyls 2 & 3, have identified this by removing leads from plugs in sequence when engine running and listening for changes.

So far - changed leads, cap, rotor arm, points & condenser, tried a coil off my GT6 on it and its still the same so coil must be ok and all other cylinders are firing ok, have correctly set dwell angle, engine starts ok and settles to a steady idle (apart from having 2 cylinders off)

  • EDIT The plugs are working on the dead cylinders, have taken them out and ran the engine and there is a decent spark* so electrical side seems ok.
No smoke, Did compression test, readings ranges from 110-120 Psi, (didnt have throttle pedal down as most guides say to do which may explain why the pressures are lower than typical values) the pressures on the duff cylinders are the same or similar to ones which are running.

Anyone have any ideas on what to look at next? could it by anything to do with the hydraulic tappets? I'm guessing its nothing to do with the carbs / mixture as its only on those specific cyls.
Thanks.

Edited by 1750Spyder on Saturday 19th July 11:45

andy43

11,580 posts

269 months

Yesterday (11:09)
quotequote all
From TVR experience (14CUX Hotwire) I d guess at leads, rotor or cap, even though you ve changed them. Chinese parts are notoriously bad for failures straight out of the box, and even a lot of the Lucas green boxed stuff is now poor quality imported stuff. What are the plugs like? Maybe mix and match your old and new parts and see if that helps? Don t suppose a P6 has plug extenders? Reroute leads perhaps?

ETA might be worth asking to move this to the TVR section.

Miserablegit

4,288 posts

124 months

Yesterday (11:31)
quotequote all
Take one of the spark plugs out on cyl 2 or 3 and check if getting spark. Alternatively put new sparkvplugs in cylinder 2 and 3.

Also - did you buy a good quality distributor cap? 2 sequential cylinders misfiring might suggest tracking on distributor cap between two adjacent contacts

Edited by Miserablegit on Saturday 19th July 11:42

1750Spyder

Original Poster:

4 posts

Yesterday (11:47)
quotequote all
Plugs ok, new set fitted and the plugs are firing on the dead cylinders.

TarquinMX5

2,249 posts

95 months

Yesterday (12:11)
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I would suggest setting out here precisely which cylinders are 'missing' as I've known different people refer to the v8 cylinder numbering differently, ie viewed from front or drivers seat, left/right bank etc..

I appreciate that you might know BUT if your numbering is different, it makes possible fault diagnosis more difficult.

Any other work done on the engine? Standard SU carbs?

Miserablegit

4,288 posts

124 months

Yesterday (12:11)
quotequote all
Are the plugs in cylinder 2 and 3 wet from fuel?
Any issues with the brake servo non return valve?


Edited by Miserablegit on Saturday 19th July 12:20

gt40steve

1,043 posts

119 months

Yesterday (12:38)
quotequote all
Remove the valve covers and make sure the valves are opening correctly ? These engines can suffer from camshaft and lifter wear.

jhonn

1,632 posts

164 months

Yesterday (12:40)
quotequote all
Are the plugs wet with fuel when you take them out? If so, and you have compression I would suspect fouled plugs. If the plugs are fouled be careful how you clean them - using a wire brush (steel or brass) can result in micro deposits on the ceramic that causes tracking when under compression.
If in doubt use new plugs in the affected cylinders - if they continue to foul you could try going one range hotter. As an aside I've found that when NGK plugs get badly fouled, no matter how well cleaned/dried, they're never the same afterwards.

Also, it's fairly obvious and apologies if already checked, but make sure that the correct leads are routed to the correct plugs, it can be easy to get wrong and is easily overlooked.

Good luck with it!

1750Spyder

Original Poster:

4 posts

Yesterday (13:15)
quotequote all
TarquinMX5 said:
I would suggest setting out here precisely which cylinders are 'missing' as I've known different people refer to the v8 cylinder numbering differently, ie viewed from front or drivers seat, left/right bank etc..

I appreciate that you might know BUT if your numbering is different, it makes possible fault diagnosis more difficult.

Any other work done on the engine? Standard SU carbs?
Hi, its as per the manual with No1 being front cyl on drivers side and No3 second one back on passenger side. All standard with nothing else done to it, 85K genuine miles, I've never had the engine apart in my 12 yrs of ownership and the previous owner who had it for 10 yrs prior to that didnt strip the engine either.

Miserablegit

4,288 posts

124 months

Yesterday (13:40)
quotequote all
I think you’ve swapped 2 and 3 cylinder leads

E-bmw

11,073 posts

167 months

Yesterday (13:43)
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jhonn said:
Are the plugs wet with fuel when you take them out?

Also, it's fairly obvious and apologies if already checked, but make sure that the correct leads are routed to the correct plugs, it can be easy to get wrong and is easily overlooked.

Good luck with it!
You beat me to it.

If you have spark & the plugs are wet then the two aren't there at the right time.

1750Spyder

Original Poster:

4 posts

Yesterday (14:13)
quotequote all
@miserablegit

The servo has been making loud whistling noises for a while now sound like an air leak, I wander if that could be affecting it, I'll try capping off the hose from the manifold and see if that has an effect. Thanks

stevieturbo

17,771 posts

262 months

Yesterday (19:16)
quotequote all
History of the engine ?

If old, worn camshaft/followers would not be uncommon. How recent or sudden did this problem present itself ?