Rover V8 rough running

[Harleyboy]

Hi, I have a 91 3.9 Range Rover V8. It has an intermittent misfire which is by nature hard to identify. Sometimes it is from cold start and others when hot and re-started. It’s really annoying me!

I have wondered if the leads could be at fault as have read that RV8’s are susceptible to bad leads? Could this be the case? Within the last year/1000 miles it has had new Powerspark distributor, new plugs, leads and coil. The leads were not expensive and I can’t recall the brand.

It only feels as though it’s dropping to 7 but there is no consistency to it. When running right it’s great!

So, is it worth changing leads to a better set or OEM spec?

Cheers

[GreenV8S]

Sounds like it has had a fair bit of work done on the ignition. Was that an attempt to fix this issue? If not, did the issue start around the time you did any of that? Any combination of wrong grade plugs, wrong rotor arm gap, wrong rotor arm, wrong plug/lead resistance, faulty/wrong coil, faulty/wrong ignition module or wrong fueling could lead to a misfire so I think you need to track this down methodically to have any chance. If you can get it to happen at idle this will be much easier to do.

[tr7v8]

Could be cross firing if the leads are poor & running too close.

[Harleyboy]

Yep, I had a really bad issue last year with permanent power loss. I frustratingly changed all the parts listed and only after spending ££££ did it get sorted....the timing was way out.

It always did have an occasional misfire but this was cured. It will now misfire on idle at times so I will try to remove leads to see which one doesn’t make a difference.

I figure it isn’t too serious as it can run perfectly.

I’ll take a look at the lead routing tomorrow. Any thoughts on going back to genuine leads?

Thanks

[paintman]

What leads are you using?

Replaced mine last year with a set of the ones that came from DLS in a Lucas box & no problems. RTC6551G.

There were no issues with the old ones - yellow 'Hot wire' marked ones - but as I was doing the top end I thought for the cost I might as well. The old ones are now the spares.

[Harleyboy]

To be honest I’m not sure but think they came with the distributor which is a power spark unit

[Steve_D]

tr7v8 said:

Could be cross firing if the leads are poor & running too close.

Leads 5 & 7 in particular.

Steve

[Harleyboy]

This is how the leads are. Definitely some that are close/touching so will re route those now

[paintman]

Check the routing of the leads & use the clips - your top pic shows 3 loose.
Might also consider buying some more & using them as bits clipped on just the leads to keep close leads in the longer runs separated.

[Harleyboy]

Thanks. All now clipped (found some in the garage) and none are touching. All running well so will take for a run later and report back - thanks for your input and help

[GreenV8S]

Ideally ensure that none of them are rubbing against other things too (such as the hose shown in that pic) although that is not always possible.

[Steve_D]

Just to be completely perverse and to demonstrate that ignition lead condition is more important we have had several customer cars into the workshop where the leads were all neatly cable tied into bundles....engines running sweetly with no issues.

Steve

[spitfire4v8]

I regularly see leads cable tied together and have done it myself. I don't think ive seen cross firing between leads for 3 or 4 decades. Does stuff like this still happen with modern leads (unless damaged)

[Mignon]

I would at least check the resistance of each lead and see if they meet the normal specs. Also when it's pitch black outside start her up and spray water on the leads from an old windowlene bottle or similar and see if you can spot any arcing. If that all checks out then most likely is an intermittent bad connection somewhere in the ignition system. Check all the HT female sockets for corrosion or damp. A quick spray with WD40 might not go amiss.