Misfire on cylinder 5

[morgant]

I picked up a Disco II wit's a misfire on cylinder 5. No overheating or signs of head gasket leak, but had visible corrosion/whitening on the spark plug connection and inside the plug wire at the plug end, so replaced the plug wire w/new genuine from our generous hosts. Didn't resolve the issue, so I tried cleaning the spark plug contact and taking it out to see if it looked fouled (a bit of carbon, but didn't seem too bad, though I'm a bit inexperienced in that area). Still misfiring on cylinder 5.

My next step is a new plug when the FLAPS opens tomorrow. Then, coil? Can just the one coil be replaced or do I have to do the whole pack?

Update: new plug didn't solve the issue. I guess I'll try a coil next, but am starting to fear that I'm putting more good money to waste and that I've got a head gasket leak. Not noticing any obvious signs like steam in exhaust, overheating, etc. Can't tell if I'm getting excess pressure in the coolant and don't think I can smell exhaust in the coolant reservoir. Any other easy at-home tests or just go for a new coil?

[mearstrae]

If the spark plug is very clean in that cylinder, it points to steam cleaning via gasket leak. Milky looking engine oil shows a leak in that direction. A hissing sound at the coolant tank shows pressure being built up. The auto parts stores have a fairly cheap checking devise to check for combustion by-products entering the cooling system, it sets on top of the reservoir and a special fluid changes color if exhaust is present. Or a coolant pressure testing set-up to see if there's over pressure in the system. I hope this is of some help... If it's just mis-firing the plug, it may be the spark plug cable, a bad plug, or the coil pack. But don't discount a bad fuel injector, which may be lean or rich in it's failure.

'99 Disco II
'95 R.R.C. Lwb (Gone...)
'76 Series III Hybrid 109
'70 Rover 3500S

[morgant]

Thanks! It ended up being a bad coil, once I got it out I could see a crack running all the way around, so I assume moisture had gotten in and caused a slow failure. Very relieved and happy to have the Disco running.

Fortunately, I was able to do the wires & coil pack without removing the intake manifold. I will be doing valve cover seals soon, but wanted to either confirm the truck as running or as bad as quickly/inexpensively as possible, so that was an interesting way to do so. :p

[mearstrae]

Cool, that proved not to be too traumatic a repair. Only Chinese acrobats should try to change coil packs left in place, LOL. Generally if a coil cracks it's due to heat. (How could that happen in that wonderful coil pack location?) Great to see the Disco on the road to recovery so soon. While doing the valve cover gaskets be sure to check over the PCV system for clogs as well as leaks. A system not venting pressure properly will cause all sorts of leaking seal areas.

'99 Disco II
'95 R.R.C. Lwb (Gone...)
'76 Series III Hybrid 109
'70 Rover 3500S

[morgant]

Agreed, changing the coil packs in place was not easy. That said, if you don't have secondary air and have small hands, unclip the wiring loom from the bulkhead, remove the right-hand (passenger-side) spark plug wire bracket and it's quite feasible (you just have to turn the coil pack in place to unscrew each coil from the bracket, then pass them out the right-hand side [[your left]]).

Thanks for the PCV tip. This Disco is replacing a Volvo 850 which are also notorious for PCV clogs, so I'm all too familiar. I think AB's valve cover seal video shows replacing the oil separator for that very reason. Any other PCV components to pay special attention to?

[mearstrae]

All the hose connections, seems that Rover isn't to fussed about using hose clamps on these connections and they start to fit loosely and leak inward and outward as they please.

'99 Disco II
'95 R.R.C. Lwb (Gone...)
'76 Series III Hybrid 109
'70 Rover 3500S