Post by Geoff on Jul 31, 2014 6:00:34 GMT
Hello, I have not communicated for several months, but here goes.
Can forum members shed some light on the following?
Ran the P5 mk3 auto last weekend through the Adelaide hills for around 2 hours of engine time, and it performed perfectly and normally. The descent back down to Adelaide was via Greenhill road (for those who don't know it, it is a windy road with several steep descent sections) for several Km and then a very gradual downhill incline through the Adelaide suburbs. Descent speed partially controlled via the B/W low range gear setting.
After a turn off Greenhill Rd, the engine began to run erratically, like a misfire on one cylinder. Anyway, to cut a long story short, I staggered home sounding like a chaff-cutter, and examined all the spark plugs. All were OK and correct colour, except for the plug on number 5 cylinder, which was black, shiny and wet with oil.
Replaced with a new plug of the same type and heat range, and the engine again ran smoothly. So, why did I have this one fouled plug, which was obviously not firing at all, hence the rough idle and running? The engine was recently (2 years ago) completely overhauled during the restoration, with new rings, valve seals, etc., and I have excellent oil pressure. Did I suck oil down via the inlet valve guide or guides during the descent? (one theory) Is there normally a large pool of oil floating around the top of the head before it drains away, and with the good oil pressure maybe there is a bit more there than "normal" to be sucked down by low manifold pressure during downhill coasting? (second theory) or just a bit of bad luck and a bit of soot lodged across the electrode and the plug ceased sparking, and over time this oiled up (third theory - now I am running out of theories...)
In the past, spark plug examination has not indicated any excessive difference between all of the plugs as far as soot, colour, or wetness is concerned. They normally have a light tan to white electrode colour and light film of dry soot on the metal parts. Plug type in use is Bosch W8CC, and I run 98 octane fuel (our Super Unleaded here, which has no ethanol in it).
Ignition is via an electronic distributor type "123 Electronic Ignition" (the Dutch one, which has performed faultlessly). There are all new HT cables.
Any clues, anyone??
I plan to run the car with the replacement spark plug for a few hours, and then remove and examine it. Maybe this will help in determining a possible cause? However, if any of you have other suggestions, I would appreciate your input.
Best regards,
Geoff.
Can forum members shed some light on the following?
Ran the P5 mk3 auto last weekend through the Adelaide hills for around 2 hours of engine time, and it performed perfectly and normally. The descent back down to Adelaide was via Greenhill road (for those who don't know it, it is a windy road with several steep descent sections) for several Km and then a very gradual downhill incline through the Adelaide suburbs. Descent speed partially controlled via the B/W low range gear setting.
After a turn off Greenhill Rd, the engine began to run erratically, like a misfire on one cylinder. Anyway, to cut a long story short, I staggered home sounding like a chaff-cutter, and examined all the spark plugs. All were OK and correct colour, except for the plug on number 5 cylinder, which was black, shiny and wet with oil.
Replaced with a new plug of the same type and heat range, and the engine again ran smoothly. So, why did I have this one fouled plug, which was obviously not firing at all, hence the rough idle and running? The engine was recently (2 years ago) completely overhauled during the restoration, with new rings, valve seals, etc., and I have excellent oil pressure. Did I suck oil down via the inlet valve guide or guides during the descent? (one theory) Is there normally a large pool of oil floating around the top of the head before it drains away, and with the good oil pressure maybe there is a bit more there than "normal" to be sucked down by low manifold pressure during downhill coasting? (second theory) or just a bit of bad luck and a bit of soot lodged across the electrode and the plug ceased sparking, and over time this oiled up (third theory - now I am running out of theories...)
In the past, spark plug examination has not indicated any excessive difference between all of the plugs as far as soot, colour, or wetness is concerned. They normally have a light tan to white electrode colour and light film of dry soot on the metal parts. Plug type in use is Bosch W8CC, and I run 98 octane fuel (our Super Unleaded here, which has no ethanol in it).
Ignition is via an electronic distributor type "123 Electronic Ignition" (the Dutch one, which has performed faultlessly). There are all new HT cables.
Any clues, anyone??
I plan to run the car with the replacement spark plug for a few hours, and then remove and examine it. Maybe this will help in determining a possible cause? However, if any of you have other suggestions, I would appreciate your input.
Best regards,
Geoff.
