My uncle has a 2000 Mercury Grand Marquis that has a check engine light that flashes once it warms up and then after about five minutes the light goes solid and stops blinking. It also loses power and feels like the car is going to die. When this happens he pulls over and stops the car for a few minutes and then it will go again for a few minutes and the process starts again. We hooked the scan tool up to it and it gave the code P0308 Cylinder 8 misfire. What would cause this? We checked the spark plugs and everything looks ok.Any advice on what to check next? I’m new to this so forgive me if this sounds like a dumb question. Thank you for your help.
wild guess . . . failing ignition coil
swap #7 and #8 coils
If you’ve now got P0307, you replace all the coils and plugs. Every single one
Pay very good attention to the female spark plug threads in the cylinder heads
Did any aluminum come out along with the spark plugs?
If so, be forewarned . . . plugs will blow out and time-serts are in your future
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A flashing Check Engine light means a major misfire is occurring. And to continue driving the vehicle with a flashing Check Engine light can damage the engine and/or the catalytic converters.
Tell him not to drive the vehicle until it can be found what’s causing the misfire.
Tester
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I think that the 2000 model didn’t get the PI heads, which had more threads for the spark plugs to hold on to. So yes, spark plug ejection is very valid concern here. The OPs car wouldn’t have the COP ignition that the later PI’s got, it should have a pair of two coil packs, one per bank, each handling four spark plug wires. So somewhat cheaper to replace than individual coils found on the later cars. OP didn’t mention the mileage but 120k-ish miles is prime time for the coils to start failing.
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Rock Auto and Napa Pro are both showing COP’s on the 2000 Merc…
If it has a dead miss, I would run a compression test on #8 cylinder while I already had the spark plug out… I have seen more than one drop a cylinder due to internal issues…
Buy yeah, swap coils, then maybe spark plugs to see if the miss follows…
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You’re right. I was always under the assumption the the non-PI 4.6L’s didn’t have COP ignition.
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My 99 Grand Marquis with 73000 miles gave me cylinder 6 misfire. Removed coil on plug to find some oil at the bottom of spark plug well. Vacuumed out the oil and cleaned coil on plug. No more misfire on follow up scans. Might take time for oil to accumulate. I should have used q-tip burn test to confirm that it was oil and not antifreeze. So how did the liquid get in the spark plug well?
Spark plug tube seal would be my guess
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Thanks. Any idea of how to get to the seal?
Thanks. How do those spark plug well seals connect to the spark plug wells? The spark plug wells are above the valve covers. Is there a passage way inside the valve covers that leads to the spark plug wells?
The (at least 1997-2004) 4.6L SOHC does NOT have tube seals, the spark plug wells are part of the cylinder head castings, the only ways to get oil into the plug wells is either:
- Spill oil when adding oil.
- Leaking from the valve cover from the top rail and leaking down to the plug wells.
- There is leakage past the threads and taper seat of the spark plug, it will leak air/fuel out of the cylinder, and there can/will be a trace of oil there, mostly from vapors recirculated to the intake manifold via the PCV system, and will slowly accumulate as the miles go by…by the time the plugs get changed, there can be quite an accumulation in the plug well.
Pic showing the intake ports with the plug wells to the upper right of the intake ports in the pic of a left (drivers side) cylinder head…
Great explanation with picture. Thanks.
I have the shop manual and there is no mention of seals.
I will check spark plug well tomorrow. If liquid present, clean it with a q-tip and use a lighter to confirm oil and not antifreeze. Tighten spark plug if required. Check valve cover bolts. Replace pcv filter.
Today 30f. Tomorrow 60f. Thanks.
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