Shorted Spark Plugs
#1
08-09-2005, 09:10 AM
Shorted Spark Plugs
All,
I could use some help. I have a 1998 F-150, 4.6L, with 180,000 miles. Four times in the last 35,000 miles I have encountered a miss while driving up a mountain incline, in hot weather. All four times, the number 3 (4?) spark plug electrode was shorted to ground. Almost like someone dropped the sparkplug. Each time a new sparkplug is used and it happans again.
Anu ideas?
-GB
I could use some help. I have a 1998 F-150, 4.6L, with 180,000 miles. Four times in the last 35,000 miles I have encountered a miss while driving up a mountain incline, in hot weather. All four times, the number 3 (4?) spark plug electrode was shorted to ground. Almost like someone dropped the sparkplug. Each time a new sparkplug is used and it happans again.
Anu ideas?
-GB
#2
08-09-2005, 10:06 AM
Postmaster
That's not good. Usually this means something hit the plug. I assume no one is dropping the plug or forcing it when they install it.
I am not familiar with this engine so my guesses are just that, guesses. Most of this info is from reading years and years, I have never had this problem except for fumblefingers.
On a really loose engine, yours might be with 180K, there may be enough slop in the piston, rod, crank assembly to have the piston touch the plug under extreme load or high RPM, not unusual during a mountain climb eh. I would be a little surprised on a modern engine since I don't think the clearances are that tight, but way back when people were doing 10 and up compression this could happen.
Alternatively it could be you have a hot spot in that cylinder that is causing you to detonate which could pound the plug. But usually there will be other symptoms too like loud knocking and eventually piston damage.
How do the plugs look otherwise? Any signs of extreme heat, melting, flecks of metal, etc?
Good Luck,
Jim Henderson
I am not familiar with this engine so my guesses are just that, guesses. Most of this info is from reading years and years, I have never had this problem except for fumblefingers.
On a really loose engine, yours might be with 180K, there may be enough slop in the piston, rod, crank assembly to have the piston touch the plug under extreme load or high RPM, not unusual during a mountain climb eh. I would be a little surprised on a modern engine since I don't think the clearances are that tight, but way back when people were doing 10 and up compression this could happen.
Alternatively it could be you have a hot spot in that cylinder that is causing you to detonate which could pound the plug. But usually there will be other symptoms too like loud knocking and eventually piston damage.
How do the plugs look otherwise? Any signs of extreme heat, melting, flecks of metal, etc?
Good Luck,
Jim Henderson
#3
08-09-2005, 11:40 AM
Jim...thanks for the response.
My trouble stems because 33,000 miles ago I replaced the engine with a rebuilt engine...just for this problem. The mechanic indicated that the piston must be deteriating(sp?). Now it is happening with the rebuilt engine. I am losing my confidence in this truck and engine.
-GB
My trouble stems because 33,000 miles ago I replaced the engine with a rebuilt engine...just for this problem. The mechanic indicated that the piston must be deteriating(sp?). Now it is happening with the rebuilt engine. I am losing my confidence in this truck and engine.
-GB
#4
08-09-2005, 03:57 PM
Post Fiend
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I would make sure the knock sensor is hooked up and working. On an OBDII vehicle you should not have any detonatation bad enough to nuke a spark plug. If it did, it certainly should pop off a DTC.
If you have a decent scanner you should be able to determine if it is working or not during a test drive. At $60+ per sensor for your year vehicle it is too expensive to replace on a whim.
Though I would be willing to bet $0.02 that the problem all along might be it is not hooked up or the harness has been cut through or burnt somewhere on that side.
It has been a while (5 years) since I worked on a 4.6, but, doesn't your engine have recessed spark plugs with coil on packs for each plug? I would look at a bad wire or faulty coil. Especially since it is happening again in the same cyl.
If you have a decent scanner you should be able to determine if it is working or not during a test drive. At $60+ per sensor for your year vehicle it is too expensive to replace on a whim.
Though I would be willing to bet $0.02 that the problem all along might be it is not hooked up or the harness has been cut through or burnt somewhere on that side.
It has been a while (5 years) since I worked on a 4.6, but, doesn't your engine have recessed spark plugs with coil on packs for each plug? I would look at a bad wire or faulty coil. Especially since it is happening again in the same cyl.
#5
08-09-2005, 05:11 PM
I stand corrected...my mechanic tells me that the short plug has been on cylinders 1,3,4,7.
The truck is now at a Ford dealership for a second opinion.
Could this be a computer problem. I did install an aftermarket chip way back at 5,000 miles...a 147,000 since I first encountered the problem.
-GB
The truck is now at a Ford dealership for a second opinion.
Could this be a computer problem. I did install an aftermarket chip way back at 5,000 miles...a 147,000 since I first encountered the problem.
-GB