#8 Cylinder Problem
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New User
Joined: Oct 2000
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#8 Cylinder Problem
I seem to remember reading a post about this some time ago, but have been unable to find it.
A friend of mine developed a bad miss on his '89 F-150 4x4 w/ 5.0 5 speed. I scoped the ignition, and found #8 (rear cylinder, driver's side) misfiring. I pulled the plug, and the center electrode was melted away. I replaced the plug, and things were fine. He told me he had just changed the plugs a couple of weeks earlier, so I figured perhaps a defective plug. 2 weeks later, same problem. I then thought that the mixture might be leaning out on that cylinder (clogged injector?) and pulled the injectors for a good cleaning. They looked fine, so I reinstalled them, switching places with #5 and #8 and replaced the plug again. 2 weeks later, same problem, on same cylinder. There are no vacuum leaks, the ignition timing is perfect, and the truck runs perfectly when the plug is first replaced, and the scope patterns appear normal. The truck has very low miles and is not abused (60,000 miles on a 14 year old truck). There are no trouble codes on the EEC system (datastream not available on '89 model). Any ideas?
A friend of mine developed a bad miss on his '89 F-150 4x4 w/ 5.0 5 speed. I scoped the ignition, and found #8 (rear cylinder, driver's side) misfiring. I pulled the plug, and the center electrode was melted away. I replaced the plug, and things were fine. He told me he had just changed the plugs a couple of weeks earlier, so I figured perhaps a defective plug. 2 weeks later, same problem. I then thought that the mixture might be leaning out on that cylinder (clogged injector?) and pulled the injectors for a good cleaning. They looked fine, so I reinstalled them, switching places with #5 and #8 and replaced the plug again. 2 weeks later, same problem, on same cylinder. There are no vacuum leaks, the ignition timing is perfect, and the truck runs perfectly when the plug is first replaced, and the scope patterns appear normal. The truck has very low miles and is not abused (60,000 miles on a 14 year old truck). There are no trouble codes on the EEC system (datastream not available on '89 model). Any ideas?
Elder User
Joined: Apr 2002
Posts: 541
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From: Victoria, Texas
#8 Cylinder Problem
They had a problem with the pcv in that cylinder intake runner causing burned #8 exhaust valves. Dont know if that is your problem, but I would suspect it is pcv related. It happenned to me.
Junior User
Joined: Apr 2003
Posts: 91
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#8 Cylinder Problem
It sounds like you may be getting too much voltage to that plug. I don't remember you saying that you had checked the wires. Something is going to ground I think. Also check and make sure that the wires are not too close causing cross fire. This is a problem that is noted on my 94 with 5.0. If the wires are too close it can cause a current to be induced at the wrong time. Just my 2 cents.
Elder User
Joined: Dec 2002
Posts: 925
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From: Quaker Hill ,CT U.S.A.
#8 Cylinder Problem
Also you may want to change the routing of the spark plug wires as I have seen that same damage on a 88 302 plug .
After the plug was removed we did a compression test and found that the #8 and only that one was running about 20 psi lower then all the rest .I know that is well with in specs.
A few months later the engine was replaced and apon dissasembly we found that the #8 cylender wall was scored bad and the piston was cracked and the rings and ringland's were cracked/broken .
Again check the routing of the wires and I would replace the wires as a extra safty measure .
These 302 and the 302s made from the mid '80's to the 94ish had this problem and it is well documented .
After the plug was removed we did a compression test and found that the #8 and only that one was running about 20 psi lower then all the rest .I know that is well with in specs.
A few months later the engine was replaced and apon dissasembly we found that the #8 cylender wall was scored bad and the piston was cracked and the rings and ringland's were cracked/broken .
Again check the routing of the wires and I would replace the wires as a extra safty measure .
These 302 and the 302s made from the mid '80's to the 94ish had this problem and it is well documented .
