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Kenny Z.
Kenny Z., Lincoln Technician
Category: Lincoln
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Experience:  Lincoln Senior Master
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Lincoln Navigator 2004 is having rough idle/hesitation

Customer Question

MY 2004 Lincoln Navigator is having slight rough idle and hesitation when cruising on the highway. Since I received a misfire code for #8 cyl 2 weeks ago, I have replaced the following: replaced all 8 ignition coils and plugs, fuel filter, replaced air filter for K&N filter, PCV valve, had Valvoline flush the fuel injectors, replaced vacuum tube elbow at the top of intake manifold. I sprayed around the intake manifold to check for air leaks (none detected). Did a compression test and received the following Data : (PSI)#1-190, #2-170, #3-180, #4-195, #5-165, #6-160, #7-150, #8-175. According to service manual, I'm within tolerance. I have noticed dry oil stains around valve cover bolts and edges near #6 & #7 cyl. I'm guessing maybe valve cover gasket is needed, but if I'm not mistaken that generally doesn't effect the engine idle But I"m not a mechanic:-).....Still, I have a slight rough idle and hesitation at cruising speeds. Any further suggestions would be greatly appreciated....
Submitted: 3 years ago.
Category: Lincoln
Expert:  Kenny Z. replied 3 years ago.
Hello and thanks for your question.

Ford had an issue with the cylinder heads on these engines. The exhaust valve seats would begin to recess causing random misfires, usually on cylinders 7 or 8. When the problem is just beginning to start it can be very difficult to pinpoint with a mechanical compression guage. As the valve rotates it will fail to seal intermittently and is next to impossible to verify with a mechanical guage. Ford has a test that is run on the Ford only IDS scanner that checks relative compression my measuring crankshaft deceleration speed. This test will often be able to identify the issue. I have also been able to verify the concern using a boroscope (reverse looking) to visually inspect the valve seats through the spark plug hole. Sometimes running cylinder bleed down tests will reveal slight leakage in the tailpipe if listened to with a stethoscope. I can't guarantee this is your problem, but I can say we have replaced many heads on these over the years for this concern. Pretty much every one that a coil or plug does not fix ends up getting a cylinder head.
Kenny Z., Lincoln Technician
Category: Lincoln
Satisfied Customers: 4854
Experience: Lincoln Senior Master
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Customer: replied 3 years ago.
Thanx for the info.
Expert:  Kenny Z. replied 3 years ago.
Your welcome.
Customer: replied 3 years ago.
quick question: if it turns out to be valve related, how much longer do I have before it gets worse?
Expert:  Kenny Z. replied 3 years ago.
Really hard to say, as the seats recede the idle will get worse and the misfire will get to the point where it is constant. Could be anywhere from 5 - 15K miles which is me guessing based on how your describing the way it is currently running.

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