4.2 spark knock question
 

Got my 156k mileage 2000 F-150 last week and it knocked pretty bad. Pulled the plugs and replaced them (they were old and gap was like a 1/4 inch). Gapped the new plugs and the knock went away. This week, I'm starting to hear the spark knock very faintly at cruise speeds (no accel or decel) of 50mph. When I accel it goes away, and when I'm cruising at 65mph I can't hear it, but that may be due to wind noise.

Question: Is my knock sensor bad or my O2 Sensor(s)? Which would you change first?

Jon:help:

You probably have carbon build up causing the ping you hear. This would be normal for the miles you have. Not sure what would be the best way to get rid of it. I would take the intake and heads off.

Like torkum said, carbon build up is probably the culprit. This limits heat transfer so the temps of the A/F mix in the cylinder become hot enough to ignite prior to the scheduled spark event - this is knock. Knock is worse is high engine load conditions, lean A/F, transient conditions, hot and dry ambient conditions, lower octane fuel or overly advanced spark timing (which you cannot control)

I really doubt the 02 sensor would make a difference unless you are running lean because of them, but that should be caught by the diagnostics. Even with a knock sensor you will sometimes hear it but typically during a quick acceleration. If the knock sensor is working properly it should limit it so you do not notice it during cruising. If this is trace knock I would not worry to much but if it is fairly sustained that can be a problem. Cheapest solution may be to use a little higher octane fuel.

There is a product called seafoam that will help loosen up those carbon deposit and burn them off. I have used it on a few rides with high milage and it help them. All you do is get the engine hot and run about half the can through a vaccum line and when it starts smoking cut it off and let it sit 10 - 15 minutes. Start it back up and it will smoke like crazy until its all burned out. It help my 83 280zx turbo, it had a rough idle and was a little sluggish on take off. Warning though there is a chance it will foul out plugs, especially old plugs...........

Seafoam sound interesting? Where can I get it and has anyone else used it?

What about just running some gas treatment or injector cleaner through it for 5-6 tank fulls (every other tank of course)?

you will never read it in a repair manual but a fuel pump that is not delvering enough volume will cause spark knock. I have encountered more than one chronic spark knocker that the only fix was a new fuel pump-typically the fuel pressure will be in spec buton the low side.

inaequate EGR flow will cause spark knock, 302 and 351 ford engined trucks where notorious for really bad spack knock due to a clogged EGR. there was nothing wrong with the engine, just the EGR system design.

I would check the basics first before i started throwing parts at the problem

On the 4.2 the lower intake has 6 EGR ports that get clogged.The only way to clean them is to remove the upper plenum to access the ports.The SeaFoam will remove some of the carbon from the piston tops if you follow the instructions on the can.But,if you pull all your plugs and inject each cylinder with SeaFoam and let it sit for awhile,you'll clean the tops alot better.Just turn the engine over to expell any that has not seeped by the rings.Install the plugs,enjoy the smoke screen,then change the oil! :fyi:

I bought the SeaFoam, but not sure how/what vacuum line to use to get it in there. Can you give me a bit more info?:coffee:

I put it in through the brake booster line just like on the can, let it sit for 5 minutes and now I have bigger problems. I have a knock, seems to be on the left side, and a check engine light. Took it to a local repair place and he seems to think the noise is in the valve train.

WHat went wrong?

Sounds like you might have had a chunk of carbon break loose and get hung between the valve and seat. How long have you run it since the seafoam treatment? The cel might be from miss fires while burning off the seafoam. Disconnect the battery and reset comp and see if the cel comes back on. I have used seafoam many times and never had any issues other thqan maybe a fouled plug.

I tried changing the plug, but didn't help. Drove it 25 miles and the knock only gets worse as it heated up.