Help with slight engine knock

[overdoseland]

Need some help in identifying whats causing a slight knock in my engine. I'm wondering if it's normal but I kinda hear a slight knocking when I accelerate. I don't hear it at higher rpms though. I hear inside the car slightly so I'm kinda confused thinking if it's normal or not. It kinda sounds like the valves or lifters a bit. I don't hear it when it's idling so I doubt its those. I notice it more when I pop the hood and rev it through the throttle cable. Sorry I forgot to mention the description of my engine. It's a 4.7 HO with 222,000 highway kms. I use mobil 1 synthetic 5w30. Run 91 and up octane fuel. I haven't changed the spark plugs for over 100,000 kms. Would worn plugs cause knocking? I also started noticing the knock as the weather got colder. The weather here is between -20 to -30 celcius. The vehicle still drives fine, don't notice any extra fuel consumption or oil loss. I don't see or hear any exhaust leaks but I haven't checked the intake manifold. The oil level is at the halfway mark sometime alittle higher at idle and near the 3/4 mark when driving. Any suggestion? Thanks!

[fredjacksonsan]

I'd check your owner's manual first, and see what the change interval for the plugs is.

Then, I'd look to see what grade of fuel the engine is designed for. If it's not 91, and instead 87 octane, that might be your issue.

Originally Posted by if manual says 87

Octane is a measurement of resistance to burning. The higher the octane, the slower the fuel ignites. So if it's designed for 87 and you're putting 91 in, then it's likely building up some carbon inside. This carbon will, over time, increase the compression ratio - end result? Knocking.

I experienced this in a car I bought in the early 90's...the previous owner had always put 93 octane in it, even though it was designed for 87. When I put 87 in it, it knocked like crazy when I gave it a lot of gas, but would idle quietly.

The end result was that I put in 1/2 and 1/2 93 and 89 for a couple of tanks, then 89 for a couple of tanks, then 1/2 and 1/2 89 and 87 for a couple of tanks, then back to 87. Once I had done that, the knocking pretty much went away.

Also, a good fuel injector cleaner will go a long way to helping clean out any deposits.

Hope that helps, post back and let us know what the manual says...I don't have the info on the 4.7.

[overdoseland]

The service interval for spark plugs is 30000 mi which is a little over 48000km. I think I'm due for a plug change. So, can worn plugs cause engine knock? As for fuel it can run on either regular unleaded or premium for maximum performance. I've always used 91 and up octane fuel. Thanks for the reply Fredjacksonsan.

[fredjacksonsan]

With worn plugs I've seen engines run rough, but not necessarily knock. Try the new plugs and let us know how it does.

As I said, not familiar with the 4.7....so 91+ octane for best performance and any grade will do.... you could still try the stepdown I suggested earlier and see if that works...and a good fuel injector cleaner might not hurt either.

[overdoseland]

I will change the plugs and see what happens but I won't be doing it till it gets warmer here as it is -40 with windchill right now in Canada. Thanks again for the suggestions and Happy Holidays!