I have a 89 EA Wagon that I bought recently that has a engine knock. I bought with the knock, so I just want to figure out what is the culprit, so I know what to expect.
The engine is a 3.9L CFI and as it warms start to make this "donk, donk" sound. It is barely audible until you pick-up the frequency, and then is quite noticeable. If the engine is cold from sitting overnight, it may take up to 5 minutes before it starts to rattle. The "donk, donk" increases frequency with engine rpms. But load or lack of makes no difference in the intensity of the sound. If you remove the oil fill cap, you can hear it very clearly and quite loud. The engine revs very smoothly with no harshness or raspiness, and has plenty of power. The fuel economy seems to be quite good, so the performance of the engine does not seem to degraded in anyway by this nemesis.
Pulling the plug wires from each cylinder makes no difference in any way. The only time it changes is when the engine is dropping rpms, it goes away completely until the rpms stabilize and then it returns. I have had engines that had bearings going out and normally when you subject them to load the rattle will increase in intensity, so I am not sure. Could it be a stuck valve, tappet, or lifter?
I am not planning any immediate action, just want to know what everyone's thought may be. I like to believe that an ounce of prevention is worth a pound of cure.
P. Lynn Miller