Questions and Answers
Jeep Cherokee Knocking Or Tapping?
Q. I have a 1992 Jeep Cherokee Laredo 2WD, 4.0 liter 6-cylinder, automatic transmission, 130,000 miles and I have a bad engine knock at idle, and at low speeds. There seems to be no loss in power, and oil pressure is up. So I replaced the spark plugs, air filter, oil and oil filter, and still no improvement. The noise seems to go away at higher RPM.
It does seem to be coming from inside the engine, and I suspect it is valve knock. I have spoken with a few other people and they have told me that once those Jeep In-line 6’s reach a certain mileage point, they do tend to knock a bit, and to just keep the proper amount of oil in. There are also many things you cannot manually adjust the infamous 4.0 liter engine, like high and low idle, lifters, and timing.
I am wondering why Jeep (or Daimler Chrysler for that matter) decided that no adjustments is better. The big question is, how bad is this engine knock, and how to get the problem taken care of?
A. I don't know why you call it the "Infamous" 4.0 liter engine. They are a good, strong engine that has been around for years and will give many miles of service with just the normal amount of care. I also don't know about them knocking or self destructing at a certain milage. I suspect that the ones who claim they knock and self destruct are the ones who change the oil and filter every five years, whether it needs it or not.
I have seen many of these engines go 200,000 miles without a problem. The reason why the valves are hydraulic is that they require no maintenance whereas mechanical valves need adjusting every 15,000 miles or so. Mechanical valves in any engine my and manufacturer is extremely rare today.
Anyway, since I can't hear the sound it's making it can be either a valve noise or a rod knock. If it is a valve tap, you will need to replace the hydraulic lifters. If it is a rod knock you can, if the crankshaft is not damaged, replace the rod bearings for now, but the engine will need an overhaul soon.
Being unable to adjust idle speed, valves and ignition timing describes every car rolling of the assembly line in the world today. And for most for many years gone by.