What is a rod knock? What causes a rod knock, how bad is a knocking rod, and how can I tell if a rod is knocking?
To get to basics, the rod (or connecting rod) is the portion of the engine which connects the piston to the crankshaft. Where the rod connects to the crankshaft there is a bearing which separates the rod itself from the crank journal. The bearing is made from a softer material than the rod or the crankshaft.
You can see most all of the internal parts of an engine in this exploded view below.
The connecting rod is #7, the crank shaft is #16, piston #4, and bearing can be found marked as #8.
When an engine is built, there are certain gap tolerances (specific distance measured in .001") which are built into it, the rod bearing to crankshaft journal being no different. In this case, the tolerance is there to allow a specific amount of oil through from a hole in the crankshaft journal and remain on the bearing surface. Oil films on the bearing and keeps the two parts from touching during normal engine operation. If the clearance gets too big from either wear or deformation, the oil will no longer film correctly and you will get the rod knock. The noise itself occurs due to metal on metal contact between the bearing and the crankshaft journal during the power stroke of the engine cycle. If the situation is really bad it can even be caused by the rod itself contacting the crankshaft journal (where the bearing has become out of place). The most common cause of rod knock is due to a spun bearing (bearing itself actually spins in the end of the rod so it becomes out of position). This usually happens due to rod cap stretch during abusive driving situations. It can also happen due to a lack of lubrication, either because of a lack of oil or because oil is flowing incorrectly. It can also be caused by a combination of any (or all) of the three.
A rod knock is a very serious issue with the engine. As you can probably tell from my write-up, it means the engine is no longer functioning correctly. It is the death knell of the engine. Once the rod knock occurs, it will not cure itself. The rod bearing must either be replaced along with fixing associated parts (you don't usually replace just one rod bearing as the entire engine would need rebuilt at that point), or the engine will need to be replaced.
A rod knock is as described ... A low pitched knocking sound which can be heard low in the engine. It will increase in frequency as the engine speeds up, but can usually be heard at any engine speed. If you hear a knocking sound which seems to disappear after the engine warms up, it's probably not a rod knock. It may be something as innocuous as a leaking exhaust gasket. If you believe you have a rod knock, you should seek a second opinion from a trusted mechanic. A rod knock is an expensive fix, so you need to make sure of what it is prior to diving in head first.
Rod knock is caused by a severe failure of one or more crankshaft connecting rod bearings. Repairs require engine removal and complete disassembly at significant expense. The most common cause is lack of engine oil, this is usually the first part in an engine to fail when it runs out of oil. The second most common cause is very hard driving. The test is simple, rev up the engine and listen for a loud knock in time with rotation; an experienced tech knows this sound all too well. Not every noise is a rod knock, so an experienced ear is needed. I saw three this week: one driven too hard, one ran out of oil and one manufactured incorrectly.
If you think your engine might have this problem do not run the engine, get it to an experienced tech.
I have only heard a rod knock, or spun bearing, once when I was asked to diagnose a friends car and although I had never heard one before I diagnosed it and it was in fact a spun rod bearing -- which was confirmed upon removal and disassembly and rebuild.
Valve tick and rod knock are often confused. Neither is a particularly good situation, but rod knock is much worse as Paulster2's response explains. Valve tick can be just because of low oil.
Valve tick is a lighter more tinny sound at a higher pitch than the lower pitched and more subdued rod knock. Rod knock will come from the low end, versus valve tick from the top end. You can use a piece of vacuum tubing as a makeshift stethoscope with one end near (no in) your ear and the other end moving towards the sound.
If you open the oil fill cap on the cylinder head and the noise gets louder/clearer, it is valve tick. Is you take the engine oil dipstick out and listen there and the sound gets louder/clearer, it is a knock from the lower end and most likely a rod bearing.
Please note that running the car with the dipstick removed will create a vacuum leak and the idle will be poor and you might throw a check engine code. But if troubleshooting a potential spun bearing, a CE code is not really your main concern. The engine will idle correctly once the dipstick is put back in, and if a code was thrown it will clear over time or with a code clearing tool.
Piston slap could also be confused with valve tick and rod knock. It would be tough to explain how the sound would be different between rod knock and piston slap, but the general rule is if you have a knock from the engine -- and especially the low end -- it isn't good and the engine won't be running long and repairs will be costly and become more costly if you continue running the engine.
Running the engine in hopes it will go away on its own will lead to a seized engine, piston through the hood, rod through the block etc.
The good news is there are a lot of knocking sounds that originate external to the engine, and if that is the case the problem isn't necessarily severe. Worn engine mounts, bad belt, pulley, etc. are examples.
The problem is misdiagnosis does happen and results in a good engine being torn down and a lot of costs sunk. That can happen either by honest mistake, or a shady shop milking a customer for unnecessary repairs.
There are companies that you can send an engine oil sample for analysis to confirm. A severe problem with the low end will have high metallic content in the oil. If you aren't sure and want to be sure, get an oil analysis done.
If you do have an engine problem in the internals which is making audible noise, don't waste your time or money on the various products sold as "engine repair". While it is possible adding product might reduce the sound or make it go away, the damage is not reversed. People do this to sell a car to an unsuspecting buyer and mask the engine knock. People don't do this as a permanent fix.
I have had rod knock for over 8 years on my daily driver. Its not good but its not the apocolypse people are claiming. It happened when the oil line broke on my astro v6 on the highway and it ran till the engine siezed up. Towed it back home and dropped the pan. The pan was full of bearing pieces. So I ordered cheap new main and rod bearings and installed them from the pan side. It was no big deal. One rod bearing was in particularly torn up shape. Thats the one that heated up the most. Replace oil and filter a few times and the engine runs smooth and strong again with great oil pressure, except for that knocking. The crank journal on that one rod must be deformed oblong and the van is NOT worth all the work of a replaced engine. So I run it with the knock and it still runs today. I dont race it. First gear is the knockyist so I lift my accelerator at 10mph so the tranny shifts up to higher gears. Then the knock is barely there. And most important, I run as thick an oil as I can find. 20w50 and when I go on long trips, I pour in a quart of motor honey. Thick oil is a shock absorber to a huge rod bearing gap that is too expensive to fix. And if Im towing a heavy load, thats even better on the bad rod because its working so hard it cant lift off the crank to smack back down again. So there you go. The poor mans cure for rod knock headaches. I never looked at the cam shaft but clearly no worse for the wear. The crank burns up WAY quicker than the camshaft. Soon I will drop the pan to see the toll of 8 years of knocking. I will install a new rod bearing of Standard size when I do, but I will have a used rod cap at the ready in case the rod cap looks like it needs replaced. No point in using oversized bearings since the crank journal is oblonged. What I need is a new high tech high temperature plastic bearing to absorb the knock impact without making a sound. Ive waited 8 years but i guess I will have to wait some more. :)