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I've seen this happen on a couple of different engines so far (BMW S62, Mitsubishi 4G63) but am not sure about how commonplace it is.

If an engine isn't used for an extended period of time, the subsequent startup results in a tapping sound that changes frequency with engine RPM. This sound then goes away after the engine is run for some time.

People usually associate this tapping sound with stuck valve lifters. I would like to better understand what causes the valve lifters to get "stuck" and what causes them to free up again without external intervention.

Pᴀᴜʟsᴛᴇʀ2
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Zaid
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4 Answers4

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I found a great article out there on the interwebz which explains this very well (and confirms my line of thinking) for the BMW M62 engine. I'm sure the explanation is pretty much the same for other engines of the same type.

Basically, the author of the article states these engines do not have rocker arms, but instead uses a cam on lifter on valve setup. The lifter in this case is a two piece design with a spring in between. When the lifter sits for too long a period without being run, the oil will drain out of it. This allows the lifter to completely collapse until the lifter can be filled again. During collapse, both sides (top and bottom) impact each other and thus creates the noise you are talking about. He states there are three conditions where this will happen more frequently:

  • If the engine oil level is low
  • If the oil is too viscous
  • If the car is only used for short journeys where the oil fails too reach temperature.

He also states this is perfectly normal and will not cause wear or failure. Anyway, there is a lot of good information and illustrations in the article, so is a good read on the subject.

Pᴀᴜʟsᴛᴇʀ2
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I've worked on the lifters in a Dodge 6G72 engine ( and also the Dodge 2.2/2.5L engines). The lifters are oil filled. There is a tiny valve on them that allows oil in. When the vehicle has not been run for a while, or runs low on oil, the lifters will loose oil. As oil pressure build up, they will 'pump up'. So the issue is not that they are sticking, it is that they are compressing all the way when usually they do not.

I've cleaned them before by placing them in gasoline and pressing the valve to release the oil. At this point, I can easily compress them by hand. Then I put them in oil and press the valve to fill them up again. They are then very hard to compress.

It sounds very similar to what Paulster2 is describing, except the 6G72 (and dodge 2.2/2.5) do use rocker arms.

rpmerf
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I'd imagine it is down to lubrication - if the engine isn't used for some time, the oil all seeps back down into the sump, and without lubrication, the lifters will stick. After running the engine again, oil has been pumped back up and worked it's way back around the lifters, lubricating them and freeing them up...

Nick C
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How do you unstick them after an extended period of inactivity that results in compression that's too low to start the engine? What I'm going to try is to remove the plugs, shut off the fuel line, put it in neutral, take the key out, put an electric grill under the oil pan, rotate the engine manually by the crankshaft pully nut, then when warmer and easier to rotate, I'll try using a drill with a 1/2" driver-bit for a while. Hopefully, this will refill and repressurize the lifters, as well as sealing the cylinder rings, thereby allowing the engine to start after reassembly. I hope it works.

Bryce Hoeltzel
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