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BMW Valve Lifters Leaking Down?

Two '97 328 BMWs, one with 150k miles and one with 230k miles.

For years, I have struggled to figure out why they run on 4-5 cylinders for a few seconds to a few minutes after starting, but only sometimes, and only after they have been parked for several days.

Now the wife’s 330 BMW with 75k miles is starting to do it.

Today in the shower I had an epiphany. (I do my best thinking in the shower)

Hydraulic lifters leaking down would do this.

It makes sense that it does not happen every time because the engine does not always stop with the bad lifters under pressure (open valve).

Any way to prove this theory?

How hard is it to replace lifters on these cars?

I hate to think that I have to buy all new lifters ($400/car), but I don’t want to do this job twice. Anyone had any experience with identifying the culprits and replacing lifters?

Unless the engines rattle when started then the lifters are pretty much guaranteed not to be the problem.

Offhand, this sounds like a pressure loss of the residual fuel pressure which can be caused by a leaky fuel injector, leaking fuel pressure regulator, or the most likely suspect; a leaking check valve in the fuel pump.

When the engine is at rest residual fuel pressure should be maintained which will allow the engine to start right up with no problems. The check ball and spring in the pump is what maintains this pressure. Minute dirt particles or wear of the ball and seat can cause fuel pressure to leak back into the fuel tank.