99 328 Jumping tacho.

[matlock328]

Hi guys,
This is not really a big issue but I'm interested in knowing what it could be. On cold starts the revs will drop, almost to a stall (very occasionally it does)then jump back up, it happens once every 5-10sec, but will stop a few minutes before full operating temp has been reach. It rarely happens in summer. In operating temp, it purs beautifully. This all happens while stationary, but if I drive straight away, I can feel minor surges whilst driving until it warms up. Any ideas?
thanks

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[Raddee]

Your VANOS-seals need replacing. The M54 DME has ways to compensate for that so it's not very noticable, but the M52TU engines start surging a bit while warming. I have the same issue, I don't think it's a very big deal. Symptoms only occur during warmup (not when completely cold or when at operating temperature).

Other things you may notice:
- you need to run the starter for longer if the engine temperature is between cold and warm.
- after starting between cold and warm, you may notice reduced power. Re-starting the engine after it has warmed up solves this.

But again; no big deal if you always start the engine either completely cold or warmed up.

[MarcoZandrini]

The first step in diagnosing a problem is to scan the engine computer (DME) for codes.

[matlock328]

Your VANOS-seals need replacing. The M54 DME has ways to compensate for that so it's not very noticable, but the M52TU engines start surging a bit while warming. I have the same issue, I don't think it's a very big deal. Symptoms only occur during warmup (not when completely cold or when at operating temperature).

Other things you may notice:
- you need to run the starter for longer if the engine temperature is between cold and warm.
- after starting between cold and warm, you may notice reduced power. Re-starting the engine after it has warmed up solves this.

But again; no big deal if you always start the engine either completely cold or warmed up.

Thanks for the info, seems like the culprit. It's been happening for a few years now and not getting any worse so I don't think it could be anything else. I'll look into it.

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