E30 325i engine shudder

I'd try cleaning the idle control valve and checking the rotor arm / distributor isn't worn out. I'd also check for air leaks in the intake hoses.

Thanks John, I might try spraying some carb cleaner around the hoses then. The main intake boot that normally fails on these seems to be quite new looking at it, so I suspect that's been replaced recently already. Good idea on the distributor too, hadn't thought of that, although you'd think that it would cause problems at higher revs too wouldn't it?

Maybe it is. It'll wear gradually so all you'll notice is a slow decline in high end power and fuel economy.

Idle misfire is often down to a poor spark - for whatever reason. The mixture is at its weakest and that needs the best spark. Check the usual things - especially plug leads.

Right, thanks. Tonight I had a quick look and have more or less discounted a vacuum leak (spraying carb cleaner around didn't have any affect and all the hoses look good) so I think it's definitely ignition. I have also noticed another symptom which I'm hoping will be part of the same problem: with the engine at idle (hot or cold), if I blip the throttle on and off quickly (most noticeable if done by hand under the bonnet) there's a definite hestitation/flatspot before the engine revs freely. In fact if you watch the needle the revs seem to even drop a fraction before it picks up. To eliminate the plugs themselves I replaced them with a newer set from my 320 and this made no difference so I'm thinking about swapping the distributor cap, rotor and ht leads next. Does that sound sensible for the symptoms? Thanks again

Rev it to 3000 rpm and then light your foot off totally. If it dips below idle and almost stalls that's a sign of a vacuum leak.

Never, never discount a vacuum leak. Go ahead and check the ignition but realize that there are many subtle ways for vacuum leaks to occur that are pretty damn hard to detect. Not as bad as with a carb, though, thankfully.

--scott