Rough idle after valve adjustment

[SpecM]

Just did a minor tune-up on my '89 325i, changed the oil, replaced spark plugs and did a valve adjustment.

Now the car idles a bit rough. Not terrable tho, its really just the sound that seems off. Runs fine under load, lots of power. Its just that the idle has a little lump to it.

Would setting the valves too tight cause an idle issue? I set them to .008in stone cold

[Brandon12V]

That's pretty tight. .010 is about as tight as I would go on cold valves. Considering you've been around for quite a while, I think you know the answer to your question. Yes, adjusting the valves too tight can do this. As your motor heats up, the eccentrics will expand slightly. The more heat rises gradually, the metal will expand until it reaches the point of molecular breakdown(melting). You know how your motor works but I'll humor ya. ;) If the valves are too tight, they will open early and close late. This can, in some extreme cases, cause a backfire in the intake or combustion leak in the exhaust manifold. If this is the issue, your valves are probably just tight enough to throw your timing a bit. Re-do the valves, cold, to manual spec. Check the gaps on your plugs as well.

[jlevie]

The slightly tighter valve setting isn't likely to be the cause of the rough idle. I use 0.006 for the intakes and 0.008 for the exhausts on my Spec E30 and it doesn't affect idle. The rough idle is more likely to be the result of intake leaks. A smoke test of the intake is the way to find the leaks.

[SpecM]

Thanks for humoring me Brandon, sometimes you just need to hear someone else say it

Yes Jim that is a valid point, but the idle Im getting is a little different. "Rough" is not a great desription, more like a bass or rumble noise at idle. Runs perfectly besides that, better than it did before the plugs/valves

[Brandon12V]

I just noticed your username.. Is this happening under race conditions or just driving on the street? If it's just on the street and it's running fine as you say, give it a few good, hard pulls and let it idle for a while. If Jim is running his m20 at that low of gaps with no issues, then maybe the REAL problem will become more prominent afterwards. We all know that pushing a car closer to it's limits will bring the little things out like a sore thumb. Are you running a wide or narrowband by chance? Definitely pull your plugs and see how they look.

[whodwho]

I find you get a bit more of a bumpy idle with valves adjusted on the tighter side.

[Brandon12V]

Also it's kinda a shot in the dark, but did you make sure to get the breather hose nice and tight after reinstalling the valve cover?

[SpecM]

I did double-check that yes ^

It is a street-driven car, I do drive it hard at times, but it is my daily

I was thinking it had to do with the valves beeing a little tight

Like I said before, it runs great, better than before the work

[nando]

a slightly tighter valve won't cause issues, but an overtightened valve definitely will. Especially if it runs like ass when cold, and then smooths out a little when warmed up.

[SpecM]

Especially if it runs like ass when cold, and then smooths out a little when warmed up.

and that would be indicative of what?

[Brandon12V]

the only other thing I can think of checking right off hand for a lumpy idle would be the ICV. There's no spec in the manual, that i know of, so test resistance under load and look for flat spots in the change.

[nando]

and that would be indicative of what?

the valve clearance expands when the engine heats up. If they are too tight the valve will actually not close all the way with the engine cold, but as the engine warms up it will start to close fully and idle will improve.

[SpecM]

^ so the lash wouldn't decrease as the metals expand?

[nando]

yeah, I don't know exactly why but it does the opposite. maybe due to the differing expansion rates of steel vs aluminum. for example the hot adjustment spec is .011" (.30mm) and the cold spec is .010" (.25mm).

[Brandon12V]

wait wait... Nando you just blew my fuckin mind... I thought the purpose behind a larger gap when hot was due to accuracy? Adjustment on a cold motor will always be more accurate than on a hot motor. Isn't this why Porsche only reccomends to adjust valves cold? I'm so confused now.. I really want to know the reasoning behind this.