[Roadsters] U20 - Rough low idle - Chain slop

[John F Sandhoff]

> How much do people's U20 chains slop around at low idle?

Personal opinion: they shouldn't. There should be enough oil pressure
to maintain the tensioner.

> Here are my symptoms: at 700 rpm's, even with a warm engine,
> the idle is swinging from 660 to 740...

My crystal ball points to a worn distributor. Put a timing light and
dwell meter on the car - is timing jumping? Is dwell drifting?

> The first thing we did was looked for vacume leaks

Good thing to check.

> Still confused, we pulled the valve cover off and ran the engine with the
> valve cover off -- messy, but a useful diagnostic. When we looked in,
> it looked like the timing chain was slopping all around and the
> tensioners weren't doing such a good job.

Messy, yes :-)
In fact, if you didn't get fairly well soaked with  oil, I suspect you
have an oil flow problem. This may be your smoking gun.

> The engine is freshly rebuilt. The tensioners have plenty of material
> left on their feet. All of the oil passages were freshly cleaned when we
> did the rebuild.  We've got three theories:
> 
> 1) The tensioner springs are shot...

Nope. The springs only maintain static tension. As soon as you get oil
pressure, the springs become quite secondary.

> 2) Because the engine is so new there's just not enough oil
> pressure to push out the tensioners are low idle.

New engine = low pressure? Nope.

> 3) We put on really loose exhaust...

Nope.

> Any ideas?

Yes, actually.

1. Verify oil pressure. You better have some. Sounds like maybe you
don't :-) Did you replace the oil pump when you did the rebuild?

2. If oil pressure and flow is good, is it possible that the gasket
between the tensioner and the block doesn't have the little hole
that allows oil flow? In reality, that sounds like the issue and is what
I'd look at...

The concern then is if the upper tensioner (which is easy to get to)
has a bad gasket, what about that buried lower one?

-- John
     John F Sandhoff   sandhoff at csus.edu   Sacramento, CA