Idles better when cold weather? Why?
 

200k+ 560sl, Like others suffers from inconsistent idle but runs well, not willing to change fuel distributors just to see if it works. Thing is, once the weather got cooler (I live in Atlanta), the idle seems to have smoothed out some (maybe the seasonal gas has some effect), but it's weird. I have a short commute and even though the engine gets up to operating temp, the idle is not nearly as rough as summer. I also notice that if I drive to lunch and it's still little warm from commute, the idle is rougher.

I dunno, could be gremlins I guess, but I'd like to see what's would maybe explain.

Thanks.

J. Boggs

I will speculate.
Your engine normally runs slightly lean.
But since it is cooler (environment), the cold start valve works for a longer time injecting more fuel, hence running better.
Try to read your DC or EHA current to see how you are running.

My guess: old and brittle plastic/rubber parts such as an idle valve or other pieces that when cold, do not let air in, but when warm expand.

Haasman

I'm with pesuazo on this. In an indirect way you're saying you have a hot idle problem. You may have an egr valve that is stuck open or a dirty throttle body or dirty injectors.

How would a "dirty injector" perform better in cold weather vs. hot?

Even though the O2 sensors regulate how much fuel is injected through feedback, the sensors can only control the average fuel injection. If one or more injectors are clogged, the flow among the injectors is not balanced, resulting in rough idle.

When cold, the injectors are injecting excess fuel (more than needed for combustion). This masks problems with injectors that are partially clogged. Once warm, the dirty injectors are injecting less than optimum amounts of fuel, resulting in rough idle.

So in a roundabout way the diagnosis may be back to a fuel dist. problem. I have one cylinder that seems to miss-black soot on the spark plug (closest to the firewall on the passenger side #??)

I was hoping that the idle valve may be the culprit and I may yet yank it a clean it out-doesn't seem that it would effect only one cylinder though does it?


Thanks.

J, Boggs

If you're finding soot on one plug, you may be on to your problem. First, make sure the spark delivery is working properly before checking the injector. A weak spark may ignite a rich mixture, but fail to ignite a leaner mixture.

Thanks ...
 

I've played around with the injectors and wires on that cylinder-swapped injectors and wires and spark plugs from the next cylinder over with no change, always the same cylinder. New cap, rotor also. I've put an inductive timing light to the wire to the bad cylinder and get sort of a stumble at idle. Sort of ****-**-**_. A little stumble that shows up as a non-rhythmic flash at the light. All other wires, show fine and rhythmic-including the offending wire when reversed with the next cylinder over.

Grasping a straws but since this is the farthest cylinder from the coil, could the spark be too weak to travel that far from a weak coil?

Thanks.

JBoggs

Do a wet and dry compression test. You could have a burnt valve.

So, explain that to me?
 

I have a compression tester but what's the wet vs dry method?

Thanks.

JBoggs

Rather than me writing reams check out:

http://www.cobra-replica.co.uk/html/cobra_52.shtml

HTH!

That makes sense..
 

Thanks.

JBoggs