What should the MCS dwell reading be at idle in closed loop? 1984 L69 T5.

Was seeing 10 degrees (6 cyl) after long drive. Idle was hanging around 900 to 1000 rpm. I'm not 100% that I had it hooked up correctly, but at this point let's assume it was. Pretty sure it was.

Throwing the question out there to get started on checking MCS operation. Being lazy by posting, instead of learning repair manual or in depth thread searches. And have not yet checked timing. Car has been running well since disconnecting failed choke thermostat. But I think it's still running rich. Thanks.

 

That idle speed is WAY WAY WAY WAY WAY WAY too high. Should be around 750ish out of gear, 600ish in. (measured of course with actual test eqpt, NOT the cute wavy pointer trim package poseur thing in the dash)

At the correct idle speed, dwell should be close to 30° on the 6-cyl scale; 20 - 40 would be the absolute max range. 10° indicates that the ECM thinks the mixture is rich and is basically maxed out trying to take fuel out.

 

I tested the meter earlier today during warmup cycle and noted readings of 30 degrees initially, probably open loop? But then when warm up to point of fan cycling it bounced as high as 50 degrees, if I recall correctly. It certainly was not at 10.

After that earlier warmup I recall it hovered between 20 and 50!! degree range, but didn't take notes or make reliable mental notes. At the time I figured I'd check it later today. Was not expecting 10 degrees. During that broad range the idling changed from pretty good and faster nearer 20 degrees to slower and rougher near 50, best I can recall. Beware as these are not reliable mental notes! Thanks.

And earlier today idle was nearer 600 to 700.

This thing has tended to maintain a high idle at intersections. Occasionally it will drop down to a reasonable idle.

 

A "hanging" high idle is often a result of the choke not getting fully open. Possible causes: the choke element has gone bad, the choke element isn't getting a full 12V to heat it, something has gone all cattywampus in the choke linkage somewhere or worn throttle shaft bushings that make it difficult for the throttle return springs to pull it all the way back to curb idle.

Sometimes, occasionally, it will idle down, especially after you "zing" the throttle, but usually it will hang on the last step of the fast idle cam in the choke linkage.

There's been hundreds of posts about this problem and it usually boils down to one of those problems.

And, obviously, a choke that isn't getting fully open can mess with the mixture and have you chasing your tail with weird dwell readings as the ECM tries in vain to find stoich in closed loop.

 

I appreciate the insight because it helps troubleshoot a number of observations over the past several months. A week ago I disconnected the choke linkage because the heating element in electric choke is shot. Car certainly runs better. Now I can begin diagnosing high idle and MCS, starting with high idle? I'll need to offer better diagnostic information than poor recollection of my previous post. Fighting a cold so garage time may not happen often this week. Will keep this string updated when I have better info. Thanks.

 

yeah 30 at idle & 30 at cruise rpm. Carb is running Lean, so ecm is making it richer @ 10. 0 is full rich wot. Put your hand over the air horn, the ecm will lean it out @ 50 to 55. I would check you air mix screws, 3 to 3 1/2 turns out. Then a comb of the Air bleed & rich stop to get it within the 30 at both if the mix screw doesn't work. (tps at .63v)

 

Laid up sick today so lot of time to research CCC carb. Looking forward to day I can get back into the garage.

Couple questions in mean time.

Will I need to drill/punch anything out to access IAB screw?

How about for rich stop screw or MCS movement measurement?

Tools needs for screws?

Thanks. Once again asking instead of researching. Lazy way out.

 

HOW did you "disconnect" the choke linkage??

If you just yanked the connecting rod that opens and closes the choke plate on top of the carb then you did NOT deactivate the high idle cam portion of the choke system.

 

Damon, you may have identified cause of high idle. I'll let you know as soon as I know. Thanks.