450SEL miss only at hot idle; idle speed is also high at same time

[WANT '71 280SEL]

I love the 450SEL, but this problem is driving me nuts because I can't figure it out. Quick history, the engine has about 15,000 miles on its rebuilt engine. All the idle air hoses were replaced at the time and are still very flexible and pliable. The car has a miss when idling, most noticeably when it's hot. I think it may have a very slight miss at cold idle, but certainly not like when it's hot. Also, it's idling between 1000-1100 rpms at hot idle. I'd adjust it down, but it idles at 725 or so up until it hits full operating temperature.

When you get the car above idle, the miss disappears. It's a random miss, so it makes random popping without any set order to it. It just has a sort of "blip" to it when idling in park or drive.

Even after driving the car for about 10 hours on the interstate, as soon as you get off stop the car, the miss returns.

Ideas? I'm thinking vacuum leak, but I'm not sure exactly how the idle circuit works on the car.

Best Regards,

David

[alabbasi]

I had a similar problem, try cleaning out the fuel injectors. You can do so with brake clean and the straw attachment. Soak them first in berrymans and then clean them up to see if it makes a difference.

If that does not work then you may need to clean your fuel distributor.

[WANT '71 280SEL]

According to the PO, the car did sit for about a year before I got it, so I'll try running some injector cleaner through. The PO did say, however, that all the injectors were replaced when the engine was rebuilt. They all look new as well.

My car doesn't have a fuel distributor since 75 was the last year of the D-Jet, so I have the fuel rails.

Best Regards,

David

[alabbasi]

Ahh, then two bottles of Tectron in the tank. Also clean your trigger points.
I can't remember if the 75 has ignition points, but if it does, then those should be cleaned and adjusted too.

[Tomguy]

Edit: Re-read the post. It could be the aux air circuit. That tricky sticky valve tends to cause a lot of issues with the D-Jet systems, second only to the points themselves. If the issue is hot only it's less likely to be the points in my experience - mine only gave issues with a COLD engine when the trigger points were fouled.

[Graham]

Could be an idle mixture problem.

You might try adjusting the ECU idle mixture knob. Note where it currently is and then try 1 or 2 clicks (or more) either way and see if idle improves. If there is no affect at all, then the idle circuit is not activating and you need to adjust the Throttle position switch.

Have you checked the exhaust CO levels? There is no real way of setting up these D-jets without an exhaust analyzer or AFR gauge.

[WANT '71 280SEL]

I had the CO level set to 4% a couple weeks ago. I don't drive the car regularly so it's hard to tell, but I think this problem occurred sometime after having the CO adjusted. I think he said its used to be around 7 or 8%.

Regards,

David

[WANT '71 280SEL]

How could I test the aux. air circuit, Tomguy?

Also, since my CO is right where it's supposed to be, wouldn't mixture be ruled out as a possibility or is there a control for specifically the idle mixture that is separate from the CO setup?

Best Regards,

David Peden

[ctaylor738]

Your aux air valve is probably stuck in the closed or near closed position. You have probably opened up the idle air to raise the idle speed during warmup, and so it's getting too much air when warm.

The test of the AAV is to remove it and look to see that it is open at room temp. Then throw it in a pot of cold water and it should open further. Then heat the water, and observe that the valve is fully closed when the water temp gets to 160 or so.

I had some success unsticking one by soaking it in penetrating oil overnight. Then when I put it back on the car, I gave it a couple of pretty good smacks on its top with a good sized hammer.

[WANT '71 280SEL]

Thanks for the tip, Chuck. Next time I'm in Atlanta around the car, I'll give it a shot and report back with the findings. I hope all is well.