450sl idle speed?

[fahrgewehr2]

Question 450 cold idle speed?

Tinkering with a '77 450SL. Car starts up ok, but cold idle speed is around 1600rpm. Drops to correct level after car has completely warmed up.

What would cause this? This version of CIS has no EHA and no potentiometer, correct? I am going to guess that the system has only mechanical injection with some sort of cold start enrichment device?

Car is a CA model.

Mike

I posted this in the T.H. section, but thought I would post here too.

[psfred]

Totally normal. There is a cold idle device that adds extra air around the throttle plate when cold.

Not adjustable, either -- makes for some interesting stop sign "events" on icy roads in the morning!

Peter

[fahrgewehr2]

Thanks peter.

So there is no way to adjust the amount of air fed to the system during this stage of operation!? Car chirps a tire upon first shift into D! 1600rpm seems so high..was this to expedite heating of the catalysts?

I see you have owned a 103 powered car as well, could you school me on the differences concerning KE I-III?

Thanks, Mike

[psfred]

Mike:

I'll check on the fast idle, but I don't remember being able to adjust it. Check the hoses (from below the air metering flap to below the throttle plate or to somewhere on the manifold) for leaks, though -- 1600 rpm is pretty high.

The KE Jetronic comes in two flavors -- the early one (1980 on) and later (1984 or so on).

The early systems use a frequency valve to control the fuel distributor differential pressure. This valve is in series with the main fuel pressure regulator and is, in fact, a modified fuel injector. It serves to regulate the pressure to control the mixture reported by the O2 sensor. If the valve fails, the system will run VERY rich.

The KE-III system uses an electrohydraulic actuator as a controlled leak between the upper an lower chambers of the fuel distributor to control the fuel mixture, with only a simple mechanical pressure regulator in the return line. Current controlled by the ECU controls the leak and hence the mixture. Much easier to diagnose, and easier to fix. There is also a "pot" that senses throttle movement for transient enrichment on throttle opening, and fuel cutoff during deceleration. Idle speed is electronically controlled, too.

Peter

[fahrgewehr2]

Excellent. Thank you.

Yes, I know all all about potentiometers, EHA's and ECM's. Replaced them all on my SE. ECM was a tough one to diagnose...

Without a feedback system on a pre '80 car, I guess mixture can only be verified with a sniffer? No on-off ratio so it's set to what the factory deemed ideal?

There is also a "pot" that senses throttle movement for transient enrichment on throttle opening, and fuel cutoff during deceleration.

I thought the decel micro switch was responsible for fuel shutoff? One should observe negative EHA current under deceleration on a KE3 car, right? I forget the number but I remember seeing it in the manual. I guess the TPS plays a part here.

Sorry for all the questions, but the fuel injection system is very interesting to me.

Mike

PS, nice collection you have there.

And I will check for air leaks in the system. Found a cracked hose already, so I am sure there are more.

[psfred]

Yup, and there is a port for the sniffer probe in front of the cats -- I think you want right at 1% CO.

The EHA cuts off the fuel by going way over lean (-20 mA on the M103 -- on some M102s it's 0 mA), triggered by the throttle "idle" switch and high rpm. TPS pot is for transient enrichment -- typically, when it's bad, you get stumbles on sudden acceleration.

K-jet meter fuel flow by differential pressure and fixed pressure drop orifices in the fuel distributor. Pretty elegant -- the pressure regulator controls the main (upper) pressure, and the plunger controls the lower pressure. As the lower chamber pressure drops, the flexible diaphram between them flexes away from the fixed pressure drop orifices and increases the fuel flow to the injectors. Slick, eh? On the KE III, the EHA "trims" the pressure by leaking between the two chambers.

Peter