130k Miles.

In cool weather, car was was VERY hard to start, and when it fires, it does it so only on a couple of cylinders and only runs smoothly after a few more seconds.

Found the Cold Start Valve not getting a ground during start sequence (Manual shows this model year is fed timed 12v by the ECU during start, but other side is permanently grounded). No Thermo Time Switch installed.

I provided a new ground for the Cold Start Injector and car seemed to start and run perfectly immediately. Then the "good starts" became intermittent (sometimes car starts perfectly, other times long crank) and there would be a cold hesitation on acceleration following hard starts. When it starts properly, you'd never know there was anything amiss. Luck of the draw on whether you get Jekell or Hyde on any particular cold start.

Thinking possible fuel contamination (a few drops of water of some solids moving about), I drained tank, and removed/drained or blew out fuel pumps (2) filter, all lines, CIS distributor, lines, injectors. Checked regulated pressure (80psi) and residual pressure (38psi held for hours). Pulled start injector and checked spray pattern (perfect), but the rubber hose it sprayed into was somewhat fouled, so removed it and cleaned it.

Removed the Auxiliary Air Valve (in this model Aux Air Valve air is sucked past the cold start injector and introduced into the intake manifold with the auxiliary air) and it looked clean, but tested with "pulsed" 12v to see if it activated (it click and rotated inside).

Still, some starts are PERFECT. Starts and runs smoothly immediately, while others are awful (long crank, and takes seconds to run smoothly).

Even when it starts and runs perfectly smoothly, the idle never varies from about 750rpm even after start-up. Shouldn't cold rpm be at least 1000rpm for 20-30 seconds after start if the auxiliary air valve is opening for cold conditions?

I did all the research I could (MB Manual, Bosch F/I manual, Internet) and never really found a detailed description of how this particular type of Aux Air Valve operated (it's different from the "old" types, or later BMW rotary vane types). It appears to be a rotary vane with a strong elecromagnetic actuator that only rotates maybe 60 degrees, and unpowered snaps back under strong spring force to slightly open, maybe 1/8" open visible through one side).

I'd love to know if the valve gets a variable voltage from the ECU, or is simply an On/Off device actuated during the Start Sequence in order to know how to "monitor" it properly during a correct start.

Like this:


Open to ideas!

Bob

I don't know the answer about the voltage to the aux air valve. I do recall something about the cold start injector being updated if the old one was butterscotch in color. I've experienced poor cold starts due to light oil fouling from worn valve seals. Replaced the seals and plugs and it worked.

Currently, after (years) the valve seal replacement, experiencing intermittent very hard/long crank starts only when hot, all cold starts excellent...go figure.

__________________
"Rudeness is a weak man's imitation of strength" - Eric Hoffer

Thanks for the reply.

Have a green cold start and a tan one.

I tested both by hooking to the fuel distributor (cold start line turned around to feed cold start facing up) and activating the fuel pump, and both produced cone-shaped very finely atomized spray.

I looked down into the rubber hose the start injector sprays into and it looked fouled with rubber shavings, so removed the hose (leads from intake manifold to aux air valve)
and trimmed out all of the rubber bits so the inside is clean, but haven't tried the car again yet, but I suspect it's more than just that.

The plugs are clean of any fouling, so hopefully it's not guide deals. The hard start vs easy start is night-and-day and can be horrible one start and great the next, so I don't know.

Getting the spray into the manifold all depends upon air flowing through the aux valve past the cold start and into the intake, so if the aux valve isn't sometimes opening enough I was thinking maybe the engine isn't "inhaling" the droplets of fuel.

...could be the fuel accumulator. My experience with a leaking diaphragm in the FA was that it caused intermittent hard starting, both cold and hot, and it gets more frequent with time.

There is a simple FA test that you can do on the car. I documented this and did a destructive failure analysis of the old FA and posted photos here. With the new FA, starting returned to normal and fast, cold and hot. You can search for threads started by me for all the details. It's worth checking out.

Duke

The fuel accumulator holds the fuel pressure after the pumps are shut off, right?

If the current fuel pressure holds at 38psi for the next start, is the accumulator supposed to hold MORE pressure than that?

Should it be holding the full 78psi?

The starting problem related to the valve seals is a "false" first start like a stumble/stall. The second attempt is a start and run well. Your symptoms seem quite different.

Good luck!!!

__________________
"Rudeness is a weak man's imitation of strength" - Eric Hoffer

When's the last time the cap/rotor were replaced on the distributor? You could be barking up the wrong tree looking at fuel.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

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1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

I thought of that, but it doesn't at all feel like a "hard" misfire one would usually get from a spark problem. It's more of a soft one one gets from a weak fuel supply if that makes sense.

Not saying it can't be electrical, of course. The plugs looked very good when removed for the compression check. The wires look new. Obviously it wouldn't hurt to check.

Sometimes cranking 15 sec vs immediate start could be a bad leak of the coil output to ground intermittently I guess.

Just invest in a ht spark tester ...Its like a pen you pop it on the ht leads as the car is running .You will see if the ht leads are faulty.

Yes, the FA should hold full system pressure after shutdown. There may be a very slight leak down rate from the FA or pressure regulator, but that should take hours if not days.

When the ignition key is turned to the on position, the fuel pumps are energized for one second, so when I take my 190E 2.6 out of six months storage every year I cycle the ignition on three or four times before cranking to build fuel pressure, and then it usually starts right up as if it only sat overnight.

When you run the FA leak test on the car if you see ANY fuel in the clear vinyl tubing the FA diaphragm is cracked, leaking, and must be replaced in order to get back to normal hot and cold starting behavior.

Duke