It's baaaack. M103 chuga-chuga at hot startup.
 

Well, we had some warm days, and the '91 300SE is back to it's old ways of idling rough for the first few seconds after a heat-soak.
Another new set of injectors and seals over the winter had no effect.

Condition:
cranks, runs, idles beautifully; except after it sits and heat-soaks for 1/2 to 2 hours. It cranks right up, but then chugs like it's running on 3 or 4cylinders for about 5 seconds. Then it smooths out and purrs. If you gas it during the chugging, it revs, while missing for a second or two, then clears right up.

This is the ONLY time this engine EVER misses a beat,
If cranked cold, it's perfect. If cranked hot after only sitting a few minutes, it's perfect.

History: I've owned the car for 16 years, it has done this for at least the last TEN YEARS during warm weather. In that time it has had all the usual tune-ups, a valve job, two sets of injectors. It has never really changed anything.
Just chuga-chuga-chuga for about 5 seconds after a heat-soak, then all is well.

Must be some sorta fuel issue, like vapor lock or leakage or boiling fuel when it sits and heat soaks?
I have resisted tampering with the fuel system, other than injectors/seals and checking the mixture and idle air settings. A couple years ago, I read up on the procedures here on the forum, and checked dwell, idle air, lambda, etc. Everything looked fine.

Is this a common ailment of the 103 engine? I hate to start digging into the fuel injection system after it's worked so well all these years, and the chuga has always been there - no better, no worse.
But it's starting to bug me.

I guess I'm hoping for someone to tell me "Yeah, they all do that when the Flux Capacitor goes out - replace it and be happy".

Any thoughts?


Tks,
DG

IMHO it's fairly common on ALL CIS Benzes. A few seconds is really no big deal, I even remember reading something on a diagnostic fiche (years ago) for M116/117 engines about this being "acceptable".

Assuming everything else is OK, you probably have a little internal bleed-off somewhere, like a fuel accumulator or even a fuel distributor. At high ambient, and with a hot engine, the fuel will vaporize more readily, especially with a loss of system pressure from a small bleed-off. This may be why you only notice it at warmer ambient.

I wouldn't worry about it too much. If you *really* want to get into it, testing the system for internal leakage is pretty straightforward.

Cheers

Thanks for the reply and the reassurance that nothing is horribly wrong with my baby.
Yes, I think you are right - some sort of hot-fuel-pressure thing.
I haven't done any pressure testing on the CIS, but have glanced over the
literature a couple of times.
Maybe if it stays cold here in Texas for awhile, and I can't play golf, I'll
look a bit deeper.
If I find anything interesting I'll pass it along.

Tks,
DG

You might try pressing accelerator pedal down somewhat, before you turn the key to start. This tip is from a respected shop owner, who explains the subtle air leaks that develop in these engines over time. I have never had this problem, cold or hot, on 2 different M103 with a whole lot of miles.

Steve

Thanks, Steve.
Yes, I've tried pressing on the gas before starting, and even cycling the key a couple of times to cycle the fuel pump. Motor jumps to life, but then immediately chuga-chuga. If you hold the pedal down a bit, it struggles to
rev while chugging, then clears up and all is good. More throttle makes the chug cycle shorter, but it's still that "running on 3 cylinders" feeling for a few seconds.

Really more annoying than anything. I really hate it when someone gets in my car and starts raving about how perfect and beautifully maintained it is, then I fire it up and it goes CHUGA-CHUGA. Kinda like finding out that beautiful blonde next door has bad breath.

DG

Did you ever figure out what was causing this problem? My car does something similar, but not quite as bad as you describe...If I hold the throttle open while cranking, it seems to help quite a bit.

John, no I never found it.
1. I bought a pressure guage on eBay, and checked out everything - no problems at all. It holds good pressure overnight, and jumps to exactly correct hi-and-lo-side pressures in an instant when the key is turned.
2. I replaced the IAC rubber hoses, checked all the vacuum lines, and it has good idle vacuum, about 17", which seems to be the norm for the 103.
It pegs the dash guage in neutral at 650 RPM.
3. I also monitored the EHA current during start and run at various speeds. I adjusted the EHA screw a bit, in both directions, and monitored the duty cycle - no effect on the chugging.
4. I replaced plug wires and cap/rotor.

So, as of today, it runs perfectly, after the initial few seconds.
Fuel system checks out fine.
Vacuum checks out fine.
EHA current and duty cycle are spot on at start, idle, and cruise.
Ignition is fine, as far as I can tell.

But to add insult to injury, over the winter, it started to no-fire on the first try. If you hold the starter on for 2-3 seconds, it will struggle to life.
But if you just nick the starter once, turn it off and nick it again, it jumps to life perfectly. EHA adjustment had zero effect on this.

Oh well, must be the flux capacitor.
Actually, I'm leaning toward something in the ignition? But, they usually fail the hotter they get. And it runs fine idling in hot traffic with the AC on.

So, onward to addressing that little R12 leak I found in the condenser.

DG

I still think it's something in the CIS system...did you test the injectors before installing them? I've heard of them being bad right out of the box... Maybe a sticking air mass meter?