Author Topic: Hard starting 280SE Coupe W111  (Read 584 times)

gcw206

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Hard starting 280SE Coupe W111
« on: August 01, 2022, 22:08:46 »
All systems were operating correctly; however, I now have a problem that I did not have a few weeks ago:
My 1969 280SE W111 Coupe always started on the first try.  Since the weather in Seattle has been into the 90 degree range recently, now
she will start the first try (as before), BUT if driven for 20-30 minutes, and then shut off, the car will not restart.  (I made no changes to any of the settings.)
The entire engine bay smells of gasoline (as if it were flooded), but there is no sign of any leakage. 
This happened again today (at home), and she will probably be fine in the AM (as before). 
I have not started any diagnosis as of yet, and need too know:  Where should I start; and what should I look at first?

Todd

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Re: Hard starting 280SE Coupe W111
« Reply #1 on: August 01, 2022, 22:35:36 »
The smell of fuel is a good first step in the diagnostic process.  My guess would be that you have injectors that are dripping, once the engine is shut off, and/or the cold start injector (in the end of the intake manifold) is dripping. 

Here's what I would do.  Dump a full can of SeaFoam into the fuel tank, with half a tank or more in the tank (you're wanting it to be fairly concentrated, for the best cleaning effect).  Take the car out on the freeway at a time when you can blast along at speed, for an hour or so........like 30-45min out, and then back.  If there are any locations in your area where there is a long grade, run the car up that grade with as much throttle as it will take and not kick down and shift if it's an automatic.  The intent being, as wide open throttle as it will take, which racks the mechanical injection pump as "open" as possible, for maximum flow volume through the injectors.  If you have a manual transmission car, leave it in 4th or 3rd, whatever allows max throttle, without lugging, or causing pinging.  Running the Seafoam through the injectors in this manner will clean them, and start to clean the carbon off the backs of the intake valves.  And enough cycles of the cold start injector will eventually clean it.

See how the car does over the next few days after the treatment.  You may find it improves over time, especially because you'll be continuing to run that SeaFoam through it as you use up that tank of fuel. 
Todd

wwheeler

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Re: Hard starting 280SE Coupe W111
« Reply #2 on: August 02, 2022, 02:54:49 »
Dirty injectors are never good and would be good to make sure they are not a problem. You can also take them out and get tested pretty easily. Somehow you need to determine if it is too lean or rich when it won't start. Can't know which direction to head until you know that. I have always found that my engine gets too much fuel when starting hot after sitting for a few minutes. Disconnecting the starting solenoid completely resolved that. I now run a manual CSV switch in the cabin. But that is another story. I am in Texas and it gets and stays hot for months.

MB made a "hot start" modification which actually added a dash of fuel during starting even when hot. I tried that and made it much worse. So in my case I determined that I was getting too much fuel when hot. Starts great cold and semi warm. It will also start ok if hot and started immediately after it is stopped. But wait 20 minutes and is hard to start. By disconnecting the start solenoid, it no longer adds fuel when starting. I have just figured out when I need to add extra fuel via the cabin switch.

I have come to believe that with wear over the near 60 years these injection pumps have been running, everybody's situation may be a bit different. That is why you need to determine what the mixture is when it is acting up before you do anything. Leaning the mixture during starting is easy by disconnecting the start solenoid. I suppose to add fuel during starting you could hot wire the CSV and spray some fuel before it cranks.     
Wallace
Texas
'68 280SE W111 coupe
'60 220SE W128 coupe
'70 Plymouth Roadrunner 440+6

gcw206

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Re: Hard starting 280SE Coupe W111
« Reply #3 on: August 15, 2022, 22:07:40 »
Wallace,
Since yours was the easiest and the cheapest solution, I decided to drive my car until up to 180 degrees, park her for 15-30 minutes, and then try to restart.
(This is the sequence that I have trouble with, and need to address.)  She did not start up right away, so I got my screwdriver, and disconnected the wire
that feeds the CSV.  I let her sit for a few minutes, and tried to start again.  This time it caught, when just before it did not.  I think that when the engine is
hot, the CSV becomes a liability.  Tomorrow I am going to wire the hot lead that runs to the CSV through a push button on/off switch mounted to the under
side of the dash, and see what difference that makes.  When cold, I will push the button, completing the circuit, and energizing the CSV.  When hot, I will
not push the button, and the CSV will not work at all.  FYI:  My TTS was replaced with a Porsche P/N 9116171170, and the specs are a little bit different
(+15 degrees difference in the high cut off temperature).  Anyway, all worked just fine until a few weeks ago.
It may still be an injector problem, but I am going to try this solution first, and go from there.
Thanks,
Galen

teahead

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Re: Hard starting 280SE Coupe W111
« Reply #4 on: August 15, 2022, 23:31:52 »
on my W113 (US automatic w/AC), my CSS isn't even hooked up.

My cold start issues was a relay.
1970 280SL auto, AC - aka "Edelweiss"

wwheeler

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Re: Hard starting 280SE Coupe W111
« Reply #5 on: August 16, 2022, 17:31:40 »
Galen,

What I disconnected was the starting solenoid on the I pump. Not the cold start valve that is on the intake. The IP solenoid richens the mixture every time the car starts. I found that when hot, it was too much fuel. Later cars, '69 or so may not have that start solenoid and have a different (smaller) cold start valve (CSV) on the intake.  The problem in disconnecting the starting solenoid is that you then do not have enough fuel when cold. That is where the manual cabin switch comes in for the CSV. I inject extra fuel to compensate for the starting solenoid being disconnected.

Hopefully that makes more sense.
Wallace
Texas
'68 280SE W111 coupe
'60 220SE W128 coupe
'70 Plymouth Roadrunner 440+6