hard start after driving

[rmmchl]

250sl  I know it's not the cold start switch. It starts right up. but when it's hot outside and I park it , after I have been driving it a while, it takes a long time to crank again. To the point that the battery becomes labored. Could it be the points? Where's dr. benz when I need him!

[hauser]

Try this the next time you have a problem.  When you get into the car put your foot all the way down on the accelerator then turn the key.  This procedure is usually not necessary with cold engine.  With my car all I do is reach in and turn the ignition and starts right up.

1969 280sl 5 spd        1999 ML320          Gainesville,Fl.

[Cees Klumper]

I have the same as you.
When my car is cold it will start right up, but during the first 5 or so seconds of running, I cannot throttle up too much because it will run very uneven.
When the engine is warm, it starts difficult, except when it has sat for only a few minutes (see below). If I then let the fuel pump buzz for a while before trying to start (say, 4 to 5 seconds) then it starts pretty quick, although not quite as fast as with the cold start. As Hauser points out, the owner's manual advises to floor the pedal when starting the engine warm but I have not found this to make a discernable difference.
If my car has sat for only a few minutes or less with a warm engine (e.g. after fueling, which as we all know is a frequent event), it will start immediately!

So this is the strange part:
engine cold: immediate start;
engine very warm and sitting very short: immediate start
engine warm, but sat for 15 minutes up to several hours: difficult start.

It seems to me that, as far as the cause for all of this, the pressure in the whole fuel delivery system slowly goes down after driving which, on a cold engine, is compensated for by all the extra fuel injected through the CSV and the cold start thermostat on the FI pump. When the engine is warm, there is less extra fuel.

Of course I could be wrong, but it would be great to get the solution to this very common 'hard warm start' problem since so many seem to experience it.

Cees Klumper in Amsterdam
'69 white 280 SL automatic

[hands_aus]

Hey Cees,
I thought the later cars , 280sls had a 2 second relay installed on the wiring of the cold start system to over come this problem.
There was a small wiring harness, part number 108 540 10 09, and the 2 second relay PART NUMBER 001 545 1624 that were sold.
I haven't bought them yet, I just checked the MB site for availability. The relay is available at
Eur 128.18, and the harness is available at Eur 31.90

Bob (Brisbane,Australia)
RHD,1967 early 250 SL, auto

[ja17]

Hello,
There are two starting aids on the injection system in your 250-SL. The starting valve is activated on the intake manifold during starts  below 95 degrees F. The amount of time that this valve squirts fuel into the intake increases with decreasing temperature. It is open up to 12 seconds when very cold.  It's function can be checked with a 12 volt test light.

Shortly after turning off  a warm engine, the coolant temperature will raise above the normal running temperature of the engine. During this "hotter than normal period" the warm up mechanism on the injection pump leans the mixture more than normal making hot starts difficult. The injection system on this model is equiped with a starting solenoid on the back of the injection pump which is controlled by a relay. The solenoid is activated at all temperatures and stays activated while the starter in engaged. This system aids starting when the engine is hot.
The function of this system can easily be checked with a 12 volt test light. The relay can be defective if the solenoid is not working.

Some of the later injection systems (from 1970) did not have the starting solenoid on the injection pumps.

In addition to the information in the previous post, weak fuel supply can cause starting problems. Check to make sure the fuel filter is clean and that you have sufficient fuel pressure and volume! Good luck, keep us up to date!

Joe Alexander
Blacklick, Ohio

[Ricardo]

I have experienced this hot start problem for the last three years, frustratingly unpredictable, occasionally not starting after fueling up, sometimes going for weeks without a problem. Tried everything I could with cleaning the fuel system, including the return line to the tank. Nothing helped. Finally this year I have discovered what the problem is with MY car. It is actually flooding and the cure was to diconnect the CSV wire and hold the pedal down. This has worked for every no start, though I have yet to determine if the relay or thermo time switch is at fault. I wonder if, as Joe says, the increase in temps after shutdown may cause old injectors to leak a little fuel into the cylinders and the CSV adds more when cranking. Mine would almost start but if I released the key too soon it would stall and restarting was impossible for at least 20-30 minutes. I now just slip the CSV wire against the side of it's hold down screw for the first start of the day and leave it disconnected the rest of the day. Not fixed, but my wife sure is happy to not have to push start me anymore. Or at least not the car.
Ricardo

[A Dalton]

<When my car is cold it will start right up, but during the first 5 or so seconds of running, I cannot throttle up too much because it will run very uneven.
>
 This can be rectified by changing to the improved ball valves       [ part 000 074 17 15]
 The early cone style valves leak by , the balls do not .

 It is also interesting to note that when they eliminated the rich solinoid [ starting rack magnet] , they   increased the balls to 4mm [ from 3] and richened up the lower part load settings one notch.

[A Dalton]

quote:Originally posted by hands_aus

Hey Cees,
I thought the later cars , 280sls had a 2 second relay installed on the wiring of the cold start system to over come this problem.
There was a small wiring harness, part number 108 540 10 09, and the 2 second relay PART NUMBER 001 545 1624 that were sold.
I haven't bought them yet, I just checked the MB site for availability. The relay is available at
Eur 128.18, and the harness is available at Eur 31.90

Bob (Brisbane,Australia)
RHD,1967 early 250 SL, auto

 This is the hot start modification kit..

 The problem with it is that it only allows a 1 sec squirt from the
cold start jet.
 Many owners , myself included , have had better luck by simply installing a push button sw in the cabin and operating it manually
[ kinda like an electric manual choke, without chocking off the intake air]
 This works great ..
 You  have two choices on the wiring...
 You can do dirct B+ and it will work anytime , or ..
 you can catch power at W of thermal/time switch and simply use a switched groung.
 The advantage here is that it will only work when cranking...
 Some actually disconnecdt the TTS and use the push button top there own likeing..
 Once you get used to it, you know the engines starting habits better than the sensor ... like the chokes of old.............

[rmmchl]

glad to hear many others have this problem. I'll have to check that solonoid on back of the injecton pump. Dr. Benz---please reply. I want to to hear your opinion on this common problem.

[Benz Dr.]

Complete fuel flow is part of the answer. When you turn the engine off, heat will travel out along the motor mount and make the fuel in the filter canister very hot. Fuel inside of the injection pump and lines gets hot as well depending on how long it's heat soaked. If you leave it long enough it will just cool off and the cold start system will take over as usual.
As long as you have a fuel pump that delivers 1 liter in 15 seconds AND the fuel is retuning to the tank you should have fewer problems. The fuel in the tank actually acts just like a cooler at the injection pump. Put your hand on the injection pump after a long drive with the engine running and it should feel cool.
There is also a check valve on the outlet of the fuel pump which should hold the fuel from draining back into the tank. On the outlet side of the injection pump there's a pressure valve that should keep fuel inside of the pump under a certain regulated pressure. All of these things need to work really well before you will get good starting under all conditions.

BTW,  BPES6 NGK plugs are the same as Bosch W7DC. I use W9DC in almost everything unless I have perfect compression. 7's are too cold for most cars if they're running a bit rich.

Daniel G Caron