Starting problem 89 300SE, symptoms, observations and advice please

Ok
I have a starting problem, observed I must say (now I think about it) when cold. It doesn't like to start when its cold. Occaisionally it fires, but
generally its a lot of cranking before it splutters into life. Once it is running, it runs perfectly. i.e no coughing or anything, even under load, so I guess I'm discounting the ignition system in this, or fuel delivery once running (read on). I note that I hear the fuel pump run under the floor for about a second as I switch on the ignition. It does this every time.
I have a number of points I would like to raise for opinion:- a) My cold start valve has failed, or my fuel pump relay is not activating this. Which is most likely?. b) My fuel pump relay is not receiving the 'I'm cranking' terminal 50 signal to tell the fuel pump to cut in again (maybe see below), or activate the cold start valve. Is there any history on this?. c) My tacho signal is not getting to the fuel pump relay to tell the pump that the engine is turning over and should re-start the fuel pump?. d) Is the CIS-E unit temperature output not telling the fuel pump relay to operate the cold start valve and what should this signal be anyway (analogue voltage or a digital [on/off] signal). e) Is the CIS-E unit working at all? I must say it seems to regulate its idle speed OK. I know the CIS can work without it. f) Is my fuel pump OK. The car has done about 180k miles and I guess its the original pump. How long do they last (but all is well once its fired up)? g) As I said, the thing is perfect once running. I have replaced the fuel pump filter (new) and the fuel pump relay with a good 2nd hand unit, though this reluctance to start is more exacerbated now the weather has turned cooler here (and I hadn't used the car for a week) in Blitey.
Any opinions on the above (Tom?).
I'm happy to start detecting, but I'd prefer to start with the probables before the possibles. Its dark and getting cold and wet and miserable out there on the drive to be tinkering whilst wifey reels off all the other 'more important' jobs to be done about the place.
Cheers... Rob.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Mercedes is supposed to retard the timing on shutdown to allow for easy cold start. What ever controls that or whatever does it isnt working possibly.
Rob. Smith wrote:

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
I would suspect the CIS-E temperature sensor is faulty. If it is not registering proper temperature for the fuel injection system, then it woudn't be able to determine precisely amount of fuel to start a cold engine.
Do you have problem starting a warm engine?
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Hi Rob,
I don't like to start when it's cold either!
Sounds like the cold start valve isn't doing what it should - 10 to 15 seconds of spraying fuel into the intake while the motor is being cranked. I don't know if it stops when the motor fires or just stops from being timed out.
The 280E version has (i) a thermostat ON below 20 C, OFF above, (ii) a thermotime relay whose ON time depending on temperature (coldness) and (iii) the cold start valve. Failure of one knocked the whole system off line. Your newer, fancy car probably differs somewhat from the 280E design. The 280E's thermotime relay was the usual culprit in cold start failure.
Hope this helps you.
Tom
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Hi Tom
You are correct about the simple elegance of my old 280E with the cold start cranking enrichment ans stat. This 300SE system is a whole lot more complex. I will need to do a bit more fault finding around the system. I guess its easy enough to measure the resistance of the valve that adds the extra fuel. A number of the tests require that the engine be cranked. I wonder If I just disconnect the starter solenoid at the junction box, so that I can pretend I'm cranking but not actually do it. In this way, I may be able to hear the valve clicking in and out (or not), and maybe hear if the fuel pump kicks in (tho this may only happen if it detects tacho pulses from the CIS-E bit of the system). Also, the electronic manual is a litle unclear (to a non Merc mechanic I guess) about the coolant temperature sensor/sensors. It talks about a sensor, but the schematics show a kind of double sensor, one part feeds the CIS-E and the other goes to the ignition amp. I guess there is another sensor still for the temp gauge on the dash. I guess I'll have a play under the hood. Just need good enough weather and sufficient permission from wifey. She's got me a list of jobs to do.
Updates when I have em.
Cheers... Rob.

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here. All logos and trade names are the property of their respective owners.