Long cold crank - 560SEL - Page 2 - PeachParts Mercedes-Benz Forum

mhingram · #16 11-22-2003, 06:14 PM

Steve, Could a fuel leak down not also effect a cold start in that there would be no gas for a cold start either and so take a while to reach pressure?
thanks

stevebfl · #17 11-22-2003, 07:45 PM

Fuel pressure is maintained in a good system for a few hours maybe in a few cases over night. System pressure is established in about a second when the pump starts.

It is not expected that the system will have pressure before a cold start. Its possible that in real cold starting that the quantity of fuel through the cold start valve could be restricted by not having full pressure at the beginning of cranking. If this were the case and was part of the problem cranking for a second or two then a pause would make some difference. I think you will find that it doesn't matter much to a properly functioning system.

mhingram · #18 11-22-2003, 08:14 PM

Thanks for the clarification Steve. Sorry to send you down the wrong path Mike.
martin

MikeTangas · #19 11-22-2003, 09:19 PM

Thanks Steve. With the sudden onset coinciding with the recent drop in temperature I was hoping for the quick and easy fix. I'll try to grab some numbers tomorrow.

In case you haven't wandered over to the Open Forum for a while, the only reason this is of concern to me right now is that I will be moving to Michigan soon. Probably before the New Year, and if it is cranking long at 40*-50*F, I really hate to see how long it will take to light off at much colder temps.

MikeTangas · #20 11-23-2003, 03:49 PM

Not sure what it means, but here are my readings I took this morning.

Key on, engine off EHA drawing 75mA.
While starting 7.0mA, after about 10 seconds reading fluctuates between 3.5 and 5.5mA. Ambient temp at the time of testing ~80*F.

Duty cycle key on, engione off, fixed at 69.7%, running and warmed 50.3% (with dips to ~46% and spikes to ~58%).

Looks like my EHA numbers are way lower than should be during start and warm up. What should my next logical step be? I will repeat the tests later today after the car has had a chance to cool off sufficiently, just to make sure I didn't screw the pooch on the first try.

EDIT: re-reading Steve's DIY article, yet again, I notice mention of an OVP issue creating hard cold starts. I do have current at the EHA and the fuse on the OVP is not blown. However, can an OVP be only "partly" bad, and cause cold hard starts?

MikeTangas · #21 11-23-2003, 09:12 PM

Well, on a whim I swapped in a back-up OVP I had laying in my parts box and re-ran the tests.

Still showing 75mA at the EHA with the key on, engine off. During start, the EHA draw went up to 78mA stayed there for about 2 seconds then dropped to 6mA, with the draw dropping to 2mA as the engine warmed. I'm sure the current draw would have dropped lower if I had kept watching. Interesting thing was the draw steadily dropped with the OVP change compared to the numbers fluctuating before the swap.

The duty cycle now fluctuates between 49.3% and 51.9%. The swings are gradual enough that one can watch as the numbers count up, then drop and start going up again. During the prior test the numbers were jumping around wildly, with the majority falling at 50.3. When I say wildly, I mean I couldn't keep track of them because they were changing so fast.

BTW, the engine fired off pretty quickly compared to the way it had been. I will know better in the morning how it starts

Am I on to something here?

stevebfl · #22 11-23-2003, 09:29 PM

Possibly, your numbers look much better now.

I didn't reply to your question about the OVP, because it is like one of the ASE test questions: no one in their right mind would suggest that a fuse could be anything other than open or closed. But if your findings prove out, the OVP is actually looking like a resistor.

By having any current at the EHA the OVP "should" be considered working.

I have seen the oVP act as a resistor before but only in the control of the idle valve. The idle valve current flow is many orders of magnitude greater at 600-1000ma. But you may have had problems there and weren't noticing it as idle does pretty well "limping home".

mhingram · #23 11-23-2003, 10:49 PM

Mike, I just got this a month ago should have read it before opening my mouth but it is excellent reading and very useful for a 560 owner.
m

http://www.amazon.com/exec/obidos/tg/detail/-/0837603005/102-3875080-6556937

MikeTangas · #24 11-24-2003, 11:36 AM

we have Joy.

This morning the ambient temp when I left the house for work was ~35*F at 0500. The car started in less than a 2 count. I will pay close attention for a while, but looks like I will order a fresh OVP.

In my haste I was looking for the "quick" fix. Thanks for pushing me to the long fix Steve, looks like the long fix was actually much quicker (but I do believe you've said that a time or two before).

I spent some time looking over the OVP I pulled from the car. The only thing I noticed was that the red top was a little loose on the body. I'm thinking moisture might have gotten inside the relay. The original cold start issue began with the temperature change, which was accompanied by rain. I think I will dissect the old OVP and see if there is any visible evidence.