W140 w M-104 hard start

[wbrian63]

I grow tired of posting questions to "other" MB forums, only to have the cretins there fail to read the details and offer suggestions that have either been tried, or are not germaine to the situation, so I post this question here, where cooler heads prevail...

92 300SE - W140 Chassis - M104 Engine - HMF-SFI fuel injection.

Car is hard to start ALL the time - cold, warm, hot - doesn't matter.

Once running, car purrs like a kitten - has good power (for a 228hp engine in a 5,000# car).

Fuel pressure is book-spec at all times. Fuel delivery not tested, but delivery volume shouldn't be an issue for startup, and if volume were slight or low, power would be absent.

Recently replaced check valves on pumps (yes, it has 2) - both were leaking down.

Even with bad check valves, pressure jumps to 60+psi immediately at key-on, it just didn't hold pressure after key-off. Now it does hold.

To get car to start, you just have to crank and crank and crank. Probably 15 seconds minimum. Will eventually catch one cylinder, then 2 then 4 then it runs smoothly.

After startup, there's no black smoke, no white smoke, no blue smoke.

I've not tested the injectors, as they're electrical, not mechanical. However, one would think that if the injectors were lazy, the car wouldn't idle or make good power - it does idle and it makes good power.

Car will restart smartly immediately after shutdown. Wait 30seconds - 1 minute, hard start returns.

Have pulled and reseated all fuses on Base Module computer - no change.

Where to look next?

[s class]

Brian, I'll try not to repeat the obvious.....

but....

a) Whilst cranking in an attempt to start, are you sure the problem is lack of fuel, not lack of spark?

b) you say pressure jumps up immediately - do I understand that you have correct pressure at the fuel rail during the cranking period?  If so, this suggests the pump and its control relay are good.

c) when you shut off the engine, how fast does the pressure decay, now that you have the check valves operating correctly?

I will continue to think...

[wbrian63]

a) I am not certain about lack of fuel vs lack of spark. However, one would think that if spark were in short supply, that once started, there'd be black smoke at the tailpipe as the excess fuel is burned off. That being said, I haven't ruled out spark - should probably check cap, rotor and general spark quality.

b) Yes - fuel pressure remains at nominal levels during cranking.

c) Spec is to hold at least 30psi for 30 minutes after shutdown. Previously, it would drop to 0 within 2 minutes. Now, after 2 minutes, pressures were still north of 40 psi, and decreasing very slowly. Don't know if we'll get the 30 for 30, but it's far improved from what it was.

I should also note that changing the check valves had no positive or negative effect on the starting problem.

Thanks for your help to this point.

[Big_Richard]

.

[wbrian63]

Good point - I don't have such a tool, but maybe now's the time to invest in one...

Someone did point out that I should pull codes - haven't done that yet - need to break out my blinker box and pin chart...

Thanks for all the suggestions thus far...

Regards

[philpot]

Hello Brian,
Are you consistently using the same brand of fuel?
Is so, try a different brand.
I have seen some issues where I live with various oil company blends.

[John Hubertz]

Looks like you aren't alone with this problem - here are a couple of threads with what appears to be quite a bit of useful info:

http://www.benzworld.org/forums/w140-s-class/1351880-ramdomly-hard-start-w140-300se-2.html

http://www.benzworld.org/forums/r129-sl-class/1648513-mercedes-1995-sl320-6-cyn-not.html

But, simple things first.  Have you sprayed starter fluid into the MAF and determined if it then starts instantly?  If it doesn't start right away with the spray, only then would I seriously start chasing spark/electronics issues.

Another thing I'd do right away would be disconnect the battery for 30 seconds and reconnect, thus resetting all the defaults in the various computers.  Note that your windows and sunroof will have to "relearn" the up position (by cycling all the switches until you hear a light click) and you will need the five digit code for your radio.


EDIT:  Here's how I'd check for initial spark while cranking (a bargain at $4.99):  http://www.harborfreight.com/inline-ignition-spark-checker-69014.html

[jbrasile]

wbrian63,

Have you checked your fuel pressure regulator?

Pull the vacuum line that goes into the regulator and see if fuel is leaking from the regulator  itself with the engine running, if so that is most likely your problem.This is a very common issue on M104's and will give similar symptoms to what you describe.

If this has been happening for a while, I would change the engine oil and filter just to be safe since the leaking fuel was sucked by the engine and has probably contaminated the oil.

Tks,

Joe