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'68 280SE (w108), '72 280SE 3.5(w108), '88 560SEL(w126), '97 E50 (W210
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179 Posts
Discussion Starter #1
Hi everyone...I've already used the search function for this with keywords "fuel pressure regulator" and "hard starting" and so far no luck, hence this post.

I've been experiencing hard warm starting lately, taking the car from 5 seconds up to start. I've looked at the other forums and online tech helps and they all seem to point to the fuel pressure regulator as the culprit. I took a look at my FPR today, noticed the vacuum hose was a bit loose, so I replaced it with a new one. However, when I pulled the vacuum hose off the FPR a small amount of fuel came out from the hose.

Based on what I've read about FPRs, I don't think its supposed to happen, is it? I believe the vacuum line is isolated and separate from the fuel line, i.e., no fuel flows through it. Maybe the diaphragm has a leak in it, hence its not holding pressure.

Folks who have experienced this, I'd appreciate it if you let me know if my assumptions are right, and if I should order an FPR or continue to look somewhere else.

By the way, I have an E50 with 32000 kms on the clock. Thanks a lot everyone!

best regards,
edward
 

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Premium Member
1999 E430
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64 Posts
triskaideka

I think your obsertion has lead you to the answer.

The purpose of the FPR is to keep the fuel pressure at the level required by the injectors to function as designed. Personally, being a recent owner of the marque, I don't know what is the pressure on the MB models, however, most injectors are rated at 43 psig or 3 Bar and I assume that is the regulated pressure.

The other function of the regulator is to keep the line pressure up for awhile after the engine has been shutoff to eliminate what is referred to as "vapor lock" or when the fuel that is static on the line absorbs the temperatue in the engine compartment and vaporizes. This vaporization, creates havoc when the car is being started as you have witnesed.

So in your case, the diafragm of the FPR has deteriorated or has a pin hole that allows the pressure to bleed down (engine off) before the engine cools allowing for vapor lock to ensue.

Sorry for the long disertation, but I figured that it will help other posters with the same problem.
 

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Registered
'68 280SE (w108), '72 280SE 3.5(w108), '88 560SEL(w126), '97 E50 (W210
Joined
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179 Posts
Discussion Starter #3
Thanks Sir boxermania. Yup, the fuel pressure regulator was definitely the culprit. The one I ordered came with the label "3.8 bar" on it. Actual replacement was around ten minutes, although I had to depressurize the system first then disconnect the battery cable.

And it did cure my warm hard start problem. And the car feels more responsive to my throttle inputs, but that could be and probably is subjective.

Happy miles to all,
edward
 

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Premium Member
1999 E430
Joined
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64 Posts
triskaideka

I'm glad the problem has been resolved, I also learned something regarding the fuel pressure on those models ~ 55 psi or 3.8 Bar.

BTW, while on assignment in Tokyo a few years back I meet and befriended some guys and gals from Dash Engineering in Cebu.....very nice and knowledgeable folks.
 

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Registered
'68 280SE (w108), '72 280SE 3.5(w108), '88 560SEL(w126), '97 E50 (W210
Joined
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179 Posts
Discussion Starter #5
Sir Boxermania,

Small world, my mechanic knows them and deals with them from time to time. If you happen to be in my corner of the globe just give me a holler!:)

happy miles to all,
edward
 
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