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Old 03-18-2003, 10:26 PM
pmizell's Avatar
Benz Zealot
Join Date: Jun 2001
Location: Houston, Texas
Posts: 838
How much fuel pressure at injectors?

Hey Guys,

I'm trying to diagnose a no-start with my 300E. Today it quit on me while running errands; prior to this, engine ran and idled flawlessly. I've checked spark, OVP fuses, and fuel pumps (2 -- I think both are turning but can't tell for sure)

I busted open bolt at pressure regulator and got a nice spray, then re-primed and tried an injector... and nothing ... just a couple of drops of fuel came out. Tried a couple more with same results.

Is this normal? Should I get a "squirt" when I break open a fuel injector? I tried some engine starter in the intake, to no avail. Engine cranks fine ... just won't catch at all.

Any help is appreciated
'01 E430, Sport 72,000 mi
'98 C280, 126,500 mi
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Old 03-19-2003, 12:42 AM
Registered User
Join Date: Jul 2001
Location: SoCal
Posts: 1,303
Of course, the injector pressure is a function of engine demand - no demand, little pressure. You can test the pressure at the output of the pressure regulator, to make certain it is high enough. The injectors have cutoff pressure valves that will block flow if differential pressure drops below about 46 psi. Differential pressure is adjusted by the air flow meter, mechanically, and electronically via the EHA.

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Old 03-25-2003, 11:19 PM
pmizell's Avatar
Benz Zealot
Join Date: Jun 2001
Location: Houston, Texas
Posts: 838
I have to believe this problem is ignition related since it happened suddenly with no performance drop prior to an absolute no start. I've poured fuel directly into the plenum and the engine will not catch at all.

As I stated I did check spark .... but it appears weak -- will replace plugs tommorrow to see if this will solve problem .. but I seriously doubt it.

I've checked distributor and cap -- perfectly clean inside. The only other part I can think of is the ignition coil. Are coils all or nothing or can they go weak causing weak spark?

Any other ideas would be greatly appreciated.

'01 E430, Sport 72,000 mi
'98 C280, 126,500 mi
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Old 03-26-2003, 08:32 AM
Join Date: Mar 1999
Location: Tucker, Ga USA
Posts: 12,153
On CIS cars the injection pressure stays constant!! The injector is set to open at approx 3.6 BAR fuel pressure. The system operates on injection quanity,
If the system operates at 5.2-5.4BAR, then there should be that much pressure at the injector to force it open.
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