M110 Residual Fuel Pressure
 

1978 280CE/ M110.984


By any chance, does anyone have any idea of how many Bar pressure /at what time duration, the Fuel System should hold. The FSM says a quick drop to -0- is indicative of a leak at the Fuel Pump, whereas, a slow leak is more likely the Seal in the Fuel Distributor Primary Pressure Regulator.


After about 5 hours, I'm down to about .8 Bar. Very Slowly.


Thanks!

30 mins is the typical kjet spec for duration with no drop in pressure. if it starts ok hot, rest pressure isn't a problem.

Porkface, Appreciate the reply.

Hard Warm Start IS the problem. That translates to 5 or 6 seconds of cranking. If I give it a little throttle, it starts in a second or so, no stumbling and no wavering RPMs. I should mention that Cold Start takes about 2 or 3 seconds. Aux Air Valve moves RPMs to about 1200 or 1300 RPM. Once warmed up (about 7 minutes), AAV begins to close and RPMs stabilize at 850.

1) Set the Timing at 10BTDC, Dwell at 40 degrees. Idle = 850 RPM.

2) Mixture set at exhaust manifold back pressure pipe (pre Cat) = About 2% CO. Measured at tail pipe (post Cat)= about .1% CO.

3) New Bosch injectors, Seals, Insulation & Guides.

4) Thermotime Switch and Cold Start Valve tested and cleaned.

5) Tested Fuel Pressures.

*At Cold start up, Control Pressure = 1.2 Bar. Once Warm = 3.5 Bar.
*System Pressure = 5.4 Bar.

*Residual Pressure (Initial) = 3.2 Bar. At 20 minutes = 2.8 Bar. At 2 hours = 2.5 Bar. At 4 hours = 1 Bar. At 18 hours (overnight) = .2 Bar.


I know that leaks (not normal) can, and leakdown (normal) will occur. Depending who you talk to, the leaks can occur at the Primary System Pressure Regulator O-Ring, the Fuel Pump Check Valve, and the Fuel Accumulator. The FSM says immediate leaks (0 Bar) are most likely the Check Valve or the Accumulator, and slow leaks may be the O-Ring in the Pressure Regulator.


From what I've read, the leakdown rate I have is normal. WHAT is causing my hard warm starts???


MM

when fully warm, twist it rich a bit-like 1/8 turn and see if it's better. too lean can do that, also. to rich can too so don't go too far. you have emissions inspections?

No Emissions for 11 years now. Virginia stopped when the car turned 30.

lived in fairfax until 83.

reason i asked is because the co looked text book and not real life. run it a hair rich and try again.

I was in Arlington 80-85. Centreville 85-02. Manassas 02-?


I gonna have to wait for a week or so to tweek it. I ordered the Pressure Regulator Repair Kit. The original is pulled out right now. Just 'fishing' for ideas.

I have the same problem and replacing the Fuel Accumulator solved problem. Just clamp return fuel line 1/4" from Fuel Accumulator to tank and see if problem is solved. This will indicate diaphragm leak in Fuel Accumulator.

he might be right. usually the rest pressure drops and it's the accum. had a 450 sl that had a stumble on start. replaced the accum. and problem gone. i suppose it had a slight leak and rest pressure didn't drop enough to see. good luck, chuck.

Thanks for the replies and suggestions. The Accum was on my original 'possible issues' list. What's throwing me off is the fact the the pressure holds within specs (20 - 30 minutes) and I still have a hard warm start. Even if the Accum is leaking down, I still have good pressure to start.


I've searched far-and-wide on this topic, and some people mention the fuel in the injectors and lines from the Fuel Dist as getting hot, and creating vapor creating the problem.

The accumulator's primary function is to maintain fuel system pressure to prevent vapor lock when the engine stops while hot. It stands to reason that if the accumulator is failing that you could develop vapor pressure that would fool you into thinking everything is rosy.

I went through warm start issues on my SL several years ago and the system pressure always seemed fine. I bit the bullet and replaced the accumulator and the problem went away. Just my own experience.

i have replaced accumulators alot. i have never been into the fuel dist for any pressure issues. good luck, chuck.