Warm up regulator question - PeachParts Mercedes-Benz Forum

knarf · #1 01-16-2006, 07:50 PM

Greeting Forum Members,

Today I tested the fuel pressures on my poorly running 1985 280sl.
I now need help in interpreting the results, CIS Kjetronic II system.
My control pressure at idle of .5 bar @ 25 degrees Farenheit I think is correct as the acceptable minimum.
My system pressure @ 20 degrees Farenheit reads only 1 bar.
The system pressure should read 5 -5.6 bar engine at idle warm or cold.
My control pressure topped out at 1.2 bar with the engine warm and begining to die due to running rich.
The control pressure should read 3.6 - 4.0 bar with the engine warm
and warm up compensator stabilized.
System maintains good constant rest pressure(engine off).
Could a worn or damaged system pressure valve be responsible for these poor pressure readings and/or does this look like a warm up regulator problem?
I suspect the former.
Any experienced help with this matter would be most appreciated.
I'd hate to order a part I don't need.

Thanks,

Knarf

t walgamuth · #2 01-16-2006, 07:52 PM

but my 84 280e euro was running very rich. like 10 mpg rich. and it was the warm up compensator.

tom w

dkveuro · #3 01-16-2006, 08:42 PM

53 psi warm idle.
Cold will be 26 to 29 psi.
Pump pressure must be above 75 psi.

Look at the CPR...there's a pin sunk slightly in the body.
Drill a 5mm hole 10mm deep in center of pin (Dowel.).
Tap drilled hole with 6x1mm bottom tap.

Fit a 6mm nut on a 6x1mm 30mm machine bolt and a washer to go between the CPR and the nut and run nut up to the bolt head.
Screw machine screw into pin on CPR with Loctite.
Run nut down to CPR and continue to tighten it until pin begines to rise.

Bridge fuel pump contacts.
With pressure guage fitted, adjust nut or tap down bolt after backing off the nut a turn until pressure correct cold. You are changing the pin height in the CPR that directly affects the fuel pressure cold.

If the warm pressure is not correct, you need a CPR.
You may find you need to tap the bolt past its origonal point if fuel pressure was way off.

.

.

raymr · #4 01-17-2006, 09:32 AM

What about the low system pressure though? Does that need to be correct before tackling the control pressure values?

MrCjames · #5 01-17-2006, 10:28 AM

What type of fuel pressure gauge are you using? If you are using a pressure gauge that has a shutoff valve, how are you connecting the two ends to the system.

When the valve is in the closed position the gauge should be reading pressure from the upper chamber of the fuel distributor, this is system pressure. When the vavle is open then it will read pressure from the warm up regulator, this is control pressure. If memory serves me correctly;
Cotrol Pressure--(.8 to 1.2 bar @10 C [11.5 to 17.5 psi]) and (3.2 to 3.8 @80 C [47 to 56 psi]).
System pressure--(5.0 to 5.8 bar [74 to 85 psi]).

Mind you, each vehicle is slightly different so you may need to adjust one or all of these values up or down to obtain optimum efficiency and optimum performance from your engine.

dkveuro · #6 01-17-2006, 11:04 AM

First determine pump stalls out at over 75 psi(Dead head.)
If not you need to find out why pressure is lower.
Filter.
Line kink, etc.
The pressures MrCjames quoted are correct, except California emmisions cars.

.

stevebfl · #7 01-22-2006, 12:01 PM

As has been pointed out your pressure readings are implausible. The posibilities for such are that you have a defective gauge or you are testing at the wrong point. Use the image included and indicate where your fuel preesure reading are taken at. For control pressure you should be reading pressure inside pipe #776. Because I don't like to mess with the running that using the tee valve does to get system pressure I use two gauges or one gauge two times to read the two pressures. (In other words the mechanism for reading system pressure with the valve is to block off control pressure till it reaches system pressure to achieve a reading. This cause the engine to run very poorly as during that moment system pressure equals control pressure not good for fuel mixture preperation)

For system pressure I have a tee fitting that reads the pressure as it enters the distributer.

knarf · #8 01-22-2006, 12:24 PM

Thanks for the reply Steve,

That is where I've been taking my readings, between the fuel line #776 as indicated in your diagram and the fuel distributor head with the shut off valve between the guage and line #776.
I'll take readings again today.

Knarf

knarf · #9 01-29-2006, 02:09 PM

Between dealing wth the weather and finding the time,
I have just completed a fuel pressure test of the fuel pump pressure delivered to the fuel distributor.
My new pump and filter maxed out at 65psi.
The car is running no differently with the new pump and filter than it did with the old pump and filter.
Assuming the new pump and filter are both good, what could cause such a low reading?
Could an obstructed fuel damper located between the tank and pump?
Bad fuel accumlator?
I know that any obstruction in the lines is another possibility.
I guess this explains my erroneous system and control pressure readings.

Thanks,

Knarf

stevebfl · #10 01-29-2006, 02:22 PM

That pressure is low, but not critically low. I could make a car work just fine with that pressure for system pressure. Since it is so close to right I assume the fuel pressure regulator is shimmed improperly. Since that pressure isn't the problem with a car that is performing poorly, I deal with it after fixing the real problem. What is your control pressure cold and warm?

knarf · #11 01-29-2006, 02:53 PM

Thanks for respondjing so quickly Steve. I think I'm close to finding the cause of my problem.
But let me clarify the source of my pressure reading.
That 65psi is the fuel pump pressure only.
This reading was taken directly from the fuel supply line that leads from the fuel pump to the fuel distributor.
I've read that this pressure should be at least 75pis.
The reading was taken with the line removed from the distributor and attached to the pressure guage.
The only readings that I get when attaching the fuel pressure guage to take system and control pressure readings have been the 1.2 bar warm system pressure reading and .5-1.2 bar cold control pressure readings previously mentioned..
The engine runs worst as engine temp rises, eventually stalling.
I'm suspecting a clogged fuel damper.

Thanks,

Knarf

stevebfl · #12 01-29-2006, 03:28 PM

Well generally speaking that pressure is meaningless, EXCEPT it is definitely TOO low. The fuel pump has an internal pressure relief for instances where it is deadheaded like you are doing. They don't exactly publish that pressure but I would guess that it was somewhere over 120psi.

The fuel pump DOES not determine the system pressure the pressure regulator does. A proper test of a fuel pump will be to measure the inlet pressure (determined by the regulator) while catching the fuel flowing through the regulator headed back to the tank. A critcal specification is the volume that flows back to the tank. On your car the pump should maintain system pressure about 75psi as you state while a liter a minute is returning to the tank.

Sounds like a fuel delivery problem. That usually means fuel pump. They way you are testing it restrictions shouldn't matter as you are not allowing flow. Even if the pump was pumping through a .020in hole it would eventually fill and pressurize the line to the blow off pressure. Is this the same gauge that read less than two bar on a running motor. I'm beginning to distrust it. No way a motor can run below 49psi. That is what it takes to open the injectors.

The other possibility is that you have no fuel in the tank or can't get it out.