CIS Pressure Question(s)
 

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

i dont know about all those readings
 

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

tom w

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.


.


.

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

K-Jetronic/CIS fuel system tests
 

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. :cool:

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.

.

Thanks to all for replying.
dkveuro,
I see that my warm up regulator is already set for this adjustment proceedure.
Will this adjustment also increase my system pressure? My fuel pump and filter are new. I have no vacuum leaks.
For the record the poor performance at operating temperature came on "all of a sudden", right after a fuel fill up. Maybe it was coincidential that it just started manifesting itself at its current level. Actually for some time I had been noticing a sooty exhaust but no noticable performance problems.
What do you mean by "First determine pump stall over 75 psi"?

raymr,
Good question.
I've ordered a system pressure valve repair kit from Phil at Fastlane I hope that this contributes to the cure.
What happens to these valves when they fail?

Cjames,
I'm testing my pressures with a Bosch CIS fuel pressure tester by ATD.
I do have the MB shop manual spec values for my 990 engine.
How does the fuel pressure damper effect pressures? I see an adjustment screw at one end.

Knarf

Pump pressure against closed line should max out above 75 psi...most will go to 100+ psi.
Don't fully understand your comment on I see that my warm up regulator is already set for this adjustment proceedure.

Rich indicates a non functioning O2 system.

May be coincedental....check tank for venting properly. A sucked on tank will collapse .

Other than that...no other ideas.

.

That adjustment will not fix the system pressure issue. The control pressure cannot exceed the system pressure, and can't bleed off enough fuel to kill the system pressure unless you have a real problem (elsewhere).

You aren't measuring the system pressure correctly! System pressure has to be at least 4bar or the injectors won't even OPEN! You did say the engine runs!??

0.5bar control pressure is too low for 25F & control pressure must be between 3.4-3.8bar WARM.

dkveuro,
My warm up regulator has the adjustment screw and lock nut installed by the a previous owner.
M.B. DOC,
I think you're correct, something's wrong with my readings if the car is running fine cold below injector opening pressure.
I suspect air in my tester line.
The guage does not have a bleed off screw to bleed air from the guage and lines after installing.
The manual describes a proceedure to purge air from from the lines that I followed but I suspect I didn't get it all out.
I'll try purging the lines again and take new readings.
P.S.
I received a system pressure valve repair kit from Fastlane today.
Ordered after 6:30 PM EST 1/16/06


Thanks to all.
I'll keep you updated.

Knarf

Warmup Regulator
 

I don't think you need a system pressure regulator. I agree with the previous post, if the system pressure really was 1 bar the engine wouldn't run. This may be a measurement error. Before putting in the pressure regulator I would measure the output of the pump, both pressure and volume tests, then re-run the system pressure test.

Once the system pressure is verified OK then and only then will the control pressure test be meaningful. Control pressure should be in the range of 3.1 to 3.5 bar (engine warm).

If the control pressure is low with engine warm then your warmup regulator will probably have to be replaced. It's possible to adjust the wamup regulator but the range of adjustability is very small. If the control pressure is 1.2 bar with the engine warm then your chances of a successful adjustment are very low. If your control pressure is a few tenths low then your chances are better. Remember that your car left the factory with the control pressure just right. If it's low now that means that something is wrong, usually (but not always!) with the warmup regulator itself. That's why our company makes lots of warmup regulator upgrades.

BTW, if the control pressure really is 1.2 bar then DON'T run the car for an extended period of time. We lost an engine in one of our test vehicles due to scuffed cylinder walls because of this problem.

Steve
UnwiredTools Support Team
www.unwiredtools.com

You might want to refer back to my first post, mainly how you’re connecting the gauge to the FI system. What will help us better understand your test values may be explaining where the gauge is connected in relation to certain fittings as well as knowing the placement of the shutoff valve. If memory serves me correctly the shutoff valve is placed between the gauge and the warm up regulator. If the shutoff is valve is placed between the gauge and the fuel distributor you will get erroneous readings.

CIS Control Pressure(s) Update
 

Again and again I have measured the system and control pressures and they read low, 1.2 - 1.5 bar max with 2 different guages.
When I turn the engine off the guage pressure rises to a resting pressure of 3 bar.
The car runs fine showing a system pressure of 1.2 bar until the engine reaches operating temperature, then the engine begins to stumble, runs rich and shuts down.
I can drive the car as long as I keep the rpms above approximately 1,000 rpms and feather the throttle at stops.
So theoretically the system pressure is actually at least 3 bar if this is the minimum pressure required to open the fuel injectors.
The fuel distributor does have a frequency valve installed.
Could this be bleeding off pressure to the guage readings?
How does the on/off ratio of the frequency valve function effect fuel pressure readings? Faulty valve?

Thanks to all for your responses,

Knarf

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.

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

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

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?

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

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.

If 2 different gauges were used and they both show the same reading, then that rules out a defective gauge.

But why would the system pressure INCREASE after shutting down the engine?

Did you take apart the pressure regulator and change the number of shims to see if that changes your readings? I think you add shims to increase pressure.

raymr,

I'm not sure why the pressure rises after the engine shuts down.
I theorize that the 42 psi rest pressure represents most of the low system pressure that was just enough to open the fuel injectors to run the engine cold and register a guage reading of only 17-19 psi until the engine stalled.
It was as if the injectors or something else is bleeding off pressure or I'm not getting enough fuel pressure to the distributor from the pump.
Would a dirty tank screen give such results?
I did add a couple shims to the pressure regulator. They caused no change in the pressure readings.

Knarf

Pressure rises because all flow stops??

Based on your test, your problem is before the fuel distributer. Do the fuel delivery test as I described. It is the factory test to evaluate what your system is doing. You can not control fuel pressure and distribution in this system unless you have a huge fuel flow. It needs to have sufficient quantity that with all manner of control and distribution the system stills puts more fuel back to the tank than it uses. Quantitatively it needs to flow a liter or more to the tank while maintaining 5 bar.

Do the test with the engine off by jumping the fuel pump relay. Read the pressure and collect the fuel exiting the fuel distributer back to the tank. My guess is you won't have any return at all, because you can't even meet the regulated pressure with a dead headed pump. make it pass the test and you will go a good ways to fixing your problems is my guess.

Good Evening (6:45 PM EST),

Today I removed the fuel tank screen.
I was hopping that it would reveal an obvious cause for my poor pressure readings.
I think it did, but it wasn't the screen itself. The removal of the screen , which looked fine I might add, revealed an empty gas tank. Albeit the rear of the car is raised about 8 inches, but no gas spilled out as I removed the screen.
I think I lost the last bit of gas in the tank while changing out and adding shims to the new system pressure regulator a few days ago.
That same day the low fuel indicator light was on so I added a few gallons of gas to the tank.
I guess that I've probably been testing pressures with 2-4 gallons of gas in a tilted tank.
What' the minimum gallon requirement in a gas tank to maintain propper fuel presures in a CIS system, 1985 280sl specifically.
Might it be a percentage of the total tank volume?


Thanks,

Knarf

Out a gas is out a gas, anything else functions. If it is there it will be pumped through the system appropriately. The quantity of gas in the tank has absolutely nothing to do with the pressure up front UNLESS internal restrictions require gravity to force fuel to the pump. A cup of fuel available will pressurize the system for a few seconds, not many.

Greeting Forum Members,

The saga continues!
This weekend I'm going to disconnect and check the fuel supply and return lines for blockages.
Is blowing them out with compressed air ok?
I would like to clean the screen in the warm up regulator.
Can that be done without removing the regulator, i.e. back flushing the regulator with Techtron or something simular?
It's located in a real tough spot to remove on the 110 engine or would this be premature given my previous pressure readings?.


Thanks,
Knarf