Manually adjusting fuel pressure in a 300E?? - PeachParts Mercedes-Benz Forum

mike690003 · #1 11-09-2003, 10:38 PM

Hello Board,

As a last ditch effort to try and put an end to my car's hesitation problem, I came upon a thread that dealt with removing the EHA and turnig a screw to adjust the fuel pressure. My details are a little sketchy, and I wanted to know if anyone had pictures and/or the specifics on how to adjust the EHA??

My car hesitates from a standstill, but the hestitation is gone above 1500rpm, If I am trying to pass at about 45mph, the hesitation will show up. I am almost certain that this is a fuel pressure problem since I have replaced a lot of parts
-OVP Relay
-Voltage Regulator
-fuel filter
-New Cap
-New Rotor
-New plugs
-New wires
-new vacuum lines
-injector seals
-new injectors
-new radiator
-new thermostat
-new vacuum sensor on top of engine(has 2 vacuum lines coming out of it)
-New coolant temp sensor
-new aux fan temp sensor
-new gas cap
-New fuel sending unit
-Cleaned idle control valve
-new 02 sensor
-fuel mixture was corrected to spec
-cleaned all electrical connections
-new air filter
-replaced #5 and #6 injector
-cleaned throttle plate
-new aux fan
-spent lots of $$$ on Techron
-Spent lots of $$$ on diagnostics

Does anyone know if the 1987 300E has manually adjustable fuel pressure?

If so, can someone provide instructions on how to adjust it?

Its worth a shot to try it.

sbourg · #2 11-10-2003, 12:05 AM

You don't say you replaced the airflow meter potentiometer. One of it's functions is signalling the ECU the need for enrichment during sudden changes in airflow - such as opening the throttle. It develops wear spots from the slider contact, causing hesitation at certain spots, and often a hunting idle too.

Steve

mike690003 · #3 11-10-2003, 12:08 AM

Airflow Potentiometer? Wow!!

Is it a costly part?

Is it a DIY job??

Where is it located??

That could be a culprit to my car's hesitation.

#4 11-10-2003, 12:23 PM

Here's a shot of the airflow pot circled in red and the connector circled in green.

In Fastlane, it's sold together with the AirFlow Meter.

Don't know if it's sold separately.

Check with Bill...

ctaylor738 · #5 11-10-2003, 12:46 PM

... including the reference to Steve Bourg's information.

300E Airflow Sensor Replacement - The Old Guy's on Viagra!

yal · #6 11-10-2003, 03:23 PM

Maybe its your fuel pressure regulator. I don't see it in your list. Someone posted that they had a hesitation problem and it was fixed by a new one.
But you really need to take this car to a pro.

mike690003 · #7 11-14-2003, 10:17 PM

Where is the fuel pressure regulator located?

How can I test if it is working correctly or not??

stevebfl · #8 11-15-2003, 08:21 AM

The adjustment you are talking about would control the leakage rate of the EHA. The EHA is a controlled leak of lower chamber pressure.

The reduced pressure in the lower chamber is called differential pressure. This is what you would be screwing with. The dynamics of the pressure make it something I wouldn't want to screw with. If it is wrong the normal reason is either an internal fuel distributer restriction or one inside the EHA itself.

The differential pressure is so sensitive that it is what is used for electronic fuel control. The EHA dumps more or less fuel depending on the current passed through it. The differential pressure is .4bar with the EHA disconnected. The action of lambda from full rich to full lean gives about a .1bar correction to either side so the range should be .3bar (lean limit) to .5bar (rich limit).

I once had a car with corrosion in a large connector carrying the wire from the starter to KE controller. The car would start and idle great and all of a sudden the thing would go pig rich and die (if one didn't hold the rpms up with black smoke streaming). Watching the EHA current found that during the event the EHA current went to over 60ma. Knowing that the only normal condition to use such a current was starting enrichment, I found the problem while tracing that wire. A current that large changes the differential pressure to over 1bar. Quite a difference from the lambda ability to correct.

drbrandini · #9 11-15-2003, 09:14 AM

Steve,

I wish you were just down the street.......Heck I'd even drive a couple of hours to let you work on my car !!

stevebfl · #10 11-15-2003, 09:31 AM

Brandon,

I'm not too knowledgable about the locations in NC, but there are a number of good shops there. One in particular First Class Automotive in Durham or Raleigh is one of the real national allstars. I know the owners well and they are right at the top technically in both the MB and BMW tech groups I belong to.

pesuazo · #11 11-15-2003, 06:31 PM

I have never been at his shop, but there is also a good contributor here from NC (Raleigh).
His log on name is Joe P (it is german, it leaks), do a search for him.

mike690003 · #12 11-15-2003, 10:39 PM

Well, I take it that the fuel pressure adjustment is better left to a pro. Will removing the r/16 (the resistor that changes the timing) solve the hesitation?? I run my car on 93 octane.

John Plut · #13 11-16-2003, 01:09 AM

My '88 260 also has a hesitation problem going on three years that I have not solved yet. I replaced the airflow sensor pot which corrected a stalling problem but it still hesitates. I also tried removing the R16 resistor which didn't help. The car would sometimes ping with the resistor out so I put it back.

The hesitation problem I have is only during cold weather - below 40 degrees F. The cars starts and runs fine when cold but as it warms up and the temp gauge hits about 60 degrees C. the car will not accelerate and pressing on the gas makes it go slower. It does this until it reaches normal operation temp. then it runs fine again. I found that the car warms up faster if I don't use the heater so I usually freeze myself just to help the hesitaton problem.

I think that the problem is fuel related and has something to do with lambda or fuel pressure. I adjusted the EHA current to spec's. but I'm wondering what or where to monitor during engine warm-up to pinpoint this problem. I have checked the coolant temperature sensor and it is fine. I have a '93 300e 2.6 that I have swapped some parts with, like OVP relay, also to no avail. I could swap the EHA but I'm afraid it might reqire some re-tuning and then I might have two things wrong which make the problem exponentially harder to solve.

Does anyone have any ideas?

Thanks,
John

mike690003 · #14 11-22-2003, 03:38 PM

Well, I have since realized tht this problem really occurs after about 10 minutes of driving. After that, when I am at a stoplight, the car will idle fine for about 10 seconds, then go through a rough idle sequence where the whole car shakes, it will then smooth out for 10 seconds, and then rough idle again. What gives??

I have a new temp sensor, so what can the problem be??

ctaylor738 · #15 11-22-2003, 07:33 PM

This sounds like a more extreme version of a problem we fixed on hbofinger's 560SEL. The lambda cycle had a huge swing, like from 30% - 70%. You could watch the ratio fall and the idle go rough, then it would snap up to 70%, smooth out and start falling.

We richened the mix about a quarter turn and that moved the cycling to 45 - 55% and the idle smoothed out.

Why don't you put a meter on Pin 3 and see what's going on?