I've been following all of your your posts here for several years and find
the people on this site extremely knowledgeable and helpful. You,
unknowlingly, hleped me fix a number of problems with my 1989 300E.
Right now I'm diagnosing a problem with the fuel pump, I think. The car
stumbles at about 83 degress celcius and wants to die but never does. I've
honed in on the upper fule pump for the following reason: I disconnected the
positive cable on both pumps, one at a time. When I disconnected the lower
pump, I heard a relatively loud buzz. This indicates to me that the upper
pump is weak and needs replacement. Do you agree with this diagnosis?
Gene, I think we are all learning from each other. Point of fact, I
didn't realize re the 2 fuel pumps. AFAIK the pumps are in parallel, I'm
a bit puzzled re the difference between the 'upper' and 'lower' pumps. do
you simply mean physical location underneath?
You're right Guenter, I'm talking just physical location. Pumps look similar
and no doubt support each other as needed. I'm guessing when the engine gets
warm, fuel demand is higher. Since only one pump is working optimally, the
cars stumbles and runs rough because the weak pump is not putting out enough
fuel. I just want to make sure my diagnosis is correct before I buy a new
Bosch pump. My previous Mercedes mechanic walked me through the test I
described earlier, but without him physically here I'm second guessing myself.
Any thoughts? Are Tiger, T.G. Lambach, or Martin still around to chime in?
Guenter Scholz wrote:
I can't tell from your problem description where to look for the
For your information, the two fuel pumps are in series. Each will build
up a pressure at the output that is a certain amount higher than the
input (naturally). The second pump in the series will therefor build up
at pressure at the out put, which is double as high as the input (if
the two pumps have same capacity). The difference in sound with only
one pump running may be because the first pump will have little flow
resistance in the line from the tank, while the second one will have
high flow resistance, (first pump, which is not running, will cause
A bad fuel pump would probably not cause a difference if engine is cold
The fuel mixture should be automatically adjusted to the correct level
by the ECU. If the ECU is not operating, the engine will run with a
preset mixture (which may be good for a cold or a hot engine, but not
I would look in that direction. First of all, you should look for the
overvoltage protection relay (OVP relay), which is located behind the
shield behind the battery. It provides power to the ECU, which of
course will not operate without power. The OVP relay has a fuse on the
top, which may be blown, or the internal relay may have bad
connections. The OVP relay has a limited lifetime, so check it first.
I replaced the OVP and fuel pump relay in Jan 2006. I changed the OVP
because it was original issue. A few minutes ago, I pulled and checked both
10 amp fuses on top of the OVP and they look good. I bought the OVP from the
The fuel pump relay was changed after a short was detected on one of the
pumps. It burned the circuit board out in the relay. I tried to solder the
circuit board with no success so I bought a new one from the dealership...
they are pretty proud of that part! Since this problem surfaced about a
month ago, Iv'e done the following:
1. New throttle microswitch
2. Injectors were cleaned and tested - all good
3. I dumped a can of Sea Foam down the the throat (1/8 can), gas tank (1/2
can) and rest down crankcase - no differance
4. New engine temp sensor (w/two green connectors)
5. Cleaned distributor cap and rotor - no oil visible inside
6. Changed plugs to non-platinum NGK after Bosch H9DC plugs looked worn.
All showed even burn with some black carbon on rims. All plugs were gapped to
manual specs, .039 in or 8mm I think.
7. Spark plug wires were all removed and checked fro cracks/damage - good
8. All vacuum lines checked and secured. Sprayed 2 cans of throttle cleaner
eveywhere I could - good
9. Air filter housing cleaned w/filter changed - good
10. Fuel filter changed - good
11. I disconnect the O2 sensor while running the engine with no change - I
have new Bosch sensor coming...looks like original sensor on there.
12. I adjusted the air/fuel mixture with an allen wrench while my mechanic
was on the phone. After several ill adjustments in both directions, I set it
back exactly where it was. I trust this mechanic with my life. In 2004 he
put in a valve seal kit, belt tensioner, distributor cap, rotor, new Beru
ignition wires, and Bosch H9DC plugs.
13. The car has always used Mobil 1 fully synthetic oil from the day it
rolled out of the dealership. I routinely use OEM parts on oil/tran changes
After all this, nothing seemed to change the problem. The car still runs
rough right at 80-83 degrees celcius. It doesn't die but will surge and
catch itself. After which it runs high for a few seconds until it does it
again. The car doesn't miss at all and you can set a glass of champagne on
the air filter cover and not spill a drop when it is not acting up.
I was leaning towards the O2 sensor before I did the check on the fuel pumps.
I've also ordered an original fuel pump off of 1985 190 from a junk yard just
to test the theory. Guys at the yard said it will fit even though the 1985
only has one pump on it. I checked new Mercedes fuel pumps for both years
(1985/1989) at BenzBin and the part numbers come up the same.
I will not give up this car for anything and will continue to troubleshoot
the problem until I'm broke! I have been a Mercedes owner for 15 years and
will always be a "used" Mercedes owner. My previous vehicle was a 1978 of
which I had a Mercedes mechanic put a brand new engine in...it was shipped
direct to Montana from Stuggart, Germany, in 1997. As a U.S. Air Force
member, I ran it all over Europe with my family, including over the mountains
of Andora...very few problems with that great car.
Any ideas on how to check the ECU?
Message posted via CarKB.com
What is your current mileage? I would change out the fuel pump that you
ordered to see if it solves your problem. If you changed out your fuel pump
relay but didn't change your fuel pumps... chances are you are burning up
your fuel pump relay too.
I would change out both your pumps even though it is really expensive. I
would try to look for cheaper source.
The odometer quit two years ago, for the second time, at 167,000 miles. I
would guess it has about 210,000 miles now. Any ideas for fixing the
odometer without sending it to the specialist again?
The fuel pump coming from the junkyard cost me $65. It was bench tested for
operation but not for pressure. I can get another one from another junk yard
for about the same price. If the fuel pump solves the problem, should I
still replace the relay? I'd rather pull the fuel pump relay and check for
scoring on the circuit board first before getting a new one from the
dealership for $250?
How do you feel about Mercedes parts from the junk yard if they test okay?
Do you think the new Bosch O2 sensor will make any difference on this 1989
300E? The engine has always struggled with maintaining a real stable idle
(stumbles slightly, no miss though). Sounds as if the engine is constantly
surging and adjusting...is this common?
While your on board, I really value your opinion, what do you know about
changing a beard heater blower motor under the dash. It still blows warm air,
but when I put the control on auto or high, it really whines and vibrates
like crazy...drives me crazy but will live with if it will cost me a bunch to
replace at the shop. I'm willing to do it myself if I had a shop manual. Do
you recommend I take this on and do you recommend a certain manual. I'm
pretty handy on basic automotive repair, especially on this car as everything
is pretty accessible.
You can fix it yourself.
I don't like used parts... they really can't do conclusive test on fuel pump
other than that they can move. I would leave the fuel pump relay alone... it
should be okay.
O2 sensor will help when you change it if it is over 70000 miles old.
The blower on 300E is located up on the top of the dash... just under the
windshield wiper... it is a pain to change it out but you gotta do what you
gotta do. Maybe your mounts are loose... Best bet is just to change it out
with a new one with new blower wheels.
Thanks for the repsonse. I've printed everything out and put it in my book
for use when I'm ready to do those repairs.
When I change the top fuel pump, is there anything that I need to be extra
careful with? I know to disconnect the battery, the + and - terminals (7mm
and 8mm) on the pump, disconnect the check valve connection from the metal
pipe that connects the filter/pumps/accumulator, and clamp the main fuel line
coming in from the tank. Probably a good idea to take the gas cap off to de-
pressurize the system. Is there a certain way that the check valve on the
pump has to be tightened as to allow all those holes to line up on the bolt?
Should I be changing the fuel accumulator while I'm in there? I haven't
noticed any problems with it. I don't have one right now, but will order if
you suggest it.
I've never checked the fuel system pressure and I'm not sure what kind of
fuel pressure regulator is on the motor or even where it is located. I have
a guy that will conduct an engine diagnostic test for $70. Does this include
the pressure test?
I just always assumed the engine idle oscillation was normal for this car.
By the way, I have been following you and Tiger for quite some time and find
you guys invaluable to Mercedes Benz owners...you would not believe how many
times you helped me understand the operation of my car or fix a problem. I
wanted to thank you guys for that. It's obvious you truly respect Mercedes.
I'm honored to receive feedback from you and I'm privileged to be part of the
T.G. Lambach wrote:
Message posted via CarKB.com
I didn't realize that your problem was at idle.
At idle the mixture is controlled primarily by the basic setting (the
adjustment you tried earlier). Apparently this setting is OK, since it
runs smoothly when the problem is not there.
Since your problem is intermittant, I would look for something loose,
which could cause change in the engine operation.
If the problem is mixture related, it is most probably a vacuum leak.
Have you checked the hoses to the idle speed air valve? Vibrations at
idle speed could cause false air to be drawn in here, causing irregular
engine operation, causing more vibration, causing more false air etc.
Or it could be at fault in the air mass meter, possibly a bad
potentiometer, which provides one of the inputs to the ECU. Or a bad
connection in any other input to the ECU.
You asked earlier how to check the ECU. You probably don't have the
testbox (providing measuring points for each pin of the connector), but
you can measure the inputs and outputs from the ECU by taking off the
plastic cover from the ECU and measure at the connector at the printed
circuit board. The most relevant points are:
- pin 1: Power input from OVP relay (11-14V)
- pin 8: O2 sensor input (fluctuates 0,1-0,9V)
- pin 13: Idle switch input (short to ground at idle)
- pin 17: Air flow input (0,5-5V according to air flow meter position)
- pin 21: Coolant temperature input (2500 ohm at 20C, 300 ohm at 80C)
- pin 25: RMP input (6-12V at idle.. not sure)
- pin 3: Idle speed air valve output (fluctuates 0-12V at idle)
- pin 10/12: EHA output (+/-20mA current loop, fluctuates +/- 3 mA at
Personally, I don't believe the fuel pumps to be the cause, because
this would mostly have effect when the engine is loaded, causing higher
But listen to Tiger and Tom, they are experienced.
GeneKelly via CarKB.com skrev:
The problem seems to be that the motor is hunting for the correct air /
fuel ratio - something is giving either a false or no signal so the ECU
goes through its map of possible settings.
I vote with Jens. This temperature sensitive oscillation is not a fuel
pump caused problem, IMHO. Jens is the electronic analytical expert.
Suggest you check the Inputs as Jens suggested and let us know the result.
Great inputs from both you and Jens. Since I've got the fuel pump and O2
sensor coming in anyway, I'll replace them first and see what happens. If
the problem is still there, I'll run the test that Jens suggested. I don't
feel that I can properly do the test myself, so I'll set up the diagnostic
test with the shop. I'll bring the data that Jens provided, just in case the
shop needs it. The shop has a $70K machine and a guy that has "some"
experience with Mercedes. I'm in Helena, MT with no Mercedes shops or
dealers. Nearest shop, private guy with shop in his garage, is 100 miles
I'm sorry about not providing exact symptoms as this case is pretty eratic.
Yesterday I took the car out after letting it warm up in park for 5 minutes.
It drove perfectly after that. Then I parked it for about two hours and
drove it after only a 20 second warm up in 30 degree F temp. It drove
perfect. I stopped again for 1 hour, let it warm up for 30 seconds and drove
off, within 2 minutes it ran terrible and wanted to die. The exhaust smelled
bad, like it wasn't running effeciently, and the economy gauge pegged right.
The oil pressure jumped up as well. I placed the car in neutral and jiggled
the gas peddle with my foot and the car surged up for a few seconds and then
went through the above sequence two more times. After about three minutes in
neutral, 83 degrees Celcius on the temp. gauge, the car ran perfect without a
problem home (20 minute drive). On other occasions, my wife said the car
exhibited these symptoms while she waited in the car, idling, while I ran
into a store. The car was fully warm then as we just exited the highway
after an hour long trip. However, the car normally exhibits these sytmptoms
when I makle a quick, short trip while the engine is still warming up (e.g.
taking the kids to school first thing in the morning, a 2 minute drive each
way). I keep the car in the garage at night with temps usually in the 50s F
inside the garage. Outside temp has been ranging from below zero to 25 F
Thanks again for all of your help.
T.G. Lambach wrote:
Message posted via CarKB.com
The symptoms are of an OVP relay failure BUT it's not the OVP relay
that's at fault, I believe the O2 sensor is THE PROBLEM.
That's because the O2 sensor is excluded during warm up but once the
motor is AT 80 degrees C operating temperature the O2 input is brought
into the engine management map calculation.
Suggest you first change the O2 sensor and test it, then look at the
fuel pump(s); this will limit the variables, not add them.
Hope this helps you.
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