I have a 1998 Mercedes E320 that generates P0170 and P0173 codes about every
third driving cycle. I suspect the air mass meter, but would like to be
certain before shelling out almost $300.
Is this air mass meter a whetstone bridge as in other air mass meters where
the element exposed to the air is a resistor? Does anyone have a schematic
diagram of this part?
It is a bridge loop circuit, but no more Hot wire ..replaced with Film Mass
The codes are fuel trim and can be caused by both vac leak or bad Fuel
Common vac leak on these is purge valve
Reg test is to remove vac line at reg. and see if fuel comes out reg vac
These are preliminary test before checking MAF [ which is also common with
these codes on that model]
Thanks, Karl. Vacuum line at the fuel pressure regulator is dry as bone.
Is the purge valve back near the fuel tank or in the engine compartment?
I don't think that it is the air mass meter any more. I took a drive with
my computer diagnostic software connected and, as far as I can tell, the air
mass meter seems to be performing. Whether it's normal or not, I don't
know. I can read its output which is about 2.1 lbs/min at a constant 55
MPH. Opening the throttle and thereby decreasing the manifold pressure it
goes up to a peak of about 6 lbs/min. At idle it is at 0.4 lbs/min. It
certainly isn't totally defective , but then again the car runs beautifully
without any evident problems except for the trouble light which comes on
after clearing the codes on about the third driving cycle.
I can't speak to your technical question, but I can tell you that my
1997 E320 began bogging when I attempted to accelerate hard and got
steadily worse. The shop I use indicated the Air Mass Meter was the
problem and, as a favor, told me the device could be bought through
distributors and installed myself. I contacted Euromeister in Castro
Valley, CA and purchased the Meter for about $170. (instead of the $340.
the dealer wanted) and installed it in about 10-minutes.
Dan Dubosky wrote:
Thanks for the response, Ken.
See my response to Karl where I point out that this car runs very well
except for the trouble light coming on.
If it is the air mass meter, it is a very marginal condition.
I'll certainly check that place it I do determine that I need an air mass
I'm watching your progress closely. My 98 e320 w/ 61,000 miles on the clock
started the same check engine light routine about 6 months ago. It goes on
and off, usually when the fuel take is between 2/3 and 1/3 full.
It started this summer, after I purchased a bad tank of gas. A respected
independent mechanic put in some additive, reset the codes and all was well
for 60 days.... then the light came on again.
For starters, I purchased a new gas cap this 24 hours ago. No status change,
Do you know what trouble code is being trigged?
Prior to this incident involving P0170 and P0173, my E320 was triggering a
P0442 code which indicates a slight leak in the evaporative emission system.
I took it to my local Mercedes dealer and they wanted to change both the
shut off valve and the purge valve at a total cost of about $500.
Admittedly, each of these devices could trigger that code. Lucky for me
they did not have the parts, so all they did is remove the trouble codes.
Subsequently, I reversed the gasket in the fuel tank cap and thereby removed
the P0442 code. That's when I purchased the diagnostic software and am now
trouble shooting the present condition.
Codes p0170 coupled witha p0442
can indicate a stuck purge valve.. very common on these.
A fuel trim code simply means that the ECU can not correct the mixture cuz it
is at its limits to do so.. [ meaning you have too much air or fuel for
The best tect for these conditions is to look at adaptation specs, but an easy
test for the DIYer is to let the engine warm up and then feel the purge valve
in ones hand for a pulsing heartbeat feel. The ecu sends
a pulsed [ 6 to 10 per/sec] electrical signal to the purge solinoid to let
canister emmisions back into the intake They pulse it back in so that the
engine does not get a big gulp of air into the intake .
[effecting too much at once]
The problem with purge valves is they simply get gummed up with gas fumes and
they no longer pulse, but rather stick open --causing an internal vac leak to
the intake which the ecu fuel trim can not adjust for
[ out of trim limit]
A trick I use on dirty purge valves is to remove the hoses and squirt some WD
40 in the valve ports and the vac lines and then retry until I get the heatbeat
Another verifying trick is to simply remove the vac line between the purge and
the intake and plug it for a couple of days to see if CE comes back on.If the
ecu readapts in the time perioid. you know the valve was sticking.
On the other end of the equation is the possibility of the fuel pressure
regulator not keeping to spec. This will also bring on same trim codes. Easy
test here is a simple F/Pressure test , both with car running and with reg vac
hos dissconnected. There is a test port for this on the fuel rail. [ this is
actually the first test one should make]
If one does not have such, you can also test by taking a reading at the 02
[ dissconnected from ecu] with a volt meter and watch for rich [ high
volts/rich] everytime you disconnect reg vac hose.] This indicates if reg is
These trim codes are usually the MAF, but not allways and a few test before
part change is the way to go..
Thanks for the really informative response!
As I indicated in an earlier posing, I am no longer getting a P0442 code
since reversing the fuel cap gasket, but I am still getting the P0170 and
Where is the purge valve located --- engine compartment or by the fuel tank?
I want to feel the beat. <grin> Would a stuck purge valve trigger a P0443
What do you mean by the adaption specs? Is this the same as the data that
is saved when the error code is triggered?
Sorry for so many questions, but I'm just trying to learn more about this
very complicated system. The internet has been great, but not as compete as
I would like.
Purge is on the inner, left fender well. Should say MOT on top.
The adaptation specs are live stream data that tells how far [+/-, in
the ecu has to correct for proper fuel mix.
The system can correct for anything from engine wear to fuel diluted crankcase
[ for those who do not believe in oil changes], but once the adaptation reaches
the corrective built-in limitation, [ram] , the CE gets triggered...
Once the remedy to the CE fault has been corrected, the ecu will re-adapt over
a drive cycle/time frame and once again get within limits specs.
However. if one has a scanner , the ecu memory can be reset to instantly bring
the adaption base line back to zero
Thanks again for the response.
The fault code P0170 triggered the engine fault light under the following
Short term trim: Bank 1= -4.7%; Bank 2= -3.9%
Long term trim: Bank 1= 34.4%; Bank 24.4%
Engine load was 20.0%
Coolant temp= 183 degrees F
intake manifold pressure.3 in Hg
Sys 1 & 2 were operating in the closed loop
I am again beginning to suspect the air mass sensor. As I said in an
earlier post, the maximum air mass that I registered on acceleration was
about 6 lbs/min. With the intake air temp of about 20 degrees C, that
corresponds to about 80 cubic feet/ min of air since air at 20 degrees C is
about .0751 lbs/cubic foot. An average for airflow versus HP is 1.5 to 1.6
cubic feet/minute for each HP. That would compute to (80/1.5=) about 54 HP
on acceleration. I DON'T THINK SO on an engine that is supposed to deliver
about 220HP at 5600RPM. I think that the air mass meter is reading low.
The engine was probably no where near 5600RPM when my wife was accelerating,
but even 100HP should be at least 150 cubic feet/ min of air. That requires
over 11 lbs/min of air and I wasn't getting any reading close to that.
Of course, I could have a large leak between the sensor and intake to the
engine, but I don't think so.
If anyone sees a tremendous hole in my analysis, please tell me. I've been
wrong before. <grin>
Low [ insuff.] vac at idle is suspect.
I think Long fuel trim CE trigger on that engine is max at 32.
Vac leaks at idle are harder to correct than large leaks at higher rpm/load.
MAF usually is accomp with accelleration stumble
Thanks again for the response, AJDalton7. You and others have been very
helpful, and I appreciate it.
Your statement that a bad MAF sensor usually results in stumble on
acceleration is confirmed by Kenneth, Hensley in an earlier posting in this
thread relating to his 1997 E320.
After reading many, many engineering reports on the Net, I believe that the
system was changed in 1998. As I understand it, the computer now stores
data from previous operations in order to run the car at least up to the
point where the oxygen sensors operate in a closed loop system to provide
the primary data to the computer for operation of the fuel jets.
Periodically, the system checks what it has been getting from the MAF sensor
in order to see if everything checks out, and if not, the check engine light
Prior to being able to remove the codes with my laptop, I twice disconnected
the battery in order to remove the codes. On so doing, I removed all of the
previously stored data in the computer, and the car stumbled badly on
acceleration at least until it developed new operating data. After that,
the car ran beautifully.
Recently, I drove the car with the computer logging all data. I pushed it
to 4675 RPM and read 10.6 lbs/min from the air mass meter. That converts to
about 141 cubic feet/min. of air flow. An average for airflow versus HP is
1.5 to 1.6 cubic feet/min for each HP developed. Dividing the 141 by 1.5,
you get a HP of 94. At that kind of near maximum load it should be about
I could have a leak beyond the MAF, but it would have to be able to deliver
another 141 cubic feet/min of air. That's not likely.
Based on this analysis, I've already ordered the air mass sensor. I'll let
all of you know if that solves the problem or if this was just another
stupid engineering analysis that comes out with the wrong conclusion.
If there is some hole in my analysis, let me know.
Sorry for the long message, but at least it may prevent someone going down
the same path if changing the air mass sensor doesn't solve the problem.
Because of the difficulty in MAF diagnosis, most techs simply read out the
adaptation numbers and install a known good maf, drive the car a few blocks,
and then look to see if adaptation starts to come back to normal.
The same can be done when suspecting
internal vac leaks [ ie EGR, Purge, etc] .. block the lines and then look at
adaptation values for a change toward norrmal after road trip...
I ordered the air mass sensor from
It cost $221 including UPS ground shipping.
I ordered on Monday and they shipped on Tuesday from a warehouse in NJ. I
haven't received it as yet.
wrote: ".... I've already ordered
I don't have the diagnostic software, nor, truth be told, the know-how at
this point to use it. However, my independent mechanic said the error codes
were related to O2 sensors. Thus, he recommended the additive.... I
purchased the new gas cap because I suspect the problem was fuel tank
I'll take it to have the codes read and post them here.
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