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Need Diagnostic Help

2K views 28 replies 8 participants last post by  Witek_M 
#1 · (Edited)
I recently finished a protracted course of maintenance on our '99 ML 320 that included;

1. New left (driver's side) catalytic converters (two cans) and resonator due to melted cat #1.

2. Two new left side Oxygen sensors.

3. New spark plugs - the (relatively expensive) NGK plugs recommended somewhere in this forum.

4. Complete fluid change in automatic transmission, cooler, torque converter.

5. Fluid change in transfer case and both differentials.

It seemed to run strong after all these repairs, but I could detect a slight misfire. Recently on a very rainy day going over a mountain pass the CEL came on (urgh)! It was the P0300 random misfire, and P0301, 302, 304, and 305 misfire on those four cylinders. When I changed the plugs it was really difficult getting the wires off the old plugs and coil packs. I took them off the coil packs to test their resistance values. All the wires had similar resistance - to my memory they were 2k ohms each, plus or minus about 100 ohms. So I reinstalled them. I suspect and hope that they are the culprit to the misfire codes and am replacing them today with a new set of Bosch wires.

However, when using my scan tool I noticed a big difference in the LTFT (Long Term Fuel Trim) between Bank 1 and Bank 2. The difference was in the "Freeze Frame Data". The difference remained after clearing the CEL and error codes - the live data showed the same discrepancy. Bank 1 LTFT stayed constant at 1.5, while Bank 2 LTFT stayed constant at 3.1. I will attach some photos showing this data. I am curious to know what the units of these numbers are. I have no idea what would cause this difference. I suspect that it's significant but don't know where to start with this one. Can anyone tell me what causes it and how to solve it?

Thank You,

John Olson
 

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#6 ·
Thanks. That's interesting.

Is this fuel injection system made by another contractor, like Bosch for instance?

Does it have a system name (Bosch Digifant was the system in an old VW I had)? I'd be interested in getting my hands on a description of how it all works together.

John
 
#7 ·
ML 320. With exception of manifold pressure sensor made by Denso all other components are Bosch. Mercedes calls it ME-SFI, BMW and Volvo Motronic, VW Digifant.

Hit your favorite search engine with those keywords. You will get all the details.
 
#10 ·
Hello Everyone,

Thanks for all the advice here.

After replacing the spark plug wires and the two oxygen sensors on the passenger side the original misfire problems I posted in this thread have disappeared. The engine is running noticeably smoother. Mysteriously we had a CEL with the cylinder no. 2 misfire code on the first trip after these latest repairs. However, since clearing these codes out they have not come back after a couple hundred miles of rigorous driving - and everything seems fine.

Can the system report a misfire like this as a legacy issue after the fact and repairs?

Thanks Again,

John
 
#11 · (Edited by Moderator)
Sounds mighty familiar to our experience....

Hello Everyone,

Thanks for all the advice here.

The engine is running noticeably smoother. Mysteriously we had a CEL with the cylinder no. 2 misfire code on the first trip after these latest repairs. However, since clearing these codes out they have not come back after a couple hundred miles of rigorous driving - and everything seems fine.

Thanks Again,

John
John:

Curious, how many miles on your ML320 ?

I think you may have weak / failing coil (s), perhaps.

We have an e320 wagon, year 2000. Same Engine as your ML.

This is the backstory:

I washed the engine. With the cover off. Big mistake. Worse than if I gave the cat a bath. (less blood, perhaps !)

I had 4 misfire codes, and no resolution until I pulled the coils & dryed behind them. There is a nice little space that can hold 1 oz of water behind each coil - how convenient.

it also appears that the exposed metal portion of coil, exactly where attaching bolt goes thru, is part of the grounding system.

That would help explain the persistent misfires, until completely dry.

(Now I think that compressed air in eactly the right spot might have dryed it well, too.)

Anyhow, I digress.

What I discovered upon coil removal was the the original coils, about 4 or 5 had pretty good cracks in the plastic material that covered the whole coil.

I wasn't sure if this was going to portend an 'operational malfunction' or not ?

So, I decided to go 'whole hog' and ordered up the 12 plugs - NGK's like you chose (6403 ?? by memory) and 12 new Bosch wires.

Got them all installed & engine ran like a top. All MIL codes gone.

Until the next humid rainly day (East Texas is always like that, seemingly), the wife calls & reports severe miss, engine barely runs.

So I go to her, open hood & let run at fast idle, to warm engine fully & air out underhood. Problem gradually goes away, for that trip.

But two codes persist, like a P0305 & P0301.

My assumption here is that the 'washing the engine' incident allowed water to invade the center of the coils, thru the cracked outer covering - and finish them off.

Only when dry climate conditions prevailed did they act 'normal'.

As soon as rainy & damp, codes re-appear & engine miss comes back.

So, the solution was to order up 2 new coils - from Amazon $37.50 each, and install them in place of the 'verified bad' coils.

So, car at 174K miles / 17 years has 4 original & 2 new coils.

That happened a year ago, no re-occurance.

But I keep a spare in the 'trunk' & tools to change it.

If money were no object, I probably would have done all 6, but too many go to 300k without trouble, so I figure they may last.

Hope this is of help.

Your comment about the 'Rainy Day misfire' brought this to mind.

Cheers - David in East Texas :wink
 
#13 · (Edited)
Need More Diagnostic Help Please!

Help!

It seems like our '99 ML320 is not going to let me rest just yet. After 1900 strong running trouble free miles our Check Engine Light (CEL) came back on. We were 15 miles into a 500 mile trip. This new CEL forced me (not wanting to through caution to the wind this time) to cancel the trip and return home.

Our OBD2 reader has five new trouble codes - PO130, 156, 150, 136, and 161. The first four codes are all "O2 Sensor Circuit" - one code for each of the four sensors. The final code 161 is for "O2 Heater Circuit Bank 2, Sensor 2". Fuel Trim numbers are way out of whack relative to the ones I noted at the beginning of this thread - I forget the exact data, but can provide it if it's important to solving this new mystery. I assume the fuel trim numbers are off because of the fault related to the oxygen sensors.

Following is our recent repair history (most noted at the beginning of this thread);
The left two cats and resonator were replaced. The recommended spark plugs (NGK), wires (Bosch), and all four oxygen sensors (cheap $25 Asian imports) were replaced. All this was undertaken and completed 1900 miles ago because of uncertainty surrounding the cause of a failed driver side cat #1 (which totally melted down presumably due to the engine running too rich).

About 600 miles ago I ran a bottle of Chevron Techron along with a full tank of gas because I read somewhere on this site that it possibly improving injector performance. I figured it could not hurt.

So I don't know where to start again. It seems unlikely that all four oxygen sensors would fail simultaneously. Is there some fuse for this circuit, or can one wire problem cause all this mayhem? Some here (even somebody in this thread) advised me against knock off oxygen sensors. However I had already purchased and installed them. I've previously had good results with other aftermarket parts. The oxygen sensors I installed are not entirely "generic" - in the sense that the connectors and pins were all direct fit to the OEM Mercedes connectors (plus they appeared almost identical in every respect).

Any advice or ideas will be much appreciated. I'll have no problem replacing all four oxygen sensors with the Bosch model (#13782 is what Rock Auto supplies for all four sensors - $65 each). However it seems an unlikely fix to the current issue - four sensor problems simultaneously after 1900 miles.

Kind Regards,

John Olson
 
#16 ·
So I don't know where to start again. It seems unlikely that all four oxygen sensors would fail simultaneously.

... I'll have no problem replacing all four oxygen sensors with the Bosch model
I think you answered your own question. I don't know why you'd fit dodgey aftermarket sensors to the most critical electronic system in the car. It's a false economy - you don't save money, you blow $100 extra.
 
#14 ·
Amazon has the 13782 for $59.

It is odd they all went kaput at once, but I have yet to see any good long term results with inexpensive or universal fit O2 sensors.

Has the carpet/floor ever gotten wet due to a blocked sunroof drain? There is an O2 sensor connector under the carpet in front foot well that has been known to affect the O2 sensor data.
 
#18 · (Edited)
There is a Bosch app for iPhone that has almost every part Bosch makes for cars. It gets updates a couple of times per year. If it is a US only app on the iTunes store setting up a VPN for the US and creating a second iTunes account works. That is how get UK only iTunes purchases that I can't get in the US.

I know there were different numbers for upstream and downstream a couple of years ago, but now they only list the 13782 sensor as the one for both.
Also checked FCP EUro and ECS tuning and it shows the same Bosch number for all four.

https://www.fcpeuro.com/products/mercedes-benz-oxygen-sensor-bosch-13782

Forgot the Amazon link. They are over $12 less per sensor from Amazon.
https://www.amazon.com/Bosch-13782-...F8&qid=1489868005&sr=8-1&keywords=Bosch+13782
 
#20 ·
Diagnosis help please?

Thanks for all the advice (opinions?). The O2 sensors I installed are no more generic than the Bosch sensors. At the minimum, both mine and the Bosch are direct fit for this ML320. Admittedly with different connectors they might fit different vehicles, but that applies to the Bosch sensors as well. Ultimately it seems very unlikely and unreasonable that all four of them would go bad at just about the same time (like withing 20 miles), and approximately 1900 miles after installation. If others had this exact problem and solved it using Bosch, that would give some credence to the advice, but it all seems to be hearsay. I don't think I need a full set of O2 sensors. I doubt I need O2 sensors that will last another 170,000 miles.

So does anybody have knowledge on how to test O2 sensors, or how to diagnose the exact problem? Is the Mercedes 'Star' system capable of showing the information/signal output of individual O2 sensors when the vehicle is running?

Possibly an entirely different problem causes all the O2 sensor codes?

John
 
#21 ·
The O2 sensors I installed are no more generic than the Bosch sensors. At the minimum, both mine and the Bosch are direct fit for this ML320.
Now you're having difficulty with the concept of "generic". By that, these members are saying that Bosch, as the OE manufacturer of the O2 sensors that came fitted from the factory, and the designers of the engine management system installed in these vehicles, are the "name brand". All other sensors that might fit the connectors and holes and are marketed as "suitable for W163" or "recommended" are called generic or aftermarket.

You must fit Bosch sensors to solve your problem. Pray that is all you need to do.

I realize you're trying to talk yourself out of losing $100 worth of 2nd-rate Chinese-made sensor copies. Why do you think you paid less than half the price of Bosch sensors?
 
#24 ·
Getting back on topic, if you have a scan tool that can show live data please drive it (hopefully with a helper and record the voltage range changes as you go from idle up to 50-60mph.
Please post the voltages. I can't look up the ranges right now. Even if the voltages are correct even a lazy O2 sensor may show the correct voltage, but not be quick enough for the ECU and then throwing the codes that set the check engine light.

Please do not take DrX's post as dramatic. He has always been straight to the point and was just strongly emphasizing how most of us feel about how important it is to use the Bosch OE sensors.
 
#27 ·
Many of us forum members know when to use OEM(Genuine MB whatever the cost) or when we can use aftermarket product. For hoses and gaskets, most of use Genuine MB products however expensive they're. O2 sensor is one of them, IMO. Many, including myself, tried to use non-OEM aftermarket product, obviously to save a few bucks here and there, but learnt the lesson hard way.

DrX has been giving me countless no nonsense advises, I really appreciate his help. I don't argue with whoever trying to help me with good advise.
 
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