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Premium Member 2001 SLK320/2000 SLK230
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Discussion Starter #1
Been getting P0170 and P0173 either current or pending for some time, cleaned MAF then replaced it with new Bosch 0280217114 Mass Air Flow Sensor . Used a bottle of Tectron.

Both air filters are new and properly seated. Codes now take a day or two to appear but keep coming back.

Monitoring MAF flow, short and long term fuel trim and leading O2 sensor for both banks.

Short Term Fuel trim is normally in the +2.0 to +3.0 range but have seen jump momentarily -8.0 to -10.0 for less than a second (both banks). Have not seen an accompanying change in either O2 sensors or MAF flow reading. Coolant temps is stable and correct. Not sure what else to monitor but something is making the trims jump occasionally.

Fuel pressure is stable at about 50 psi.

Any suggestions ? Ran about 150 miles yesterday and didn't appear until in stop and go traffic on return. Idles smoothly at 600 rpm light on or off, good power to redline, 24mpg on last tank.
 

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Administrator 2009 SLK 55 AMG/Founding Member 2006
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Fuel filter?
Bad fuel?
 

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Premium Member 2001 SLK320/2000 SLK230
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Discussion Starter #3
Just tested the fuel pressure and stable a bit over 50 psi. Filled up at least four times now at different places, mostly 93 PON but starting to alternate with 89 at 1/2 tank to average 91.

Manifold Intake Pressure (MAP) is varying as expected.

Is intermittent which is hardest to find.
 

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Premium Member 2001 SLK320/2000 SLK230
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Discussion Starter #5
Saw that. Been monitoring just about everything but no real outliers except occasional high negative short term trim excursions (both banks, not at same time) - most of the time everything looks nermal. (engine off so not many readings just showing parameters being monitored - is one of three different screens I use.).
 

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Hmmm....

You don't have the patience and vigilance of the ECU logging function, so it's pure luck (after all, stick to driving safely!) if you catch intermittent moments of being outside limits. The ECU has (likely, I don't KNOW for sure) an allowance regarding time and level of error before throwing codes. It may even collect towards an error, needing a few out-of-allowances values before triggering.

But generally, it can be one (or both) of either actual errors, due to metering problems (example: too little fuel, due to problems with injectors, presuure variations, delivery problems etc etc) OR problems with the monitoring devices (perhaps the lambda sensors AKA oxygen sensors) fooling the monitoring system.

The car is obviously thinking it gets too little fuel sometimes. :)
 

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The octane numbers of the fuel you use are quite low. In the UK our standard fuel is 95 RON and the super is 98 RON.
I use an additive in mine that adds about 2 RON and helps to keep things clean at the same time.
A lot of people I know with these cars seek out the fuel stations selling the 98 RON.
 

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Registered 2006 SLK55 AMG
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252 Posts
Just a guess: Follow the wires from your O2 sensors and ensure they aren’t resting on your exhaust. I’m wondering if you might be experiencing an intermittent short owing to the insulator being slowly burned away?

tn
 

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Premium Member 2001 SLK320/2000 SLK230
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Discussion Starter #9
No, in the US we use the average between the Research Octane Number and the Motor Octane Number which is quite a bit lower. This is known here as the PON (Pump Octane Number) and is generally 5 points lower than the Research Method (RON) in the 80-90 ranger

So 87 here is about 91 RON, 89 is closer to 94 and 93 is abut 98.

Now the US owners manual is quite specific: "Use Premium unleaded gasoline: Posted Octane Index 91 (average of 96 RON/86MON". I achieve this by alternating 89 and 93 PON at 1/2 tank fillups and use more 89 for long distance cruising that does not need to get into Performance Enrichment (PE) or high revs.

Currently 93 is about U$0.30/gal (15%) more than 89.

Understand that gas is gas and all has the same BTU/lb. The difference in octane is that the higher the octane the slower it burns which requires more advance but is also more resistant to detonation. Today we have direct injection engines that can have high (11-13:1) compression or boost and run on low octane (87 PON). My '12 Pentastar produces almost 300 hp from 3.6 liters, 11:1 compression and on 87 PON.

So I could use pure 93 in my 3.2s (have two) but the ignition maps are optimized for slightly faster burning 91 so why bother ?

ps in the bad old days we would tweak the distributer by advance to crack (detonation) on a quick throttle opening, then back off two degrees. 38 degrees total advance over initial was good for up to 6000 rpm. Today all of that is handled by the computer. There are ways to tweak that a bit (many use resisters, I prefer to reprogram the computer) but for me the 3.2 has "enough" so I tend to tune for reliability/longetivity.

More than you ever wanted to know ?

BTW Torque Pro can be set to keep logs of selected parameters as a .csv and at about a once per second refresh rate that I can export to and scan on my PC.
 

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Have you changed the fuel filter recently? I had fuel trim codes occur shortly after I replaced my fuel filter. Double checked that I installed the right way... but I also had a MAF code. Both were intermittent. I ended up putting in a new MAF (first of two so far), and the trim codes also disappeared. You have a new MAF.

It's always bothered me that I think the trim codes were brought on by the fuel filter change. Three years later, I replaced 3 ft of rusted fuel line from the filter forward. It was holed and dripping fuel. I also replaced the injectors with remaned units at that time. Maybe the original trim codes were due to rust or dirt dislodged when I replaced the fuel filter. .....just my thoughts on a possible cause.
 
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