I've been getting misfires on my 1997 E320 (80k miles). The engine seems to run fine on the freeway, but runs rough

waiting at lights. The RPMs also surge, especially if it's in park.

My AutoEnginuity tester shows a variety of codes. Usually P0300 (random misfire), plus P0301-4 (misfire cylinder

1-4). Here's what I've done, in chronological order:

- Action - DTC
Initial problem - - P0300, P0303
reset DTC - Replaced all plugs with Bosch F8DC4 - P0300, P0303, P0304
reset DTC - Swapped coils on cylinder 2 and 4 - P0300, P0303, P0304
reset DTC - Swapped plugs on cylinder 1 and 3 - P0300, P0303, P0304
reset DTC - Swapped plug boots on cylinder 1 and 3 - P0300, P0301, P0304
reset DTC - Replaced 3 plug wires & 6 boots Bosch 09386 - P0300, P0301, P0303, P0304

This is very frustrating - the problem seems to be getting worse with each part replacement, and the parts left to

replace are getting expensive!

Any suggestions?

Some possibly related information:

1) I replaced the head gasket (and all the vacuum lines) about three months ago. This misfire problem started in

the last 3 weeks, so I don't think they're related. The wiring harness looks OK, although I have not unwrapped the

harness to check the wires in detail.

2) I've been having a P0170 - Fuel Trim DTC off and on for over a year. It takes about 1 month to pop up after a

reset, so diagnosis is slow. I've replaced the Mass Air Flowmeter, the fuel header pressure regulator, and the

injector o-rings. Fuel header holds pressure for > 30 minutes after motor turned off. Injectors don't drip.

Thanks.

What you described happened to my wagon when the MAF went south, but you said you changed that out. Is it a whole new unit or did you clean it?

__________________
dtf
1994 E320 Wagon (Died @ 308,669 miles)
1995 E300 Diesel (228,000)
1999 E300 Turbodiesel ( died @ 255,000)
2006 Toyota Tundra SR5 AC 4X4 (115,000 miles) rusted frame - sold to chop shop
2011 Audi A4 Avant (165,000 miles) Seized engine - donated to Salvation Army
BMW 330 xi 6 speed manual (175,034 miles)
2014 E350 4Matic Wagon 128,000 miles
2018 Dodge Ram 21,000 miles

I first tried to clean the old unit. I then replaced it with a used MAF from a wrecked MB C model (same p/n). There's always a possiblity that it's also defective. I can pull up a graph of the MAF output on the OBDII reader, and I know that both the old and new MAF have about the same readings, and that both vary with load on the engine. I'll try swapping them out again tonight to see if there's any difference. The MAF swap predated the misfire problem by a few months.

On my c class, i swapped the MAF out with one that was workin fine in a e class. However it still faulted and errored. Banged a new one in and presto. Its work a try - its the most likely culprit.

I swapped it out with the "old" MAF. DTC light went out and car seems to be running smoother. I'll give it a day or so to see if I get the light again.

Thanks!

FYI the MAF indicates 0.42 lb/min at 750 RPM, unloaded and 3.20 lb/min at 4,000 RPM unloaded. I'll have to drive on the highway to check it under load later in the week. The scale for the MAF is 0-86.7 lb/min, so I'm wondering if the reading I'm getting is a bit low.

I put in the "old" MAF. Car ran fine for a few days. Yesterday, I got the rough idle again. I checked the codes, and show misfires on 4 of the 6 cylinders. I had read somewhere that if you unplug the MAF, the car will adjust, so I did that to see if the rough idle went away. Nope.

I happened to have the DTC code reader hooked up at the time, so I looked at the signals from the MAF. Here's what's odd - the inlet air temperature showed -70 (and 95 when the MAF was connected), which is what I expected. However, the air flow showed EXACTLY THE SAME, even changing with engine RPM, whether the MAF was connected or not!

I'm starting to wonder if I have a wiring problem. I plan to check the wiring with a voltmeter over the weeked to see if I have a short.

I also figured out what the MAF should read, based on this article
http://www.installuniversity.com/install_university/installu_articles/volumetric_efficiency/ve_computation_9.012000.htm

Based on 80% volumetric efficiency, I made this table:

Speed Estimated
rpm lb/min
0 0.0
1000 3.2
2000 6.5
3000 9.7
4000 13.0
5000 16.2
6000 19.4
7000 22.7

This will let me know if the numbers I see are reasonable.

I replaced the MAF with a brand new unit. The check engine light came back on. Arghhhh!

I did some testing with the old MAFs, since I have three now. I found out some interesting things:

1) The car will run if you unplug the MAF (in place). Interestingly enough, my scanner shows a signal from the MAF that still varies with engine speed, even with it unplugged.

2) I left the MAF in place, and set the second MAF on top of the motor, plugged in. Started car. Monitored MAF signal with scanner. Blew air (from about 2' away) into MAF. Scanner showed higher MAF output. Engine would speed up, then start running rough. Suddenly, engine would smooth out, and MAF reading would return to normal. After that, no amount of air into MAF would change signal.

Based on this, I think that the car bypasses the MAF if it detects an abnormal signal, and provides a synthetic signal. Sure makes it hard to detect a MAF failure!

OK, so back to the problem.

Still getting same codes - P0300 (misfire), P0303 (Cyl 3 misfire), P0170 (fuel trim). Exact codes vary from reset to reset.

I looked for vacuum leaks, but didn't find anything. Checked MOT valve, which seems to be working.

Freeze frame data from scanner:

Diagnostic Trouble Codes
P0303 Cylinder 3 Misfire Detected

Freeze Frame (Frame 0) for DTC P0000
Calculated Load: 18 % Coolant Temperature: 172 F
Engine RPM: 725 r/min
Fuel System Status Bank One: Closed (With O2 Fault)
Fuel System Status Bank Two: Not Reported
Intake Man. Abs. Pres.: 15.6 in.hg
Long Term FT B1: 9.36 % Short Term FT B1: 24.96 %

The O2 fault thing had me concerned. Looked at O2 sensor outputs. Upstream of catalytic converter, varies from .1V to .8V when I race the engine and looks OK. Downstream, varies slightly. (I wonder if this is a synthetic signal, though)

Any ideas?

The fact that you have a fuel trim error combined with a misfire suggests to richer mixture?

If you O2 sensor is cyclying up and down I would forget about this. The fuel trim error is simply saying the mixture is outside the range that the computer can adjust to.

If you have a new MAF in, then thats one thing eliminated.

I dont know where you would look next, however things that can cause over rich mixture include temp sensor for inject, air temp sensor, throttle actuator?

The two sensors are relitavely cheep.

As for the signal from the MAF, I would make the assumption that the computer goes to limp home mode when it senses major error. I might be wrong.

Come on guys, back me up, or shoot me down!! lol


PS. on a side note, this sounds like a simular probelm i had in a 95 c200. It was giving lamda out of range and maf faliure, but it turned out that the computer wasnt getting enough voltage.

I thought one of the fuel injectors might be clogged. First, I hooked an oscilloscope to each of the injectors to see what the signal looked like. Each one had an identical signal - 50V for .16ms, followed by a rapid voltage drop for the next .24ms. I then unplugged each injector in turn to see if maybe one didn't have an effect. Each slowed the idle slightly. However, I noticed a whistling when I unplugged the cable from one. I found that when I wiggled it, it seemed to be leaking. I used a mechanic's stethoscope to verify that there was a leak on #3. I also found a small leak on #6. That was odd, because I had replaced all the o-rings just a few months back.

I replaced the o-rings again ($21 - ouch), plus rotated the injectors to different cylinders to see if the problem moved.

Success! The idle is no longer rough, and the check engine light has not come back on. I'm a bid dubious that this is the entire problem - I won't be surprised that it comes back on in a few weeks.

Since I now have three MAFs, I decided to test them to see what sort of signal they put out. To create flow through the MAF, I put it up against an ordinary box fan, and ran it on low and high speed.

Here's the pinout on the connector:

rounded end ( 1 2 3 4 5 | flat end

1 - IAT (temperature)
2 - 12 VDC
3 - ground
4 - 5 VDC
5 - MAF signal

Results:

rate pin 5 - MAF
no flow 1.0 to 1.1 VDC
lo 2.0 to 2.1 VDC
hi 2.2 to 2.3 VDC

All three made the same signal.

that seems like an expensive lesson learned.

Just curious, what is the o-ring to that you replaced?

Your 97 E320, is it still the m104 inline six or the m112 v6?

I can tell you we just had a misfire problem on our vw eurovan, it was the fuel injector, and it ran poorly at idle but ran basically ok at speed. If your misfire jumps around, it is probably not injectors. How hard to change injectors on whatever 6-cyl you have? On the vw, about 18 hours the first time around, then 7 hours the second.

__________________
________________________
2002 Mercedes-Benz E320 4Matic
1995 Mercedes-Benz E320 Wagon
2005 Land Rover LR3 V8 SE
1999 Audi A8 4.2L quattro

The 6 o-rings that I replaced were the ones between the fuel injectors and the inlet manifold. There are another 6 o-rings between the fuel injectors and the fuel header that I did not replace.

I measured the old and new o-rings with a micrometer to see if they were the wrong size. The old o-rings had some ovality in cross-section, probably from compression, but seemed to be about the right size. Maybe they were old stock or the wrong elastomeric material.

The engine is the inline 6. It takes about 1 hour to change out the injectors - the only problem is a few bolts on the injector fuel header that are a challenge to get out.

I'm still getting a smooth idle, so I think this has fixed the rough idle problem.

Not that I'm surprised - but I got a Check Engine light in the middle of a highway trip. The good thing is it's a different one than I've gotten before:

P0133 - Sensor Circuit Slow Response (Bank 1 Sensor 1)

I guess that means I have an old O2 sensor. I've reset the DTC to see if I get the same code again, since the O2 sensor seems to be responding when I pull up the reading on a graph. I'm a bit surprised it's this one, since the Bank 1 Sensor 2 reading does not change much, and I would have thought it was the bad one.

Do these things just get old and have to be replaced? 1987 MB has 90k miles.

The second O2 sensor is used to monitor converter efficiency. When all is well with the system, the front sensors will swing rich/lean in response to the fuel delivery, and the second sensors will meander about showing constantly lean. When the second sensor pattern starts to look like the front sensor, then the converter is no longer doing its job.

__________________
1989 300 SEL that mostly works, but needs TLC