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.