G Wagon - M104 RPM drop under slight acceleration

[Roscoes_Wetsuit]

G’day from sunny Australia!

I recently picked up a 1994 SWB G Wagon (Japan delivered) with a M104 engine.
During my short ownership I’ve rebuilt the top end ( with guidance from Gerry’s post, an absolute legend) including head gasket, water pump and related seals and sensors.

However I’m still having an issue with the revs dropping and in some cases stalling with slight throttle input. If im heavy on the throttle there’s no issue but under gentle input the revs drops and the engine stalls or nearly stalls then revs surge back up. There is also a strong fuel smell from the exhaust which leads me to believe it is running rich.

So far to try and solve this problem I’ve also replaced the 02 sensor, crankshaft and camshaft position sensors, plugs, coils, fuel filter, thermostat and a smoke test to find and fix any vaccum leaks and I’ve even opened the throttle body to check for signs of broken wires (visually all looks okay even tho it’s a 94 year throttle body).

This issue is driving me mad and I can’t think of that else may be causing the issue. I’ve hooked up a test light which is showing codes 24,35 and 13 on pin 8.

Any thoughts as to what the issue maybe? Wiring harness has not been replaced by the seems but doesn’t seem to be in horrible shape. I know this is going to have to be replaced at some point but it is a lot of coin and I’m hoping this issue is something else.

 

[195910]

Some issues to look at if no vacuum leaks found:

- Intake manifold flap (black square in the middle of the intake manifold), might be stuck slightly open.
- Oil contaminated throttle valve
- EGR and other internal vacuum leaks.

 

[gerryvz]

It should NOT be running rich. You may have a bad coil pack - a common failure on the M104. I have had one or two go bad on my G320 over the past 11 years.

I would center in on the ETA (especially if it is original — buy a used unit with date of 1998 or later on eBay or other source).

Has your upper wiring harness been replaced?

I would also check your MAF - get and swap in a spare unit. It’s a good spare to have anyway. It’s shared with pedestrian HFM M104 models.

 

[The G320 Driver]

I’m Not familiar with the fault Codes because I’m using the HFM Scan to read the faults on my G320
May someone got the G320 M104.996 fault list on pdf for me. I’ve seen some ytube videos of 320 sedan that’s it.
From my side some questions, may you did it already.
Did you remove the injection plugs during idle run to see any rev. drops or taken out the coil plugs coming from ecu to avoid ecu faults.
Did you done some measurements on the plugs coming from Ecu and injecting voltage during engine running. Injecting voltage is lower then coils but use to be more less same under idle run on all of them.
Did you take out the injector rail there is a strainer on the fitting where the fuel hose is connected coming from fuel pump.
Did you take out the injectors for cleaning and testing the solenoids for proper operation even checking the injecting behavior, comparing with the others.
may they got droppings under 3 bar pressure because of wear and tear. Take an hose ca. 100mm long connect to the inlet with a clamp easy fix don’t brake the inlet connection and fill out the hose with fuel and set the compressor pressure 3 bar and test them for any drops.
One last thing did you replace or measure the 2 kOhm connector (extension) between coils and plugs.
Hope this helps you a bit.
Best wishes to down under.

 

[Roscoes_Wetsuit]

Thanks for everyone’s replies! I spent the weekend diagnosing the G and I believe I found the culprit. The brand new spark plug wires that I had just installed had failed. Cylinder 1 spark plug wire was no longer attached, I swapped it out for a spare one I had and these seemed to fix the issue all together! No more rough acceleration/ running rich and after clearing codes and a test drive they haven’t come back. Hopefully this was the only issue and the fix lasts!

Also Gerry, I dont know if my upper harness was replaced before my ownership as the car has limited history and spent time in Japan, NZ and now Aus. I have inspected the wiring and it seems to be in decent nick apart from the wiring going to what I think is the smog pump located in the passenger side fender. From what I can tell ETA is also original but has been inspected and looks to be in good condition. I’ll keep an eye on this and plan to replace both eta and harness sometime down the track.

 

[Roscoes_Wetsuit]

I’m Not familiar with the fault Codes because I’m using the HFM Scan to read the faults on my G320
May someone got the G320 M104.996 fault list on pdf for me. I’ve seen some ytube videos of 320 sedan that’s it.
From my side some questions, may you did it already.
Did you remove the injection plugs during idle run to see any rev. drops or taken out the coil plugs coming from ecu to avoid ecu faults.
Did you done some measurements on the plugs coming from Ecu and injecting voltage during engine running. Injecting voltage is lower then coils but use to be more less same under idle run on all of them.
Did you take out the injector rail there is a strainer on the fitting where the fuel hose is connected coming from fuel pump.
Did you take out the injectors for cleaning and testing the solenoids for proper operation even checking the injecting behavior, comparing with the others.
may they got droppings under 3 bar pressure because of wear and tear. Take an hose ca. 100mm long connect to the inlet with a clamp easy fix don’t brake the inlet connection and fill out the hose with fuel and set the compressor pressure 3 bar and test them for any drops.
One last thing did you replace or measure the 2 kOhm connector (extension) between coils and plugs.
Hope this helps you a bit.
Best wishes to down under.

Hi G320 driver,

I had cleaned and tested all injectors during the top end rebuilt about a month ago. Seems I found the issue in a broken spark plug wire. Can you share the fault code PDF you have? I have come across multiple PDFs and am unsure if I have the correct one.

 

[gsxr]

What brand plug wires had that failure at brand new?

 

[Roscoes_Wetsuit]

What brand plug wires had that failure at brand new?

They are the Karlyn-Sti Spark Plug Wire Set. Supposed to be okay quality from the research I did before buying. Seems like the wire was pulled out, unsure if this was user error when installing but more likely a faulty part.

 

[gsxr]

Karlyn-Sti are not OEMs for Mercedes ignition parts (same with Bremi or Facet). Best to stick with Bosch or Beru.

 

[The G320 Driver]

Hi G320 driver,

I had cleaned and tested all injectors during the top end rebuilt about a month ago. Seems I found the issue in a broken spark plug wire. Can you share the fault code PDF you have? I have come across multiple PDFs and am unsure if I have the correct one.

Hi Roscoe,
Pls see PN for your request.