M104 - Rough Idle after Rebuild

[Seangbadee]

I recently completed a rebuild on my M104 engine and wanted to share the experience while also seeking some advice. The engine and transmission are coupled, and overall, the car drives really well with smooth power delivery and no hesitation under load.
The one issue I can’t get past is idle quality. The car just won’t idle smoothly. It sputters and feels unstable at idle, although once you’re driving it’s fine.

Here’s what I’ve done so far:

• Sent the injectors out for diagnosis and cleaning​

• Sent the throttle body out for service​

• Replaced the fuel filter (thought it might be clogged no change)​

• Replaced the fuel pressure regulator (still no improvement)​

At one point the CEL came on and when I checked I got 7 blink codes which indicate “ignition system defective.” At this point, I’m stumped. I can’t tell if I’ve overlooked something during the rebuild or if there’s another weak link I’m not considering.

Has anyone run into this before on an M104 after a rebuild? Could this be vacuum related, ignition, or maybe something electronic I’m missing? Could the crankshaft position sensor be the culprit? Any suggestions or troubleshooting ideas would be much appreciated.

 

[gerryvz]

Interesting.

Without finishing your post, I immediately thought “ignition system”.

Any other codes?

How is your ignition? New plugs? New wires? New coils?

If you didn’t replace those three items as part of your rebuild, I would. You should be able to test your wires for resistance.

Coils either usually work or don’t — you will get rough running, but also a fuel smell from the exhaust and of course, two cylinder not firing. I don’t think a coil is your problem, though per my own G320 rebuild (ongoing) I have three new coils I bought.

Plugs would be a good idea to replace if you didn’t. Gap them to 1.0-1.1 mm.

Vacuum leak would likely result in a high idle, not so much rough running.

Last thing I would look at would be MAF, and also whether your ETA was indeed properly rebuilt, but those would be “later” things after you ensure the ignition system is working well.

 

[gsxr]

Sean, check the plugs & boots for any signs of carbon tracks. IIRC you have new OE plugs. There have been some reports of defective new dealer plugs, so a long shot might be swapping the plugs out for a different set (even old/used plugs as a test). Set the gap around 1.0mm.

Next up would be checking blink codes from the HFM system, that will show more specific ignition faults (blink codes 21-23). To get the digital codes (055 to 062) you need a digital scanner (SDS/HHT-Win, MT2500, or possibly HFM Scan).

 

[Seangbadee]

Interesting.

Without finishing your post, I immediately thought “ignition system”.

Any other codes?

How is your ignition? New plugs? New wires? New coils?

If you didn’t replace those three items as part of your rebuild, I would. You should be able to test your wires for resistance.

Coils either usually work or don’t — you will get rough running, but also a fuel smell from the exhaust and of course, two cylinder not firing. I don’t think a coil is your problem, though per my own G320 rebuild (ongoing) I have three new coils I bought.

Plugs would be a good idea to replace if you didn’t. Gap them to 1.0-1.1 mm.

Vacuum leak would likely result in a high idle, not so much rough running.

Last thing I would look at would be MAF, and also whether your ETA was indeed properly rebuilt, but those would be “later” things after you ensure the ignition system is working well.

The plugs are new, swapped them out again but didn't re gap them. I changed out the boots, so those are all new. I swapped the coils (old working ones), but the problem was still there. I remember that I changed the crankshaft sensor and had to put a .5mm washer because it was touching the flywheel. I have not changed out the wires. Do you have a part number for these? MAF is good and working, and even when I changed it the problem still remained.
I'm stuck!!!

 

[JC220]

An easy way to check for ignition system arcing is simply to run the engine in complete darkness and observe for blue sparks closely. Do that before buying any more parts for it including ignition leads.

 

[emerydc8]

I remember having an idle stumble after a top-end refresh I did on a Chevrolet Gen VI big block and I was totally out of ideas so I took it to an indy. He was an older guy probably near retirement. The first thing he did was take out a spray bottle to spray/mist the plug wires while I watched. I felt pretty stupid because the diagnosis was obvious—current was jumping from a wire near the plug boot to the adjacent exhaust manifold heat shielding an inch away. The mist really made it visible. Like @JC220 suggested, checking for current jumps in the dark would be the way to go. As part of the top-end refresh I had replaced the old plug wires with a new set of factory AC Delco wires ($350) so the last thing I expected was that they would be bad right out of the box. The dealer tried to push back when I brought them back but I showed him the diagnostic invoice. A new set of wires fixed the idle problem.

I had an idle stumble issue on my M119 about 11 years ago and after replacing virtually everything that could have affected idle I showed my list to a Mercedes tech the day before my appointment. He said changing wires was likely what he would recommend when he got the car. I went home and changed the wires with a custom-cut set of @gsxr specials. This fixed the idle problem. I tried misting the wires at night but between the plastic covers on the cam covers and the other plastic obscuring the wires I couldn’t see any shorts.

 

[Seangbadee]

An easy way to check for ignition system arcing is simply to run the engine in complete darkness and observe for blue sparks closely. Do that before buying any more parts for it including ignition leads.

I will. Thank you

 

[Seangbadee]

I remember having an idle stumble after a top-end refresh I did on a Chevrolet Gen VI big block and I was totally out of ideas so I took it to an indy. He was an older guy probably near retirement. The first thing he did was take out a spray bottle to spray/mist the plug wires while I watched. I felt pretty stupid because the diagnosis was obvious—current was jumping from a wire near the plug boot to the adjacent exhaust manifold heat shielding an inch away. The mist really made it visible. Like @JC220 suggested, checking for current jumps in the dark would be the way to go. As part of the top-end refresh I had replaced the old plug wires with a new set of factory AC Delco wires ($350) so the last thing I expected was that they would be bad right out of the box. The dealer tried to push back when I brought them back but I showed him the diagnostic invoice. A new set of wires fixed the idle problem.

I had an idle stumble issue on my M119 about 11 years ago and after replacing virtually everything that could have affected idle I showed my list to a Mercedes tech the day before my appointment. He said changing wires was likely what he would recommend when he got the car. I went home and changed the wires with a custom-cut set of @gsxr specials. This fixed the idle problem. I tried misting the wires at night but between the plastic covers on the cam covers and the other plastic obscuring the wires I couldn’t see any shorts.

@gsxr specials! What are those?

 

[emerydc8]

It was a secret @gsxr production run from 2014. IIRC I loaned Dave my factory crimper and he selflessly sent me a set which are still in use. I don’t know if the wire and terminals are still available anymore from the dealer. I suspect not, considering so many mission-critical parts are now NLA.

 

[Seangbadee]

It was a secret @gsxr production run from 2014. IIRC I loaned Dave my factory crimper and he selflessly sent me a set which are still in use. I don’t know if the wire and terminals are still available anymore from the dealer. I suspect not, considering so many mission-critical parts are now NLA.

 

[Seangbadee]

I will. Thank you

I ran the 'arc test' myself last night, started the car in the dark and didn’t see any arcing, so that's one thing ruled out.
Today, after changing out the ETA, I took the car for a 19 mile highway drive, and overall it ran pretty well. But as I slowed down near a stop, the engine started to "gargle," almost like it was misfiring. It’s not a constant issue, but definitely noticeable during deceleration. So, I think I can conclude that this is not an ETA issue.
So,I'm back to the drawing board. Next weekend, I plan to pull the new crankshaft position sensor and reinstall the original one to see if that makes any difference. Still chasing down the root cause, but hopefully getting closer.

 

[emerydc8]

Is the engine wiring harness in good shape?

The reason I ask is that on deceleration the engine drops forward and down, away from where the harness connects near the firewall, so if your harness insulation is in bad shape you might notice this more on deceleration due to disturbing the harness.

EDIT: I just noticed this is an M104 and I’m not familiar with how deceleration might affect engine movement. Could it be the fan hitting the lower shroud causing the gargling? How are the motor mounts?

 

[Seangbadee]

The mounts are in great condition and so is the harness. It's not the fan, it feels more like a misfire.

 

[195910]

The mounts are in great condition and so is the harness. It's not the fan, it feels more like a misfire.

Can you connect the car to star and post a short video of the live data ? Smooth running function and idle when warm

 

[Seangbadee]

Can you connect the car to star and post a short video of the live data ? Smooth running function and idle when warm

I do not have the diagnostic tools. I'm relying on blink codes

 

[195910]

I do not have the diagnostic tools. I'm relying on blink codes

It won't provide the required readings. Its the same with the M119 misfire issues where no fault codes are registered.

 

[Alphasud40]

I would revisit your cam timing. My gut tells me you have a camshaft off. Not sure if HFM is sophisticated enough to flag a cam crank correlation issue. The 7 blink might encompass that correlation issue.

 

[J. M. van Swaay]

Make sure you have the right plugs. The 104 uses a waste spark ignition system. Any thing other than Bosch F8DC4 (if I remember correctly) will cause idle problems. DO NOT use iridium, resister, high performance, yada yada yada plugs. The very basic plugs are the only thing that works….

 

[gerryvz]

Yes, the M104 uses the same plug as the M119. F8DC4, non-resistor.

 

[Seangbadee]

I would revisit your cam timing. My gut tells me you have a camshaft off. Not sure if HFM is sophisticated enough to flag a cam crank correlation issue. The 7 blink might encompass that correlation issue.

@Alphasud40, yesterday I retraced my steps and reviewed the cam timing to make sure everything was aligned properly. It turned out to be spot on, so there was no need to reset anything. The car starts up fine and drives well on the interstate, but then it suddenly behaves like it's misfiring, almost as if it briefly loses spark to one of the plugs. Now I'm starting to wonder if this could be related to the crankshaft position sensor, which I recently replaced.

 

[Seangbadee]

Make sure you have the right plugs. The 104 uses a waste spark ignition system. Any thing other than Bosch F8DC4 (if I remember correctly) will cause idle problems. DO NOT use iridium, resister, high performance, yada yada yada plugs. The very basic plugs are the only thing that works….

I'm running F8DC4 plugs, which have never given me any issues.

 

[J. M. van Swaay]

OK, correct plugs are confirmed. Just reread everything you have already done, it seems to me you have been pretty logical in your approach. I once had an idle problem on my E320 wagon that I spent months looking for. Turned out to be a cracked/broken hose nipple on the underside of the black plastic intake manifold resulting in a vacuum leak. If I remember correctly there are two vac hose fittings on the bottom of this manifold, the one that was cracked was the smaller of the two and it was almost impossible to see that it was cracked. If I remember correctly the idle problem was not present at cold start up but would present after a short drive. Maybe this was because the HFM was still in “open loop” mode at start and then would switch to closed loop after a short warm up…. but then again I’m not really sure how the system works. Might be worth a very careful look with a flashlight and inspection mirror. Try moving the hose slightly to see if the nipple is still attached. I don’t remember if any codes were present.

 

[Seangbadee]

@J. M. van Swaay, after my post this morning, I removed the new crankshaft position sensor and reinstalled the previous one, as this one wasn’t faulty. I started the car again, but the issue still persists. So I’m going to take your advice and inspect the entire intake manifold, including the two vacuum nipples underneath. I also plan to take Gerry's advice, and replace the coils and the coil wires, and hope that resolves the problem. My injectors haven’t shown any signs of failure, but at this point I’m open to replacing them as well

 

[195910]

Do you have access to an Automotive oscilloscope ? should be available at classic cars workshop.

You can then determine which cylinder runs poorly

 

[Seangbadee]

@Alphasud40, yesterday I retraced my steps and reviewed the cam timing to make sure everything was aligned properly. It turned out to be spot on, so there was no need to reset anything. The car starts up fine and drives well on the interstate, but then it suddenly behaves like it's misfiring, almost as if it briefly loses spark to one of the plugs. Now I'm starting to wonder if this could be related to the crankshaft position sensor, which I recently replaced.

Do you have access to an Automotive oscilloscope ? should be available at classic cars workshop.

You can then determine which cylinder runs poorly

Let me look into this. Thanks.

 

[Seangbadee]

After a refreshing three week break, I finally found my way back to the garage and got my hands on my car again. I pulled out the OBD1 code blinker and went hunting for any stored trouble codes. To my surprise, more than 20 DTCs showed up. Most were tied to the multi port fuel injection system on Socket 8, with a few extra codes on Socket 6.

These documents proved to be extremely helpful:

https://r129.co/content/Mercedes-OBD1-Fault-Codes-For-MB-Code-Reader.pdf

https://www.w124performance.com/docs/mb/diagnostics/DTC_List_W124_M104.pdf

After clearing everything out, I took the car for a good drive to see how it would behave. The improvement was immediate, the idle is smoother, the transmission shifts are buttery, and the whole car just feels happier on the road. It’s moments like this that remind me why I enjoy working on these machines in the first place.

Part of what motivated me is the quoted cost for diagnostics on these older Mercedes. In Atlanta, rates hover around $225 an hour with a two hour minimum, which adds up. Every step teaches me something new about this car and gives me a deeper appreciation for its engineering.

Next up, I plan to take a few spirited drives and check the codes again. Hopefully everything stays clear, but either way, I’m feeling encouraged. The progress so far has been worth it.

Thanks to everyone who chimed in with suggestions. I really appreciate all the help in getting my car working .
Now, I can complete the work on the headliner

 

[emerydc8]

The only thing you did was clear the codes and now it runs fine? I didn’t think that clearing codes would fix anything.

 

[Seangbadee]

The only thing you did was clear the codes and now it runs fine? I didn’t think that clearing codes would fix anything.

I went through just about everything based on the initial blink codes. Replaced the plugs, wires, coils, ETA, MAF sensor, and even the OVP relay, but nothing changed. After talking with a specialist at MBS in Atlanta, who confirmed what Dave said about using an OBD1 blink code reader, I cleared the codes myself. Once I did that, everything finally settled down.

 

[Seangbadee]

I went through just about everything based on the initial blink codes. Replaced the plugs, wires, coils, ETA, MAF sensor, and even the OVP relay, but nothing changed. After talking with a specialist at MBS in Atlanta, who confirmed what Dave said about using an OBD1 blink code reader, I cleared the codes myself. Once I did that, everything finally settled down.

I went through just about everything based on the initial blink codes. Replaced the plugs, wires, coils, ETA, MAF sensor, and even the OVP relay, but nothing changed. After talking with a specialist at MBS in Atlanta, who confirmed what Dave said about using an OBD1 blink code reader, I cleared the codes myself. Once I did that, everything finally settled down.

I put all my original parts back in the car, and the new ones are back on my shelf as spares.

 

[Seangbadee]

The only thing you did was clear the codes and now it runs fine? I didn’t think that clearing codes would fix anything.

@emerydc8 I drove my1995 E320 with the M104 engine a few days ago and noticed another misfire, this time on socket 8 with DTC 21, and DTC 36. It turned out the culprit was a bad purge valve that had been stuck open the entire time. I’m not sure how or when it happened, but at idle there was continuous suction through the valve, sending too much fuel vapor into the engine and disrupting the air-fuel ratio. Once I replaced the purge valve (part #000-470-21-93), the misfires disappeared. I’ve been driving it for the past three days with no issues at idle. I also checked the new purge valve, and it’s functioning as intended.

 

[gsxr]

Sean, nice find on the purge valve! I've seen several of these blasted things stuck open (either fully or partly), which causes a moderate to major vacuum leak, depending on where the valve "stuck". The result is generally a bunch of unmetered air entering the intake tract, most often causing idle problems.

However: The valve may have gotten stuck due to carbon particles from the canister inside the fender. The canister can have the internal screen fail, allowing carbon to get ingested into the valve. I replaced a purge valve or two and found the replacements jammed up, before figuring this out. Definitely check into this, if carbon bits are escaping from the canister, it will need to be replaced.

P/N A1244700359, spendy at $120 list / $90 discount. AFAICT this is still available but may need to come out of Germany, online sites incorrectly show it as Backorders or Currently Unavailable, which I believe is wrong.

 

[Seangbadee]

Sean, nice find on the purge valve! I've seen several of these blasted things stuck open (either fully or partly), which causes a moderate to major vacuum leak, depending on where the valve "stuck". The result is generally a bunch of unmetered air entering the intake tract, most often causing idle problems.

However: The valve may have gotten stuck due to carbon particles from the canister inside the fender. The canister can have the internal screen fail, allowing carbon to get ingested into the valve. I replaced a purge valve or two and found the replacements jammed up, before figuring this out. Definitely check into this, if carbon bits are escaping from the canister, it will need to be replaced.

P/N A1244700359, spendy at $120 list / $90 discount. AFAICT this is still available but may need to come out of Germany, online sites incorrectly show it as Backorders or Currently Unavailable, which I believe is wrong.

View attachment 227341

I just missed one on eBay. They show up as NLA on Revolution MB sites

 

[Alphasud40]

I have noticed other manufacturers place a filter before the purge valve. Toyota comes to mind. The next time I see one I will post the part number. Also I have used the little transparent fuel filter used on air-cooled Beetles.

 

[gsxr]

I have noticed other manufacturers place a filter before the purge valve. Toyota comes to mind. The next time I see one I will post the part number. Also I have used the little transparent fuel filter used on air-cooled Beetles.

Good idea! A simple filter would also work... and, a clear filter would let you see if carbon is escaping.

 

[emerydc8]

Good find @Seangbadee!

 

[gsxr]

I just missed one on eBay. They show up as NLA on Revolution MB sites

I'm not seeing NLA at RevPart sites, yet. MBCC shows available (click here).

RevParts usually has 3 different terms that show on their website. They do not all mean the same thing:

Discontinued = NLA (however, if a part comes back from the dead, RevParts can be slow to reverse this)
Currently Unavailable = Probably Available, but may not be stocked in USA, and needs to come out of Germany.
Backordered = Same as "Currently Unavailable".

In all cases, you must contact a live person at a dealership and have them check worldwide inventory in Paragon. RevParts websites DO NOT have any connection to real-time inventory and OFTEN do not reflect reality...

 

[gerryvz]

I replaced this MOT valve on my M119 in 2020 when I did my engine-bay refresh. My valve had failed.

Ordered a new one from MB and yes, it wasn't cheap. But it felt good to get the system working as it should be.

You can test its operation with a simple 9-volt battery and listen for a "click."

As I said at the time, I'd say that 90%+ of people with cars with these valves.....they are probably not working correctly (either stuck open, or closed). My M119 didn't have any ill effects or running differences from it not working, that I could tell, before or after replacement. My 1995 gray-market G-wagen's M104 (currently in the middle of rebuild) does not have an "MOT" purge valve.

My key M119 purge valve related posts are here:

Comparo - old vs. new

Install of new valve (M119)

Testing valve (posts #394-396 + #400)


I have never heard or experienced any running differences with any of the three M104s I have owned after clearing codes. That is new to me.

 

[Seangbadee]

I replaced this MOT valve on my M119 in 2020 when I did my engine-bay refresh. My valve had failed.

Ordered a new one from MB and yes, it wasn't cheap. But it felt good to get the system working as it should be.

You can test its operation with a simple 9-volt battery and listen for a "click."

As I said at the time, I'd say that 90%+ of people with cars with these valves.....they are probably not working correctly (either stuck open, or closed). My M119 didn't have any ill effects or running differences from it not working, that I could tell, before or after replacement. My 1995 gray-market G-wagen's M104 (currently in the middle of rebuild) does not have an "MOT" purge valve.

My key M119 purge valve related posts are here:

Comparo - old vs. new

Install of new valve (M119)

Testing valve (posts #394-396 + #400)


I have never heard or experienced any running differences with any of the three M104s I have owned after clearing codes. That is new to me.

Yes, @gerryvz, the purge valve was not cheap @ $118.

 

[Seangbadee]

I'm not seeing NLA at RevPart sites, yet. MBCC shows available (click here).

RevParts usually has 3 different terms that show on their website. They do not all mean the same thing:

Discontinued = NLA (however, if a part comes back from the dead, RevParts can be slow to reverse this)
Currently Unavailable = Probably Available, but may not be stocked in USA, and needs to come out of Germany.
Backordered = Same as "Currently Unavailable".

In all cases, you must contact a live person at a dealership and have them check worldwide inventory in Paragon. RevParts websites DO NOT have any connection to real-time inventory and OFTEN do not reflect reality...



View attachment 227342 View attachment 227343 View attachment 227344

I called MB Warwick and they said there were none in the US or Germany. MB Paramus checked today, and there are none available.

 

[gsxr]

I called MB Warwick and they said there were none in the US or Germany. MB Paramus checked today, and there are none available.

Crap. Well that's not a good sign.

Forgot to mention, when a dealer parts person tell you none available anywhere, also ask for the specific NLA codes. If none are present, it may be a temporary OOS issue, where there are more on the way from the manufacturer. But if zero available AND also there are codes which say "discontinued permanently"... uh-oh. If it's a very recent NLA, the MBCC website may not have updated yet, it takes a month or two for the site to catch up after inventory is depleted.

 

[Kridre]

You can try tapping the MOT valve lightly on pice of wood or something. I fixed mine by doing this and applying 9V few times.

Few pieces of the carbon balls fell out of it and this what was probably preventing the valve form functioning properly.

Adding the in line filter is good idea.