1997 E420 - Rough Idle

[Maxweisen]

I've got a persistent rough idle on a 1997 E420 restoration project and I could use some help.

A little background on the vehicle:
- 1997 E420 with M119.985 engine
- Build Date September, 1997



I bought this car with hopes of restoring it to its former glory.

I started by assessing what needed attention:
- The engine felt like it was misfiring and down on power.
- Obvious rough idle. Would dip below 500 RPM.
- Engine operating temp was high.
- Smoke tested the engine and found lots of vacuum leaks.
- Smoke tested the exhaust and found 2 major exhaust leaks.
- Tested fuel pressure and it came back within spec (3.5 Bar)
- And tested fuel pressure after 30 minutes post shutoff and it is within spec (3.0 Bar)
- Checked the wiring harness and it does NOT have the biodegradable insulation

Here's a list of what I have replaced and repaired:
- Started with oil & filter change (Mobil 1 0W40)
- ETA re-manufactured
- Replaced 1 of 4 mismatched O2 sensor with Bosch OEM
- Replaced ignition coils and boots
- Replaced fuel injectors
- Replaced spark plugs
- Replaced MAF with Bosch replacement
- Replaced crankshaft position sensor
- Welded exhaust at leak locations. Verified by smoke test.
- Replaced water pump and coolant
- Replaced air intake filter
- Replaced all failed rubber vacuum connection & Cohline lines. Verified by smoke test.
- Replaced the fuel filter (original fuel filter date stamped 1997 was still on the car!)
- Serviced transmission with new fluid and filter

After all of this work, the car runs significantly better. It also now passes California smog!
Albeit the rough idle has improved, it is not perfect. There are times where the idle drops to ~450 RPM still.

I called a local German shop and asked for their opinion. Based on the actions I have taken, they suggested that the ECU could be the culprit.
Has anyone had any issues with '97 ECU's? I am located in Southern California. Does anyone have experience with an ECU shop in that region they could refer me to?

I am out of ideas and I need the help of the brilliant minds here on this forum!

Thank you for taking the time to read

Max

 

[emerydc8]

I think the 1997 is a different animal than the 94-95 E420, but I would consider changing plug wires too. On the 124.034 (M119 engine), increasing the spark plug gap to 1 mm has been known to help with a rough idle. 450 RPM doesn’t seem abnormally low if the car is in gear. I would see if you can find a used ECU module on eBay rather than trying to fix your existing module.

 

[Maxweisen]

@emerydc8
You are correct. The 1997 M119 is a bit different than the earlier M119 engines. Especially when it comes to the ignition system. No cap and rotor.

The "wires" that connect to the ignition coils are integrated into the wiring harness.
I've outlined the connectors with red squares and traced the path of the wiring harness in this schematic from the factory service manual:


Thanks for the tip regarding the plug gap. I will try setting the gap to 1mm and see what happens.

 

[gsxr]

One thing about the ECU. It would be VERY unlikely to be an ECU failure. Also, the ECU (Bosch ME 1.0) is coded/locked to the VIN, so you cannot easily swap in another one. You'd have to get a spare from an identical car (210.072 with M119.985), send the ECU out to have it unlocked, and swap it in to see if it works. Likely a $600+ experiment that may not have any effect on the idle speed.

How are you measuring RPM? Only from the tach? 450rpm is awfully low, but the tachs are not always accurate.

 

[Maxweisen]

One thing about the ECU. It would be VERY unlikely to be an ECU failure. Also, the ECU (Bosch ME 1.0) is coded/locked to the VIN, so you cannot easily swap in another one. You'd have to get a spare from an identical car (210.072 with M119.985), send the ECU out to have it unlocked, and swap it in to see if it works. Likely a $600+ experiment that may not have any effect on the idle speed.

I would like to avoid a $600 experiment that may not help!

How are you measuring RPM? Only from the tach? 450rpm is awfully low, but the tachs are not always accurate.

2 methods. Watching the tach needle bounce around along with the rough idle. I also have my OBD2 scanner hooked up and saw some RPM values in the ~400's range.

 

[gsxr]

OK - the live RPM data from OBD-2 should be accurate. What is the idle RPM when at operating temp, both in P/N, and also in gear?

Is the tach needle still bouncing, along with a rough idle, after all the work you've done so far?

What brand/type/part number are the spark plugs?

Any fault codes?

 

[Maxweisen]

What is the idle RPM when at operating temp, both in P/N, and also in gear?

RPM in P/N: ~650
RPM in D: ~500-450

Is the tach needle still bouncing, along with a rough idle, after all the work you've done so far?

Yes. You can feel the rough idle. It has significantly improved but it does have moments where it dips way low (450) and immediately jumps back to a bumpy ~500RPM in Drive. While moving the car feels great.

What brand/type/part number are the spark plugs?

Spark Plug Details:

Mercedes Bosch Spark Plug - Bosch F8DC4

Mercedes Bosch Spark Plug - Bosch F8DC4

www.fcpeuro.com

Bosch, Copper, F8DC4

---
No fault codes!

Also might be worth noting that I sent the borescope into each cylinder and they look almost perfect after 138,000 miles.

 

[gsxr]

I wonder if you have an intermittent spark plug rubber boot causing problems. These are replaceable separately from the coils.

You said it has new coils+boots... what brand were installed?

 

[Maxweisen]

You said it has new coils+boots... what brand were installed?

Bosch coils + Beru boots. All from FCP. This did provide some improvement to the idle situation.

 

[alabbasi]

I ran a 97 E420 for about 6 years and had two issues with rough idle
1) Big vacuum hose from the intake that runs to the back of the passenger side valve cover that just rotted out
2) When I time chain replaced at around 100k miles, the new chain was a tooth off. Fortunately it did not cause any damage and was set right, but you might want to check for chain stretch.

On my 98 SL500, the head gasket let go at 77,000 miles and gave me a rough idle.

 

[gsxr]

Good point about checking the cam timing. The timing chain rarely stretches enough to require replacement, but broken chain rails are VERY common, and will cause the cam timing to be off significantly. Need to check all 4 camshafts, pin them 1 at a time and read the number off the balancer. 2-4° off is normal-ish, beyond that some rails may be broken. Need to check the rails anyway if this has never been done, if original, it's almost certain that multiple need replacement.

 

[Maxweisen]

I ran a 97 E420 for about 6 years and had two issues with rough idle
1) Big vacuum hose from the intake that runs to the back of the passenger side valve cover that just rotted out
2) When I time chain replaced at around 100k miles, the new chain was a tooth off. Fortunately it did not cause any damage and was set right, but you might want to check for chain stretch.

On my 98 SL500, the head gasket let go at 77,000 miles and gave me a rough idle.

@alabbasi -
1. I have replaced that rubber hose
2. See images and info regarding my timing check below

Good point about checking the cam timing. The timing chain rarely stretches enough to require replacement, but broken chain rails are VERY common, and will cause the cam timing to be off significantly. Need to check all 4 camshafts, pin them 1 at a time and read the number off the balancer. 2-4° off is normal-ish, beyond that some rails may be broken.

@gsxr & @alabbasi
Using this factory service manual document as a guide, I pulled off the valve covers and checked the timing.
With the help of my special tool (a 7/32 drill bit ) you can see from the images below, with the crankshaft pulley set to the 45 degree mark the holes in the cam sprocket lined up.
Also note the exhaust cam lobe was pointed straight up as shown in the diagram from the FSM link above.



Driver's side exhaust cam


Driver's side intake cam



Passenger's side intake cam


Passenger's side exhaust cam

Need to check the rails anyway if this has never been done, if original, it's almost certain that multiple need replacement.

I failed to mention in the original post, I replaced the upper guides when I replaced the plastic oiler tubes with OEM metal tubes (shout out to @aldedmon)

---

I think it's safe to say the engine is in time. Is there any other data point I can check?

 

[gsxr]

If all 4 cams pinned with the crank at 45°, that is great, the chain and rails are all good. But yes, you are running out of items to check.

At this point I'm wondering about a defective new part (possibly a plug boot, or spark plug). This is a long shot though. Ideally, you'd like to pinpoint which cylinder(s) are misfiring and focus on those specifically. I can't recall if SDS / HHT-Win will show live data for individual cylinder voltage to identify a misfiring cylinder... or, if that data is available via OBD-2 scanners.

 

[Maxweisen]

@gsxr Is this the port you are referring to for diagnostics?

 

[alabbasi]

Given the all the parts you replaced, the smoke testing and the timing checks, it might be that you don't have a rough idle and might be imagining all of this.

Any cracks in the harmonic balancer? What kind of shape are the engine mounts in?

 

[Maxweisen]

Given the all the parts you replaced, the smoke testing and the timing checks, it might be that you don't have a rough idle and might be imagining all of this.

Any cracks in the harmonic balancer? What kind of shape are the engine mounts in?

I wish that were true, but the car has dipped into the ~400s RPM. You can see it on the tach (and OBD) and feel it in the cabin. It’s not right.

When I have the car buttoned up I will post a video.

Regarding the harmonic balancer it looked good while I was rotating the engine today checking timing.

Engine mounts are original and I do have a replacement set. Have not gotten around to replacing as it looks like quite a job.

 

[alabbasi]

I can't remember doing the mounts on an m119, I've done them on the M113 E55 that replaced the 97 E420 and it was not too bad on a lift.

 

[gsxr]

@gsxr Is this the port you are referring to for diagnostics?

Yes, the factory diagnostic tool connects to that 38-pin port.

 

[Maxweisen]

Alright guys. I got her put back together and fired up.


Above is a live graph of the RPM from the E420.

Some details about this graph:
- Engine is at operating temperature
- Beginning of the graph is with the car in PARK
- The downward slope is when I shift into DRIVE
- The dip to 403 RPM is the random stumble that happens periodically
- The tachometer needle is not steady and wiggles between 500 - 550 RPM

Another note that might be useful - the power steering pump is noisy and puts a strain on the RPM when turning the wheel while parked. Not sure if a bad power steering pump can cause issues at idle.

I have an appointment on Monday with a shop that has the diagnostic scanner. I love being able to DIY my way to a solution but I think I need some additional data.

 

[nocfn]

my spike in foolishness was one out of the box spark plug that was bad. Also, making sure that all of the additional slots were made to the Distributor caps, and the wd40 was liberally applied to the attachment points and the components for the ignition. After that, the car has run fine since.

 

[emerydc8]

If it’s dipping down like that when shifting to drive have you looked into replacing the neutral safety switch? This used to be SOP when having a problem like this on the 124.034/.036 until they became NLA. I don’t know if the NSS for yours is still available but if it hasn’t been replaced it might not be a bad idea to do so, especially since they will eventually become NLA too (if not already).

 

[gsxr]

Jon, he has a 1997 E420 with 722.6... those don't have a NSS. The NAG box sends gear position data directly to the TCU.

 

[emerydc8]

Okay, I give. That’s obviously a totally different animal than the E420 I drive.

 

[gerryvz]

Moving this thread to Off-Topic Discussions because:

1. It pertains to the W210 chassis, which is not a focus of this forum
2. It pertains to the .98x variant of the M119, which is not a focus of this forum
3. The 400E/E420 sub-forum is specific to the W124 chassis with the .97x (LH injection) variant of the M119