Smooth idle on cold start, then rough + surging (Video)

[SL500_WPB]

Hi all, let me start by saying that this is for a 1994 SL500. I've had better responses and engagement over here versus BenzWorld so hope this is okay.

Short Background: I had a coolant issue a while back that was determined to be due to a leaking heater core. After this was discovered by a local classic Mercedes shop, they recommended blocking off the coolant passages that lead to/from the heater core and so that's what we went ahead and did. Ever since picking up the car, it has been giving me this new rough/surging idle issue. At first, it was once every few minutes... When idling, the car would suddenly cut out and then the revs would surge up to keep it alive. This was back towards the end of last year and I haven't driven the car since.

Present: Now, I get about 1 minute of smooth idle on a cold start and then the car idles rough enough that it's basically undriveable. It also eventually dies after a few minutes. Rather than try to describe the exact symptoms, i've recorded the short video below:

Theory: Being a 1994 car, it suffers from the degrading wire insulation. I have a hunch that while messing around on the backside of the engine where the coolant passages that lead to the heater core are , they probably nudged or bumped into the throttle body cabling and now some wires are touching/interfering with each other. I would think that since I have clean (enough) idle for at least the first minute, there's probably nothing wrong with spark plugs or the associated wiring. Additionally, upper wiring harness has been replaced.

Attempted Solutions:

1. Rotors, Caps, Insulator Shields. There was no history on when these were replaced so I went ahead and took care of this today. None of the original components were all that bad in my opinion (see pics). Unfortunately, no change. Maybe the idle stays smoother for a bit longer, but it still eventually gets bad.
2. Vacuum Lines. Admittedly, I don't quite know the best way to test the vacuum system but on a visual look, all seems to be good.

Has anyone here experienced symptoms similar to those in the video? What would be the best next step? ETA?

 

[gsxr]

Could be a vacuum leak, perform a smoke test on the intake to check for any major leaks. I don't think this would cause what is seen in your video, but it should be checked anyway. Most of the vac tubing was TOAST ten years ago or more, and all needs replacement if original. Vac pipe is cheap, but installing is tedious. The one piece in your photo is visibly old.

Your old distributor caps looked fine, btw. Caps/rotors/etc cause misfiring, and that doesn't appear to be what is happening in your video. If you get misfiring after 10-15 minutes, add vent slots in the new caps per the other forum thread on this topic.

Your ETA is original, March 1994 date code. If it has never been rebuilt, it needs to go to Don Roden at RFC Electronics, ASAP. The ETA is your throttle-by-wire and also is what maintains idle speed control. This alone could be the cause of your idle woes. Replace the three PCV hoses to the passenger valve cover if R&R'ing the ETA. They should be soft rubber, not feel like plastic. Ditto for the smaller ones to the driver valve cover.

The upper engine harness shouldn't cause the problem in your video, and since it's been replaced, there's nothing to look at there.

Pull fault codes next, specifically from pin #7, the E-GAS module (throttle body control module).

 

[emerydc8]

What happens at idle if you brake with your left foot and apply a tiny amount of pressure on the gas pedal with your right? Will it stabilize? Does it pulsate like this in park, drive, neutral, or reverse? Or does the gear position make no difference at all?

 

[SL500_WPB]

Could be a vacuum leak, perform a smoke test on the intake to check for any major leaks. I dont think this would cause what is seen in your video, but it should be checked anyway. Most of the vac tubing was TOAST ten years ago or more, and all needs replacement if original. Vac pipe is cheap, but installing is tedious. The one piece in your photo is visibly old.

Your old distributor caps looked fine, btw. Caps/rotors/etc cause misfiring, and that doesnt appear to be what is happening in your video. If you get misfiring after 10-15 minutes, add vent slots in the new caps per the other forum thread on this topic.

Your ETA is original, March 1994 date code. If it has never been rebuilt, it needs to go to Don Roden at RFC Electronics, ASAP. The ETA is your throttle-by-wire and also is what maintains idle speed control. This alone could be the cause of your idle woes. Replace the three PCV hoses to the passenger valve cover if R&Ring the ETA. They should be soft rubber, not feel like plastic. Ditto for the smaller ones to the driver valve cover.

The upper engine harness shouldnt cause the problem in your video, and since its been replaced, theres nothing to look at there.

Pull fault codes next, specifically from pin #7, the E-GAS module (throttle body control module).

Admittedly, I am quite the beginner mechanic. Prior to this car, I hadn't even performed an oil change lol. For someone without a smoke kit, is there a more DIY method? I have seen some videos suggest spraying Brake Cleaner along all the vacuum lines and connections and listening for changes to idle. Would that suffice?

I agree with your take on the distributor caps. This doesn't seem like random misfiring... it's more like random engine cutouts and then a surging reaction to keep the engine alive, with the cutouts getting progressively worse over the course of 2-3 minutes before the engine dies.

Understood that the ETA needs to be rebuilt. Just want to make sure I eliminate any cheaper possibilities before biting the bullet for an ETA rebuild.

Pulling codes, I got the following:

Socket 6 - Initially Codes 21(Pressure Switch Charge) and 30(CAN Data Line to Electronic accelerator/cruise control/idle speed control module). After clearing and letting the engine run, only Code 21 . This is probably the culprit for my ASR light that comes on at about the 3 minute mark, similar to Post #11 here by gerryvz. But i'll deal with that another time.

Socket 7 - Initially Codes 2 (Cruise Control/Idle Speed Control Module) and 6 (Backup Lamp Switch). After clearing and letting the engine run, only Code 6 .

What happens at idle if you brake with your left foot and apply a tiny amount of pressure on the gas pedal with your right? Will it stabilize? Does it pulsate like this in park, drive, neutral, or reverse? Or does the gear position make no difference at all?

The random engine cutouts/drop in revs are felt regardless of gear. Back when I drove this car last, I also recall feeling the cutouts regardless of speed.

Am I correct in saying that the 'surge' isn't a symptom? It's more just an engine reaction to the cutting out and almost stalling? The main symptom is the random cutouts that are absent at cold start and continue to get worse over the course of a few minutes?

In any case, here's another video from today of the random cutouts in revs:

 

[emerydc8]

Unless you have a spare mass air flow sensor to swap and compare,, or you can't find any vacuum leaks, I'm with @gsxr--Send your ETA to Don Roden for a rebuild. It sounds like it needs rebuilt anyway since it's the original.

 

[jftu105]

Read my thread on "Being rich is not good ....". I had the same symptom as you shown in the video. It turns out that the EGR valve was bad. Read the entire thread to see the steps I took to eventually get the problem solved.

jftu105

 

[gsxr]

Admittedly, I am quite the beginner mechanic. Prior to this car, I hadnt even performed an oil change lol. For someone without a smoke kit, is there a more DIY method? I have seen some videos suggest spraying Brake Cleaner along all the vacuum lines and connections and listening for changes to idle. Would that suffice?

Spraying solvent can find some leaks, but it's not as comprehensive as a proper smoke test. Doesn't hurt to try. You can buy a cheap smoke tester, or build one at home - Google for details. Again, I'd replace every piece of vacuum tube you can find with new, it will cost about $20 and is worth every penny if the old stuff snaps like twigs when you touch it.

Understood that the ETA needs to be rebuilt. Just want to make sure I eliminate any cheaper possibilities before biting the bullet for an ETA rebuild.

Keep in mind that the ETA is a throttle-by-wire setup and could, possibly, be the root cause of the RPM jumping and surging.

Pulling codes, I got the following:

Socket 6 - Initially Codes 21(Pressure Switch Charge) and 30(CAN Data Line to Electronic accelerator/cruise control/idle speed control module). After clearing and letting the engine run, only Code 21 . This is probably the culprit for my ASR light that comes on at about the 3 minute mark, similar to Post #11 here by gerryvz. But ill deal with that another time.

Yep - code #21 is a different issue, unrelated to the idle RPM. Code #30 is a generic fault that appears any time you get limp mode. Ignore code #30.

Socket 7 - Initially Codes 2 (Cruise Control/Idle Speed Control Module) and 6 (Backup Lamp Switch). After clearing and letting the engine run, only Code 6 .

Code #2 is related to limp mode, likely appears any time you encounter limp mode. #6 is not just a backup lamp switch / NSS, it's also the gear position indicator switch - this can cause intermittent limp mode. If the NSS is old or original, replacing it would be recommended, note it requires adjustment when installing.

The random engine cutouts/drop in revs are felt regardless of gear. Back when I drove this car last, I also recall feeling the cutouts regardless of speed.

Am I correct in saying that the surge isnt a symptom? Its more just an engine reaction to the cutting out and almost stalling? The main symptom is the random cutouts that are absent at cold start and continue to get worse over the course of a few minutes?

In any case, heres another video from today of the random cutouts in revs:

Your video is titled "rough idle" but I'm seeing a dead smooth idle, with significant RPM fluctuations. I'm very, very suspicious of your ETA. If there is no record of it being rebuilt, bite the bullet and send to Don Roden. Or he can send you a rebuilt one so you don't have any down time, and you return the old one as a core.


To test EGR, with the engine at operating temp and idling, apply vacuum to the EGR valve. The idle should immediately get ROUGH, like shaking the engine badly. Remove vacuum, idle should smooth out. If so, the EGR valve is likely OK.

 

[SL500_WPB]

Spraying solvent can find some leaks, but its not as comprehensive as a proper smoke test. Doesnt hurt to try. You can buy a cheap smoke tester, or build one at home - Google for details. Again, Id replace every piece of vacuum tube you can find with new, it will cost about $20 and is worth every penny if the old stuff snaps like twigs when you touch it.




Keep in mind that the ETA is a throttle-by-wire setup and could, possibly, be the root cause of the RPM jumping and surging.





Yep - code #21 is a different issue, unrelated to the idle RPM. Code #30 is a generic fault that appears any time you get limp mode. Ignore code #30.




Code #2 is related to limp mode, likely appears any time you encounter limp mode. #6 is not just a backup lamp switch / NSS, its also the gear position indicator switch - this can cause intermittent limp mode. If the NSS is old or original, replacing it would be recommended, note it requires adjustment when installing.





Your video is titled rough idle but Im seeing a dead smooth idle, with significant RPM fluctuations. Im very, very suspicious of your ETA. If there is no record of it being rebuilt, bite the bullet and send to Don Roden. Or he can send you a rebuilt one so you dont have any down time, and you return the old one as a core.


To test EGR, with the engine at operating temp and idling, apply vacuum to the EGR valve. The idle should immediately get ROUGH, like shaking the engine badly. Remove vacuum, idle should smooth out. If so, the EGR valve is likely OK.

Thanks all for the replies! Apologies for the delays, it has been a busy month.

What I am getting is that bottom-line, the ETA is long overdue for replacement and there is a strong possibility that it is the source of my problems. Additionally, none of the DTC codes are pointing to an alternate cause. I still believe that what could have happened is that the wiring loom (the long one that extends out of the throttle body) was disturbed while the mechanic was working on the backside of the engine to plug the heater core lines.

I'm going to go ahead and begin the removal process of the ETA this weekend, get it rebuilt by Don Roden, and then see if the problem continues. Additionally, i'll look into starting the replacement of any of the easier vacuum lines, as you suggest. Main priority at this point is just getting it back up and running and then I can take my time with all the preventive maintenance items.

Victor's 9 Part Series seems to be the best guide on ETA removal. So i'll follow that and also replace all that he recommends during this process.

As to testing the EGR Valve- i'm going to go ahead and rent a hand vacuum pump and see if the valve is holding a vacuum. I will also attempt to pull a vacuum with the engine running but it may be tough since the car stalls once it gets up to that operating temp.

 

[SL500_WPB]

Read my thread on Being rich is not good ..... I had the same symptom as you shown in the video. It turns out that the EGR valve was bad. Read the entire thread to see the steps I took to eventually get the problem solved.

jftu105

Spraying solvent can find some leaks, but its not as comprehensive as a proper smoke test. Doesnt hurt to try. You can buy a cheap smoke tester, or build one at home - Google for details. Again, Id replace every piece of vacuum tube you can find with new, it will cost about $20 and is worth every penny if the old stuff snaps like twigs when you touch it.


Your video is titled rough idle but Im seeing a dead smooth idle, with significant RPM fluctuations. Im very, very suspicious of your ETA. If there is no record of it being rebuilt, bite the bullet and send to Don Roden. Or he can send you a rebuilt one so you dont have any down time, and you return the old one as a core.


To test EGR, with the engine at operating temp and idling, apply vacuum to the EGR valve. The idle should immediately get ROUGH, like shaking the engine badly. Remove vacuum, idle should smooth out. If so, the EGR valve is likely OK.

Couple Weekend Updates:

EGR TESTING - EGR was tested in two manners. With the engine off, I applied a vacuum of about 20 inHg. Valve held steady at the vacuum and there was an audible *click* on releasing the vacuum. With the engine running and close to operating temp, applied the same vacuum. I am fairly certain there was an immediate rough idle when applying the vacuum, which went away went removing vacuum. It is hard to say for 100% certain as the idle gets rough on it's own without me doing anything. EGR tests as good.

VACUUM LEAKS- Although not the ideal test, sprayed brake cleaner around visible vacuum lines. No notable change to idle.

Additionally, I recorded today the first start in about a month. As stated by GSXR, idle is dead smooth with significant fluctuations. (Video shows the dead smooth part on cold start + reversing car into garage + putting it back in park).

 

[SL500_WPB]

Good news/Bad news:

Just as I was getting ready to remove the ETA, I figured that I might try investigating one more thing- the Mass Air Flow Sensor. I'm going to provide a video showing the engine performance with the MAF disconnected (4 minute video) and then immediately following that, a short clip of engine performance with the MAF plugged back in.

UNPLUGGED (No Fluctuations):

PLUGGED IN (Immediate Fluctuations):

My only question now is- does this strongly hit at the MAF being the problem or is it possible that the engine would idle smooth even if the ETA was still the real problem?

I wanted to take the car for a quick spin to see how long I could keep the smooth idle going but I was hit with another problem. The last time I drove this car, I had a shop (that supposedly specializes in older mercs) plug the heater core lines because they diagnosed that my coolant leak at the time was from the heater core. Now, I never got to actually drive the car following that because of this idle issue that popped up the day I got it back.

Now that i've been able to have it run for 4 straight minutes, I see that coolant is flowing directly out of the open line that supposedly comes from the heater core (see pics and video below). Engine side, one line was capped on the passenger side and one line capped on the driver side. There HAS to be another coolant line directing coolant to the firewall side because by the time the 4 minutes of running was up, my coolant was low in the tank. What a nightmare.

So 2 questions now:

1- Would a new MAF potentially fix the issue? There's a local Pull & Pay here in West Palm Beach with a few SLs that I could snag a MAF from to test this out.

2- How bad was this 'fix' performed by the shop? Is there simply an additional line going to the heater core from the engine that I need to plug? Or should I tow it back to get them to fix their mistake?

 

[gsxr]

Oversimplifying, if the engine runs better with the MAF disconnected, it's a good possibility the MAF is defective. If you can get one or more inexpensive MAF's to experiment, that would be fantastic. Note it MUST be from another M119 with LH-SFI, meaning if it's an R129, it has to be 1993-1995 500SL/SL500 only. Not 1990-1992, not 1996-up.

If there is a leak in the heater core, the heater core hoses at BOTH sides of the engine must be disconnected / blocked. Can't tell from your video if they did this, so check the other side too. I'm not sure if this will cause any other problems with the cooling system.