Poor running at low RPM

[Big E]

Been having an issue w/ my 400E that has otherwise been a spectacular car over the last 4 years (and 30k!) of ownership. Now sitting just shy of 220k, so it's a driver.

A few months ago, took it to the store, all was good. Came back out, started it up, and immediately noticed it was not idling right. Got it home but it had no power. Like, none. Needed to floor it to get up my street, which is indeed a hill, but this was a bit ridiculous. Did a little troubleshooting a few days later, which included running it again, but didn't find anything immediately. Another couple of days and then it wouldn't start at all. Fuel pressure checked out ok, and fuel filter was changed like 2 years ago.

Since I've had the car, I've been gradually working through maintenance items as time / money / etc allow. When it stopped starting altogether, I traced that to a non-firing coil, so I went ahead with a big overhaul of the ignition system. In went new plugs, wires, and coils. Caps, rotors and the infamous dust shield behind them went in about a year ago. I also decided to poke the beast and examine the upper harness, and it was indeed original. I really can't believe the thing ran at all given its state, let alone 30k near-flawless miles in the last 4 years. So, new upper harness went in. With all this replaced, it started right up, but still wasn't running right. It was all needed anyway, so I'm fine with that.

Given the state of the harness, I then confirmed the ETA was also original, so off that went to Don Roden for a rebuild. Before I removed that, I did a smoke test but didn't find any obvious vac leaks. Nevertheless I replaced all of the rubber vent lines on the top of the engine (which were of course hard as a rock). I also realized the throttle cable was held on by 2 zip ties, and had been for at least 4 years (!), so in went a new throttle cable. Removing the ETA resulted in a few more broken vac lines, so I replaced them as well.

Now, with all of that done, it's still not right. Better, but not right. It starts up, maybe a little less snappy than it should, but still starts every time. However, in P or N, it idles around 1500 rpm. I need to adjust the throttle Bowden cable a bit (was too tight, now too slack), but that hasn't changed the idle much, if at all. Sitting in D, it will idle closer to 1000-1100, but not very smooth and with a bit of hunting.

Once driving, below about 1500rpm it is a dog. No power at all, totally bogged down. But get it above about 2000rpm – shifting, waiting for it to finally get there, whatever – and it reverts to its glorious M119 self and feels like new, pulling hard the whole way until you're well into extra-legal speeds.

So I'm running out of ideas of what to investigate. Ideas?

Full disclosure, in the maybe 5 miles I've driven it for testing, I've been running with out the air filters and associated ducting, purely so I don't have to take it all apart over and over. I can't imagine that would cause any drivability issues, but maybe I'm wrong?

 

[gerryvz]

Realistically, the air filters and two zoom tubes shouldn't affect too much in terms of running smoothness and such, but are you running it with the air-box installed (even without filters)? If not, I would definitely install the airbox, and preferably just put the filters in and run it as it should be run. It's not good to run an engine without air filters. The airbox should have a calming and evening effect for the incoming airflow, as well. I would also install the zoom tubes, because your intake air temp sensor is on the driver's side behind the headlight, and relies on the incoming air going from the headlight area into that zoom tube to go into the airbox.

It really doesn't take but more than 30 seconds to remove the airbox and zoom tubes, and then replace them again.

Adjusting the throttle cable is the very first thing you should do.

The second thing you should do is to pull the codes, delete them, drive the car a good 50 miles, and see what if anything comes back.

Third, if it's idling fast, I would definitely give it a smoke test. Have a shop with a proper smoke machine do it. Sounds like you have done everything else that would alleviate running issues. A fast idle means that unmetered air is getting into the intake system. Is the MAF rubber boot atop the ETA in good shape?

I'm also wondering if you (again) have a bad coil or EZL.

Have you checked (or had a shop check) the operation of the MAF?

 

[gsxr]

Questions:

1) When you said "without air filters", do you mean with the entire airbox removed (bare MAF), or no filters installed in the airbox? There is a huge difference between these two scenarios. Remember the airbox pops off completely intact, in 5 seconds. If you have been unclipping the rectangles at each side, you don't need to do that.

2) DEFINITELY need codes. Ignore whatever code diarrhea you first read (seriously, don't even bother writing them down). With all codes cleared, drive the car for 10-20 mins if possible. Then pull codes again. Report which codes have returned.

3) Any CEL? If you have a 1992 model it may not have a CEL.

4) Have the fuel pumps ever been replaced, or are they original / 25+ years old?


I doubt it's a coil as that may affect high RPM power, but it's a possibility. MAF can be ruled out by test driving with MAF disconnected. Fuel pressure has to be checked under load, but if you get full power at high RPM (not intermittent), the pumps may be ok.

 

[Big E]

Huh. All the stuff I've done on that car, and I never even noticed that you can pop the entire airbox off at once. How have I not noticed that! Yeah, I should go ahead and put those back on then.

I will have to do the codes, not sure how that escaped my mind. I made a DIY code reader when all this started and got nothing out of it, so I guess I sorta forgot about it. Or, I made it wrong. Could be that too!

I have not done the pumps in my 4 years, so there's probably a good chance they're original. But once above 2000rpm, it does not hiccup at all with the pedal buried to the floor. So that seems unlikely.

I'll do the smoke test again as well. My little homemade smoke machine pumps out plenty of smoke, so that should catch that. There's just not much left that I've not touched. Basically anything rubber - including the boot for the ETA - is new.

Thanks for the ideas.

 

[Kyiv]

I never even noticed that you can pop the entire airbox off at once

Airbox has two anchoring points:


These anchoring points slide onto two anchors on each side of the engine. It's likely one or both of yours might be missing of they have never been replaced. Sliding the airbox on or off these anchors is a 30 second affair. No need to remove the filters!

(Passenger side)

 

[gsxr]

The engine gets VERY unhappy with the airbox off, as Gerry described above. Definitely re-install that and see if there is any difference.

An intermittent fuel pump can cause a major loss of power, with no misfiring. But if your problem is entirely RPM based and not intermittent, I agree, it's unlikely to be fuel delivery (probably not ignition either).

 

[Big E]

I'm really gonna feel like an idiot if it's because of the airbox!

 

[Big E]

Well between the weather, my schedule, and life in general, progress on this had slowed to nothing until a few weeks ago. With the roads finally cleared of salt, the rebuilt ETA installed, a couple of cracking vacuum elbows replaced, and the airbag properly affixed (how did I not noticed how easy that is!!??), things are better but not correct.

The summary is, it starts right up, idles at around 1200 in P/N when cold, drives perfectly through warmup and for the first 10-20 min once the temp gauge is sitting at ~80C. Then, upon giving it some gas (say, 1/2 throttle) from about 1700 rpm, it bogs down, badly. RPMs will go down to maybe 1200. Let up on the gas to 1/4 throttle, and the revs will rise back up and if you keep it there it will get eventually get above 2000rpm and then you can give it more gas and it's good. I'm honestly starting to think it's the transmission shifting an extra gear? I dunno. Seems odd. And when I then put it into P/N, after it's misbehaved, it idles at like 1800.

Anyway, I apparently screwed up my DIY code reader, so I got fed up and bought the one from mercedescodereader.com, which worked perfectly btw. I disconnected the battery overnight, then reconnected it, started it up, drove it for about 25 min, and then shut it down and pulled the codes.

Here's what I got:



So none of those seem like a smoking gun exactly, but I'm not yet well versed in all their meanings. I'm also wondering if I should clear everything first (rather than disconnect battery), then drive 20 min again, then pull codes?

Thoughts?

 

[gsxr]

What you have there is "code diarrhea". Clear everything, drive the car, and check again. Codes that return are the ones to focus on. Disconnecting the battery DOES NOT clear codes, or do much of anything really. Make sure you are using the code translation PDF file from W124performance or in the De-Coding section of this forum; most vendors include a generic file that isn't accurate.

If the ETA is rebuilt by Don/RFC, and you still have idle issues, triple-check the throttle cable adjustment. If it's perfect, next is swapping the T/LLR module, assuming your 400E does not have ASR.

Lack of power is likely a separate issue from the high idle. Does it behave the same at all RPM's / speeds? For example, can you limp it up to 55mph and when you try to accelerate, it also bogs, with a smooth lack of power (no misfiring)? Or does it have full power once above 2000rpm, same as before?

You're nearing the point of needing live data from a digital scanner.

 

[Big E]

Ok, got it, clear codes first. Will do.

I'll also recheck the throttle cable. ETA was rebuilt by Don, and I replaced the throttle cable (was held on by zip ties!), and I thought I got it dialed in, but maybe not.

As far as I can remember, once it's above about 2000 rpm it always behaves properly. I have a small loop around my house that I drive that lets me get it up to 65+, but I basically just make laps since I don't want to stray too far from home with it atm.

Thanks.

 

[Big E]

I was able to clear all the codes except for pin 16, the air conditioning module. For some reason, it would never clear, just kept repeating all 30 blinks (of course it was a 30-blink code, not a 3-blink-er!).

Anyway, got that done, went out and drove it for about 20 minutes, as usual about 15 min in it started exhibiting its now-usual behavior. Checked the codes again, and the only module that had anything set was pin 7, the cruise / idle speed control module. It gave me 6 blinks, which according to the docs is "starter lock-out / backup lamp switch". Which seems distinctly unhelpful in resolving whatever it is that's going on. Every other pin returned a single blink (except for pin 16, AC, which is still on 30 blinks).

So, I'm open to suggestions at this point, including either finding a relatively-local shop (or someone - I'm near DC) that can read the live data, or acquiring such a setup, depending on price.

Anyone?

 

[gsxr]

Ignore the HVAC codes on pin 16, they won't affect engine operation.

Does the car have ASR? Either way, the "starter lock-out / backup lamp switch" is also the gear position switch, sending a signal to the throttle control computer. Common failure, but causes limp mode on cars with ASR. I'm not sure what it will do on a non-ASR model. About $100 for a new one and it must be adjusted correctly during installation. More info here, although that post is for a 6-cyl car, most of the same info applies to the 400E.

 

[Big E]

Hmm. No ASR, and no obvious signs like Gerry described in that linked post. I could certainly replace that switch, but seems wildly unrelated to my current issues (but then, what do I know!).

May have to make a few phone calls to find a shop that will still work on these cars, or at least has a compatible scanner.

 

[gsxr]

The NSS is likely the cause of code #6 on pin #7 (T/LLR). But it is probably NOT the cause of your engine running issues.

Assuming the primary issue is still poor throttle response and low power below ~2000rpm, and it pulls hard to redline above that point, something else is going on.

Re-reading, you never reported if the fuel pumps are original or not... the only valid test is measuring rail pressure under load, or confirming pressure is normal at 40-50psi at the rail when the lack of power occurs. This is a long shot, but if those pumps are 30 years old, they should be replaced proactively anyway.

 

[Seangbadee]

The NSS is likely the cause of code #6 on pin #7 (T/LLR). But it is probably NOT the cause of your engine running issues.

Assuming the primary issue is still poor throttle response and low power below ~2000rpm, and it pulls hard to redline above that point, something else is going on.

Re-reading, you never reported if the fuel pumps are original or not... the only valid test is measuring rail pressure under load, or confirming pressure is normal at 40-50psi at the rail when the lack of power occurs. This is a long shot, but if those pumps are 30 years old, they should be replaced proactively anyway.

This may or may not be related, but I had a similar problem with my 91 300E. It would run well until it warmed up, then the throttle response would be poor until I released it. It would seem to pick up a bit, and the moment I depressed the gas pedal, the car would stutter. What was the issue? A blocked fuel filter! The dirt inside was unimaginable. Yes, the pump could also cause similar symptoms.

 

[Big E]

I have not hooked up my fuel pressure gauge to the engine and driven round with it - not 100% certain if I can rig it so I can see while driving - but you're right that would be a wise thing to check. I replaced the fuel filter about 16 months and 10k mi ago, so that should be ok, but I have never replaced the pump. I have no real records on the car prior to my ownership except the VMI, which seems weirdly disjointed - service notes from '93-97, then a bunch of stuff from '08-'10. No fuel pump anywhere in there, so if I had to put money on it, yeah, its probably original. And yeah, that would be wise to replace, regardless of whether that's the current issue.

 

[gsxr]

FYI, there are two fuel pumps on the 400E. Less than $100 each for new Bosch. Try to get the gas tank near empty before replacement, for safety...

 

[Big E]

I don't want to get too overconfident here, but a pair of new fuel pumps seems to have fixed the issue! When I pulled the cover there seemed to be an oily film on the lower pump, which I have no explanation for, but I'm pretty certain those pumps were original. Both said Made in Germany, maybe one of the numbers is a date code. I dunno.

The only issue was that on the new pumps - Bosch, same part number - the size of the fitting (the check valve actually) where the banjo fitting connects seems to have changed slightly, and with the new pumps and a single crush washer, the banjo fitting would not sit flush. I didn't have my mic convenient, but it seems the hole in the banjo fitting on the hard lines is actually slightly smaller on the outside, and it was hung up on that. It only needed, eh, call it 0.5mm, so a 2nd crush washer did the trick.

Two separate drives of my test loop, which had always uncovered the problem around the start of lap 2, showed no issues after 3 laps each time.

Now, about those rubber lines. I already ordered new rubber hangers for the whole assembly, but I guess I need to pony up for both lines that connect to pump 1. I can clearly see the 11/92 mfr date on the line to the tank

 

[Big E]

Well, it took way too long to get this sorted and then for me to post the update here, but my 400E is back to proper running. After much time trying to isolate the problem myself and replacing a lot of stuff - which, frankly, was needed anyway as preventative maintenance - I finally threw a Hail Mary and took it over to Mitch Carr in Kensington, MD, which a few friends had recommended. They had it sorted quickly and for a not horrendous cost, all things consider.

The 3 main issue were a badly leaking purge valve and a coolant temp sensor that was reading abnormally low as the engine warmed up. I had checked the CTS but I think I only did it with the engine off. And I was confident I had a vac leak but couldn't identify where it was coming from. A little puzzled I never noticed it at the purge valve.

Oh well, live and learn. Nice to have her back...