Idle surge turns into stumble, damn! - PeachParts Mercedes-Benz Forum

New2MB · #1 07-31-2007, 09:31 PM

I've got the classic W124 ('93 300TE) idle surge problem, had it for a few months now. Oscillates from about 500 to 1200 rpm almost always when idling but not as often when in drive.

This wasn't a big problem but now it has turned into what I consider to be a safety issue. 1 out of 10 times when I accelerate the car falls flat on it's face, just kind of chugs like it's not getting enough fuel and after maybe 2 seconds it jumps to life and takes off like nothing ever happened. Runs perfectly at all other times.

The only band aid I've found so far is to do somewhat of a brake torque, I'm not talking burning out, just enough gas to keep the rpm's up prior to accelerating.

I haven't been able to find any vacuum leaks, the car runs the same with the MAFS disconnected, OVP seems good.

Any ideas?

mbdoc · #2 08-01-2007, 08:31 AM

Most likely problem is the mass air flow sensor. It is responsible for 95% of acceleration enrichment. Of course the fuel pressure regulator is the other 5%

New2MB · #3 08-01-2007, 11:06 AM

Should the issue change or go away if I drive it with the MAFS disconnected?

I figure that I'll pull the battery cable first to reset the ECU.

jcyuhn · #4 08-01-2007, 01:38 PM

What you should do is build the Arthur Dalton code reader and pull the error codes. Then clear them all and see what comes back in a day or two. Post up the results.

I have a '93 124 with the 2.8 liter M104. Last year it had a nasty idle surge, stumbling on acceleration, and generally lousy running. After evaluating the codes I decided both the MAF and O2 sensor were shot.

Replacing the MAF completely cured the driveability issues, but the idle surge remained.

Replacing the O2 sensor corrected the idle surge. It is impressively smooth now.

If yours runs exactly the same with and without the MAF disconnected, that's a good sign it's shot and is being ignored by the ECU. But I wouldn't spend the couple hundred on a MAF without first building the $10 code reader to be certain.

- JimY

New2MB · #5 08-01-2007, 06:21 PM

Thanks, I thought I'd heard something about a homemade code reader. For anyone else reading this I found the diagram here and here.

If the ECU is ignoring the MAFS how can the MAFS be causing a problem?

Interesting that the surge and stumble developed only weeks apart, I hope it's the MAFS and O2 sensor, that'd be an easy fix.

I'd already bought intake gaskets anticipating a leak or at least some teardown for inspection.

New2MB · #6 08-05-2007, 09:44 AM

Well I pulled up the carpet to see my oxygen sensor connector and what do you know, it's covered in green corrosion. The connector area was so corroded that I broke one of the wires off and had to solder it back together.

After seeing this I thought for sure that a new oxygen sensor would have some effect on the idle surge and stumble problem. So I got to work and changed the sensor out but unfortunately when I got it back together I found that it runs exactly as it did before, nothing has changed at all.

I have yet to get back in there and see if I have continuity across the wire that I soldered so I guess there's still a slight chance.

Either way at least I found and fixed the corroded wire connector.

Next up is the MAFS I suppose, as long as I can get it at wholesale $. I'll probably make a code reader first though so I'm not replacing random parts all the time.

crhenkel · #7 08-06-2007, 01:15 AM

With the O2 sensor disconnected, the computer just went in to "o2 sensor not yet warmed up mode" like when you first start the car and the sensor is still not to operating temp. You might get beter gas milage now, because the computer was just using the default signal since the o2 sensor was never "warm enough to send a signal" I would definitley follow up on the trouble code reader

New2MB · #8 08-14-2007, 08:28 PM

I checked the connection to the oxygen sensor and it's good so next up is the code reader and maybe a MAFS if I can get a free one. But first it's time for a vacation.

ASaltyDog · #9 08-14-2007, 10:40 PM

Sometimes with older motors you've just got to clean out the entire throttle body. It soots up, gunks up, clogs PCV, breather tubes, gets all over temp/press sensors etc.

In my travels, most seemed to be clogged by combustion residue that didn't get pulled out of the car [normally] with regular oil changes. If dirty oil is left in the car for any period of time it will certainly give you trouble. Oil can only hold so much dirt and the excess finds its way to the intake system through ventilation design. Not to mention internal stuff like valve stems, seats etc.

True, newer oils can resist breakdown for 10k miles or so, but oil can still only hold so much dirt by volume. How many teardown mechanics out here can site sludge and dirt as a culprit of such a motor? I'd bet quite a few.

It's ALWAYS the first place I investigate on a 119

psfred · #10 08-14-2007, 11:42 PM

Check for a vacuum leak -- I'm not familar with the LH-Jet injection system, but likely you have a bad mass air flow sensor, it's dirty, or you have a vacuum leak. Throttle lag is quite typical of a major vaccum leak -- this is often the idle control valve on KD-Jet systems, but I dont' know what or where it is on an LH-Jet system.

Check the injectors for leaking seals by spraying carb or brake cleaner on them. If the idle changes while you spray, the seals are leaking, replace all of them.

Make sure all three wires on the O2 sensor are good -- one is the actual voltage signal (0-1V) and the other two are heater connections -- bad heater connections usually set a check engine light, but not always, and until the sensor is hot, it doesn't read right.

Peter

New2MB · #11 09-10-2007, 12:36 PM

you win this round of "guess that cause."

I swapped MAFS's with a '94 e320 and the problem instantly disappeared.

I noticed that in '94 MB changed the orientation of the MAFS such that it is 180 degrees off from the orientation in '93.

Why? Because the MAFS as designed in '93 was very close to the exhaust and the high heat would have an impact on the longevity of the delicate circutry of the MAFS.

So I changed the orientation of mine even though it's already toast.

Now I just need to get Bosch to give me a new one!

Thanks for all of the help everyone!

Falsapartenza85 · #12 09-10-2007, 09:25 PM

This surging idle problem on the M104's seams to come from a circuit of failing items all relating to the Fuel Injection. Just to save yourself the headache, replace the following, b/c even if it is just one item right now, the rest will fail shortly after:

Replace:
-both O2 sensors
-Mass Air Flow (MAF) meter
-CTS Coolant Temp Sensor

VOILA! the problems will go away for another 100,000mi

waynesb · #13 09-11-2007, 03:44 AM

This sounds similar to my issue. I'm a newbie having bought a '94 SL280 with the M104 engine last Sunday, (my first Merc), and walked headlong into a badly running car. I can't complain too much as I didn't pay too much for it, (NZ$12,500 about US$9k, it's pretty untidy) but it would have been nice to see it coming...

So after reading multiple threads during the week about badly running M104 engined cars decided I'd better take Arthur Dalton's advice and replace the suppressor between the coils and the spark plugs, and the plugs themselves, (I know, only F8DC4 plugs. I gapped them at 1mm. They were selling cheap because they're a deleted item so I bought 12! ;-). While doing that I noticed all the HT leads to the other plugs had white points where it looked like they were shorting so I bought a set of those too.

Anyway having replaced leads, suppressors and plugs, the car is running better but it still "coughs".

I can now get it to happen regularly. If I slow down to about 40km/h and then gently depress the accelerator, it'll hesitate and cough and then slowly pick up. If I bury my boot it'll only stumble a little, (hardly noticeable), and then GO like a cut cat! The cough sounds like it might be fuel going off in the exhaust but it's not REALLY loud, just quite noticeable.

Is there anything else I should try before replacing the MAS?

I haven't priced one yet from my friendly discount Mercedes parts guy, but I can feel the pain in my wallet all ready! Still if I'd seen all this coming I might have lost my metal and not bought the car and it's such a cool car! ;-)

New2MB · #14 09-12-2007, 10:34 PM

Quote:

Originally Posted by waynesb

This sounds similar to my issue. I'm a newbie having bought a '94 SL280 with the M104 engine last Sunday, (my first Merc), and walked headlong into a badly running car. I can't complain too much as I didn't pay too much for it, (NZ$12,500 about US$9k, it's pretty untidy) but it would have been nice to see it coming...

So after reading multiple threads during the week about badly running M104 engined cars decided I'd better take Arthur Dalton's advice and replace the suppressor between the coils and the spark plugs, and the plugs themselves, (I know, only F8DC4 plugs. I gapped them at 1mm. They were selling cheap because they're a deleted item so I bought 12! ;-). While doing that I noticed all the HT leads to the other plugs had white points where it looked like they were shorting so I bought a set of those too.

Anyway having replaced leads, suppressors and plugs, the car is running better but it still "coughs".

I can now get it to happen regularly. If I slow down to about 40km/h and then gently depress the accelerator, it'll hesitate and cough and then slowly pick up. If I bury my boot it'll only stumble a little, (hardly noticeable), and then GO like a cut cat! The cough sounds like it might be fuel going off in the exhaust but it's not REALLY loud, just quite noticeable.

Is there anything else I should try before replacing the MAS?

I haven't priced one yet from my friendly discount Mercedes parts guy, but I can feel the pain in my wallet all ready! Still if I'd seen all this coming I might have lost my metal and not bought the car and it's such a cool car! ;-)

From what I've heard the MAFS is going to run you about $350 so I think I would investigate further before replacing.

Is the idle consistent?

waynesb · #15 09-13-2007, 01:08 AM

It was but now it's going up and down, (yep a tell tale sign), but there are a couple of complications. I started a new thread on it with all the details as of last night.

http://www.peachparts.com/shopforum/showthread.php?t=199688