420sel Misfiring/Running Rough when cold

1979-1991 280SE, 300SE, 300SEL, 380SE, 380SEL, 380SEC, 420SE, 420SEL, 420SEC, 500SE, 500SEL, 500SEC, 560SEL, 560SEC
Post Reply
User avatar
pinky420
A Class
Posts: 10
Joined: Tue 27 Nov, 2007 7:30 pm
Model you own: w126
Location: Merton, Victoria

420sel Misfiring/Running Rough when cold

Post by pinky420 » Thu 24 Sep, 2009 2:41 pm

Well the heading says it all, but here are the symptoms:

When i start the car in the morning (or after not being started for several hours) it runs dead smooth for 10-20 sec, then it starts misfiring, occasional and not in any pattern. But when I put it in drive, with the brake on (so the engine has a bit of load) the misfires get more frequent.
When i first drive off, the misfires will be there, sometimes quite frequent, sometimes not so frequent, but enough that the car feels much slower to accelerate.
The missing happens throughout the whole rev range from 500rpm all the way up to 3000 (didnt wanna push it any further while it was doing this)
Stop at traffic lights, at idle, the misfire will happen maybe once every 2-5 seconds, but a couple of times it has felt like it missed on every revolution.

BUT

As soon as the temp gauge crosses 60, the prob goes away, totally. The car runs like a dream, dead smooth, perfect idle speed. Perfect everything.

So its gotta be a cold start/run control of some sort, but i cant work it out.

420SEL 1989 Aust delivered
329,000kms
serviced every 5,000
I use 95 or 98 fuel
Plugs are good,
Leads are good,
Strong spark,
Air filter is clean,

Little Help Please :D

420 SE
OZBENZ Admin
Posts: 2247
Joined: Tue 30 Aug, 2005 6:54 pm
Model you own: w126
Location: Gold Coast - Q, AU, & Doha, Qatar

Re: 420sel Misfiring/Running Rough when cold

Post by 420 SE » Thu 24 Sep, 2009 7:40 pm

Possibly cold start valve or idle speed air valve.
You do not have the required permissions to view the files attached to this post.
Ross
"I don't know, it's just some random dude..."

220 SEb - 1962 (Hilda - sadly gone to which there is no return)
420 SE - 1988 (Lucy) - back into service!
ML 500 - 2008 (the Doha desert Dame) - retired
ML 400 - 2015 (the new Doha desert Dame, or is that Princess?)
A couple of others not worth mentioning

Beware of fundamentalists
#73

The 126 - undoubtedly the last, best large body MB

User avatar
pinky420
A Class
Posts: 10
Joined: Tue 27 Nov, 2007 7:30 pm
Model you own: w126
Location: Merton, Victoria

Re: 420sel Misfiring/Running Rough when cold

Post by pinky420 » Thu 24 Sep, 2009 9:36 pm

thanx. how would i go about testing these? jus unplug em and see what happens or what?

User avatar
Simon Dove
170s
Posts: 175
Joined: Tue 20 Dec, 2005 10:49 pm
Model you own: w126
Location: Kensington Grove, QLD

Re: 420sel Misfiring/Running Rough when cold

Post by Simon Dove » Fri 25 Sep, 2009 3:17 pm

Move to Katherine, NT....it will take only seconds to get to 60degC :wink: :wink:

I would give the idle speed control valve a good clean (Y6), my 420 was having low idle, occasional miss issues a while ago and when i cleaned this valve a fair bit of crud came out and the the idle speed picked up a tad. I cleaned mine by soaking the mechanical end in kerosene overnight and using compressed air.

Also check the idle control unit under the front passemgers side carpet on the firewall, 20+ years of grunge can play havoc with electrics.

Good luck

Simon
71 W109 300SEL 3.5
78 W123 300D
88 W126 420SEL

User avatar
Hendrik
Mercedes Demigod
Posts: 5497
Joined: Thu 28 Jul, 2005 11:33 am
Model you own: w124
Location: Hawthorndene SA

Re: 420sel Misfiring/Running Rough when cold

Post by Hendrik » Sat 26 Sep, 2009 10:51 am

420 SE wrote:Possibly cold start valve or idle speed air valve.
The cold start valve only helps the motor start, has nothing to do with post start enrichment.
Might be a crook sensor telling the motor BS.
Not sure about the M117 but usually the idle speed valve is tested by pinching the hose to see if things change.
There are more scientific tests, such as measuring the voltage to it and removing it and applying 12v.

User avatar
pinky420
A Class
Posts: 10
Joined: Tue 27 Nov, 2007 7:30 pm
Model you own: w126
Location: Merton, Victoria

Re: 420sel Misfiring/Running Rough when cold

Post by pinky420 » Sat 26 Sep, 2009 11:24 am

I dont see how the idle speed valve would cause a misfire at and ABOVE idle, because the idle itself is spot on.

User avatar
Mercmad
Mercedes Demigod
Posts: 7986
Joined: Sun 11 Sep, 2005 9:01 pm
Model you own: w109
Location: Brissy
Contact:

Re: 420sel Misfiring/Running Rough when cold

Post by Mercmad » Sun 27 Sep, 2009 9:27 am

It's a typical problem with older M116 and M117 V8's. Start by replacing the plug leads .
The plug ends are the main cause of a misfire and because a set of plug leads,cap and rotor ( always replace the whole lot as a set) are expensive ,I find owners messing with totally unrelated stuff in an attempt to fix something by not spending money on the replacing the crook parts.
A check of the entire EGR system is also in order. Replace any worn ,seized or perished parts or eliminate the whole set up entirely .

User avatar
Hendrik
Mercedes Demigod
Posts: 5497
Joined: Thu 28 Jul, 2005 11:33 am
Model you own: w124
Location: Hawthorndene SA

Re: 420sel Misfiring/Running Rough when cold

Post by Hendrik » Sun 27 Sep, 2009 11:34 am

Mercmad wrote:It's a typical problem with older M116 and M117 V8's. Start by replacing the plug leads .
The plug ends are the main cause of a misfire and because a set of plug leads,cap and rotor ( always replace the whole lot as a set) are expensive ,I find owners messing with totally unrelated stuff in an attempt to fix something by not spending money on the replacing the crook parts.
A check of the entire EGR system is also in order. Replace any worn ,seized or perished parts or eliminate the whole set up entirely .
The thing is that the car works well above 60C, which leads me to think it is not ignition related. Does the ignition timing change with engine temperature?
We have to focus on the things that change between the cold, warm and hot engine engine running.
As I understand the system, more fuel is going into the motor when it is cold , or in other words the motor is running richer to compensate for the lack of heat.
Perhaps the problem is that there is no cold engine enrichment and thus it runs crappy when the engine temp is below 60. That's why I suggested testing the sensors that tell the system what the engine temperature is.
Might be that the ECU is dead and the engine is running in K jetronic mode, the OVP might be the culprit there.
This is basically a matter of testing the components that make up the enrichment circuit, starting with power to the ECU.
I am not familiar with the setup in the 126 chassis but from memory in other KE cars you pull the top off the ECU and check if there is 12v at terminal 1 of the ECU plug, this tells you that the OVP is feeding power to the ECU.
Alternatively you can use the diagnostic plug to check things but without documentation you are flying blind.
There however the problem is that the official MB guide to diagnose the engine management system works on the basis that you have access to the correct testing equipment.
I have a fair bit of literature relating to the cars i own or have owned but this does not include the V8's or 126 chassis.

User avatar
Mercmad
Mercedes Demigod
Posts: 7986
Joined: Sun 11 Sep, 2005 9:01 pm
Model you own: w109
Location: Brissy
Contact:

Re: 420sel Misfiring/Running Rough when cold

Post by Mercmad » Sun 27 Sep, 2009 12:03 pm

Trust me on this. What you describe is typical of these engines.
Always do the basics first before touching the Kjet. it's a very simple system that wil run OK for years.
In fact I would begin by removing the cam covers and checking the timing chain and rails
Then a comprehensive overhaul of the ignition .
The power terminal block by the battery will need to be cleaned ( I've never seen one yet that didn't) .
Then an examination of the EGR.
The EGR introduces exhaust gas into the intake manifold. A shitty idea which gums up the intake ports. On 560's it's not unusual to see them completely blocked.
Mercedes even have a tool and a set proceedure for cleaning the intake tracts without removing the manifold.
The Fuel tank system has a charcoal canister which is connected by thick black plastic tube to the intake. Any perished connectors here also cause misfires.
Next, get the engine idleing at the running condition where the problem is occuring and with the air cleaner off, spray engine start,propane from a MAPP gun or other flammable gas around the intake manifold. Any Change in engine behavior indicates a leak. This can either be the airflow meter cracked underneath or injector seals leaking ($10 for a set ) .
Your engine should have electronic idle control,not temp controlled idle like a 380.
Although there were some odd ones sold in OZ with the older style idle warm up valve ( tube like device clamped in rubber clamp with a hose on each end.
Any airleaks here will do the same thing,cuase an odd misfire at different engine temps.
Do all this FIRST before worrying about the Kjet.

User avatar
Hendrik
Mercedes Demigod
Posts: 5497
Joined: Thu 28 Jul, 2005 11:33 am
Model you own: w124
Location: Hawthorndene SA

Re: 420sel Misfiring/Running Rough when cold

Post by Hendrik » Sun 27 Sep, 2009 12:28 pm

Mercmad wrote:Trust me on this. What you describe is typical of these engines.
Always do the basics first before touching the Kjet. it's a very simple system that wil run OK for years.
In fact I would begin by removing the cam covers and checking the timing chain and rails
Ummnh, an 89 runs the KE jetronic and as far as I know has no EGR set up, well my 89 300TE doesn't.
The KE is not a simple system and uses a lot of sensors to adjust engine fuel delivery.
And how does valve timing affect the issue of poor cold engine running?

User avatar
pinky420
A Class
Posts: 10
Joined: Tue 27 Nov, 2007 7:30 pm
Model you own: w126
Location: Merton, Victoria

Re: 420sel Misfiring/Running Rough when cold

Post by pinky420 » Sun 27 Sep, 2009 7:33 pm

Drove the car yesterday morning, all good. no missing at all.
Drove it out to a 21st. still all good.
Drove it home. problem returned. It was freezing cold outside which may have played a part. Once the car warmed up though, it was fine.

I KNOW the ignition system is fine. and if leads were breaking down, or any of the other ignition components, it would occur when the engine bay is hotter, heat = greater electrical resistance.
Plugs are gapped perfectly, although the plugs did show signs of lean mixture when i pulled them straight after it started missing.
Fairly sure theres no EGR on mine. Well i cant find any EGR components.
And i'll check for intake leaks, even though a cracked hose or manifold leak would cause missing irrespective of engine temp, and it would cause lopes in idle, which isnt happening.
And who was worrying bout the kjet itself? i'm more inclined to think a sensor has died, because its behaving much like other cars where there was no cold enrichment.
Havent been able to have much of a go at it due to the crappy weather and no shed to work in, so its just playing in my mind

User avatar
pinky420
A Class
Posts: 10
Joined: Tue 27 Nov, 2007 7:30 pm
Model you own: w126
Location: Merton, Victoria

Re: 420sel Misfiring/Running Rough when cold

Post by pinky420 » Mon 14 Dec, 2009 9:15 pm

Still having this problem, cant seem to find the cause. i feel like i've checked everything.
i found that the cap and rotor were a little worse for wear, so replaced them, but that didnt get rid of the missing, which i reckon is getting worse.
I've check for vac leaks, chaecked condition of vac lines, i've tested sensors, checked voltage levels at ecu, idle computer....... what have i missed?

Bombardier
250s
Posts: 263
Joined: Thu 27 Sep, 2007 4:45 pm
Model you own: w115
Location: Rockhampton

Re: 420sel Misfiring/Running Rough when cold

Post by Bombardier » Mon 14 Dec, 2009 9:32 pm

I am reaching here but have you disconnected the earth leads and cleaned their respective contacts( chassis, engine etc)?

Colder weather will cause more resistance and may well be the issue.

User avatar
Ivanerrol
Mercedes Demigod
Posts: 5638
Joined: Sat 12 May, 2007 3:21 pm
Model you own: w204
Location: Melbourne

Re: 420sel Misfiring/Running Rough when cold

Post by Ivanerrol » Tue 15 Dec, 2009 11:32 am

When was the last tome you changed the fuel filters?
Have you checked for small fuel leaks at the fuel pump/filter/accumulator area?
W211 - E240
W204 - C280
W210 - E240
Departed
W202 - C200, C180, C180
W126 - 380SE , 380SE (Ex SA Import), 560SEL
W124 - 300e, 260e (ex Japan)
W111 220s (Indonesia) 4 speed manual column shift
W123 230
W116 450SEL
W140 420SEL
W210 E240
W209 CLK 240
W201 190e 2.6 (ex U.K.)

User avatar
pinky420
A Class
Posts: 10
Joined: Tue 27 Nov, 2007 7:30 pm
Model you own: w126
Location: Merton, Victoria

Re: 420sel Misfiring/Running Rough when cold

Post by pinky420 » Mon 22 Feb, 2010 10:10 pm

I'm still having this problem..... i'm considering pushing the car off a cliff.

I replaced the fuel filter, which seemed nearly blocked.
I checked the coil, the crank angle sensor, ignition timing, dwell angle, checked rate of advance, rechecked the leads,
Tested the ignition control unit, checked all the other control units.

What more can i do? I'm basically broke now, yet i've achieved nothing.

harley
S Class
Posts: 88
Joined: Wed 11 Nov, 2009 11:45 am
Model you own: w126
Location: Sydney

Re: 420sel Misfiring/Running Rough when cold

Post by harley » Fri 18 Jun, 2010 4:19 pm

Leave a can of propane or butane in the boot and when the problem starts up again, quickly spray a little at a time into the air intake and see whether the engine likes it or not. At least you will confirm if it is a fuel problem, and if so, if it is too rich or too lean. Put a length of vacuum hose on the end of the can nozzle to make it easier to administer. This setup will also let you find any vacuum leaks, it worked great for me. There are various switch-over valves that operate at 50-70 degrees so this would happen around the same time as you are describing the problem. You really don't want to go replacing parts until you know the cause, you can spend a lot of cashola on nothing that way.

10-20 seconds is the length of time that the cold start valve will open for so that might be significant to the 10-20 seconds after starting that the car goes well. I had just K-Jet, not KE, so I don't know how the ECU works exactly, but I know it uses a frequency valve to bleed fuel pressure instead of a warm-up regulator, and these can go bad, but it seems like it would be a constant if it went out.

What is the economy/vac gauge saying at the following times:

-cold startup
-cold warmup (with problems) in gear
-cold warmup in N/P
-idle warmed up in gear
-idle warmed up in N/P

Off the top of my head this is what I could think of that might cause what you were talking about
-incorrectly routed EGR - your car for sure came with EGR, it is on the right hand side exhaust if you're looking at the engine. The vac hose should match the pdf routing shown previously
-bad idle control valve
-bad sensor (has trouble transitioning between cold and hot)
-ECU problems

You can clean your idle valve by taking it out and using a degreaser to clean inside the tube area, don't immerse the motor in water through. I used an ultrasonic cleaner and came out like new.

If you want to really test your leads, I thought mine looked okay, but really weren't. Spray a little water from a spray bottle on them at night and look at the fireworks it they aren't any good. My new wires do not react at all, and feel much more flexible.

Try the gas thing and see what happens, I think that will be the biggest indicator.
90 420 SEL
85 500SE
94 Supra Aero-Top

User avatar
monobenz
E Class
Posts: 60
Joined: Tue 16 Feb, 2010 6:31 pm
Model you own: w126
Location: rockingham perth wa

Re: 420sel Misfiring/Running Rough when cold

Post by monobenz » Wed 23 Jun, 2010 10:25 pm

i feel for you my car i have replaced heaps and still it misfires,try replacing the filter at the engine end ,crack all the fuel lines
around engine looking for crap(while running and cold)but thinking about it it could be a injector seal or vac leak somewhere
were there is metal that could exspand and seal a vac leak eg inlet man ,hope that helps good luck. :roll:
"I seen what i wanted,threw a tantrum,and got it......."

1986 300se silver (Bugsy) Daily drive.

Brendon Monaghan

BensBenz
Dernburg Wagon
Posts: 5
Joined: Mon 27 Nov, 2017 9:28 pm
Model you own: w126
Location: Frankston Victoria

Re: 420sel Misfiring/Running Rough when cold

Post by BensBenz » Thu 30 Nov, 2017 8:47 am

Sorry to drag a post from the dark ages back to life but was this issue ever resolved? The description is 100% identical to mine :(

User avatar
TheMadRacoon
C 111
Posts: 807
Joined: Sun 25 Nov, 2012 11:34 am
Model you own: w107
Location: Melbourne

Re: 420sel Misfiring/Running Rough when cold

Post by TheMadRacoon » Thu 30 Nov, 2017 6:01 pm

A couple of observations from my d-jet M116 (350SLC). I haven't read the entire thread in a long time so I may be going off here, and this is a layman's perspective (apologies if already covered) but....

1. Misfire by definition indicates something may be wrong with timing. Of course rotor, dist cap, leads etc are part of timing because if degraded they stop spark of sufficient strength occuring at the right time. The other component in this is the timing chain as Mercmad mentioned. After timing chain related work (guides, tensioner) and no other change my car sounded better, performed better and stopped leaving a brown smokescreen when flooring it. I didn't have a misfire prior, but as noted timing is important.

2. This may not relate to your KE-Jet and your distributor: once, after the car came up to temperature it started misfiring and got worse. Car was cutting out and bucking. Lifted the distributor cap and there was a broken wire. Can't recall exactly but it was going from a pin (soldered) on the plate below the rotor and looped down outside and to below the plate. Anyway, soldered it back together and it ran perfectly. Prior, the ends were just touching, like a dry joint. Obviously vibration and heat would move the ends apart.

3. Anything temperature related can also be caused by expansion. So a split tube may close up as your engine warms up. A bad electrical joint may be better when engine bay is hotter.
Emad,
TheMadRacoon
1975 350SLC Astral Silver (725) / Blue (2012, 284,900 km - present.... a real Benz, getting better and better)
1988 190E Deep Blue (900) / Cream (2006, 190,000 km - 2007, ~ 215,000 km .... FSH and still spent big $$$)
1974 280E Reed Green (860) / Bone (1993, 316,700 km - 2004, ~490,000 km and still A1)

Post Reply

Return to “W126”