Mercedes-Benz Forum banner

s500 engine misfire when warm

18K views 97 replies 12 participants last post by  Yamae 
#1 ·
Hi All,

When I bought my car I knew it had a misfire that had the previous owner stumped. So much so that he gave up on it and parked it up for two years which is where I come in....

The symptoms are;

When the car is cold it starts up fine, idles smoothly and revs up as it should.
If I let it run up to temperature it then starts to have a slight miss at idle, only really noticeable by blip in the exhaust note. If you stamp on the throttle it hiccups before revving up but it will then rev cleanly. I haven't taken it out on the road yet but I've been told it will then die under load. The car will start easily when hot.

The previous owner had several 'specialists' look at it and spent lots of his money and changed the MAF, leads, dizzy caps and rotors.

So I did some research and thought it was the ignition module (EZT) so I found another one and swapped it out today but the car was exactly the same... so it's not that.

I don't think it is the fuel supply as that's not really affected by heat so it must be either a sensor or perhaps some wiring that is overheating and being affected by resistance?

Any ideas?

Cheers
 
See less See more
#2 ·
What year is the car? If it's the 1994 Coupe that you have in your signature, check your distributor caps and rotors. I know you have written that the "specialists" changed them, but check them anyway.

My car does the same thing and I've narrowed it down to those items. Haven't changed them yet because I'm wanting to change the camshaft oil seals, which I'm wanting to do right.
 
#3 ·
Hi,

it is a December 1993 coupe.

I will check them but not sure what I will be checking other than clean contacts? A bad cylinder wouldn't make the engine stop which is what makes me think it is an electronic signal.
 
#8 ·
I think I've found the problem....the engine wiring loom is shot. Whilst some of the wiring has been replaced they obviously didn't do the engine one and it still has a 1993 date sticker.

Here is a picture of one of the fuel injector wires !

 
#14 ·
The car isn't being used at the moment as I'm still getting the issues sorted from it being left standing outside for a couple of years.

In the UK a car needs an MOT certificate, an annual road worthiness test before it can be used on the road so I have to make sure all the electrics work and it's mechanically sound.

I'm not in an particular rush to get it back out on the road especially with it being winter so I'm happy to take my time to get everything fixed properly. :smile
 
  • Like
Reactions: Jayare
#15 ·
Ok so I've replaced the loom with a good 2009 manufactured item which cost me £140 from the US.
All fitted but still have exactly the same problem! I can't believe thats not fixed it as the old loom was in a terrible state and would have been shorting all over the place.

So when the engine is warm at idle it hunts occassionally. If I blip the throttle it stutters briefly before revving up. If I apply the throttle smootly it revs up cleanly...

I'll check the dizzy caps for moisture but does anyone have any other ideas?

Cheers
Richard
 
#16 ·
Check all earth points you can find ,remove and polish them up regrease and fit back in place. Al the fuses in the box want removing one at a time .Lookin for any that have corrosion on them .. With bad wires under the bonnet ,or hood for our fellow members it would have blown one or even 2 of the fuses you never know. Even relays /Good luck
 
#19 ·
it's not really a misfire....

OK, so I've now had the chance to drive the car on the road so I now understand a bit more about the issues it has.

So it's not really a misfire, more a reluctance to accelerate.

It does seem to be a heat related as it get's worse the more you drive it. The problem is this, when you put your foot down it coughs a bit and almost dies before picking up. If I am gentle on the throttle I can tease the revs up but then around 4000rpm it's stuttering doesn't want to go much past this.

It feels like fuel starvation. I don't think it is the fuel pump as I guess this would be an issue from cold too. I'm not sure it is an issue with the fuses as this part of the engine bay isn't getting that warm.

Is there a fuel pressure regulator somewhere?
 
#21 ·
Thanks.

Is this something that can cause problems ? I'm fishing in the dark at the moment as I'm not sure what it is that is causing the fault. It's going to be something simple I'm sure....hopefully :eek
 
#22 ·
Is this something that can cause problems ? I'm fishing in the dark at the moment as I'm not sure what it is that is causing the fault. It's going to be something simple I'm sure....hopefully
Imagine dark place with sign over the entrance in shape of mercedes star, and the sign says: Welcome to hell of rough idle. :)

...and there is me, in one darkest (even darker than total dark) corner of this place, who is waiting for newcomers ... and you just came in ... and I ask you with rough (as my idle) and sad voice: "hey buddy, tell me the secret ..."

:) Ok, that was for fun, I hope you understand the message :). If I will live 90 or even more years, it will be only to maybe hear how to solve rough idle in w140. As for your question, some will say it is possible, some will say it is not. Check it. Many times the vacuum tube from fuel pressure regulator to intake manifold is broken or regulator itself has a broken diaphragm which is evident through fuel (smell) in vacuum line (rubber tube) which i mentioned first.
 
#23 ·
Imagine dark place with sign over the entrance in shape of mercedes star, and the sign says: Welcome to hell of rough idle. :)

...and there is me, in one darkest (even darker than total dark) corner of this place, who is waiting for newcomers ... and you just came in ... and I ask you with rough (as my idle) and sad voice: "hey buddy, tell me the secret ..."
You must be an actor in your spare time :smile

I haven't had my S500 for long but it is proving to be hard work, problems that should be easy to fix and are in 'normal' cars aren't.... the thing is, when you finally work out what the problem was it seems so obvious...
 
#24 ·
i learned this from real actors :).
i agree, these cars are cool but hard to diagnose. Mine is 1992 so i do not know how it is owning a car with OBDII, but OBDI is almost useless. And I can tell you that guessing which part is maybe bad is not a good approach. Too much money through the window, too much time spent for nothing. i wish I had an answer for you ... but the answer my friend is blowing in the wind like Bob says... :)
 
#25 ·
Also a problem when it's cold...

I've spent some more time on the car today.

I took it out for a run from cold and the problem is there, it stutters a bit when you put your foot down and will only rev up to 3-4000 rpm. So it's nothing to do with the engine being hot....

I checked all the vacuum pipes and they all seemed ok other than the one from a round sender on the front left of the engine which goes to a couple of switches? behine the left hand side headlight. This pipe didn't seem to have any vacuum. Does any know what this is/does ?







I also checked the wiring and cleaned all the fuses and I didn't see anything out the ordinary.

I guess it could also be the fuel pump so I really need to check the fuel pressure, do you measure it off the connector on the front of the fuel rail ?

I had a bang on the exhaust cats but they seem solid with no rattles - it still could be blocked cats though so if the fuel system is ok I'll drop the exhaust system off it.

What fun ! :eek

Cheers
 
#26 · (Edited)
in the first picture there is EGR valve exactly in the middle of it. the vacuum line which comes into it must not have any vacuum at idle ! in the second picture there is upshift vacuum delay element. it controls automatic transmission.

i can guess that EGR might cause some problems to you. there is a valve inside which should be closed at idle and open at higher revs (do not know exactly when it opens but i assume at slightly more gas than at idle). you can take EGR valve off of the car and clean it with brake cleaner. Check its diaphragm with sucking air with your mouth for instance. I do not think the another valve from second picture is related to your troubles but you never know ...

I read that only proper way to check fuel pump is to remove its output line and measure quantity of fuel which comes out in certain time. i can check for correct data. Another thing is measuring fuel pressure at fuel rail like you indicated but this is more or less just to diagnose rough idle of the car which is not your case. Namely, fuel pressure is not strictly related to fuel volume flow (hydrodynamics laws). pressure can be high, fuel flow can be low and consequently you have troubles especially at high revs.

measuring vacuum at intake manifold can maybe tell you something about your exhaust system. if not, then measuring pressure before cats (where o2 sensor is) can tell you condition of exhaust for sure.
 
#27 ·
Thank you for your reply. I will take a look at the EGR. And that's a good tip on the exhaust, I will see if I can work out how to measure the exhaust pressure which will be easier than removing the system.

One thing I did notice was a slight smell of fuel when I tested the Fuel Pressure Regulator vacuum, reading up on it it says this can be a sign of a damaged diaphragm.
 
#28 ·
if you remove the vacuum line from egr valve (and put on another line) and put it in your mouth then you can make a test: with engine idling you suck the air and create vacuum. when so, the idle must become rough. this is sign that diaphragm is good and most probably the valve functions well. but i would check it. it has three screws only and you can use old gasket if not particularly unlucky.

check google to see what vacuum values in the intake manifold can tell you about your engine and exhaust. i bought vacuum gauge for cars for 15 eur on amazon uk and instructions are enclosed. you can do a lot of diagnosis for small money. For instance:

https://www.amazon.co.uk/Draper-590...d=1484633381&sr=8-3&keywords=vacuum+gauge+car

it is hard for me to say anything about fuel smell and fuel regulator. it is engine area and fuel smell is pretty normal for it if not drastic. ckeck the inside of teh vacuum tube. if it is wet after couple of minutes of idling, then you have got it.
 
#29 ·
I've been doing a bit more reading up on this and there can also be an issue with the throttle body wiring so I've just checked it and sure enough it's knackered



So now I need to find myself a good non-ASR throttle body! :frown

I'll replace that before delving into the fuel delivery.
 
#30 ·
yes, so you still have original wire harness. try to remove wires only. it is ca 15 EUR part for my car for instance. then open the throttle body and check/repair wires. namely if you will buy used throttle body then you must be very lucky to get a good one.
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top