W124 300TE-24 Cold idle problems

[ms500]

Hi, I bought myself a 1991 300TE-24 Auto estate a few weeks ago. I'm very pleased with it, but there are a few issues that need looking at, one of which is the idle after a cold start.

When started from cold, it will start quickly but cannot idle on its own; I have to keep blipping the throttle or the revs fall and the car will stall itself, there is also a smell of petrol from the exhaust when it is started from cold. After a few minutes the car idles fine and the petrol smell goes away.

Also, when I try to start it warm, it takes a few seconds to start. The starter motor turns normally but it just takes longer to fire, though once started it idles smoothly and runs fine.

Can anyone help?
Thanks.

[Pentoman]

This may be the idle control valve or one of the many inputs that make it work.

You'll find, I'm 99% sure, that this car will run without the ECU plugged in, but it will be a bit like you describe - poor idling and flat when cold.

It has a basic diagnosis, checked by finding the round underbonnet diagnostic port and using a meter between holes 3 and ground. Does it have a catalyst/o2 sensor? (Does underbonnet electric switches say "KAT" on it). With this information you can do the diagnosis and it'll tell you if the electric functions are working.

Probably the most common reason for it is the OVP being broken (the solders break) or the fuse in it being broken. Good to get one for peace of mind. A broken OVP will show up a ASD problem - does the ASD light go out with the ignition? A missing or very broken OVP shows an ABS light too.

The OVP powers the ECU so without out it you get no electrics and no idle control or start enrichment.

Your problem could also be fuel pressure or engine compression related as well though, or other usual suspects such as ignition. Worth checking those maybe first - a friendly garage could confirm those are ok.

[ms500]

Pentoman said:

This may be the idle control valve or one of the many inputs that make it work.

You'll find, I'm 99% sure, that this car will run without the ECU plugged in, but it will be a bit like you describe - poor idling and flat when cold.

It has a basic diagnosis, checked by finding the round underbonnet diagnostic port and using a meter between holes 3 and ground. Does it have a catalyst/o2 sensor? (Does underbonnet electric switches say "KAT" on it). With this information you can do the diagnosis and it'll tell you if the electric functions are working.

It does have a catalyst, and there is a small square box on the driver's side under the bonnet which has numbered pin holes, is this the diagnostic port?

Pentoman said:

Probably the most common reason for it is the OVP being broken (the solders break) or the fuse in it being broken. Good to get one for peace of mind. A broken OVP will show up a ASD problem - does the ASD light go out with the ignition? A missing or very broken OVP shows an ABS light too.

All the warning lights come on when I put the key in the ignition, then they all go out once the engine has started. The ABS light isn't on.

I've looked at the compartment behind the battery and can see a few black boxes with 'Bosch' written on them, but I can't see which one is the OVP relay. How much would a replacement be?

Edited by ms500 on Tuesday 29th July 13:20

[Pentoman]

ms500 said:

Pentoman said:

This may be the idle control valve or one of the many inputs that make it work.

You'll find, I'm 99% sure, that this car will run without the ECU plugged in, but it will be a bit like you describe - poor idling and flat when cold.

It has a basic diagnosis, checked by finding the round underbonnet diagnostic port and using a meter between holes 3 and ground. Does it have a catalyst/o2 sensor? (Does underbonnet electric switches say "KAT" on it). With this information you can do the diagnosis and it'll tell you if the electric functions are working.

It does have a catalyst, and there is a small square box on the driver's side under the bonnet which has numbered pin holes, is this the diagnostic port?

Pentoman said:

Probably the most common reason for it is the OVP being broken (the solders break) or the fuse in it being broken. Good to get one for peace of mind. A broken OVP will show up a ASD problem - does the ASD light go out with the ignition? A missing or very broken OVP shows an ABS light too.

All the warning lights come on when I put the key in the ignition, then they all go out once the engine has started. The ABS light isn't on.

I've looked at the compartment behind the battery and can see a few black boxes with 'Bosch' written on them, but I can't see which one is the OVP relay. How much would a replacement be?

Edited by ms500 on Tuesday 29th July 13:20

Ok so do you have the orange wheelspin warning light in the speedo and does it come on with ignition and go off with starting? If so I believe your OVP is OK.

Behind the battery (and I've never looked on a 24v) you'll have an ABS ECU (part no 008 545 5132 or similar and the engine ECU (usually taller) then alongside those a fuel pump relay (size of a cigarette box) and OVP relay (half the size of a tube of toothpaste, a not round) which has a fuse on top, in fact probably 2 fuses on top.

However if your OVP is ok (you can test it by seeing if you get 12v to pin 1 of the engine ECU) it's likely to be something else.
There is some diagnosis on the square diagnostic box you've found - is there anything in pin hole 3? On mine I just have connectors in pin holes 1 (ground) and 5 (for ASD diagnosis) but apparently 3 can be engine diagnosis. However try to look for a round plug, usually on the front wing, with 9 smaller holes, and a screw cap over it. about 1+ inch diameter. I know how to diagnosis with these and it lets you know if the computer is unhappy with any of the signals its getting, very basic but you'll know if a sensor has failed badly or a wire is broken.

[ms500]

Pentoman said:

ms500 said:

Pentoman said:

This may be the idle control valve or one of the many inputs that make it work.

You'll find, I'm 99% sure, that this car will run without the ECU plugged in, but it will be a bit like you describe - poor idling and flat when cold.

It has a basic diagnosis, checked by finding the round underbonnet diagnostic port and using a meter between holes 3 and ground. Does it have a catalyst/o2 sensor? (Does underbonnet electric switches say "KAT" on it). With this information you can do the diagnosis and it'll tell you if the electric functions are working.

It does have a catalyst, and there is a small square box on the driver's side under the bonnet which has numbered pin holes, is this the diagnostic port?

Pentoman said:

Probably the most common reason for it is the OVP being broken (the solders break) or the fuse in it being broken. Good to get one for peace of mind. A broken OVP will show up a ASD problem - does the ASD light go out with the ignition? A missing or very broken OVP shows an ABS light too.

All the warning lights come on when I put the key in the ignition, then they all go out once the engine has started. The ABS light isn't on.

I've looked at the compartment behind the battery and can see a few black boxes with 'Bosch' written on them, but I can't see which one is the OVP relay. How much would a replacement be?

Edited by ms500 on Tuesday 29th July 13:20

However try to look for a round plug, usually on the front wing, with 9 smaller holes, and a screw cap over it. about 1+ inch diameter. I know how to diagnosis with these and it lets you know if the computer is unhappy with any of the signals its getting, very basic but you'll know if a sensor has failed badly or a wire is broken.

OK, I've found that diagnostic port now, it was on the left front wing under the bonnet. Will I need a code reader or can I just use a Multimeter?

[bolide]

The 300-24 valve engine is a unique one and the injection & ignition isn't the same as the later 280/320 engines

If it runs badly when cold & improves when warm then the temperature sensor may well be the problem

I'd follow Pentoman's advice

Nick Froome
www.w124.co.uk

[Pentoman]

ms500 said:

Pentoman said:

ms500 said:

Pentoman said:

This may be the idle control valve or one of the many inputs that make it work.

You'll find, I'm 99% sure, that this car will run without the ECU plugged in, but it will be a bit like you describe - poor idling and flat when cold.

It has a basic diagnosis, checked by finding the round underbonnet diagnostic port and using a meter between holes 3 and ground. Does it have a catalyst/o2 sensor? (Does underbonnet electric switches say "KAT" on it). With this information you can do the diagnosis and it'll tell you if the electric functions are working.

It does have a catalyst, and there is a small square box on the driver's side under the bonnet which has numbered pin holes, is this the diagnostic port?

Pentoman said:

Probably the most common reason for it is the OVP being broken (the solders break) or the fuse in it being broken. Good to get one for peace of mind. A broken OVP will show up a ASD problem - does the ASD light go out with the ignition? A missing or very broken OVP shows an ABS light too.

All the warning lights come on when I put the key in the ignition, then they all go out once the engine has started. The ABS light isn't on.

I've looked at the compartment behind the battery and can see a few black boxes with 'Bosch' written on them, but I can't see which one is the OVP relay. How much would a replacement be?

Edited by ms500 on Tuesday 29th July 13:20

However try to look for a round plug, usually on the front wing, with 9 smaller holes, and a screw cap over it. about 1+ inch diameter. I know how to diagnosis with these and it lets you know if the computer is unhappy with any of the signals its getting, very basic but you'll know if a sensor has failed badly or a wire is broken.

OK, I've found that diagnostic port now, it was on the left front wing under the bonnet. Will I need a code reader or can I just use a Multimeter?

No a multimeter does the trick. Ideally use a multimeter with "dwell" on it. However if you don't have dwell you can calculate it like this:

Dwell = ( 1 - (Measured voltage / Battery voltage) ) x 100.

Voila. You can test you've got the right port and its working by unplugging the coolant temp sensor or air flow position sensor or something and the value will change. That is, if the whole thing hasn't got no power as I described above in which case unplugging sensors will do nothing. But if that's the case you'll know because you'll have 100% or 0% dwell.


This is the thread you need to visit for hte specific diagnostic instructions
http://s14.zetaboards.com/mercedes_190_club/topic/...
Should be able to read that without registering but if not

mercedes190.co.uk said:

This is for anyone who is having troubles with engine running.

Ideally you need an automotive multimeter with dwell setting on it, to read off the % given from the ECU.

- You will need to unscrew the cap off the round diagnostic socket.

- You will then see a number of connecting holes.

- You will need to connect the multimeter to hole 3 and earth the other wire from the multimeter on the vehicle body.

- With the engine running at idle you should get a reading.

- If there is no fault you should get a reading which fluctuates 50 (+10 or -10)

- If there is a fault you will get a reading which stays the same and does not change.

- Now see the tables below

[ms500]

Pentoman said:

ms500 said:

Pentoman said:

ms500 said:

Pentoman said:

This may be the idle control valve or one of the many inputs that make it work.

You'll find, I'm 99% sure, that this car will run without the ECU plugged in, but it will be a bit like you describe - poor idling and flat when cold.

It has a basic diagnosis, checked by finding the round underbonnet diagnostic port and using a meter between holes 3 and ground. Does it have a catalyst/o2 sensor? (Does underbonnet electric switches say "KAT" on it). With this information you can do the diagnosis and it'll tell you if the electric functions are working.

It does have a catalyst, and there is a small square box on the driver's side under the bonnet which has numbered pin holes, is this the diagnostic port?

Pentoman said:

Probably the most common reason for it is the OVP being broken (the solders break) or the fuse in it being broken. Good to get one for peace of mind. A broken OVP will show up a ASD problem - does the ASD light go out with the ignition? A missing or very broken OVP shows an ABS light too.

All the warning lights come on when I put the key in the ignition, then they all go out once the engine has started. The ABS light isn't on.

I've looked at the compartment behind the battery and can see a few black boxes with 'Bosch' written on them, but I can't see which one is the OVP relay. How much would a replacement be?

Edited by ms500 on Tuesday 29th July 13:20

However try to look for a round plug, usually on the front wing, with 9 smaller holes, and a screw cap over it. about 1+ inch diameter. I know how to diagnosis with these and it lets you know if the computer is unhappy with any of the signals its getting, very basic but you'll know if a sensor has failed badly or a wire is broken.

OK, I've found that diagnostic port now, it was on the left front wing under the bonnet. Will I need a code reader or can I just use a Multimeter?

No a multimeter does the trick. Ideally use a multimeter with "dwell" on it. However if you don't have dwell you can calculate it like this:

Dwell = ( 1 - (Measured voltage / Battery voltage) ) x 100.

Voila. You can test you've got the right port and its working by unplugging the coolant temp sensor or air flow position sensor or something and the value will change. That is, if the whole thing hasn't got no power as I described above in which case unplugging sensors will do nothing. But if that's the case you'll know because you'll have 100% or 0% dwell.


This is the thread you need to visit for hte specific diagnostic instructions
http://s14.zetaboards.com/mercedes_190_club/topic/...
Should be able to read that without registering but if not

mercedes190.co.uk said:

This is for anyone who is having troubles with engine running.

Ideally you need an automotive multimeter with dwell setting on it, to read off the % given from the ECU.

- You will need to unscrew the cap off the round diagnostic socket.

- You will then see a number of connecting holes.

- You will need to connect the multimeter to hole 3 and earth the other wire from the multimeter on the vehicle body.

- With the engine running at idle you should get a reading.

- If there is no fault you should get a reading which fluctuates 50 (+10 or -10)

- If there is a fault you will get a reading which stays the same and does not change.

- Now see the tables below

Thanks for this, I'll try it.

[ms500]

OK, I put the +ve terminal of the multimeter in pin 3 of the diagnostic socket, and the -ve terminal on the battery post while the engine was idling.

My meter doesn't have a dwell function but it measured between 3-5v which was fluctuating, does this mean anything?

[ms500]

I just built myself a blink tester to use on the ECU pin of the 16-pin square diagnostic socket, and it gave me 17 flashes.

After checking the list of codes, I understand this to mean a bad injector in cylinder 4. There were no other problem codes.

Could this be the cause of my problems?