Topic: 350SE cold running and warm start problems

[Berggreen]

Hello,

I am the (otherwise) happy owner of a 350SE automatic from 12/1977 with a M116.985 engine with K-jettronic ignition system and 327.000 km on the clock. I bought the car back in February, and most of the season went with body repairs and repainting. But now I have put my attention to the engine, and the following jobs have been done up to now to make it run better:

1) K-jettronic system was exchanged and sourced from a donor 116.985-engine with (a claimed) 160.000 km on the clock. This included also the warm-up regulator unit.
2) All injection nozzles were replaced to new ones from BOSCH.
3) Fuel filter (the large one) was replaced to a new one.
4) All spark plugs were replaced with new NGK BP6ES plugs. Old plugs all looked fine - no oil on the plugs.
5) Spark plug wires were relatively new already, but I measured that they all have a resistance of around 1 kOhm. We removed the metal shield from the plugs at the nr. 1 and 5 spark plugs respectively, because we registered a spark between the 1 & 2 and 5 & 6 plugs, because these plugs are placed very close to each other in the M116/M117 engines.
6) Distributor cap was changed to a new one from BOSCH.
7) Rotor with rpm limiter was changed to a new original from BOSCH.
Ignition coil was changed to a new from BOSCH.
9) Both ballast resistors (0.4 and 0.6 Ohm) were changed to new ones.
10) I have adjusted the warm idle (750-800 rpm) CO% to around 2.0%CO.
11) Compression checked out nicely with 164-179 psi measured across all eight cylinders (even with a cold engine).

But even though all this made the engine a lot more responsive, when running warm, I still have two big problems, which I cannot figure out how to improve:

A) It fires up fine cold and very shortly goes to around 1000 rpm as it should. But then after 2-3 seconds it drops down to around 700 rpm and runs very unevenly. First after a couple of minutes with continuous driving, it starts running better and more evenly.

B) When warm and after standing turned off for a few minutes (or longer) it is extremely difficult to start. It never fires first time no matter what I do with the gas pedal. Only the second time it fires after cranking for 5-10 seconds and only with the gas pedal all the way to the floor! I can add that it behaved in the same way also before I changed the K-jettronic system and warm-up regulator, though as both systems were used they could suffer from the same problem, but just seems unlikely.

I would appreciate some tips on how to proceed from here to cure the two problems (A & B) above.

I look forwards to hear from you and thanks for your help.

Cheers,

Christian

[BCK1963]

Hello Christian,

as far as I can tell you have done a very good an comprehensive job on Tuning up the ignition System. But the gas system leaves some space for improvement.
And particularly an aged K-Jet system can exhibit some nasty behaviour.
You have replaced a very 'old' system by an 'old' System. This doesn't necessarily mean that it will not work but it still brings About the likeliness of some bugs.

Bug B) seems to be the usual suspect in the gas pump unit. This unit behind the rear axle consists of the gas pump, the gas filter and a cylinder which maintains the gas pressure for a certain after shutting down the engine. The german word is Druckspeicher, the english term may be pressure Reservoir or retainer. This thing commonly Fails after a while and after 320.000 km is is highly probable. The consequence is that even after some minutes of stand still the pressure is down and you must build it up again by cranking before it fires up.
I have read repeatedly in this Forum that fellows have the opinion to check step by step and replace only what is necessary. There is some reason behind that logic but you should be very patient when following that path. When I bought my car I had a similar problem and I decided to replace the entire gas pump unit.
You have done the entire ignition System and to me it makes sense, in particular with respect to the mileage on your car to do that job now and have no Problems in the next years. In particular if you don't know the history of the car and those components very well tracking bugs can be very tidious and it is good to have a 'solid ground' to start from when Looking for the next bugs.
Other suspects which may contribute more or less to your described problem are:
inhibited gas flow due to blocked filters.
 injection nozzles
old and brittle rubber hoses

Bug A) sounds to me like sitting in the warm-up components like the warm-up Regulator or a Valve which supplies additional air for a limited time during the warm-up Phase. In german:  Zusatzluftschieber. This thing is notorious for failure and can be repaired only with difficulty, if at all.

As I have never experienced bug A) personally, I am not a specialist in that.

     Regards and good luck       Bernd

[Berggreen]

Hello Christian,

as far as I can tell you have done a very good an comprehensive job on Tuning up the ignition System. But the gas system leaves some space for improvement.
And particularly an aged K-Jet system can exhibit some nasty behaviour.
You have replaced a very 'old' system by an 'old' System. This doesn't necessarily mean that it will not work but it still brings About the likeliness of some bugs.

Bug B) seems to be the usual suspect in the gas pump unit. This unit behind the rear axle consists of the gas pump, the gas filter and a cylinder which maintains the gas pressure for a certain after shutting down the engine. The german word is Druckspeicher, the english term may be pressure Reservoir or retainer. This thing commonly Fails after a while and after 320.000 km is is highly probable. The consequence is that even after some minutes of stand still the pressure is down and you must build it up again by cranking before it fires up.
I have read repeatedly in this Forum that fellows have the opinion to check step by step and replace only what is necessary. There is some reason behind that logic but you should be very patient when following that path. When I bought my car I had a similar problem and I decided to replace the entire gas pump unit.
You have done the entire ignition System and to me it makes sense, in particular with respect to the mileage on your car to do that job now and have no Problems in the next years. In particular if you don't know the history of the car and those components very well tracking bugs can be very tidious and it is good to have a 'solid ground' to start from when Looking for the next bugs.
Other suspects which may contribute more or less to your described problem are:
inhibited gas flow due to blocked filters.
 injection nozzles
old and brittle rubber hoses

Bug A) sounds to me like sitting in the warm-up components like the warm-up Regulator or a Valve which supplies additional air for a limited time during the warm-up Phase. In german:  Zusatzluftschieber. This thing is notorious for failure and can be repaired only with difficulty, if at all.

As I have never experienced bug A) personally, I am not a specialist in that.

     Regards and good luck       Bernd

Hi Bernd

Thank you very much for your very nice reply and great input and help - this is highly appreciated!

I actually forgot to add to the list in my original posting, that I also changed all the injection nozzles - the old ones were very black and worn - as well as the large fuel filter. So these parts at least are out of the game, and I updated the list in my original posting.

For Bug B - it sounds very much like the "smoking gun" with that Druckspeicher (pressure retainer), as it feels very much like the engine is not getting any fuel to start with and only by forcing fuel forward with a fully deployed gas pedal the engine gets enough fuel to start up. Are there any ways of checking if my current Druckspeicher is ok, or should I just buy a new one and install?

For Bug A - thanks I will definitely check the air-valve out in the service manual and see what can be done to test if it is working ok.. Maybe someone else in the forum knows how to test it? Also, maybe I have just to bleed the funds to have one of my warm-up regulators renovated professionally, so that I am sure it checks out without problems.

Thanks again,

Christian

[Type17]

Problem A sounds like there is a problem with the warm-up regulator (WUR) or the air bypass valve (ABV). With the WUR issue, the engine gets enough fuel for the initial few seconds because extra fuel is supplied by the cold-start injector, but when it (correctly) cuts out after a few seconds, the mixture is wrong because the WUR isn't allowing the correct amount of fuel for the engine's cold temperature (too little fuel/mixture too weak). This problem gradually goes away as the engine warms up, when less fuel is required anyway. The control pressure is influenced by the WUR during warm-up, and can be measured at different temperatures with the correct gauge.


If it is the ABV, the symptoms can be different, depending on how the valve has failed - when the engine is cold, this valve is normally fully open and closes in the first few minutes, controlled by the rising temperature of the coolant, to match the lowering of the fuel supply by the WUR.
If the ABV has failed open (mechanism has failed) the engine will start normally, but the mixture will become weaker and weaker as the fuel supply is lowered by the WUR, but the valve doesn't lower the air supply to match.
If it has failed closed (jammed) the car will be hard to start (too much fuel, over rich mixture), but will improve as the fuel supply is lowered by the WUR.


Problem B sounds like a dead fuel accumulator - this is a device under the car, near the fuel pump and filter. It keeps the fuel pressure in the system high for a few hours after engine switchoff. If the fuel pressure is allowed to drop, the residual heat from the engine vaporises the fuel in the fuel metering head and surrounding pipework, making starting impossible until fresh (liquid) fuel arrives from the tank (about 8-10 seconds of cranking with the throttle open). A working accumulator keeps the fuel pressure (and therefore the fuel's boiling point) high enough to prevent vaporisation during the first few hours - by the time the fuel pressure has dropped, the engine temperature has dropped anyway (even with a dead accumulator, the fuel also cools and condenses to liquid again, which is why cold starting is not affected by this issue).

[Berggreen]

Problem A sounds like there is a problem with the warm-up regulator (WUR) or the air bypass valve (ABV). With the WUR issue, the engine gets enough fuel for the initial few seconds because extra fuel is supplied by the cold-start injector, but when it (correctly) cuts out after a few seconds, the mixture is wrong because the WUR isn't allowing the correct amount of fuel for the engine's cold temperature (too little fuel/mixture too weak). This problem gradually goes away as the engine warms up, when less fuel is required anyway. The control pressure is influenced by the WUR during warm-up, and can be measured at different temperatures with the correct gauge.


If it is the ABV, the symptoms can be different, depending on how the valve has failed - when the engine is cold, this valve is normally fully open and closes in the first few minutes by a bi-metallic strip, lowering the air supply, to match the lowering of the fuel supply by the WUR.
If the ABV has failed open (no power or bi-metallic strip has failed) the engine will start normally, but the mixture will become weaker and weaker as the fuel supply is lowered by the WUR, but the valve doesn't lower the air supply to match.
If it has failed closed (jammed) the car will be hard to start (too much fuel, over rich mixture), but will improve as the fuel supply is lowered by the WUR.


Problem B sounds like a dead fuel accumulator - this is a device under the car, near the fuel pump and filter. It keeps the fuel pressure in the system high for a few hours after engine switchoff. If the fuel pressure is allowed to drop, the residual heat from the engine vaporises the fuel in the fuel metering head and surrounding pipework, making starting impossible until fresh (liquid) fuel arrives from the tank (about 8-10 seconds of cranking with the throttle open). A working accumulator keeps the fuel pressure (and therefore the fuel's boiling point) high enough to prevent vaporisation during the first few hours - by the time the fuel pressure has dropped, the engine temperature has dropped anyway (even with a dead accumulator, the fuel also cools and condenses to liquid again, which is why cold starting is not affected by this issue).

Thanks a lot for your your input.

Looks like you fully agree with Bernd that the smoking gun is the fuel accumulator for problem B and the WUR or ABV for the problem A.

This gives me some very good leads to investigate further. Thanks a lot for your help!

Cheers,

Christian

[raueda1]

Hello Christian,
>>>snip<<<

I have read repeatedly in this Forum that fellows have the opinion to check step by step and replace only what is necessary. There is some reason behind that logic but you should be very patient when following that path. >>>snip<<<
     Regards and good luck       Bernd

I guess I'm one of those fellows.  True, patience is needed but you can be sure that the system will end up being right.  Anyway, it sounds like the time has come to get a fuel system pressure gauge. You'll then be able to diagnose exactly what's going on and solve it.  Otherwise it's really just guesswork.  There are numerous threads on the topic, just do a little searching.  Regardless, it does sound like pressure accumulator isn't working.  If the old one is indeed old just replace it on principle.  Cheers,

[Berggreen]

Thanks a lot for all your input.

I am trying to locate the different suspect parts, we have been discussing, and have found the following parts:

Fuel accumulator:

1) A0004760121 (nr. 371 on this drawing: http://mb-teilekatalog.info/view_SubGroupAction.php?lang=G&mode=BM&class=1&aggtyp=FG&catalog=040&model=116028&group=47&subgrp=001
Is this the one?

2) But there seem to be a second fuel accumulator A0004760421/A0004760821, 379 in the same drawings. Will I also need to change that one and do I actually have both in my 350SE from 1977?


Air bypass valve (ABV):

1) A0000940765 (nr. 585 in this drawing: http://mb-teilekatalog.info/view_SubGroupAction.php?lang=G&mode=BM&class=1&aggtyp=M&catalog=042&model=116985&group=07&subgrp=070)
Is this the right one?

2) But there is also this valve A0000940865 (nr. 588 in the same drawing above) and A0001404460 (579 in the same drawing as above). Could also be one of them, or?

Thanks for your help!

Cheers,

Christian

[Type17]

Accumulator: Looking at Google images of the two part numbers, one of them has two screw-on connectors, and the other only has one, so you should look under your car and see which one you need.


ABV: maybe look at the one already in your car - also check its part number - it may have been superseded, but the system or Google should tell you which one replaced it.

[Berggreen]

Accumulator: Looking at Google images of the two part numbers, one of them has two screw-on connectors, and the other only has one, so you should look under your car and see which one you need.


ABV: maybe look at the one already in your car - also check its part number - it may have been superseded, but the system or Google should tell you which one replaced it.

Thanks a lot, I will check the fuel accumulator under the car to find the right type. But I assume there should only be one of them on each car and not two. Correct?

I have to confess that I am not really sure, where the ABV is located on the engine. Can you maybe help me?

Cheers,

Christian

[Type17]

Yes, there's only one accumulator in each car.


I'm honestly not sure which item it is - my car has an M116.985, but is a 1976 model, and I think the part is different.


However I can't check easily as my car is stored about 40km away, and we have a 5km travel limit in force right now (Covid-19).

[Berggreen]

Yes, there's only one accumulator in each car.


I'm honestly not sure which item it is - my car has an M116.985, but is a 1976 model, and I think the part is different.


However I can't check easily as my car is stored about 40km away, and we have a 5km travel limit in force right now (Covid-19).

Thanks, good that we have the fuel accumulator straightened out at least.

I think we have the same engine, if yours is also a M116.985 with K-jettronic. However, it changed in 1976 from the M116.983 with D-jettronic, but if you have a M116.985 it will have to be with K-jettronic.

There was a change in 1978 to an updated version of the K-jettronic, but I am not quite sure what the changes were, other than it seems to have 200 HP instead of 194 HP on the early K-jettronics, like ours. I can make some pictures of my engine bay, and maybe you can maybe point on the part in the picture.

Luckily we don't have the COVID movement restrictions here in Denmark - yet! But assemblies cannot be more than 10 people, and we have to stay within the Danish borders. :-/

Thanks again,

Christian
 

[ptashek]

There was a change in 1978 to an updated version of the K-jettronic, but I am not quite sure what the changes were

From Daimler archives:

Between November 1975 and February 1976 the fuel injection system of the 2.8-liter, 3.5-liter and 4.5-liter injection-engines was changed in order to correspond to the now higher exhaust-emission standards in most European countries. The electronically controlled Bosch "D-Jetronic" was replaced by the newly developed mechanically controlled Bosch "K-Jetronic". In all three cases the conversion was connected to slight power reductions. In the 2.8-liter engine as well as in the 3.5-liter engine the compression was lowered at the same time. In order to facilitate maintenance both V8-engines were equipped with a breakerless transistor ignition and a hydraulic valve-clearance compensation.

Parallel to the 2.8-liter injection engine the compression was also lowered in the carburettor engine. Here, too, this led to a reduced potential of performance. Two years later, since April 1978, the original performance of all three models with injection engine was reached again. With the 2.8-liter injection engine - in contrast to the carburettor engine - the compression was increased to its old value. With the two V8-models the former performance was basically achieved by changing the exhaust-emission system.

[Berggreen]

There was a change in 1978 to an updated version of the K-jettronic, but I am not quite sure what the changes were

From Daimler archives:

With the two V8-models the former performance was basically achieved by changing the exhaust-emission system.

Thanks, that is very interesting!

Do you know what was changes in the exhaust-emission system?

The reason I ask is, because the M116.985 donor-engine I have, originally was installed in a 350SLC from the early 80'es. Thus, it must then be of the late post-1978 version. But when I installed it in my car, I checked the BOSCH part numbers of the two main parts of the K-jettronic - the "spider" and the "toilet seat" - and they both had the exact same BOSCH part numbers (spider 0438100012 & toilet seat 0438120062) as the same two parts on my original K-jettronic system, which must be of the early K-jettronic version.

So I am curious to hear which parts were modified to increase the HP?

Cheers,

Christian

[Type17]

ptashek's details mean that our engines should be the same (I thought that only the '76 cars had less than 200 BHP)


Looking at the illustrations you linked above, it is part 333. It's different than I thought because it is closed by the temperature of the coolant, rather than via an electrical bi-metallic strip (Most of my previous experience with K-Jet was on VW's, where it is electrical - every day's a school day! I edited my post above to reflect this, for the benefit of future readers of this thread)


It's located under the front of the air filter container, near the WUR - here's a picture from another thread on here - in the third pic down, it's the silver cylinder just to the right of the WUR, and above the blue handle of the oil dipstick: https://forum.w116.org/mechanicals/450-m117-timing-chain-replacement/ (RIP to WGB, who started that thread)


(In the first illustration, there's a new one at the top of the picture)

[Berggreen]

ptashek's details mean that our engines should be the same (I thought that only the '76 cars had less than 200 BHP)


Looking at the illustrations you linked above, it is part 333. It's different than I thought because it is closed by the temperature of the coolant, rather than via an electrical bi-metallic strip (Most of my previous experience with K-Jet was on VW's, where it is electrical - every day's a school day! I edited my post above to reflect this, for the benefit of future readers of this thread)


It's located under the front of the air filter container, near the WUR - here's a picture from another thread on here - in the third pic down, it's the silver cylinder just to the right of the WUR, and above the blue handle of the oil dipstick: https://forum.w116.org/mechanicals/450-m117-timing-chain-replacement/ (RIP to WGB, who started that thread)


(In the first illustration, there's a new one at the top of the picture)

Fantastic, thank you very much! Yes, we learn everyday, and I also learned a LOT today about the fuel supply system on our K-jet systems thanks to you guys!

Cheers,

Christian