Early M104 24V starting hiccups

[guyonabuffalo]

I m sure many of you have experienced a similar starting issue.

https://www.youtube.com/watch?v=MjeiaEtW2WY

I plan on replacing the following -

Distributor cap
Spark Plugs
Ignition coil (this car only has one)
Front Crank position sensor (are these known for going bad)?

Considering the Rear CPS too but not sure how much more difficult that will be to access. Is it better to replace these in pairs?

Any suggestions or advice from the more experienced folks with this engine would be greatly appreciated.

The car is a 1992 300CE with M104.980 engine. Other than this starting issue and the occasional lumpy idle when the engine is cold(sat overnight or days), it runs great. This starting issue is sometimes random, sometimes the revs will drop like in the video but the car wont stall, it will figure things out and idle normally.

Thank you in advance for your advice and suggestions

[duxthe1]

I doubt any of the ignition parts you previously mentioned will have any effect. The crank-stall-crank and run behavior is almost intrinsic to the CIS systems. The systems run at a pretty high fuel pressure and if there is any leak down beyond the specified rest pressure you get that behavior. Injectors leak down, fuel distributor plunger leaks, accumulator leaks, and the fuel pump check valve(s) leak. There is a cold start valve that sprays fuel independent of the rest of the system that should take up the slack on startup but in a 20 year career, I've seen exactly one that was bad.


If you really want to fix it, you need to determine what the worst offenders are and remedy the worst first.

[guyonabuffalo]

Thanks duxthe1,

I thought that too. there is a fairly loud "air leak" type sounds from around the back of the car. I suspect it is coming from the fuel pump/accumulator area. Are these fuel pumps known for being loud? What are some tests that I can do to find the obvious culprits?

A friend of mine suspected an exhaust leak - while I can sometimes smell exhaust at idle in traffic, I cant really pinpoint if it is leaking. There is a metal plate above a section of the exhaust that is loose, looks like it hit something and got a little bent and possibly loosened.

The "air leak" sounds could possibly be a fuel pump but I cant be sure until I check.
If that is the case, is there a cheap fix or should I just replace the pump? I am only asking because I dont know much about gas cars. My first Mercedes is a W123 300D diesel and I (thanks to this and another forum) have become quite knowledgable and experienced at solving its problems. Gas engines however; are a whole different thing.

Any additional advice would be sincerely appreciated.

[Markus]

Rather than throw parts at the issue (unless they need to be replaced anyway) why not check fuel pressures and duty cycle first? Slight enrichment might just solve your issue. As a technique, depressing the accelerator momentarily while cranking might also be of benefit.

Why is your oil pressure reading 0 at idle? INOP gauge or sensor?

[guyonabuffalo]

Thanks for the suggestion Markus.

The oil pressure sender is bad. I got a new one - will be installing soon.

As far as replacing parts, the spark plugs were replaced in 2012. The distributor much earlier, so I am changing those anyway. The car also sat for 1 year in 2017.

I will definitely look into the duty cycle and update here.

[ytech15]

A fuel filter replacement cured my cold start issue (dies first try), same M104.980.

[guyonabuffalo]

duxthe1 was right. The spark plug/distributorcap/rotor made zero difference to the starting issue

I checked my duty cycle and it goes between 76-80 when in closed loop mode. Apparently this is normal for CA cars. Is this ok?

Thanks for the advice Ytech15.

I am planning on replacing all or most of the engine bay vacuum lines/connectors as many look old/yellow or are wet.

There is a local enthusiast selling a few fuel accumulators. I might get them all and try the best one as well as do a fuel filter change. Hopefully that will better results.

[ytech15]

Originally Posted by guyonabuffalo

I checked my duty cycle and it goes between 76-80 when in closed loop mode. Apparently this is normal for CA cars. Is this ok?

With ignition switched on (engine not running) the duty cycle should be about 85%.
A duty cycle at idle fluctuating around a mean value of about 47% is ideal.

Originally Posted by guyonabuffalo

There is a local enthusiast selling a few fuel accumulators. I might get them all and try the best one as well as do a fuel filter change. Hopefully that will better results.

A faulty accumulator will usually result a hot starting problem.

[guyonabuffalo]

Checked Duty cycle again, yesterday. Used the cycle measurement method described in detail by a forum member.

I am around 49% at idle. Which would indicate its running fine. However once it goes into fluctuating mode, the variance is pretty wide. Goes between 9-18V.

[97 SL320]

What you are calling the front CPS is a _Camshaft_ sensor. This is just tells the computer is cyl 1 is on the compression or exhaust stroke and on the CIS M104 to confirm the intake cam advance is working. For the later ME M104 is it used to sync the injector pulses for a smoother idle / diagnose miss fire for the OBD2 system. With either motor, the engine should still start with a non working cam sensor.

Do you have a lab type oscilloscope? If so hang it on the rear sensor, this is the _Crankshaft sensor. This tells the ignition when to fire and gives RPM info to the ignition module and other systems.

I'd hang a test light on the fuel pump to see if it is shutting off. Another test would be to manually run the pump for 30 sec then, while leaving it powered up, try to start.

[ytech15]

Originally Posted by guyonabuffalo

Checked Duty cycle again, yesterday. Used the cycle measurement method described in detail by a forum member.

I am around 49% at idle. Which would indicate its running fine. However once it goes into fluctuating mode, the variance is pretty wide. Goes between 9-18V.

Checking the Duty Cycle while suspecting the fuel system isn’t very useful. The first thing that comes in to mind while facing this condition is that the system is leaking down pressure, it can be a leaky injector , a faulty cold start valve or even a leak inside the FD- keeping this in mind; starting with the basics such as a filter replacement is a must as it’s usually overdue for service and relatively cheap compared to the other components of the CIS E system.

[guyonabuffalo]

Coming from a diesel background, I have extremely limited experience with gas engines.

Today, I heard what sounded like liquid being sprayed and then come to a drip somewhere in the engine after I shut it off. I was replacing vacuum lines and connectors and a Voltage regulator. Is this normal, or is the liquid dripping sound possibly some kind of leakdown happening?

Also, I replaced the OVP, voltage regulator to no improvement. Car still starts like it used to and drives like it used to. Keep in mind - I was going to replace these parts anyway since they had never been replaced or even checked. (voltage regulator brushes were worn down substantially).

Next up, Fuel filter. Are Fuel pressure regulators known to go bad?