OM603 350 SDL morning start problem - PeachParts Mercedes-Benz Forum

phlfly · #1 11-23-2017, 11:28 AM

I have question:
I have 91 SDL 350 with 961 motor, I replaced nozzles (from Mercedes source), replaced glow plugs (all reads about 1 ohm), new (but aftermarket ) glow plug relay.
So an engine starts right the way even to day at 25 F and runs smooth for first few1 min. I like to warm up for few minutes before driving it, so after 2-3 min it will run like "misfiring", unstable, and getting smoke blue smoke (diesel ) not a lot but enough to see. When I started to drive, it will smooth out and putting good power. While engine still lower than 80 F, at stop light it will "berb", like one misfire every 10-15 sec, even oil gauge will flux. (My rmp gauge is not working).
Oil pressure at hot (just above 80F) right at 2 mark or just little below.

All nozzles are put in spec about 2,000 psi. Al tested and properly torqued.

What would it be a problem? Is relay is not staying long enough. At least it's felt like that.
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Diseasel300 · #2 11-23-2017, 12:23 PM

On the OM603.96x, the glow plug relay shuts off as soon as the starter is engaged. The OM603.97x introduced "afterglow" where the glow plugs stay on for a while after the engine is running. You don't specify if you have the 96 or 97 style glow plug relay.

It really doesn't matter, the engine should start and run fine when cold, even at 25˚ without the afterglow. It's normal for it to chug a bit for up to 30 seconds or so after starting without afterglow, but with afterglow it shouldn't start stumbling once the plugs shut off.

First things first, fix your RPM gauge (not RMP). Do you have an Antilock light lit? You could easily have an OVP problem, which will disable the tach and ABS. If the tach isn't working, your EDS computer may not be working either, in which case your idle will be fairly low - ~500-550RPM which will definitely cause rough running and chugging at idle & in gear until the engine reaches close to operating temperature.

It's doubtful you have air in the fuel - that usually shows up during the starting process or immediately after. Not 2-3 minutes after the engine is started.

Done a compression test? If you have iffy compression and a lower than normal idle, you can expect some roughness.

phlfly · #3 11-23-2017, 04:20 PM

I don't have an anti lock brake light. I will check anti lock if they are working today later.
So I guess I saw that relay sits between windshield and engine compartment. Is there way to check if relay good? It seems to me looking fairly new, may be few years old.
RPM gauge is working it could be three problem : sensor at engine, capacitors at gauge or OVP relay ? I guess. Is there way to check gauge for good working condition. I have special tool to remove a gauges.

My idle is bad only it's cold.
An engine compression? Can be but don't believe because the way car feels at any speed, uphill it will pull and even downshift properly. If it was compression than it would be slow and smoke an oil. But I'm adding about qt for few thousand miles.

Diseasel300 · #4 11-23-2017, 06:35 PM

When you turn the key to pre-glow, does the Anti-Lock light illuminate? If not, the bulb is probably burnt out. It should come on half-brightness with the rest of the "idiot lights" when you're pre-glowing and go out when the engine is started. The OVP relay is right beneath the windshield near the brake booster on the main firewall.

Does your A/C work? Cruise control work? If so, the crank position sensor is good and the Tach signal from the EDS computer is good and likely the tach itself is what has issues. If neither work, suspect the EDS computer or the crank position sensor. The crank position sensors aren't really known for dying on these cars.

Poor idle when cold is due to poor fuelling or poor compression. The 603 is a strong enough engine that it should be able to start without glow plugs down to around freezing. Mine's not the best example and it still fired up fine, albeit a bit rough for ~30 seconds when the glow plug relay died last winter.

You haven't confirmed if you have an after-glow relay or not. If you have the after-glow relay and it starts running rough once the afterglow kicks off, suspect mechanical issues with the engine or serious fuel supply problems. There's no excuse for an after-glow system to run rough. You can have low compression without using oil and without being slow. A 10% difference in compression between 2 cylinders is enough to cause a rough idle.

Low compression or unbalanced compression is the biggest issue when cold starting and cold idle since the fuel does not vaporize and combust as well in the low compression cylinders as it does in the normal compression ones. Once the engine warms up to operating temperature, the fuel vaporizes properly and most smoothness issues largely go away.

phlfly · #5 11-23-2017, 08:42 PM

I have this glow plug :

https://www.ebay.com/itm/NEW-Mercedes-W140-W126-350SD-300SD-350SDL-S350-Diesel-Glow-Plug-Relay/151364568887?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649

during prestart up I have ABS light come up and goes away soon as car is running. Cruise control is working. AC is another issue. It's working , well compressor and producing super cold air, but working only with short plug, as I bought this way. I got new relay as you told me , but it didn't fix a problem, it will not turn compressor on, but it may be too cold now. I will try to nail air conditioner close to summer time.
Would be air conditioner not working because of this OV relay?

phlfly · #6 11-23-2017, 09:23 PM

One more as I said it starts but after few minute will misfire or unstable idle. Some days is not so bad some days is worst.

JHZR2 · #7 02-08-2021, 08:11 PM

Was this ever resolved?

Diseasel300 · #8 02-08-2021, 08:15 PM

Quote:

Originally Posted by JHZR2

Was this ever resolved?

OP was last active on this forum 1/29/19

JHZR2 · #9 01-27-2022, 01:56 PM

Quote:

Originally Posted by Diseasel300

Does your A/C work? Cruise control work? If so, the crank position sensor is good and the Tach signal from the EDS computer is good and likely the tach itself is what has issues. If neither work, suspect the EDS computer or the crank position sensor. The crank position sensors aren't really known for dying on these cars.

Can you walk through the crank position sensor, EDS and tach?

Mine is a 93 but think it’s about the same. Want EDS working right so I don’t have a low idle.

Does the CPS feed the EDS and then that corresponds with the hvac and tach? Or does the tach get its own signal?

I have a working tach. Coolant sensors seem to be working ok, I get the 5V signal and measure resistance on the sensor.

But low idle. AC probably locked out. Seems these things may be related.