Need help with V12 not starting
 

Hey guys,
Just out of the blue my 92 600SEL refuses to start. The engine cranks but it doesn't fire when I try to start it. The Palm scanner didn't reveal any useful codes and gave me some with "no descriptions". Any ideas what to look for and where to start troubleshooting? Thanks for your help in advance.

I think you should start with the usual diagnostics for a no-start condition.

1) are you getting fuel?
2) are you getting a spark?
3) does the car even crank?
4) any unusual noises when cranking?

Today I checked and verified the sparks from coil to the spark plugs, any idea how to check the fuel delivery? In the meantime I ran the scanner again and jogged the codes but not sure how reliable they are.

DM: code 06, Idle speed control incorrect, fault from EA/CC/ISC
DM: code 05, EGR incorrect, fault from fuel control system
SPS: code 30, no description
SPS: code 32, no description
RST: code 36, 37, 40, no descriptions
CF: code 36, control unit defective (N57)
CF: code 37, volt too low (9v) (t.30E F4-11)
CF: code 40, power supply terminal, 30B, control unit

Do you happen to know how to check the fuel delivery since I'm getting sparks at the plugs?

M120 (Early production) Cranks with No Start
 

Good Morning

You might want to check the engines compression. On some of the 1992-94 engines the pressure relief valve in the oil pump would stick resulting in excessive oil pressure. The high pressure causes the lifters to pump up and the valves remain open. The engine suffers from a temporary state of No Compression. There was a little known blurb in some of the "Diagnostic Literature" circulated through the dealerships at that time. A replacement valve was/is the cure for this temporary setback. The replacement is easily identified by a shoulder located on the end face of the valve. The modified end eliminates the sticking problem at the bottom of the bore in the oil pump housing.

If you discover the engine has no compression remove the spark plugs. Place a battery charger on the battery and charge to full capacity. To avoid unwanted damage to the vehicle/you/or the ignition controllers electrically disconnect BOTH EZL ignition control units. Once the battery is fully charged begin cranking the engine at 1 minute intervals. Allow the starter motor to rest for 5 minutes after cranking for 1 minute. Alternate this process until compression returns. This may take an hour or so.

If I remember correctly the 140 chassis was more challenging because the location of the oil pump. You need to drop the sub frame in order to remove the main oil pan to gain unobstructed access to the relief valve.

Thanks for the suggestion. How/when do I know the compression is back?

When using a compression gauge you'll begin to see the reading(s) rise above 100 PSI. The engine should start once you get above 100 PSI. Don't be alarmed by the amount of smoke you'll see after start up, it is nothing to worry about.

Afterward you'll want to run the engine pretty hard to clear it out. You may want to change the oil as well because some dilution can/does occur.