W126 420SEL Stalled- Won't Start
 

Hi Everyone,
Last week my '88 420SEL stalled while accelerating from a red light and it won't restart. I have checked the following :

- opened an injector tube, ignition on, fuel sprays temporarily. While cranking the engine, fuel is sprayed.
- jumping the fuel pump relay causes fuel spray
- checked the spark plugs, they are new. Wet with gasoline.
- checked for spark on both banks. Seems OK.
- I inspected the distributor cap although it is new. In good shape.
- Checked the ignition coil résistance between the three connection. All as per manual.
- Position sensor ohm OK.
- Checked the cylinder compression on both banks, it's about 10 bars. I am not sure if that's high enough.

What else can I check? Can there be water in the fuel? How can I check for that?

Thanks for any help.

The ignition has to jump a 1/4 gap and be blue if spark looks orange may be weak coil , make sure both fuel pumps are operating

zorin:

Re: Ignition
Turn the engine (in the direction of normal rotation) until the top dead center (TDC) mark on the harmonic dampener aligns with the pointer. It will be on either #1 or #6. Remove distributor cap. The rotor must be pointed either directly to a line scribed on the rim of the dist. housing, or 180 deg. away from the mark. If not pointed as described, a fault may have occurred in the dist. drive components, including the timing chain, sprockets, and guides. Also check the rotor for security on the dist. shaft, and for free movement of the rotor centrifugal advance mechanism (will the rotor turn ~10-15 deg., and then spring back?).

Re: Fuel
One pump will be sufficient to show a flow at a loosened injector line, but does not provide enough pressure to open the check valves. One pump will, however, provide enough pressure for the cold start valve to supply enough fuel for a very brief start.

Ensure that spark timing and quality are correct before chasing fuel system faults.

Engines need fuel, spark, air...

A favorite trick of mine is for someone to spray engine starting fluid (get at automotive store) into the engine air intake as someone else starts the engine. You will get a response from the engine if there is a fuel problem.

And as stated above, some fuel pressure is not enough, it needs to be a certain pressure - properly test with a fuel pressure tester (Harbor Freight sells a kit which includes Bosch Jetronic adapters for not too much $$.)

Thanks for the replies. I will run the checks.
BTW, I noticed the oil level is a bit higher than the mark. Would that have caused any damage?

Thanks again.

From what I understand overfilling the oil can cause oil leaks from the overpressure. I doubt it would cause it not to start.

The high oil level is a potential clue; if the diaphragm in the fuel pressure regulator (FPR) has failed, the leaked fuel is discharged thru the leak line to one of the hoses of the crankcase ventilation system, and thence to the crankcase. A slowly failing FPR diaphragm would leak fuel while still maintaining sufficient pressure for the engine to run. When the diaphragm finally fails completely, the engine stops.

When I checked the sparks, I noticed that they were not blue but orange. They seem weak. Although the resistance between the three connectors are exactly what they should be. So what can cause a weak spark?

1) What three connectors do you refer to?
2) What is the voltage at the positive [+] terminal of the coil while cranking the engine?

Hi Frank

Between the two terminal that wires are connected with nuts, I have about 0.3 ohm.
Between any of these terminals to the connection that the distributor cap cable plugs to, there is about 10k ohms.

While cranking the voltage between the terminal with larger nut and the grounded pole of the battery is about 8 volts or less .

Thanks for all the help.

The coil might be going bad or have failed if the spark is weak/orange. I've had it happen before on my 300E and had to replace it. Is it the original coil?

The coil was changed couple of years ago. Although I have ordered another one, in the meantime I tried the ignition coils from a 300E and a 1994 420SEL that is off my friend's cars. Same problem. The engine cranks but does not start. I checked the spark right at the wire going to the distributor cap, it seems week. The cranking voltage on the positive connection is 8-9 volts.

z:

The previous question about the primary voltage was to determine if that is the cause of a weak secondary spark. It is! At 8-9V. primary, expect a weak spark. In the second reply of this thread, it was suggested that you check the rotor position, vis-a-vis the crank position. Did you? Have you used a timing light to insure that the sparks are occurring at the correct point?

I checked the distributor, it looks okay and it moves freely to advance.
I will check the ignition points tomorrow. I am not sure how to exactly turn the engine manually, socket and a wrench? It doesn't seem easy to fit a socket in front of the engine.
If the cranking voltage of 8 volts is low, I am hoping the new ignition coil will fix that. I should receive it tomorrow.

Thanks.

If the Reference Resistor is bad would it cause the cranking voltage at positive terminal of coil to be that low?