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?

I replaced the coil with a new one today. Still no go. Cranking but not starting. I check the spark plugs, they seem a little wet with gasoline. I also tried pressing the accelerator pedal all the way down to stop the flow of gasoline.

While cranking, the intake butterfly goes down by about half a mm and it resists when pressing down.

At TDC, the rotor was pointing at the Cylinder 1 (three scribed marks) .
Reference resistor is about 750 ohm.

Would a blocked catalytical converter prevent the engine from starting?

In your first post you state that you were "accelerating from a red light". You may have to take this matter up with Santa; you have been naughty. Please, for the sake of the rest of us, restrict your accelerating to green lights.

The reference resistor will not affect starting.

If the engine was running, and the car accelerating when the engine quit, it is possible, though not not very likely, that the cat suddenly became 100% blocked. Usually cat blockage is a gradual process. Have someone hold a piece of paper against the exhaust outlets while cranking the engine with the throttle open. There should be detectable puffing of air out of the pipes.

Perchance, had you refueled just before the engine quit?

:) , yeah, i meant the traffic lights :)

I had not just refueled at the time. I also sucked out gasoline (about the volume of small water bottle ) and checked it for water, which would accumulate in the bottom. I did not see any sign of water. I also poured on a piece newspaper and lit it. It flamed without the paper turning into ash, until fuel was burnt up and the paper started turning into ash.

BTW, outside temperature is -5 F here! I am thinking if the spark plugs are too wet that might prevent the combustion?

What would cause the low voltage (8-9 volts) at the positive terminal of the ignition coil while cranking? Before cranking I make sure the battery is charged to about 13 V.

Thanks for all the inputs.

Reduced primary voltage is a normal condition when cranking; the starter motor draws so much power that the system voltage is reduced. When a battery is in top condition voltage will stay above 10V., however, with age and low temps the voltage decreases. Below ~9V. spark quality falls off quite a bit. The problem is compounded by the decreased vapor pressure of the fuel at low temps; winter blend fuels have higher vapor pressures. Wet and sooty spark plugs also bleed off spark energy.

It would seem necessary to try to describe the shutdown of the engine; was it as though the ignition had been turned off, resulting in a smooth and quiet shut off, or was there misfiring, popping, rough running, or backfiring? Although it seems obvious, if you can determine what caused the shutdown, you will probably have found the reason for not starting again. The wet plugs and weak spark may contribute to the problem, but what caused the initial failure?

Fixed! Finally.
 

After a lengthy battle with problem of cranking but not starting, thanks to the input and help of members of this group, I managed to fix the problem, or I should say problems. :)

I guess I was so unlucky that many problems happened around the same time and the fact that cold weather arrived compounded the problem. Here is the list and the fixes:

- I had 4 defective spark plugs. I am not sure what made them go bad so quickly as they were just a year old. I replaced them. I will replace the rest after the holidays.

- There were 3 bad coils. Their resistance was few Mega ohms! I replaced them.

- Replaced the ignition coil, since originally I thought that was the problem.

- The plugs were wet and I guess the engine was flooded because of all the cranking I have been doing, even though I was occasionally flooring the gas pedal to stop the flow of gas to engine. So I pulled out all the plugs and dried, cleaned and re-gaped them. And while the plugs were out, I cranked the engine to push out the gasoline from cylinders.

- After performing all these fixes, the engine was cranking but still no start. It seemed that the battery was draining quicker as the result of cranking. So I used booster cables and used the battery from my other car while the rpm was raised and kept above idle speed. This made the engine start and keep running. Plugging the block heater helped as well. I guess the battery was no longer in top condition after all this, although I had mostly kept it charging.

In summary, the problems were: 4 bad plugs, 3 bad spark plug wires, probably a bad ignition coil, flooded engine, and a battery that was not good enough to start the engine in this cold weather.

Thanks again for all the help.
and
Happy New Year.