My 450sl starts fine and will run for about 5 minutes until it gets
warmed up. At that point it starts to run so lean that is stalls.
I have tested the fuel pressure 73psi (both warm and cold) and it is
within specs. I did a fuel volume test both when it was cold and warm
- ok. The pump and filter were replaced about 2, 000 miles ago.
I tested the control pressure from the warm up regulator to the fuel
distributor. Cold the pressure was 22 psi, warm 50 psi. Both within
When I warm the car up and it will not run, if I hold the mass air
flow senser down a little enriching the mixture the car will run.
I have not tested the lambda control, but not sure that if it was
failing could lean the car out so much it would not run.
Tyger, the mixture must allways be adjusted with the car at normal
temp, isn't it?
My 2 cents: If you find lambda after the car is at normal temp,
measure the fuel pressure at the intake fule line: this pressure must
remain for 30 minutes or near that even with the car not running; if
not, you have a bad fuel pressure accumulator.
No, not necessirly... if you change the distributor, the car will not start
up, so you have to fiddle around with the mixture setting until the car
Once the car is warm, you do the final adjusting. In this case, I am
assuming the car is running lethargically during the warm up... so by the
time it is warm, the engine will die from too rich of fuel mixture.
He/she didn't mention hard starting issue... which is what you are thinking
of. and the technique you are talking about is for static fuel pressure that
will tell you why you have hot start issue.
As Tiger says, the basic mixture is set by the screw next to the air
flap (air mass meter). This should be adjusted when engine is warm.
During warm up (2-3 minutes) a warm-up regulator will enrichen the
mixture and gradually decrease the enrichment with the rise in engine
Your situation could be, that your basic setting is too lean, but due
to the warm-up enrichement, it will run until warm, i.e. until
....on the other hand, if the engine previously has been running well
with the present setting of the mixture screw, you should not change
it but instead look the cause of the change!
Definately you are running too lean, since pressing the air flap
provides better running condition.
I can see two possible reasons for the lean mixture:
1) If you have an intake manifold vacuum leak, which allow air to
bypass the air meter (flap).
2) Defective oxygen sensor, lamda control circuit or frequency valve.
At normal operating the output voltage from the oxygen sensor will
fluctuate between 0.1 and 0.9 volts, where 0.1 volts represents too
lean mixture and 0.9 volts represents too rich mixture. If the oxygen
sensor is stuck in the "rich position", it will cause the frequency
valve to lean the mixture. Or if the frequency valve is stuck in the
"lean position" (either by itself or by wrong signal from the control
unit), you would have the same symptom.
When both above things are cleared, the output of the oxygen sensor
should fluctuate as said with a duty cycle around 50%. The ealier
mentioned mixture screw is used to adjust this.... but of course only
when everything else is OK.
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