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1989 Mercedes Benz 300sl intermittent hot start

Tiny
3SILVERFOX
  • MEMBER
  • 1989 MERCEDES BENZ 300SL
Engine Mechanical problem
1989 Mercedes Benz 300sl 6 cyl Two Wheel Drive 110000 miles

always starts from colds ometimes a bit reluctant, about once in 10 times. When hot refuses to start about once in 5 times, at such times if I keep cranking for about 15 secs it will sometimes start. When running ig goes faultless. I have replaced --fuel pump relay-distributor cap-leads-rotor-plugs-VOPrelay- battery-idle valve. Plus hours checking all wiring. Car does not have lambda sensor or a/c

updated 2-2-09
Thanks for the quick reply--I have replaced the coldstart thermistor in the cyl head- fuel pressure is 4bar running, switch tests out OK. I am told there is a one way valve on the fuel system, that this could be draining it down when hot. Do you know where it is located.
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Saturday, January 31st, 2009 AT 6:46 PM

8 Replies

Tiny
RASMATAZ
  • MECHANIC
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Test the fuel pressure to rule out the fuel pump and also the ignition switch also check if it has a cold start valve and also test the coolant temperature sensor
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Saturday, January 31st, 2009 AT 7:54 PM
Tiny
3SILVERFOX
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Thanks for the reply--I have checked the fuel pump pressure3.5bar-checked switch continuity and resistance all OK-replaced coolant thermistor. It has 4 connectors inside and the plug that goes into it does not seem to have a groove so that it goes in one way only, so it goes in either way. I am told there is a one way valve on the fuel system, maybe if it is worn the fuel may be draining back to tank. What do you think, do you know where its located. Still having the same problem.---John
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Monday, February 2nd, 2009 AT 5:24 AM
Tiny
RASMATAZ
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Fuel pressure
3.5bar is about 50psi

See specifications


https://www.2carpros.com/forum/automotive_pictures/12900_fuel_pressure_5.jpg

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Monday, February 2nd, 2009 AT 7:24 AM
Tiny
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HI -thanks for the reply--I have removed one of the fuel injectors and installed a blank---on connecting the injector pipe to the injector and ru nning the engine seems to be an excellent spray both in quantity and spray pattern. The system on this car is the KJetronic by Bosch. So I am going to have to look elsewhere, The cold start injector is coupled to a thermal switch and it all looks OK, I am thinking the fault maybe somewhere in the hotstart system somewhere. Do you have a wiring and position of components diagram. Thanks John
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Monday, February 2nd, 2009 AT 9:22 AM
Tiny
RASMATAZ
  • MECHANIC
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If I can remember back the Bosch K type is a mechanical continuous injection system CIS

Your fuel pressure should be 77-80 psi, what is the fuel pressure now?

Also check the cold start injector spray pattern

This injectiion system has fuel accumulator/mixture control unit you might want to check this components out. I don't have a schematic diagram on this type of injection in the Mit1 auto references. I'm very sorry this is as far as I can take you

FUEL DELIVERY
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Tech1lesson learned

NOTE:For fuel system pressure testing, refer to the appropriate BASIC TESTING article.

AFTER-START ENRICHMENT TEST
Connect meter adapter cable to differential pressure regulator. Set multimeter to mA scale. Unplug O2 sensor under right front floor mat. Current reading at operating temperature should be 7-9 mA. If reading is not correct, test coolant temperature sensor as previously described. If reading is correct, go to next step.
Disconnect Green wire of ignition switching unit (located on firewall) or install Protective Plug (102 589 02 21 00) into diagnostic socket. Simulate coolant temperature of 68 °F (20 °C) by connecting 2500-ohm resistor wire between coolant temperature sensor lead and ground. Crank engine for about 3 seconds and let key return to ON position. DO NOT turn ignition off.
After 4 seconds (including 3 seconds of cranking), current reading should increase to 24 mA. After 20 seconds, current reading should drop to 9-14 mA, which is basic warm-up current value. If readings are correct, test is complete. If readings are not correct, go to next step.
Connect voltmeter between ECU connector terminal No. 24 and ground. Crank starter motor. Cranking voltage signal (terminal No. 50) should be 10 volts. If canking voltage signal is not 10 volts, check and repair wiring. If voltage is correct, go to next step.
Check wiring between ECU connector and differential pressure regulator connector. See WIRING DIAGRAM article for wiring color and terminal numbers. If resistance reading is zero ohms, circuit is good and ECU should be replaced. If reading indicates open circuit, repair wiring
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Monday, February 2nd, 2009 AT 8:36 PM
Tiny
3SILVERFOX
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Changed fuel pump and ecu for ones known to be good-made no difference so changed them back to originals. Car drives and idles perfectly. I am of the opinion that it may have something to do with tha accumulator, but apart from changing it, no way to test it, and these items are expensive. Maybe it may have to go to the merc shop, I did not want to do this as I have always repaired all of my own carsjohn
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Wednesday, February 4th, 2009 AT 6:59 AM
Tiny
3SILVERFOX
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I have cured the fault on my car by my own efforts. I am afraid your advice was of no help at all. Of course it is unfair to expect your people to know everything about old type Mercs, and the Kjetronic is a very complex system, and one cannot go replacing parts one after another, because of the expense involved. That being the case I would expect you to refund my donation. The fault I finaly traced it to the fuel accumulator, which, unless you have a special service tool, cannot be repaired--so I replaced it with a new one, and I have not haqd any problems since. Thank you for your efforts.
John
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Friday, February 6th, 2009 AT 1:14 PM
Tiny
THIS IS MIKE
  • MECHANIC
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We are sorry we could not help you with this problem and are have refunded your donation. We hope we can help you next time you need us.
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Friday, February 6th, 2009 AT 1:39 PM

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