Problems with petrol injector pump on 1968 250 SE Mercedes Benz W108 series automatic - Mercedes-Benz Forum

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post #1 of 16 (permalink) Old 06-20-2016, 01:53 PM Thread Starter
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Problems with petrol injector pump on 1968 250 SE Mercedes Benz W108 series automatic

My next door neighbour has completed a total restoration of a 1968 250 SE, W108 series Mercedes Benz automatic sedan including a total rebuild of the 6 cylinder petrol engine. I have been helping to get the engine running but there seem to be serious trouble with the petrol injection pump system. Although the engine will start it runs very roughly and pours lots of black smoke from the exhaust. After having the engine running, the spark plugs become sooty.
We have checked and double checked all the linkage adjustments and fuel supply hoses in accordance with the workshop manuals and installed new injectors but can't overcome the rough running or get the engine to idle.
We suspect the injector pump is injecting far too much fuel to the injectors as there are now signs of fuel in the sump oil but we can't find any sign of leakage from the pump to the main oil system.

Unfortunately, we haven't been able to find anybody out here in country Australia with experience or testing facilities for the Bosh injector pump (80327416 PESAKL 708120R21 ) and are working in the dark. I have read several posts by "bostosh" on the injector pump on "124wagon_boy's" 1966 250 SE coupe and have done most of the tests he suggested but have not solved the problem.
What could cause fuel in the oil? Could it come from leakage within the injector pump or could it could it come from excess fuel through the injectors

I would welcome any suggestions or assistance anyone can offer to help get this beautifully restored vehicle back on the road.
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post #2 of 16 (permalink) Old 06-21-2016, 05:30 AM
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It sounds like you have a defect in the cold start system which includes a cold start solenoid valve located on the intake manifold, a cold start relay and a temperature sensor located in the water path of the cylinder head.
Begin by disconnecting the wire to the solenoid valve. If the problem is still there it is not the cold start system and pump timing will need to be corrected. If the problem is solved you may wish to isolate the relay and temp sensor for proper function to eliminate the cold start valve as the culprit.
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post #3 of 16 (permalink) Old 06-21-2016, 03:51 PM Thread Starter
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Hi Yakl G
Thanks for your reply. Over the many weeks we have been trying to fix the problem, we have disconnected the wire to the cold start to avoid squirting fuel into the manifold each time we crank the engine to start but although the motor will still start it still runs rough. We have even tested that the pulse sent to the cold start solenoid is a pulse and not continuous. We have even closed off the fuel feed line to the cold start injector to eliminate any possibility of fuel being injected into the manifold while the engine is running but it makes no difference. We are still hopeful of rectifying the problem.

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post #4 of 16 (permalink) Old 06-22-2016, 09:57 AM
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The lubrication of the FI pump is independent of the engine lubrication and may not be the source of fuel in the sump.
A logical place to start is with a fuel pressure reading of the system which includes the fuel pump, lines and pressure regulator.
Leaking injectors can be responsible for excessive fuel as well.
Be very cautious of a leaking injector which could cause hydro-static lock in a cylinder that could result in a broken connecting rod going through the block on start up.
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post #5 of 16 (permalink) Old 06-22-2016, 04:55 PM
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I believe this injection pump oil is fed by the engine and not filled separately like the earlier 230s or the 6.3.
Will

Last edited by Baybear; 06-23-2016 at 07:25 AM. Reason: clarification
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post #6 of 16 (permalink) Old 06-23-2016, 10:48 AM
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The earlier pumps on the W108 still had their own oil, later ones shared the engine oil. Not sure what year they converted pumps. If it has the T handle dipstick on the back, it's independent of the engine. My 67 is still the earlier version.

Last edited by kdjones99; 06-24-2016 at 07:47 AM.
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post #7 of 16 (permalink) Old 06-23-2016, 01:00 PM
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It is my recollection that only diesel FI pump lubrication was integrated into the engine lubrication path.
To be sure look for an oil return line to the engine block by means of banjo bolt configuration to indicate your FI pump is lubricated by the engine oil path.
In any event a measure of the fuel pump pressure is in order before moving forward with replacing any components.
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post #8 of 16 (permalink) Old 06-23-2016, 05:41 PM Thread Starter
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Thanks for your responses. Our injector pump does not have a dip stick but does have an oil pipe to the engine block. We didn't know there was two types of lubrication We have been confused because the workshop manual we are working to stated a pre determined volume of oil had to be added to the pump when refitting. It stated the volume was critical for correct operation of the pump. We could not figure why the pump also had an oil feed as well as there was no indication that there were two types of pumps.
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post #9 of 16 (permalink) Old 06-29-2016, 04:27 AM
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Hi Bill,
I live in Brisbane and Mermad 6.3 does too - he has a fair bit of experience with these pumps, and has the Bosch contacts on who to send them too if they need specialist repair. So drop me a PM and I can pass on Ron's details.
Cheers, Drew
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post #10 of 16 (permalink) Old 06-29-2016, 02:23 PM Thread Starter
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drew56cus
Hi
What a welcome surprise to receive your post. I am very new to this forum and don't know how to make contact with you. What is a PM?

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