Problems with petrol injector pump on 1968 250 SE Mercedes Benz W108 series automatic - Page 2 - Mercedes-Benz Forum

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post #11 of 16 (permalink) Old 07-09-2016, 04:25 PM
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A few quick questions. Doid you take the pump off? Is it back in the correct place ? Pump timng is noty real important but it helps .
Disconnect the cold start valve completely . No fuel supply to it. Connect a pressure gauge to the cold start supply line and turn on the key. If the pressure is greater than 14PSi (1 bar) you have a blockage,probably at the return line to the tank.
Now with the throttle link disconnected at the pump, start the engine. how does it run now?

Theres a mountain of things you have to know about MFi to get them running properly, but the first is never touch the pump!.
If the pump is indeed leaking fuel into the oil, the element seals have gone .
MFi pumps can be quickly and expertly done at prime Fuel injection ,Archerfeild. Derek is the man in Charge and really knows his stuff. Prime Fuel
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post #12 of 16 (permalink) Old 07-09-2016, 11:54 PM Thread Starter
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Hi mercmad6.3
Many thanks for your response. When the car was bought many years ago it was not in good condition!!! The engine was in pieces ( large pieces), in the boot, the injector pump was separate, still with the fuel pipes connected. The engine was subsequently completely reconditioned and re built and is now considered in good shape. The control mechanism on the injector pump was not touched but the individual piston had to be "unstuck" and re installed. We certainly have disconnected the cold start injector valve and even plugged off the fuel line to it to try to improve the running. No change was noted. We have also checked the pressure at the fuel line to the cold start valve and had 20 psi, the same as the fuel pump pressure. 19psi is the only pressure level we can find mentioned in the workshop manual and that is for the fuel pump itself.
What should the working pressure be in the pump? Is that pressure controlled by the pressure relief valve in the return line? It would make sense to have a pressure in the pump at some value less than the pressure output from the fuel pump. If 14psi is the working pressure, we can adjust the relief valve to suit.
At least you have given us another line of attack. Thank you.

Bill A
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post #13 of 16 (permalink) Old 07-11-2016, 03:34 PM
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19PSI means the small line into the fuel tank is blocked. There is return valve on the out put end of the injection pump, this may need cleaning.But the actual pressure to the injectors is around 200PSi. If any of the pump elements were seized you would be better off getting the entire pump rebuilt. Note" you set the engine to 60 after TDC to install the pump which has a small notch on the drive nut . this notch and the mark on the pump body must be aligned. If it's running super rich ,either the cold start solenoid or the fuel rack is seized which again is a job for the specialists who set the pump on a test bench. I have a complete used pump which was working fine if you want to save money ,$500 .
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post #14 of 16 (permalink) Old 07-11-2016, 10:57 PM Thread Starter
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Hi Ron,
We know the return fuel line is clear but we suspect that the relief valve in the return line is set too high, hence the normal fuel pressure in the pump is so high. If 14psi in the cold start fuel line is too high the normal pressure in the pump housing must be somewhat lower than 14psi under normal working conditions. Do you know what that normal working pressure should be?

The pistons in the pump were certainly not seized but needed a good clean. They were each cleaned and replaced in their housings and we believe back into the correct position on the control rack. We have wondered if they were all one notch out on the rack whether the engine would still run roughly but with excess fuel burning causing the black smoke and the excess fuel finding its way into the sump. It's a theory but don't know enough about the at rest position for the Pistons be able to check.

We could be interested in that pump you have but need some more info about it.
I do appreciate your assistance.

Bill A
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post #15 of 16 (permalink) Old 07-12-2016, 12:48 AM
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The pressure is constant between the pump and the cold start valve . The higher pressure won't make it run rich but it causes vapour lock in hot weather .The return valve wont cause a high reading though unless you have a blockage. Crushed fuel lines under the car are common where someone has lifted the car on a two post and lifted under the floor.
if you have touched the plunger elements with bare fingers there is every chance they are marked now ,and you can't really set them by guess work . The rack is worked by the governor via the cold start and cold running links ,any maladjustment there will affect the running,hence the need to get them set up on the test bench. As i mentioned before there is a oil pressure seal which lubricates the plungers but also prevents fuel leaking into the base of the pump and then into the engine where it does a great job of wearing everything out.
if there is a lot of petrol smelling oil in the governor case you need to get the pump rebuilt . A governor floating liquid makes the pump slow to operate .
The pump i have is still on the engine, which was removed from a car because some clown had bolted a manual flywheel on and the imbalance wrecked the crank and the block. The rest of the engine is OK. You are welcome to come over and have a look ,I'm at rocklea,ph 0402413373.
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post #16 of 16 (permalink) Old 07-12-2016, 01:11 PM Thread Starter
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Hi Ron,
Thanks for your latest post. I am going away fo a couple of weeks so will be out of contact. If we wish to come over to see the pump I will ring you to make a suitable time when I return.

Bill A
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