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Before these threads about the IP fuel pressure came about I was trying to figure out a miss fire issue I had with my fathers 300dt. I like reading the FSM as there is a ton of good info in it, but with out the index its a bit of hit or miss to get the right page. Well I found the page relating to the lift pump pressure and decided to test it with a gauge. I made an adapter and low and behold the pressure was <0.6bar at idle.
FSM says to stretch the spring to 27mm, so thats what I did. The spring was around the 20mm length and I thought WOW 7mm strech thats a lot. But the silly Germans say to do it and I did. The pressure went up to ~1.0bar, at idle.
I was not happy with the pressure when I crimped the return hose, so I took the lift pump apart and honed the check valve faces. Put it back together and when I clamped off the return hose pressure went past the 30PSI the gauge could read.
It did not help the miss fire but it was within MB's spec so I was happy.
I did the samething to my IP relief spring about 30min after I finished with the 300dt. That spring was in the 20mm length range too. Thats was ~18k ago, I just added a gauge to see if the spring was not keeping the pressure up. It still is, but I fear my lift pump check valves will have to be honed soon, as the pressure with the return line clamped is just above spec.
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