Trying to diagnose why i have air bubbles in the fuel return line (back of IP returning to spin on filter) of my 300D.

At idle no bubbles show up but as throttle is increased the bubbles begin and increase as throttle increases to almost a froth. No fuel leak visible from above will check underside of IP later today/tomorrow.

No bubbles visible in other fuel lines.
Pressure relief valve is tight (location of fuel return line banjo bolt).
Car idles fine, no shaking or rocking as throttle increases.

I believe this problem existed before, but here is work just performed:
Replaced camshaft - 2 lobes were badly pitted and discolored (lobes 8 & 9)
Replaced rear rocker arm spindle - bent probably a 3/16" over the rear third
Replace cam towers 4 and 5 associated with bent cam spindle
Replaced rocker arms 8 & 9 - discolored and faces showed wear
Replaced timing chain - 2mm method showed 4+ degrees...so while i was in there

Injection Pump in not original to the car, was bench tested/adjusted prior to install several years ago. Car sounds and runs better after above work but still sluggish.

thanks,

chris

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1985 300D, 250k
1980 300SD, 180K

Do you have the original primer pump? Or has it been replaced with the new style? Sounds like when its being throttled the pressure is building up and your sucking in air.

Here's my payback! Diesel Primer Pump Replacement

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cooljjay - Yes, newer style bosch primer pump is installed...but i will check nut for tightness.

thanks,

chris

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1985 300D, 250k
1980 300SD, 180K

Another place I would check are the rubber lines under the fuel tank above the rear. If ones is cracked and seeping, it could also cause air to get into the lines....they are usually over looked

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The Hand Primer and the Hoses that Connect the Plastic Fuel Filter are on the Suction Side.
See if you have any wet Fuel Lines from the Tank on forward.

On Mine one of the Metal Tubings under the Car Rusted through.

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84 300D, 82 Volvo 244Gl Diesel

cooljjay - I did replace all those lines several years ago but you never know - will check them. If they were leaking i believe bubbles would show up going into the pre-filter?


Diesel911 - Was just under the car repairing a small hole in center muffler (car in garage) and did not notice any wetness. Will trace rubber and metal tank to fuel pump lines. If there is air entering the system prior to IP, wouldn't the bubbles show up leaving the spin on filter in route to IP?

thanks,

chris

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1985 300D, 250k
1980 300SD, 180K

Hmmm I wonder if you are missing a crush washer some where? I am also wondering if its normal, not really air bubbles but fuel voids? I would think you would have an idle problem.

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What is a fuel void?

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'79 240D

Tomorrow i will take a look at how my 116 (300SD) fuel return line segment behaves from the pressure relief to main fuel filter as it has the same 617 engine and similar injection pump while working the throttle.

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1985 300D, 250k
1980 300SD, 180K

Bubble.

Your problem may also be a restricted pickup screen in the fuel tank. Feed your lift pump with a container of fuel to see if you still have the bubbles at higher rpms. This may help you decide where to look.

I am not a great fan of reversing the fuel pickup and return lines in the engine compartment. The fuel tank should be half full to do this and it is another method of clearing or condeming the fuel pickup from the tank.

I did replace the screen about 4 -5 years ago along with all the fuel supply/return hoses. FWIW original screen was clean. Still could be an issue.

thanks,

chris

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1985 300D, 250k
1980 300SD, 180K

Found bottom pre-filter clamp slightly loose but not wet. Also tightened top pre-filter clamp. All other fuel connections look good.

Still need to check rear fuel hoses at tank.

Need to wait a couple more hours to give my JB Weld center muffler patch a full 24 before testing with engine running.

thanks,


chris

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1985 300D, 250k
1980 300SD, 180K

That is part of the Mystery.
All of the Lines with the Exception or the 2 sections of Hose (cheap to replace with 5/16" ID Fuel Hose from any parts store as it is rated for Diesel Fuel) that attache the Plastic Filter to the the Lift Pump on are under pressure. So in theory there is no way Air can get into the Lines from that point on.

Do you know if you have an MW or M Type Fuel Injection Pump?

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84 300D, 82 Volvo 244Gl Diesel

Several times I have chased air issues in the fuel system. In a few instances I used segments of clear hose to provide visual inspection. So if you want to eliminate the possibility of a section of the system as culprit. Install a section of clear hose. no air, then move on to another section until you isolate where the air is getting in.

Either way its a pain in the butt. Good luck.

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85 300D 3 pedal. Current project.
83 300TD (need rear wiper assembly dead or alive)
84 300SD Daily driver
85 300TD almost 400k miles and driven daily.
98 E300D *sold
86 300SDL *sold and made flawless 10 hour journey to new home.

The consensus on this issue appears to be that the improbable is possible, which I agree with. Along that line of thinking, I suggest it is worth also taking a good look at the IP return line from the relief valve, as well as all connections and seals on the fuel filter stand. I suspect that the problem lies somewhere in that area, probably at one end of the clear line.

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'79 240D