Bypassing the FDC. I’m new, be nice.

[Pa Well Driller]

I have a 1972 M35A2 with the multi fuel engine. I have had this rig for over 20 years….and it’s made me money most everyday. Lately I’ve noticed the making oil problem. Found many posts saying to bypass the fuel density compensator. However after more than a hour looking for the correct method of doing so, I can’t not come up with it. Could someone please help a fella out.

 

[Guyfang]

You might want to ID the truck, so as to help people, help you.

 

[Pa Well Driller]

You might want to ID the truck, so as to help people, help you.

Thank You, I did an edit.

 

[Pa Well Driller]

I found it. For anyone else having the same trouble. It’s in “The Deuce TM’s” The very last one as of 4/26/2025. Called ”FDC bypassing”.

 

[cattlerepairman]

I found it. For anyone else having the same trouble. It’s in “The Deuce TM’s” The very last one as of 4/26/2025. Called ”FDC bypassing”.

There is info on this site. One comment is to not use the hose loop as described, but to just plug the ports. Reason: eliminates a failure point.


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[Floridianson]

Just reminder when we bypass the FDC it's a good idea to put a pyro on there cuz you're going to have to adjust that fuel. Most cases we have to reduce the fuel to keep the egt's down. Also if and when you bypass you have to see still keep an eye on that oil cuz the booster pump can also put fuel into the crankcase. Also we have to be real careful when we go to adjust the main fuel. That will be the twin nuts. You have to use very thin wrenches and do not let the inner nut move. While holding the inner nut you break the jam nut or the outside nut loose. If you let the inner nut turn along with the jam nut you will break that stud and parts can be hard to find. You don't have tighten the jam nut every time you make an adjustment just leave it loose until you're done with your egts. Once you find egt you want to run that's when you can tighten the jam nut while holding the inner nut still Also it's a good idea to take a black magic marker and mark one of the flats on the inner nut so you have a reference point.

 

[Pa Well Driller]

Just reminder when we bypass the FDC it's a good idea to put a pyro on there cuz you're going to have to adjust that fuel. Most cases we have to reduce the fuel to keep the egt's down. Also if and when you bypass you have to see still keep an eye on that oil cuz the booster pump can also put fuel into the crankcase. Also we have to be real careful when we go to adjust the main fuel. That will be the twin nuts. You have to use very thin wrenches and do not let the inner nut move. While holding the inner nut you break the jam nut or the outside nut loose. If you let the inner nut turn along with the jam nut you will break that stud and parts can be hard to find. You don't have tighten the jam nut every time you make an adjustment just leave it loose until you're done with your egts. Once you find egt you want to run that's when you can tighten the jam nut while holding the inner nut still Also it's a good idea to take a black magic marker and mark one of the flats on the inner nut so you have a reference point.

@Floridianson
Thanks for the input. A little back story. In 19 years and over 70,000 miles I have never touched the injection pump. A couple months ago I noticed the making oil problem. Oil was thinner and had a slight fuel smell to it. And for a while now maybe 6-8 months it seemed like it was putting out blacker than normal smoke. The double nuts you are talking about, are just in front of the FDC, correct? I understand the inner nut controls the adjustment and the outer is just a jam nut. My understanding is that turning the inner nut clockwise ( making the (what is seen from the outside) stud longer) adds fuel. Is this correct? After breaking the jam nut loose, I turned the inner nut counter clockwise 8 flats, so 1 1/3 turns. Making the stud sticking out shorter, hoping to take some or her fuel away. While I was there I pulled the fuel shut off cover off and turned on the electric fuel pump. I definitely have a decent drip coming out of the shaft. I called Memphis Equipment and they sell no internal parts for these pumps. Do you know of a supplier ? I found a TM on here with a diagram. Looks like I need a preformed packing (Part K) .Would you mind if I direct message you? I’m new here but I assume that can be done on here. I would love to pick your brain.

 

[fsearls92]

You might also want to replace the o-rings on the hydraulic head of the IP as this can be a cause of making oil too. If you do make sure to use Viton o-rings.