Oil Rising in Caterpillar D4 Diesel Engine

Saturday January 10, 2026

Subject: Oil Level Rising in My Caterpillar D4 Engine

Hello to all members of the ACMOC community. I need help understanding a problem I’m having with my tractor. With the engine running the dipstick of my diesel engine appears to show the oil level rising. My machine is a Caterpillar D4 tractor S/N: 7U22732 (built in 1953). The machine starts and runs normally and the coolant level appears to be constant. However I suspect that diesel fuel is leaking into the crankcase through the fuel injector pump assembly (this includes the fuel filter, fuel pump housing and the fuel transfer pump group). I’ve attached three diagrams to this posting in an attempt to understand and identify what critical seals might be compromised. Could someone in the ACMOC community tell me which seals in these diagrams might be suspect and should be replaced? I would also like to know more about the diesel fuel pathway and exactly how it flows from the filter assembly to the fuel injector pump. Many thanks from David in Shingletown, CA.

Saturday January 10, 2026

Subject: Oil Level Rising in My Caterpillar D4 Engine

Hello to all members of the ACMOC community. I need help understanding a problem I’m having with my tractor. With the engine running the dipstick of my diesel engine appears to show the oil level rising. My machine is a Caterpillar D4 tractor S/N: 7U22732 (built in 1953). The machine starts and runs normally and the coolant level appears to be constant. However I suspect that diesel fuel is leaking into the crankcase through the fuel injector pump assembly (this includes the fuel filter, fuel pump housing and the fuel transfer pump group). I’ve attached three diagrams to this posting in an attempt to understand and identify what critical seals might be compromised. Could someone in the ACMOC community tell me which seals in these diagrams might be suspect and should be replaced? I would also like to know more about the diesel fuel pathway and exactly how it flows from the filter assembly to the fuel injector pump. Many thanks from David in Shingletown, CA.

It could be possible to leak thru the front of transfer pump seal, I think it can leak past the fuel tower to pump housing also to enter engine? im sure another well experienced member will clarify it shortly.

transfer pump shaft seal is a common failure spot.

does the oil smell like fuel?

there is supposed to be a couple of weep holes on the injector pump they might be blocked. mud wasps love tiny holes.

I attempted to describe the solution with text and pictures but this crappy sight decided to dump my post.

I've also found that this crappy sight did not auto-renew my subscription.

I'm out of here, if you want response I suggest you use the www.heavyequipmentforum where the system actually works.

The o ring seal from the fuel tower to the injection pump housing is likely where your problem is. It's always the front one.

Picture of pump and what the failed o ring looks like. The rear plug is usually good but I always change both after going so far into removing the pump

Yes a big fat o ring is the right part, to replace the old tired 0 ring that looks square now.

OLD MAGNET, please dont go away. The much less educated of us need your knowledge. You have helped myself and many others over the years. I agree the site is not very user friendly but many of us depend on it. Grant.

I just did this exact repair on my D2 5U. It was not easy but i am not an experienced mechanic. It took me two days at about 4 or 5 hours each day. The seals are cheap and the Cat dealer has them in stock. Make sure to change the oil and oil filter after the repair.