• Hi Guest!
    You can help ensure that British Car Forum (BCF) continues to provide a great place to engage in the British car hobby! If you find BCF a beneficial community, please consider supporting our efforts with a subscription.
    There are some perks with a member upgrade!
    **Upgrade Now**
    (PS: Subscribers don't see this gawd-aweful banner
Tips
Tips

MGB Fuel in the oil

Share this page

drooartz

drooartz

Moderator
Staff member
Gold
Country flag
Offline
So onto the next thing to sort out with my MGB. Since the engine rebuild (2 years ago, maybe 2500 miles) I've noticed that the oil level was occasionally going up. Not good. Car has been off the road for a year so hasn't been driven much since I noticed this.

I drove the car about 100 miles last weekend. The oil level before I set off was about 1/2 between the min and max levels. When I checked the day after the oil level was up over the full mark. So I have an issue somewhere.

To see what might be going on, I sent an oil sample off to Blackstone labs for a looksie (before last weekend's drive). The report back showed 0 coolant (woohoo!) but did show some fuel in the oil. Not more than they consider problematic, but it was present. Their comment was that if the level keeps going up, to check the fuel system. So here I am.

Where do I start looking?
 
Thats a tough one, since the most obvious and common method of getting fuel in the oil is themechanical fuel pump with isn't part of the MGB's system. Does it happen while you're driving, or after being parked for a while. If it happens while parked, I'd suspect fuel dripping into the cylinders through the carbs and past the rings - but even that seems unlikely because the float bowls on the SU's are below the jets, and once the key is off there's no pressure to force fuel past the bowls. If it happens while running, then possibly a bad valve guide seal on an intake valve allowing fuel to enter the crankcase but normally the vacuum of the intake would cause it to suck oil out instead.

The fact that he level goes up substantially on the stick overnight makes me still think about fuel seeping thru the carbs into a cylinder, and then getting past the rings. That seems like more fuel than could be accounted for in just the float bowls. Is your fuel pump somehow accidentally wired to an always-on power source where its possible to be slowly pushing fuel past the float valves over the course of the night?
 
Does seem to happen after a long drive. I did check the car once during the drive last weekend and the oil level was still OK if I remember correctly, so I am thinking it's from sitting after a drive. I don't think the pump is wired incorrectly, but I'll have to check it. I've not touched that wiring, though the shop that did the work on my car (curse them) may have. Easy enough to check

I was thinking something in the carbs as well. Hadn't thought about the fuel pump. Could the gas tank be getting pressurized somehow and causing the same thing?

I'll dig into later this week or this weekend. Thanks for a few things to look for.
 
I don't know if this is related, but that drive last weekend was about 40 miles one way. When I got to that half way point, the car seemed to diesel when I shut if off, took a little for it to actually stop running. May be unrelated, but that is not a normal thing for this car.
 
Pressure in the tank? Possible - easy to check by taking the cap off. Other thing you could try would be to disconnect the fuel line right at the carbs when you park (stick the end in a small glass or metal jar/can - nothing that fuel will dissolve), and see if you end up with fuel there over time?
 
That's a good idea. I'll give that a try when I get some time.
 
Back
Top