Fuel Percolation/Leak Down? 283SBC Fuel-in-Oil Woes

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RWBeringer4x4

RWBeringer4x4

Mechanically Challenged
SILVER Star
10 Year
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Hey all -

For as long as I've had this truck, I have been winding up with fuel in my oil. For background, this 40 is running an old 1965 283 SBC V8, which runs fairly hot (but not dangerously so). It has an Edelbrock SP2P intake with a Rochester Quadrajet off a 1967/8 Buick. I believe I'm running a bit rich. It is cooled by the stock FJ40 radiator and a clutch-less rigid fan.

Here is what I've tried, so far, to address the fuel-in-oil issue:

1. Rebuilt the quadrajet using all ethanol-safe gaskets and seals. Bored out the leaky welch plugs, tapped the holes, and replaced them with allen-threaded inserts, then covered with marine-grade epoxy. Set the float on the low side. This thing should not be leaking fuel...

2. Just a couple weeks ago, replaced the fuel pump (stock, mechanical, 5.5-6.5 gph), in case the other had failed and was leaking fuel into the crank case. It showed no evidence of failure, but suffice it to say it has a new fuel pump.

Changed the oil after the fuel pump was replaced, drove it 50 or so miles, and the oil is smelling gassy again. I'm starting to wonder if I have fuel percolation/boiling issue.

I'm thinking this because after the truck has been run at temperature for a while, and then sits for about 10 minutes (ie: I run into a store) it is difficult to start. Only starts with lots of cranking and in some cases holding the throttle open. When it does start, I get a big puff of black smoke - seems like it's flooding after the engine shuts down.

I'm not sure there's a way to confirm this theory - any ideas?

FUEL LINES: The fuel lines are 100% rubber after the pump. I'm thinking of going back to metal - partially for safety, and partially because I've heard has hot rubber lines cool, the contract, and push a bunch of fuel into the car. Maybe an old wive's tale? When routing hard fuel line, how far off the heads/ intake manifold should I keep it? Is there a rule of thumb?

PHENOLIC SPACER OR HEAT SHIELD?: Anyone used one of these? I don't care about performance gains, but they also apparently insulate the carb from engine heat. Not sure if a heat shield or a phenolic spacer is the best route. The internet seems divided.

Any thoughts? If I've ruled out the fuel pump and the welch plugs, is there any other way the engine could be flooding/getting fuel in the oil when shut down and hot? Looking at you, @SuperBuickGuy and anyone else with V8 experience.
 
Also forgot to mention - There are no vapor/fuel return lines or any emissions controls whatsoever.

Here's some shots of the lines, for visual aid!

At the Carb - you can see i have a fairly thick gasket under there already...

Fuel Line 1.webp


Currently routed over the passenger side valve cover, through an inline filter (the filter in the carb is also present).

Fuel Line 2.webp


At the fuel pump:
Fuel Line 3.webp


The callenge with routing hard line at the fuel pump will absolutely be getting it between the engine and the radiator hose...about 1" of clearance...
 
Last edited:
Add a shroud to the radiator and it will lower the engine temperature.

What do your plugs looks like?

Not sure how you get gas in the oil if not leaking from the backside of the fuel pump. Fuel coming out of any carb vent would end up inside the intake and could be causing a flooded condition.
 
What happens after you run it for a while and then pull the gas cap. Is the tank pressurized? A sealed off 40 tank can push fuel through the carb needle and down the throat of the carb.
 
B y r o n said:
Add a shroud to the radiator and it will lower the engine temperature.

What do your plugs looks like?

Not sure how you get gas in the oil if not leaking from the backside of the fuel pump. Fuel coming out of any carb vent would end up inside the intake and could be causing a flooded condition.
Click to expand...

A shroud is on the list. This whole engine is going to get moved backward 4 inches or so next summer. Right now, it's way too far forward and there's no clearance for a shroud.

Couldn't the gas run from the intake down into the cylinders...and from there around the rings and into the oil? Given that it isn't as viscous as oil I could see it slipping past the rings (and probably destroying all the oil on the rings at the same time).

As for the plugs - I just replaced them. The last set was pretty foul - some oil on a few of them (probably leaking valve stems, compression was fine). However, I have no idea how long they were in there - they're the same ones that were in it when I bought it.

Plugs.webp


Mace said:
What happens after you run it for a while and then pull the gas cap. Is the tank pressurized? A sealed off 40 tank can push fuel through the carb needle and down the throat of the carb.
Click to expand...

@Mace - I've never noticed the tank being pressurized, but I haven't checked recently. There's never been a "woosh" when I crack the tank to get fuel. I'll definitely check it out next time I get gas, but I believe it's a vented cap. Were gas caps still vented in 75?
 
brian said:
no, by 75 the charcoal canister was in use.
Click to expand...

Ah, didn't realize those two things were mutually exclusive. My canister is still there, currently connected to nothing on the engine side but it looks like the hard line to the tank is still intact. Maybe the tank is venting through the canister?

image.webp


Both the upper and lower rubber hoses you see are cut.
 
Any other thoughts on this?

Does anyone have any experience with phenolic spacers/heat shields if the issue is, indeed, fuel percolation?

Any guidance as to how far away to keep hard fuel lines from head/valve cover/intake manifold surfaces? Looking at several pictures online it looks like there is often less than an inch or so of clearance.
 
Float setting? How does the needle valve look on the float assembly? If its worn it might allow the float bowl to overfill from the pressure in the line.

Do you have any other evidence that the truck is running rich? Just the plugs and it might be the heat range of the plug leaving them sooty. Might look into what others are running for this engine.

Does the carb have a fuel shutoff valve? If so replace it.

Are there any change in colder weather?

take a look here:
http://forums.aaca.org/topic/155995-can-boiling-point-of-gas-be-raised/

Also you could try Water Wetter to increase cooling efficiency for a test,

Pete
 
peteinjp said:
Float setting? How does the needle valve look on the float assembly? If its worn it might allow the float bowl to overfill from the pressure in the line.

Do you have any other evidence that the truck is running rich? Just the plugs and it might be the heat range of the plug leaving them sooty. Might look into what others are running for this engine.

Does the carb have a fuel shutoff valve? If so replace it.

Are there any change in colder weather?

take a look here:
http://forums.aaca.org/topic/155995-can-boiling-point-of-gas-be-raised/

Also you could try Water Wetter to increase cooling efficiency for a test,

Pete
Click to expand...

I replaced the needle and seat when I rebuilt the carb - It has only been run about 500 miles since the rebuild. I set the float on the low-side of nominal for the carb. I forget the exact setting but it shouldn't be sitting too high.

Only evidence I have of the truck running rich is the fact that it will smog you out of a two car garage in about 5 minutes - really gets the eyes burning! General black soot from the tailpipe as well. It doesn't kick out black smoke unless it has been sitting but it's a rather smelly old beast.

The Quadrajet is really WAY too much carb for a 283 - so I wouldn't be surprised if it's running a bit rich. It's got vacuum secondaries so it only pulls in the air it needs, but I'm not sure what jets/rods I'm running. The Q-jet provides air "on demand" but fuel is still managed by the idle circuit and jets, which might be too large. Other people run Q-jets on 283's but I haven't been able to find a consensus on proper jetting.

I don't think there's a fuel shut-off valve, at least not one I can remember from the rebuild.

Cold weather - I'm not sure. They salt the roads here in the winter, so the truck stays in the garage.
 
If your engine tends to run marginally hot, that may be hot enough to heat sink after shutdown and boil off the fuel in the float chamber, especially if you are running ethanol. The expanding fuel could then overflow into the Venturi tubes , draining down into the intake runners and then the cylinders/rings. Thus: gassy smelling oil.

Heat sink could also expand fuel in the fuel lines close to engine and push too much gas into float chamber: same problem. Setting up a return line might help some. Kinda hard to prove this theory. If you wait 1/2 after shutdown, you could pull air cleaner and look into throat of carb for signs of wet fuel. Unfortunately, if you open the throttle to inspect the intake then the power valve will squirt fresh fuel in. John
 
inkpot said:
If your engine tends to run marginally hot, that may be hot enough to heat sink after shutdown and boil off the fuel in the float chamber, especially if you are running ethanol. The expanding fuel could then overflow into the Venturi tubes , draining down into the intake runners and then the cylinders/rings. Thus: gassy smelling oil.

Heat sink could also expand fuel in the fuel lines close to engine and push too much gas into float chamber: same problem. Setting up a return line might help some. Kinda hard to prove this theory. If you wait 1/2 after shutdown, you could pull air cleaner and look into throat of carb for signs of wet fuel. Unfortunately, if you open the throttle to inspect the intake then the power valve will squirt fresh fuel in. John
Click to expand...

Yeah - the heat sink thing is my exact theory. I'm wondering if a phenolic spacer and hard lines will help it. The spacer to help insulate the carb from the hot engine, and the hard lines to prevent as much fuel expansion.

I suppose I could LISTEN for leak down or boiling fuel in the carb after shutdown. Not sure if that's really plausible or not...
 
Yeah, the primary air dam hangs open. I figure I should be able to see fuel dribbling into the primary venturis if it's boiling after shutdown.
 
I say give the spacer a try.

Seems like there must be some kind of aluminized sleeve you could put over the fuel lines where the rest near the engine…
 
peteinjp said:
I say give the spacer a try.

Seems like there must be some kind of aluminized sleeve you could put over the fuel lines where the rest near the engine…
Click to expand...

Yeah, there's protective heat tape, which apparently is good up to 1100 degrees...

I think I'll start with just running hard line from the pump to the carb - that's how it's "supposed" to be anyway. This shouldn't allow for as much thermal expansion in the lines.

Then I'll add the phenolic spacer under the carb.

If the problem persists, I'll probably just live with it until I can move the engine back and get a fan shroud installed and get the sub-par cooling issue solved (hopefully next summer).

Before I do ANY of this, however, I'm going to check and see if I can find any evidence of fuel boiling, and double check the engine timing again, to make sure that it isn't the timing resulting in a rich condition.
 
Redline Water wetter? Worth a shot?

I've seen some fan shrouds that go right up against the radiator. East enough to make- maybe not worth the effort required to install..

Pete
 
Well, I'm embarrassed to report I found part of the problem. In the three years I've owned the truck, I never really fooled with the distributor. It's a HEI unit and had always worked ok so, "if it ain't broke..."

Well, turns out the distribtor wasn't clamped. It was free to rotate. In short, something tells me my ignition timing is off, which could be the source of my rich running condition. It doesn't solve the flooding at shut-down issue, however.

Of course, the second I broke out the timing light last night, it promptly broke so I haven't been able to verify the timing. Once I get a new light, the search will continue.
 
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