1970 Cutlass Supreme
350 completely rebuilt. (me) No Hi Performance stuff.
2 barrel carburetor (rebuilt by a pro shop)
New Carter fuel pump (3 line pump)
New fuel lines from tank to carb.
Inline glass fuel filter.
I've got about 250 miles on everything and it runs and drives good.
BUT, the other day after I shut it off I was watching the fuel filter and noticed a small air bubble travel through the filter towards the carb. It eventually drained the filter overnight.
With the engine running afterwards the filter will fill with fuel but have a small void and not completely fill 100%.
Then the following day after I drove the car I observed the fuel filter at idle. I can see the fuel rushing through the filter but it won't fill the filter but it still runs fine.
I've checked checked the oil many times to see if the engine is "making oil" and everything looks good.
There's no obvious puddles or odors of leaking gas and I've retightened every clamp and fitting.
This might be a "red herring" but the fuel pump is over 3 years old but only has maybe 6 hours on it.
Fuel pump gone bad?
Where is this fuel draining back to?
Thanks

 

More posts have been written because of these transparent fuel filters than anything else.

First, if the car starts and runs reliably, you don't have a problem.
Second, the factory fuel filter in the carb has a check valve to prevent drainback. Clearly this aftermarket filter does not.
Third, the air pocket won't effect anything. Don't lose any sleep over it. This isn't a high pressure diesel injector where air matters.
Finally, if you have a mechanical fuel pump, the check valve in the outlet port of the pump should limit drainback like this. As the pump ages, the valves start to leak. This is compounded by ethanol causing deterioration, especially if the pump is older.

 

If the car starts the next morning quickly, this a non-issue and there is no problem.

 

Hmmm,
Starting quickly might be a matter of opinion.
The engine does have to crank maybe 6 - 8 seconds before it'll want to fire on the first start of the day. It's not instantaneous like does after running a bit.
I can tolerate the time it takes on the initial start.
My concern is, if it is draining back is the fuel getting into the oil
The other concern is the fuel pump failing and leaving me stranded.
I probably pull the glass filter and test it for leakage and maybe replace it with a solid metal one.

Thanks for the replies.

 

People are spoiled by EFI systems with high pressure electric fuel pumps that prime the system before the starter motor turns. For carbureted cars that are not driven regularly, the fuel in the fuel bowl evaporates. This is normal. Cranking is necessary to refill the bowl. I'm guessing all your other vehicles are EFI. You do pump the gas pedal once or twice to set the choke and prime the motor before a cold start, right?

If the fuel is draining back, it is going into the fuel filter and from there back into the fuel lines. The only way it can get into the oil is if the diaphragm in the fuel pump is bad. If that were the case, your dipstick would reek of gasoline. Also, the oil level would keep going up.

If the fuel pump hasn't been replaced in a while, it would be prudent to get a new one as a preventative measure.

 

Rubber hoses between components can be pinched closed to narrow the location of leaks, e.g., fuel pump supply side.

Where is the glass filter? If it is on the supply side of the pump and installed with rubber hose consider eliminating it and going to steel tube.

Good luck!!!

 

Where is the glass filter? - supply side between the pump and carburetor. It's one of those take apart deals that will be replaced later this week.
When I put the car together I used the late model rubber fuel injection hose in hopes of avoiding ethanol breakdown of the regular rubber fuel in the short sections where needed.
That fuel injection hose is STIFF and somewhat resistant to complying with my wishes, lol. I read the thread about the Wix 33003 filter and will try that along with the more compliant rubber hose. FWIW, I'm using new Ideal brand worm drive clamps.
I'm going to test the glass filter as well and report back.
Thanks everyone for your suggestions.

 

Remove the glass filter and toss it in the trash, it’s a fire hazard.

 

Follow up on drain back problems.
First of all I tested the glass style fuel filter with air and soapy water and no leaks.
I sent the carb back to the rebuilder and they replaced the booster, accelerator pump and the needle and seat.
Reinstalled the carb and the car runs good but the fuel still drained back as viewed through the clear filter.
Installed a brand new Carter three line pump and the drain back is the same. Although, maybe twice the filter has shown that the fuel line stayed full overnight.
I've put three fuel pumps in a vice in their proper orientation, set up a gravity feed to the line that feeds the carb and all three drain back fuel out the return line in less than a minute,
eventually draining the makeshift reservoir.
Two were new pumps.
Is this supposed to happen?
Is the function of the return line similar to that of an EFI system where any unused fuel is returned to the tank?
The problem is bad enough that the engine will start but sometimes stall because the pump hasn't caught up to the gap in fuel.
Is there any merit to the idea of converting to a two line pump?
I realize it's hard to diagnose these issues without actually being here.


.

 

The stock filter in the carb has a check valve to prevent drainback. Apparently your aftermarket clear filter does not.

 

Interesting read. There shouldn't be any filter between the fuel pump & the carburetor, except the original on the carb. Use a steel line. A solid metal WIX filter is good before the pump. Rubber fuel line on the engine is a fire hazard, as is a glass filter. Watching the fuel level through a clear filter is an exercise in futility. I replaced my fuel pump (seeped) & installed a new filter, at the carb. The car will start after 3 to 4 seconds cranking when sitting a week. Sometimes it takes longer. That builds up some good oil pressure. If it fires & stalls, mine does this after sitting 1/2 a day, crank & start again. Be sure your choke is adjusted correctly, plate closing cold, & pull off working. Then be sure it gets set when starting. You seem to be over thinking this.

 

A very late follow up.
I'll try to be concise. The fuel in my car does drain back to some extent but through a very unscientific experiment (today's fuel), evaporation is a huge factor.
When I had the carb on the bench, I filled the bowl with fuel, took measurements and then replaced the lid (screws and all) to simulate the carb on the engine.
I let 24 hours pass and the evaporation rate was 40%.! So, if the engine isn't started for weeks, I've got an empty fuel bowl. Hence the cranking to get the engine to start,

The final situation is : removed all external filters and replaced with a straight line, pump > carb. I'm relying on the GM in carb filter and the sock in the fuel tank.
6/8 seconds of cranking to fill the bowl on initial start up and the engine fires. My car now runs and drives fine.

A side note, after I got the carb back from the "pro rebuilder" , the car would start and run/drive ok. BUT, it finally let me down in a very busy gas station. It would run ,only if I filled the carb bowl externally, and I managed to get the car onto to the side road. After I trailered the car back home, I ordered a good rebuild kit and meticulously rebuilt the carb myself. The problem was a sticking needle in the seat.

I should've listened to Joe, he's a smart guy.

 

Old post, but I have a similar situation and I believe the fuel pump is the issue. I have a 1970 Cutlass Supreme with a new built motor 3 years ago. I also have a clear fuel filter between the fuel pump and carb, mainly because my Quickfuel carb does not have a built in filter. The fuel always drained back after a few hours which bugged me and I believe led to slower start ups. Last weekend I was driving and it felt like it ran out of gas, even thought I had a full tank. I nursed it over to the side of the road and saw there was no fuel in the clear filter, clearly the fuel pump went out or I had a restriction. Got towed home........

The fuel pump that went out was the Carter muscle car mechanical pump like the o/p had. I put in a new Delphi pump and it works great. It is notable that the fuel no longer drains back out of the fuel line any more. I think my Carter pump was bad from day one, perhaps that was part of your issue as well. I think there is an issue with how much the carter bleeds back through the return line? Maybe the carter return line expects there to be a backflow valve like in the q-jet filter?



Pat

 

Thank fasteddy !
Would you happen to have the PN for that Delphi fuel pump?
Currently the car is doing okay. I just put up with the somewhat annoying lengthy start up due to the fuel leaving the bowl.
If the car sits for a week or so, it'll take maybe three five second hits of the starter and it fires. After it starts everything is okay,,,

On a related note.
A few months after I got the car running correctly I'm cruising around on a Saturday afternoon beaming with confidence and suddenly the car quits running,
Fortunately, I was only a couple miles from my shop and a buddy trailered me back home AGAIN! The problem this time was internal failure of the new Carter fuel pump.
NAPA refunded my money (warranty) and I went to another vendor and purchased a new pump and so far so good.

 

This is the part #

Delphi MF0006

Was $23 shipped from Amazon. That was available quick and cheap. I may have gone with a higher quality rebuildable aftermarket, but with my aluminum heads they won't likely fit.

I wonder if this is a common issue? Nice you got a refund but that did not get you home. It is a shame we have so few or no options for quality replacement parts, everything is crap from China...

My car had less that 500 miles on it when the carter pump failed in less than 2 years.

Pat

 

Just for informational purposes, my car has a Carter pump from NAPA and it’s been working fine for several years now.

 

The OEM filter in the carb has a check valve that prevents drainback. Those aftermarket filters obviously do not. Without the valve in the filter, the fuel column in the line must be held up by the check valve in the outlet port of the pump. The important thing to consider here is head pressure of that column of fuel. The check valve at the pump hardly has any pressure to retain. The one at the pump obviously has a lot more, so even a small leak rate will empty the fuel line. These valves deteriorate with use.

 

I don't recall the anti drain back fuel filters being used in 1970, pretty sure they came in later years. Point is that if it is draining back it is likely something other than a fuel filter such as the mentioned check valve in the fuel pump.

 

Fasteddy, thanks for the PN.
I might just buy one and have it as a spare.

An FYI about the NAPA/Carter pump failure. Something inside the pump physically broke, the arm lost tension and when moved by hand you could hear a clicking noise.
I was very tempted to cut the retaining ring and open it up just to see what broke, but, that would've voided any warranty so I refrained.
As the NAPA guy was giving me my money back he moved the arm and experienced the clicking sound and loss of tension. He acknowledged the issue, turned to another counterman and said, "Sometimes that link inside breaks". Maybe mine was a fluke but when the problem is recognized as happening more than once I'm not instilled with confidence, (gun shy)