Fuel filter not full - potential vapor lock? - Team Camaro Tech
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post #1 of 13 (permalink) Old May 18th, 19, 04:04 PM Thread Starter
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Stanford
 
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Fuel filter not full - potential vapor lock?

Do you know how full a fuel filter should look during idle, 1900 RPM cruise or full throttle? I recorded the vid linked below showing both fuel filter (on left half of screen) and fuel bowl (sight glass) levels. I sent the vid starting at 8:50 when vehicle's warmed up & we go WOT to show what's going on. (If you want to see the fuel levels when the car was cold, that's at the 0:00 start of the clip.)


The car runs great in cruise & WOT, but when it was warmer May 4, I noticed the air/fuel ratio at idle would creep from 15:1 to mid 17s when idling or off-throttle decel. Which is why I've started looking into potential vapor lock since the car will also drain the bowls if parked for a week after being parked when warm.

Figured I should ask around if this behavior is normal for a '69 Camaro with the Carter high-flow mechanical pump, Demon 750 carb, and a new fuel tank & hard lines installed in 2004 (stored with Sta-bil anytime it was parked over a week).

If it's not normal, I'll drain & drop the tank to look into whether the sock might be clogging, or whether there's a safe way to install another vent to reduce pressure. Removing the gas cap will add more fuel to the filter - I've tried a new vented and a vented locking cap w/same results.

Last resort might be adding an electric fuel pump in-tank and maybe a bigger (1/2") line toward the engine & use the stock 3/8" line as a return... but I'm no fan of adding complexity, running wires & relays if we don't have to, as I'd like to stay carbureted since this thing handles autocross & street driving so well (12.5:1 air fuel when WOT, 14.8-15:1 in cruise & idle).


'69 Z/28 - Dad's orig. owner, 44K mi., match #s DZ302 & M20 preserved in glass case. LT-1 w/ 210 AFRs, 0.640" lift 253/259 @ .050" Erson roller and Tremec TKO600 in car for more fun & 11.7s @ 121 MPH on drag radials & pump gas.
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post #2 of 13 (permalink) Old May 18th, 19, 04:34 PM
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Re: Fuel filter not full - potential vapor lock?

Having "air" in a fuel filter is normal. The air inside is pressurized just like the fuel and there is no way of getting rid of it. Even if you did it will just reappear again as gas vaporizes at about 120 degrees, and it is a lot hotter than that under the car, and hood.

This gas vapor is only a problem on the suction side of the pump. Your fuel lines to the carb are pressurized (this is the side the filter needs to be on: between the fuel pump and the carb inlet)

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post #3 of 13 (permalink) Old May 18th, 19, 07:15 PM
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Re: Fuel filter not full - potential vapor lock?

Yep. No biggy.
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post #4 of 13 (permalink) Old May 18th, 19, 07:31 PM Thread Starter
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Re: Fuel filter not full - potential vapor lock?

Quote:
Originally Posted by Larger Dave View Post
Having "air" in a fuel filter is normal. The air inside is pressurized just like the fuel and there is no way of getting rid of it. Even if you did it will just reappear again as gas vaporizes at about 120 degrees, and it is a lot hotter than that under the car, and hood.

This gas vapor is only a problem on the suction side of the pump. Your fuel lines to the carb are pressurized (this is the side the filter needs to be on: between the fuel pump and the carb inlet)

Big Dave
Thanks for the quick response. I neglected to say that the filter in this vid is actually on the intake/suction side of the Carter mechanical fuel pump, and that the outlet side only has a hard-line & fittings all the way to the Demon 750.

Do you recommend me moving the filter to the outlet side of fuel pump instead? Is there any increased risk of leakage (since that filter would be under pressure on pump's outlet vs. it being in suction where it is currently)?

Part of the reason I did the right side of that vid of the carb at the same time (synced with left half of vid showing fuel filter) was under the assumption that fuel should be pressurized between the pump outlet and carb's needle/seat, hopefully btwn 5-7 PSI. Thus whatever's past the needle/seat inside the bowl isn't under pressure.

Thanks again for your help - funny that it's been running fine for 15 yrs and just now am I to the point of tracing why the air/fuel ratio at idle goes lean when the car runs above 180F coolant temp.

'69 Z/28 - Dad's orig. owner, 44K mi., match #s DZ302 & M20 preserved in glass case. LT-1 w/ 210 AFRs, 0.640" lift 253/259 @ .050" Erson roller and Tremec TKO600 in car for more fun & 11.7s @ 121 MPH on drag radials & pump gas.
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post #5 of 13 (permalink) Old May 18th, 19, 08:23 PM
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Dave
 
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Re: Fuel filter not full - potential vapor lock?

Past the needle and seat (in the fuel bowl) the fuel is at atmospheric pressure as the bowls are vented. If you place a fuel filter before the pump you could encounter vapor lock depending upon temperature at the mounting location, elevation above the fuel inlet, and the resistance offered by the filter (usually it is a 100 micron filter known as a strainer or screen to catch debris and rust flakes). After the pump you use a fine filter (10 micron paper, fiberglass, or stainless steel wool) to filter out anything that might get caught inside the carb passageways.

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post #6 of 13 (permalink) Old May 19th, 19, 03:23 AM
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Re: Fuel filter not full - potential vapor lock?

I have a filter before and after the pump along with what is in the holley.
I have never encountered fuel starvation
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post #7 of 13 (permalink) Old May 19th, 19, 06:08 AM
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Re: Fuel filter not full - potential vapor lock?

First,
You should never put a filter in the inlet tubing as it induces a restriction that lowers the boiling point of the fuel, which can cause vapor lock. The tank strainer is all you need.

Second,
Those glass filters are junk... they look pretty, but don't flow well. I had one on my Camaro & it was starving for fuel.
Summit sells a nice polished conventional style one that is made out of stainless.

For EFI applications, you should run something in the 6-10 micron rating.
For Carb applications, a 40 is typically sufficient... no need to add unnecessary restriction.
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post #8 of 13 (permalink) Old May 19th, 19, 09:11 AM
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kevin
 
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Re: Fuel filter not full - potential vapor lock?

I guess it depends where it is placed. i am running this if it makes a difference 130+ GPH Mechanical Fuel Pump

Street/Strip Carbureted Applications Fits Small Block Chevy V8s Compatible with Gasoline

PART# 12-327-13
I have my filter on the passenger side subframe where the door hinges. as I said I have had no issues since 2011 . no problem hitting the red line of 6200.
hell maybe I am just lucky
I might add I use these

Last edited by flat tire; May 19th, 19 at 09:47 AM.
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post #9 of 13 (permalink) Old May 19th, 19, 09:36 AM
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Re: Fuel filter not full - potential vapor lock?

Quote:
Originally Posted by Lonnie P View Post
First,
You should never put a filter in the inlet tubing as it induces a restriction that lowers the boiling point of the fuel, which can cause vapor lock. The tank strainer is all you need.

Second,
Those glass filters are junk... they look pretty, but don't flow well. I had one on my Camaro & it was starving for fuel.
Summit sells a nice polished conventional style one that is made out of stainless.

For EFI applications, you should run something in the 6-10 micron rating.
For Carb applications, a 40 is typically sufficient... no need to add unnecessary restriction.
X2, Couldn't agree more!!! Get rid of the GLASS FILTER!!!. They break and leak gas causing fires. Ask me how I know LOL

1969 X66 396 LeMans Blue, M20, 373 (Sold)
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post #10 of 13 (permalink) Old May 19th, 19, 09:54 AM
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Re: Fuel filter not full - potential vapor lock?

OP

You might want to run a FP gauge to confirm your FP is stable when you see your AF ratio going south to rule in/out if fuel delivery is a problem

I have one with hose that I tape gauge to windshield to check FP on a sustained WOT pull to confirm FP does not "fade"

+ 1 on dumping the glass or plastic bowl fuel filters and use a metal one between fuel pump and carb

FWIW I also run a thick (about 3/8") gasket spacer under carb to minimize chance of vapor lock/heat soak/fuel boiling away in bowls
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post #11 of 13 (permalink) Old May 19th, 19, 11:31 AM
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Re: Fuel filter not full - potential vapor lock?

OK, I just checked my fuel pressure after two of the filters I posted in a previous post, so 8# is insufficent ?
just asking

Last edited by flat tire; May 19th, 19 at 12:49 PM.
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post #12 of 13 (permalink) Old May 19th, 19, 07:12 PM
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Re: Fuel filter not full - potential vapor lock?

If you have a Holley carb, 8# is too much. LOL.
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post #13 of 13 (permalink) Old May 20th, 19, 04:39 AM
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Re: Fuel filter not full - potential vapor lock?

Quote:
Originally Posted by DT View Post
X2, Couldn't agree more!!! Get rid of the GLASS FILTER!!!. They break and leak gas causing fires. Ask me how I know LOL
Correct. Those glass filters are garbage. Years ago one failed on a friend's 'Cuda and the car burned up on the roadside.
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