David Posted March 28, 2011 Share Posted March 28, 2011 Hello FGMC! I hope everyone is enjoying there Sunday evening.... So, need a little help...have a 70 SS MC that I'm having some issues with.....the car normally runs pretty good, however there have been a handful of times that the cars feels like there is temporaray loss of power while driving....almost feels like you are running out of gas or have a loose wire, then all of sudden the power comes back....we've ruled out loose wires, carb, timing and now we're brainstorming......one friend suggested vapor lock, however I don't have a lot of experience with VL...any ideas on how to confirm if this is the problem? And last, when the gas tank was cleaned and leak-checked during the rebuild of the car a couple of gas return lines were plugged....I was told we did not need these lines since the car was no longer stock and would no longer be required to meet CA smog emissoin requirements....I think one of the lines may have gone to the charcoal canister, but I'm not certain....so, long story short, could the lack of a good gas tank vent be the source of vapor lock or loss of power? Let me knwo what you guys think....and many thanks as always!!! David Quote Link to comment Share on other sites More sharing options...
robs70monte Posted March 28, 2011 Share Posted March 28, 2011 What kinda carb are you running sounds like a Q-jet with the power valve poped out,this doesn't sound like vapor lock my 70 monte tank has a few vents sounds carbish Quote Link to comment Share on other sites More sharing options...
zmanabba Posted March 28, 2011 Share Posted March 28, 2011 Another thing you might want to check is what type of gas cap you have now. If you plugged all of the vents then the tank has to have air come in to the tank to replace the fuel that is used. So if you have an unvented gas cap there is no way for air to get in. With fuel prices going the way they are this would be a good time to upgrade to a locking one anyway lol. David Quote Link to comment Share on other sites More sharing options...
wallaby Posted March 28, 2011 Share Posted March 28, 2011 If you have all your canister lines plugged off, you'll need to use a vented gas cap. Usualy a problem shows up after a long period of being under load...a lengthy drive uphill, or at highway speeds, that kind of thing. Another item to check is the stupid inlet filter in the carburetor. It's located behind the 1" fitting where the fuel line goes in if you have a Quadrajet. The stock type filters in there are a sintered bronze type, and they clog up pretty easy. Pull out that filter and see if you can easily blow through it...chances are good that you can't. Replacement filters can be had in a paper style, and I think they clog less often than the bronze type. Just be careful with the carb threads in that area when you do the filter; it is a weak area of the Quadrajet and it's common to strip out the threads. If you are the type that worries, you can remove the filter in the inlet completely, and just rely on the in-line filter near the fuel pump to do the job. The less you have to mess with that inlet fitting, the better. Quote Link to comment Share on other sites More sharing options...
David Posted March 29, 2011 Author Share Posted March 29, 2011 Thanks for the tips and advice guys... To answer some of your questions.... I'm using an 870 Holley Street Avenger Carb....filters look good and Holley claims this Carb has power valves that will not blow out....I'm thinking it's not a carb issue, because I have tried 3 different carbs now and the problem is still there...the lack of power does not happen often, but when it does, it really leaves me wondering what the heck is wrong.... The Gas Tank Cap is not vented, however it does not fit tight either. I'll probably replace it to see if that helps.... Any other ideas? Thanks! David Quote Link to comment Share on other sites More sharing options...
Mo's70MCs Posted March 29, 2011 Share Posted March 29, 2011 LIQUID VAPOR SEPARATOR(ECS) This is part of the Evaporation Control System (ECS) its design was to prevent fuel vapors in the fuel system from entering the atmosphere. The Liquid-Vapor Separator-a very important piece in the closed fuel system, the system somewhat primitive by today's standards is still in use! Our system consists of fuel tank, fuel (non-vented) tank filler cap, liquid vapor separator, charcoal canister. Three vent lines from the gas tank lead into the separator, and one line from the separator to charcoal canister. These lines are different heights(inside separator) in the fuel system and will always remain vented regardless of vehicle attitude. The separator catches liquid fuel and returns it to fuel tank while allowing vapors back to the charcoal canister. A definite needed part in the fuel evaporation system design, common to our cars. "Ditto with the fuel cap", if you cap off the system it needs a vented cap, I'd say after sometime the fuel tank will start to pressurize (vacuum) itself and start sucking the fuel back. Have you removed the gas cap on a warm day & does it feel pressurized? Quote Link to comment Share on other sites More sharing options...
David Posted March 30, 2011 Author Share Posted March 30, 2011 Rod, Thanks for the detailed explanation, all very good information. And yeah, I have noticed that the tank felt pressurized a couple of times, but it happened so infrequently that I never really thought much of it...should have done something about it lookin back..... Hey, one last question would uncapping some of lines going from the tank to the seperator or the line from the seperator to the canister help vent the fuel system? Good/bad idea? Or maybe I would start leaking fuel...? Thanks again for all help guys! David Quote Link to comment Share on other sites More sharing options...
Mo's70MCs Posted March 30, 2011 Share Posted March 30, 2011 I can't say that it would hurt, as long as you use a vented cap. Just like Mark suggested. Funny thing all my 70s' have the separator.. don't know if it was just 70 related or common to all three years? Maybe someone will chime-in with the answer. Just like the in-line fuel filter with the 1/4" return line.. just in front of the motor..Don't know if that was just Ca. emission control. Does yours have it? Quote Link to comment Share on other sites More sharing options...
David Posted April 1, 2011 Author Share Posted April 1, 2011 Yeah mine has the seperator and 1/4" return line as well.... I changed the cap and am trying to put some miles on it to see if it happens again.... Thanks for all the assitance! D Quote Link to comment Share on other sites More sharing options...
montefrazer Posted April 6, 2011 Share Posted April 6, 2011 I can't say that it would hurt, as long as you use a vented cap. Just like Mark suggested. Funny thing all my 70s' have the separator. don't know if it was just 70 related or common to all three years? Maybe someone will chime-in with the answer." Only California sold 70s have the seprator system and it uses a special gas cap. The other states have no vent on the tank for 70 except the vented anti surge gas cap. All 71s and 72s have the separator system. This system is explained on the pink B 4 page in the 70 owners manual. Quote Link to comment Share on other sites More sharing options...
58impala Posted April 7, 2011 Share Posted April 7, 2011 If you can,replace fuel line with a clear hose by carb. to see if you are getting any air bubbles. Then check temperature of fuel line from carb. to tank. It may be vapor locking. My 83' Chev. truck started doing the same thing and would even quit when pulling heavy loads. Found air bubbles in fuel line, inline pressure gauge showed after 20 minutes of idling pressure dropped to 2 psi. Replaced fuel pump, installed electric pump, no change. Replaced all rubber hose, caps, checked pickup tubes in tanks, no change. Checked temperature of lines and found 236 to 276 degrees between headers and tranfer case. Installed thermo sleeve on lines on the frame rail, mounted away from frame with nylon washers and built sheilds from engine cross member to transfer case. No air bubbles, fuel pressure doesn't drop at all now. Only took about 8 months to figure out and solve. The only thing we can think of, is the good gas we get now boils at a lower point. I installed the 454, headers and exhuast about 16 years ago and just started having that problem a year ago. Hope this helps. Cameron 1 Quote Link to comment Share on other sites More sharing options...
Mo's70MCs Posted April 7, 2011 Share Posted April 7, 2011 Great advice..Cameron. Don't know if this applies..70 Monte had the special "intake" carb shield just below the base of the carb, along with that special 1/4" fuel filter return line. If memory serves me right, I read somewhere that G.M. engineers had experienced VL issues with the QJ and had incorporated these measures to minimize the VL problem. hope this helps Quote Link to comment Share on other sites More sharing options...
jamato Posted April 13, 2011 Share Posted April 13, 2011 I also have a related question to this topic. I have a 72 MC with the extra vent lines from gas tank capped off. I do have the 1/4" return line still hooked up. Is there a vented gas cap avail. for the 72? Also how can i tell what kind of cap i have? there r no markings on it. Quote Link to comment Share on other sites More sharing options...
JP_AstroMan Posted April 19, 2021 Share Posted April 19, 2021 I am having power loss while driving- any temp, any speed. Thinking it's a fuel venting issue. No idea. Killing me slowly. Quote Link to comment Share on other sites More sharing options...
jft69z Posted April 19, 2021 Share Posted April 19, 2021 Crack the gas cap, do you hear air rushing in? Also may try leaving the cap cracked, take it for a spin to see if the problem still exists. Keep in mind you don't want a lot of fuel pouring out the tank, people behind you may not appreciate it a whole lot, lol. Quote Link to comment Share on other sites More sharing options...
JP_AstroMan Posted April 19, 2021 Share Posted April 19, 2021 I wouldn't be able to hear it with the new Delta 44's. I did try leaving the gas cap cracked and it actually seemed to run worse. Quote Link to comment Share on other sites More sharing options...
Glen Posted April 27, 2021 Share Posted April 27, 2021 I had the same pressurized gas cap problem as my tank isn’t vented. The tank would actually contract with a loud “thunk” as I removed the cap after driving, especially when there was only a little gas (more vapor) in it. I ordered and installed a “vented” cap and still had the same issues I fixed this problem by drilling six 1/16” holes in the cap. Now when I back into the garage and turn the car off I can hear the cap hissing. Problem solved, just don’t squat down by the rear plate with a lit cigarette in your mouth to check your handy work. Quote Link to comment Share on other sites More sharing options...
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