Vapor lock or vacuum in tank........ - Chevelle Tech
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Troubleshooting Diagnosing problems done here.

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  #1  
Old May 9th, 09, 4:14 PM
WakkoWarner WakkoWarner is offline
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Brian
 
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Location: Mesa, AZ
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Default Vapor lock or vacuum in tank........

I was coming home Thursday & my car died. I ended up running the battery down trying to get it restarted (the front fuel bowl was completely empty). I ordered a new fuel pump, thinking that I had another one go bad. After it sat there for a couple of days (I got the battery charged back up), I cranked on it for a little bit & got it started. I have had a problem off & on when accelerating (mostly when the outside temperature is over 100) that it starts to nose dive & I have to let off the gas & put it into neutral until it catches back up. My engine is a 377 that puts out about 450 hp. I am running a 110 gph Holley mechanical fuel pump. I believe the factory fuel line from the tank is a 5/16, but I am running a short piece of 3/8 line to the pump & 3/8 up to the carb. Does this sound like it vapor locked on me or could it have pulled a vacuum on the tank?? I do run a locking gas cap & I do not have a vapor recovery canister (it fell apart after 38 years & I couldn't find a replacement part - this problem persisted even when the canister was in place). If it did vapor lock on me, would running a line from the fuel log hooked up to the carb to the inlet side of the fuel pump help alleviate that problem?? Also, sometimes after turning when I go to accelerate, the engine hesitates. I am running a 31 squirter on my Speed Demon 750 vacuum secondary. I don't know if maybe it needs a larger squirter or not (I just started adjusting the Idle-ease screw in the base plate, so I don't know if that will help or not). Any input will be greatly appreciated. Thanks in advance.
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1971 Malibu, Full Rally interior (Factory SS Type), 377 roller cammed small block, Turbo 400, 3.42 10 bolt posi (Buick), - All your base are belong to us.

My Chevelle w/ open headers: http://www.youtube.com/watch?v=Q61TTYXmIsc

My Chevelle w/ 2.5 pipes + X crossover/18 inch glass packs:
http://www.youtube.com/watch?v=YFm_ISn_UDA
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  #2  
Old May 9th, 09, 7:26 PM
WakkoWarner WakkoWarner is offline
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Brian
 
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Default Re: Vapor lock or vacuum in tank........

I ran some errands this afternoon & it started to die on me again. I got it home & within a few minutes of idling, it died & the front fuel bowl was empty again. I took the gas cap off the tank & tried to get it started again, but it didn't work. So now I am starting to lean towards a fuel pump that is going out. If it turns out to be a bad fuel pump I will be very disappointed (I tire of spending a lot of money on a part that is just going to fail again within 2-3 years - this is what happens when you outsource your manufacturing to places like China & Korea). I had a 60 gph Holley fail on me a couple of years ago (after only about 2 years worth of use) & the current 110 gph unit has been on the car for just over 2 years now. If anyone has any other ideas as to what might be causing this problem, your input will be greatly appreciated. Thanks in advance.
__________________
1971 Malibu, Full Rally interior (Factory SS Type), 377 roller cammed small block, Turbo 400, 3.42 10 bolt posi (Buick), - All your base are belong to us.

My Chevelle w/ open headers: http://www.youtube.com/watch?v=Q61TTYXmIsc

My Chevelle w/ 2.5 pipes + X crossover/18 inch glass packs:
http://www.youtube.com/watch?v=YFm_ISn_UDA
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  #3  
Old May 10th, 09, 9:53 AM
S10 Racer S10 Racer is offline
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Michael
 
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Default Re: Vapor lock or vacuum in tank........

Are you running a bypass fuel regulator? If not, that may explain why your pumps are not lasting. Make sure the fuel level in the bowls are set correctly and there is no blockage in the lines. If you do not have a bypass type regulator, you need to install one and run a return line back to the tank. On an electric pump, it uses the fuel to help keep it cool. If the pump is "dead-headed", it will produce alot of heat and cause damage.
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  #4  
Old May 10th, 09, 10:07 AM
Keith Tedford Keith Tedford is offline
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Default Re: Vapor lock or vacuum in tank........

I've always just stuck to the stock GM fuel pumps and have had zero problems over the years. Can't say about aftermarket quality. I see that you are also running a mechanical unit. I have replaced three electric fuel pumps on our newer cars though. I've found, over the years, that electrical components have caused me most of my problems so I steer clear of them as much as possible. Is there any chance that the sock on the end of the fuel pickup in the tank is plugged. Some times the inside of soft fuel lines can deteriorate and plug the lines. Another thing to check. Judging from other peoples experiences with brakes and fuel delivery, I consider myself fortunate in not having problems........yet.
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  #5  
Old May 13th, 09, 11:39 PM
WakkoWarner WakkoWarner is offline
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Brian
 
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Default Re: Vapor lock or vacuum in tank........

I am at my wits end with this problem. I installed a new fuel pump, but the problem was still there (I did discover, however, that the new pump was not internally regulated, so it was pushing 14 psi to the carb. I had to put my regulator back on the car & the pressure was holding steady at about 6 3/4 psi.). So I redid the feed hose to the pump (it had kind of a tight bend in it, so I was concerned that maybe it was sucking itself flat) & also replaced the short hose coming from the pickup assembly to the fuel pump feed line (back by the tank). Now the car won't pull fuel at all. I was concerned that maybe the fuel pump push rod had ground down the lobe on the cam, but when I had the pump out, I examined the rod & the cam end was nice & shiny - no signs of any bad wear at all (I have a Comp roller cam - non billet - & the fuel pump push rod is the Moroso light weight version with the chromoly ends. My builder said it would be fine & would not damage the camshaft.). Where do I go from here?? Would it hurt to put some air to the pickup assembly in the tank to see if maybe the sock has become clogged?? The pickup assembly has only been in the car about 10 years, so I would hope that the sock is not clogged, or worse, disintegrated. If anyone has any ideas as to what might be causing this, I would truly appreciate your input. Thanks in advance.
__________________
1971 Malibu, Full Rally interior (Factory SS Type), 377 roller cammed small block, Turbo 400, 3.42 10 bolt posi (Buick), - All your base are belong to us.

My Chevelle w/ open headers: http://www.youtube.com/watch?v=Q61TTYXmIsc

My Chevelle w/ 2.5 pipes + X crossover/18 inch glass packs:
http://www.youtube.com/watch?v=YFm_ISn_UDA
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  #6  
Old May 14th, 09, 3:46 PM
WakkoWarner WakkoWarner is offline
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Brian
 
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Default Re: Vapor lock or vacuum in tank........

Okay. I dropped the tank this morning. The sock was a little bent (which I think it normal from the way it sits in the tank), so it it possible that it might have weakened to the point of being sucked up against the pickup tube. I changed the sock out & reoriented it so that the seam was on top, which should help to prevent it from bending so easily. I blew out the line from the tank to the pump - it is completely clear. I still couldn't get any fuel up to the pump. I removed the new pump & it looked like the rod was riding in the dimple of the arm instead of on the raised portion. I have reinstalled the Holley pump & will try it again when the battery is fully charged. I also checked the motion of the fuel pump rod & it seems to move about half an inch. I hope that the lobe on the camshaft is okay, but as I said in a previous post, the camshaft end of the rod was shiny & smooth - no signs that it was tearing anything up. If anyone has any other ideas I would greatly appreciate your input. Thanks in advance.
__________________
1971 Malibu, Full Rally interior (Factory SS Type), 377 roller cammed small block, Turbo 400, 3.42 10 bolt posi (Buick), - All your base are belong to us.

My Chevelle w/ open headers: http://www.youtube.com/watch?v=Q61TTYXmIsc

My Chevelle w/ 2.5 pipes + X crossover/18 inch glass packs:
http://www.youtube.com/watch?v=YFm_ISn_UDA
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  #7  
Old May 18th, 09, 6:01 PM
WakkoWarner WakkoWarner is offline
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Brian
 
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Default Re: Vapor lock or vacuum in tank........

Nobody has any ideas on this one?? I was hoping to figure out the cause before installing an electric pump. There has to be a reason that it is happening. Could the fuel pump lobe on the cam get worn down without tearing up the rod?? Would using a 90 degree fitting on the fuel pump prevent it from pulling the fuel from the tank to the pump??
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1971 Malibu, Full Rally interior (Factory SS Type), 377 roller cammed small block, Turbo 400, 3.42 10 bolt posi (Buick), - All your base are belong to us.

My Chevelle w/ open headers: http://www.youtube.com/watch?v=Q61TTYXmIsc

My Chevelle w/ 2.5 pipes + X crossover/18 inch glass packs:
http://www.youtube.com/watch?v=YFm_ISn_UDA
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  #8  
Old Jun 2nd, 09, 4:04 PM
WakkoWarner WakkoWarner is offline
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Brian
 
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Posts: 650
Default Re: Vapor lock or vacuum in tank........

I ended up installing an electric pump, which solved the problem. I am thinking that the fuel pump lobe on the camshaft got worn down by the rod (contrary to what my builder told me). So, no more running out of fuel, which makes me happy.
__________________
1971 Malibu, Full Rally interior (Factory SS Type), 377 roller cammed small block, Turbo 400, 3.42 10 bolt posi (Buick), - All your base are belong to us.

My Chevelle w/ open headers: http://www.youtube.com/watch?v=Q61TTYXmIsc

My Chevelle w/ 2.5 pipes + X crossover/18 inch glass packs:
http://www.youtube.com/watch?v=YFm_ISn_UDA
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  #9  
Old Jun 5th, 09, 7:26 PM
Turbine Turbine is online now
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Chris
 
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Default Re: Vapor lock or vacuum in tank........

only thing I would think if that solved the problem, and your correct about the pump lobe on the cam...

where did that metal go?
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  #10  
Old Jun 8th, 09, 10:31 PM
SWHEATON SWHEATON is offline
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Smile Re: Vapor lock or vacuum in tank........

Clogged strainer/sock on fuel pickupin take and or maybe need a vented fuel cap and mech f-punmp ispulling a vacuum on the tanks then starving for fuel?

BTW,you can easily check fuel pump lobe on cam.

You can have someone spin motor over slowly by hand with plugs out while holding the f-pump rod by the sides to see just how much travel it has. But dont put pinger on end of rod asit could get piched hard or even broken whoile trying this out,you have to hold it by its sides to do that check.

If it bearly moves thats the issue but ifit hase a decent pump stroke then thats not the issue but the new elec f-pumps may be better ar sucking etc bandaing maybe a partially clogged strainer sock.

Any,here are some oher thought if the issue comes back.

Is it hot enough out at the times it dies and or is motor running hot ehough when it dies to make you suspect vapor lock?

Is this only happening at higher rpms when on it hard and not when just driving at normal cruise?

If you get the tank vented and its still doing it can the stock heavy f-pump rod & try a light weight fuel pump to see if the very heavy stock f-pump rod is floating the pump arm at higher rpms resulting in fuel starvation.

But again only try this if your sure the tank is vented properly and strainer sock in fuel tank is not clogged.

To test tank venting just get it to die on you and get out of car ASAP and remove the fuel cap to see if theres a vacuum being pulled on the tank when you remove the cap which you will hear right off the bat when you remove the fuel tank cap if thats the case.

If that happens the motor should also refire when you spin it over enough to pull fuel from tank to carb to refie it,it should run fine then untill it pulls vacuum on the tank again if thats the case. If yes get a vented cap or correct-fix the tank vent.

Or drill a small hole in the fuel tank cap and forget about it like i did over 30 yrs ago when the vent kept messing up and dropping fuel all over the hot exhaust pipes in my 69 chevelle.

Tank has vented fine since then with no issues with small hole in cap.have no fuel out the hole issues from the cap either. I fill the tank unitll the pump shuts down 1 time and thats it,i never fill to tippy top.

Scott
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1969 CHEVELLE SS396,ORIGINAL #'S MATCH,GOT IN 1978,(In 2001 rblt/bored original 396 .030 to 402)/M20/12BOLT/3:31'S
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  #11  
Old Jul 12th, 09, 10:04 AM
427 2X4 427 2X4 is offline
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Default Re: Vapor lock or vacuum in tank........

Tried to fire my 396 for cam break in y-day but could not get any fuel to the carb. Using a holley mechanical pump with a tube to a small carry tank of 103 oct. The pump wont pull the fuel out. The pump is good. Could the rod be sticking on the downstroke and not returning because the the pump arm spring is not strong enough? I think the rod is sticking while at the bottom of its stroke. Does anything need to be primed?
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  #12  
Old Jul 12th, 09, 1:41 PM
mochevy69 mochevy69 is offline
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Larry
 
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Default Re: Vapor lock or vacuum in tank........

Clogged or collapsed in-line filters maybe?? If installed backwards they will clog quicker.
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  #13  
Old Jul 12th, 09, 3:58 PM
427 2X4 427 2X4 is offline
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Default Re: Vapor lock or vacuum in tank........

Filter is in correct and brand new.
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  #14  
Old Jan 8th, 10, 6:11 PM
david bull david bull is offline
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Default Re: Vapor lock or vacuum in tank........

pull the pump and make sure the pushrod for the fuel pump slides up and down freely.I ran into this on a 435hp 69 vette years ago.When I bought it,it had an electric pump which I removed and then figured out why it was there.Pushrod(bent,no ideal why) was stuck at the bottom of its stroke.This could also cause erratic fuel delivery problems.DBULL
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