Lars Grimsrud -Can we talk about Fuel ? - long post
06-12-2007, 10:23 AM
#1
Race Director
Thread Starter
Lars Grimsrud -Can we talk about Fuel ? - long post
When Lars speaks anybody with a brain and a body temp of around 98 degrees listens. I only have 1/2 a brain so I read his stuff over several times so I don’t miss anything. Lars you said something in one of your posts that turned what I always believed about stock mechanical fuel pumps up side down. My concern is forum members that read this thread would be changing their fuel pumps and not fixing their problem. Here is what you said and a link to the thread it appeared in: “ More common on the fuel disappearance problem is that the check valve in your fuel pump is leaking. When this happens, since the needle/seat in a Q-Jet is in the bottom of the bowl, the fuel in the carb will be siphoned out of the carb from the fuel draining back through the pump.”
Lars
http://forums.corvetteforum.com/show....php?t=1725724
How can this be? The check valve is barely that, if it were the inlet to the fuel pump from the tank would hold a vacuum and it wont and the outlet from the pump going to the carb would hold pressure and it wont. The thread was about a 1975 C3 and so you would have to factor in the return line, which is an open tube from the pump reservoir to the fuel tank with a .070 orifice, and at the tank end it drops in from the top with no down tube to create a siphon. Siphoning requires a tube full of fuel like the one coming “down” from the carb to the pump and the one going in the top of the tank and going “down” to the bottom of tank (75 and newer). We know the fuel is leaving the carb and ending up in the tank but what path is it taking? Is the siphon from the tank end through the fuel pump which is trying to empty the pump can (reservoir) which is being fed from the carb and what’s left in the return line draining back into the can from the tank end? Or is the siphon force coming from the line coming “down” from the carb forcing fuel back into the can and then back through the return line into the tank? Or both? Those C3s without a return line has to be going through the inlet line. The 74 and older models must be using the siphoning force created by the downward tube coming from carb because those tanks have their outlet on the bottom of the tank (no down tube going into the tank to create siphon).Noonie brought up the new seat that had no slots at the bottom as the GM fix –makes sense-I thought it was the check valve in the paper filter. Maybe both? Also height of bottom of fuel bowl or top of new seat and bottom of tube going into tank play a role also in this. Anyone know these dimensions on a flat surface?
One other question Lars on the subject of fuel tanks with safety in mind. The problem of the fuel tank building pressure. Some guys are doing what I feel is a band-aid fix for this problem. They are putting on fuel caps with the vent that goes both directions or making their own by drilling a hole in the cap. While in most cases this is not a risk, but with the cars that “are” building pressure this could be dangerous. For instance (and some of these guys race these cars) in the event of a rollover these caps will be spraying fuel because of the pressure in the tank. These fuel systems are designed to contain most of the fuel in a rollover and don’t need any added spilled fuel to make the situation any worse. I’ve only had a few C3s building tank pressure. One time it was the line coming from the fuel tank was not tight and was allowing the pump to suck in air and sending most of it back to the tank via the return line and pressurizing the tank. This one was a 69 and on the return line at the top by the filter I put a small piece of clear tubing so I could see the air going through. The second time I think it was a bad diaphragm doing the same thing but I didn’t tear the pump apart I just replaced it because it was slightly wet around the vent hole. This link is the thread where this was discussed.
http://forums.corvetteforum.com/show....php?t=1713824
I know this all seems a little **** (I am) but I think getting to the bottom of things can be a great help for future problems, understanding and to make ours cars safer. I try really hard and spend time trying to give out accurate info and going back and correcting it if it was wrong. I do think that accuracy on the CF is important because of the number of people this forum influences that are not mechanics and can get into real trouble and not know it. I know none of this will make your cars faster or look better but it really scares me when safety designs are being overlooked or disabled. Sorry for the long boring post but I felt the need to at least try to open a discussion about some of these less than exciting issue.Be Safe
Roger
Lars
http://forums.corvetteforum.com/show....php?t=1725724
How can this be? The check valve is barely that, if it were the inlet to the fuel pump from the tank would hold a vacuum and it wont and the outlet from the pump going to the carb would hold pressure and it wont. The thread was about a 1975 C3 and so you would have to factor in the return line, which is an open tube from the pump reservoir to the fuel tank with a .070 orifice, and at the tank end it drops in from the top with no down tube to create a siphon. Siphoning requires a tube full of fuel like the one coming “down” from the carb to the pump and the one going in the top of the tank and going “down” to the bottom of tank (75 and newer). We know the fuel is leaving the carb and ending up in the tank but what path is it taking? Is the siphon from the tank end through the fuel pump which is trying to empty the pump can (reservoir) which is being fed from the carb and what’s left in the return line draining back into the can from the tank end? Or is the siphon force coming from the line coming “down” from the carb forcing fuel back into the can and then back through the return line into the tank? Or both? Those C3s without a return line has to be going through the inlet line. The 74 and older models must be using the siphoning force created by the downward tube coming from carb because those tanks have their outlet on the bottom of the tank (no down tube going into the tank to create siphon).Noonie brought up the new seat that had no slots at the bottom as the GM fix –makes sense-I thought it was the check valve in the paper filter. Maybe both? Also height of bottom of fuel bowl or top of new seat and bottom of tube going into tank play a role also in this. Anyone know these dimensions on a flat surface?
One other question Lars on the subject of fuel tanks with safety in mind. The problem of the fuel tank building pressure. Some guys are doing what I feel is a band-aid fix for this problem. They are putting on fuel caps with the vent that goes both directions or making their own by drilling a hole in the cap. While in most cases this is not a risk, but with the cars that “are” building pressure this could be dangerous. For instance (and some of these guys race these cars) in the event of a rollover these caps will be spraying fuel because of the pressure in the tank. These fuel systems are designed to contain most of the fuel in a rollover and don’t need any added spilled fuel to make the situation any worse. I’ve only had a few C3s building tank pressure. One time it was the line coming from the fuel tank was not tight and was allowing the pump to suck in air and sending most of it back to the tank via the return line and pressurizing the tank. This one was a 69 and on the return line at the top by the filter I put a small piece of clear tubing so I could see the air going through. The second time I think it was a bad diaphragm doing the same thing but I didn’t tear the pump apart I just replaced it because it was slightly wet around the vent hole. This link is the thread where this was discussed.
http://forums.corvetteforum.com/show....php?t=1713824
I know this all seems a little **** (I am) but I think getting to the bottom of things can be a great help for future problems, understanding and to make ours cars safer. I try really hard and spend time trying to give out accurate info and going back and correcting it if it was wrong. I do think that accuracy on the CF is important because of the number of people this forum influences that are not mechanics and can get into real trouble and not know it. I know none of this will make your cars faster or look better but it really scares me when safety designs are being overlooked or disabled. Sorry for the long boring post but I felt the need to at least try to open a discussion about some of these less than exciting issue.Be Safe
Roger
Last edited by ...Roger...; 06-12-2007 at 10:27 AM. Reason: spelling
06-12-2007, 11:32 AM
#2
Team Owner
Since I didn't live long with the Qj on my '72 here I can't answer anything about that fuel drain problem.....BUT, over the years I have owned many a Olde Tyme Pontiac, with QJ and AFB/etc.....and have had problems with dry carbs....IMO, what cures it was blocking the heat rise crossover with plaster of paris, let it cure overnight, bolt it back in place and i'ts case closed....too damn much heat under the carb, even with eliminating the heat riser valve, another trick which worked once or twice for me, without the plaster trick....
you can put all the insulators you want under the Qjet, but you got a iron intake....it wins....hotter 'n hell....burns the paint off, what you think that gas won'd boil right outta that carb??
the fuel bowls are basically a terminal leak problems.....I have tried various glues/epoxies, hammering, tapping, screws, all sorts of crap over the years, never did find a 110% fix....some epoxies are better than other for that......
lotsa times I would just ditch the mechanical pumps due to hot running and fuel perk, hell with it, put a electric pump...Carter being my favorite, in the trunk and push the fuel forward.....cured a whole lot of problems with the cars over the years.....never had one give trouble after THAT fix....
you can put all the insulators you want under the Qjet, but you got a iron intake....it wins....hotter 'n hell....burns the paint off, what you think that gas won'd boil right outta that carb??
the fuel bowls are basically a terminal leak problems.....I have tried various glues/epoxies, hammering, tapping, screws, all sorts of crap over the years, never did find a 110% fix....some epoxies are better than other for that......
lotsa times I would just ditch the mechanical pumps due to hot running and fuel perk, hell with it, put a electric pump...Carter being my favorite, in the trunk and push the fuel forward.....cured a whole lot of problems with the cars over the years.....never had one give trouble after THAT fix....
06-12-2007, 11:52 AM
#3
Race Director
FYI-This doesn't answer your questions but this method works.
Here is what I did to my wifes' 77 daily driver.
A previous owner had installed a non return mechanical fuel pump.
Not wanting to change it or use a regulator, and since I was adding an inline fuel filter, I made a tee with a .030 hole for the return line. Before I connected it, I shot a little air into the return line to check for blockage and ended up with at least 5 gal of gas all over the floor in a matter of seconds.
The weather here is hot all the time so I mounted the carb with a thick gasket, stainless heat shield, and then a thin gasket. Also used the barrel N&S assembly.
Because of the open tee, I assume the fuel drains back a little in the pressure line. Haven't bothered to check, but the bowl doesn't empty. The lines will fill quickly after the engine runs for a few seconds. I wanted to keep the fuel pressure under 5 psi, havent checked that either, but it runs very well under all hot conditions.
The bowl will evaporate if left for 2 weeks though.
Here is what I did to my wifes' 77 daily driver.
A previous owner had installed a non return mechanical fuel pump.
Not wanting to change it or use a regulator, and since I was adding an inline fuel filter, I made a tee with a .030 hole for the return line. Before I connected it, I shot a little air into the return line to check for blockage and ended up with at least 5 gal of gas all over the floor in a matter of seconds.
The weather here is hot all the time so I mounted the carb with a thick gasket, stainless heat shield, and then a thin gasket. Also used the barrel N&S assembly.
Because of the open tee, I assume the fuel drains back a little in the pressure line. Haven't bothered to check, but the bowl doesn't empty. The lines will fill quickly after the engine runs for a few seconds. I wanted to keep the fuel pressure under 5 psi, havent checked that either, but it runs very well under all hot conditions.
The bowl will evaporate if left for 2 weeks though.
06-12-2007, 11:58 AM
#4
Le Mans Master
Member Since: Jan 2006
Location: Boca Raton Florida
Posts: 9,192
Likes: 0
Received 24 Likes
on
23 Posts
Roger, couple questions...
regarding fuel tanks Presurizing, I can't see the fuel pump having anything to do with the fuel tank building pressure. It sucks fuel out of one line, runs it to the carb where some (most) of it used and returns the rest (less than it takes out) back to the tank, for a net loss of gas in the tank, If the tank is sealed or unvented, that would create a vacuume would it not? I think the pressure in the tank is caused by heat and agitation of the fuel causing it to gas off in the tank.
Or did I read your post completely wrong?(it happens)
I think I am with Gene on the dry carb thing, most of it is caused by fuel percolation from heat soak. It does happen sometimes because of a bad pump valve, seen that, but I think a lot of that is heat related too, heat soaks the line from pump to carb, presurizing it against the bowl valve seat and forcing fuel back thru the pump valves to the tank and assisted by siphon to the tank. I also think your correct about the return line path being involved, but I think that is heat related too.
interesting stuff
regarding fuel tanks Presurizing, I can't see the fuel pump having anything to do with the fuel tank building pressure. It sucks fuel out of one line, runs it to the carb where some (most) of it used and returns the rest (less than it takes out) back to the tank, for a net loss of gas in the tank, If the tank is sealed or unvented, that would create a vacuume would it not? I think the pressure in the tank is caused by heat and agitation of the fuel causing it to gas off in the tank.
Or did I read your post completely wrong?(it happens)
I think I am with Gene on the dry carb thing, most of it is caused by fuel percolation from heat soak. It does happen sometimes because of a bad pump valve, seen that, but I think a lot of that is heat related too, heat soaks the line from pump to carb, presurizing it against the bowl valve seat and forcing fuel back thru the pump valves to the tank and assisted by siphon to the tank. I also think your correct about the return line path being involved, but I think that is heat related too.
interesting stuff
06-12-2007, 12:21 PM
#5
noonie: can you give details on how you built the bypass tee? i've calculated about the same size orifice required for my app; just can't think of a good way to make it.
If the vapor canister is working, even this wouldn't be an issue would it?
SIXFOOTER: I think the pressure in the tank is caused by heat and agitation of the fuel causing it to gas off in the tank.
06-12-2007, 01:57 PM
#6
Tech Contributor
Member Since: Aug 1999
Location: At my Bar drinking and wrenching in Lafayette Colorado
Posts: 13,654
Received 4,926 Likes
on
1,931 Posts
Not ignoring you - I'm at work and can't type a long post right now... try to get back with some thoughts ASAP....
06-12-2007, 02:15 PM
#7
Race Director
Member Since: Jan 2000
Location: Corsicana, Tx
Posts: 12,616
Received 1,877 Likes
on
915 Posts
2020 C2 of the Year - Modified Winner
2020 Corvette of the Year (performance mods)
C2 of Year Winner (performance mods) 2019
2017 C2 of Year Finalist
I also remember way back when that there were fuel filters at the carb entrances with a check valve...and some without. Often people bought the one without because it was cheaper.
Then they had issues with empty carbs. Re-installing the correct filter at the carb inlet solved the issue.
JIM
Then they had issues with empty carbs. Re-installing the correct filter at the carb inlet solved the issue.
JIM
06-12-2007, 02:32 PM
#8
Race Director
Started out with a straight 3/8 brake line, bent it to fit at the fuel pump and come out in front of the head as the oem one did, but left the filter end just cut off a little long.
Used a piece of 3/4" cold rolled, 3/4" long with a 3/8" hole drilled in it. Mounted it to the fuel pump and adjusted the height and marked it for the 5/16" return with a spring loaded center punch. Brazed the 3/8" line to the cold rolled and after letting it cool slowly, drilled the 5/16" for the return line. Then brazed the return piece with a bead expansion for the hose and clamp. Used a double flare tool for making the bead. Then closed up the end by brazing.
Lastly, drilled a .030" hole in the closed return line. Then, cut the open end for length and flaired for the 6an for the filter. Tested it with the engine running and a long hose in a bucket and it shoots a good stream.
I made the tee big enough for a threaded schrader adapter to be threaded into the tee so I could use a quality fuel pressure test gauge as is used on modern FI setups.
I looked quickly, but couldn't locate the schrader setup, so I quit there. If you know of a good source, let me know.
Ultimately, I would like to keep the fuel pressure as little as possible and still have adequate flow. With the .030" hole, it will run at 4200rpm for over a mile and not starve for fuel, but always cab be closed smaller or drilled bigger.
I used the Holley regulator for years, but found it was not accurate or reliable.
06-12-2007, 03:16 PM
#9
Race Director
Member Since: Oct 2004
Location: Cape Cod, Mass.
Posts: 18,764
Received 4,553 Likes
on
2,160 Posts
2023 C3 of the Year Finalist - Unmodified
2021 C8 of the Year Finalist Unmodified
2020 Corvette of the Year Finalist (performance mods)
2019 C1 of Year Winner (performance mods)
2017 Corvette of the Year Finalist
2016 C2 of Year
2015 C3 of Year Finalist
I also remember way back when that there were fuel filters at the carb entrances with a check valve...and some without. Often people bought the one without because it was cheaper.
Then they had issues with empty carbs. Re-installing the correct filter at the carb inlet solved the issue.
JIM
Then they had issues with empty carbs. Re-installing the correct filter at the carb inlet solved the issue.
JIM
06-12-2007, 03:28 PM
#10
Team Owner
HEHEHE....I forgot to post about the tank + pressure issue....i'ts funny as hell, especially with DPFI....I finally figger out what caused it, you will laugh your asses off.....
on the uper drivers end of the tank is a check valve with a float in it, the black plastic box mounted on tank externally....now, what happens is if you fill the tank completely...the thing goes into float, and closes the valve, no problemo....except the tank is completely full, well if you drive for a while, that float, which is supposed to stop full liqiid gas from hitting the charcoal canister behind the driver's fender well up front....that float ball is held in place by positive tank pressure....why?? as you are running the engine consistantly for several miles, the tank drains, but the fuel is constantly heated by going up front, and then returning on an over pressure situation, THAT and the exhaust adds to the vapor pressures in the tank....so the tank is slowly draining and developing positive pressure which holds the float ball in position closing off the vapor escape hatch.....
I have been very careful about my observations over quite some time with this, noting the pressures when stopping for gas, and noting the driving habits....i'ts highly intermittant...frankly it's funny as hell....
all this EPA type crap....
any of you think this is funny, try all that series of capillary tubes in a early C4, like my '87, you think this is comical...try THAT one....
nutzo time waster and WE pay for it....
06-12-2007, 03:56 PM
#11
Race Director
Thread Starter
I should have put this in my original post.The lack of a good check valve is in the newer can type pumps,the old pumps that you can rebuild have a great check valve.Actually the new pumps have the same type of valve but its an assembly.The actual valve isnt bad.The valve mounts in a large tube with an o-ring around it and that o-ring always leaks even new ones.I wonder if this (what I refer to as a defect) is the reason they came up with the check valve in the filter that 427 refered to was to compensate for the pump design?
mrvette-when you used the plaster did you also use the little metal blockoffs in the gasket also?
noonie-Yes I have used the T return also instead of a regulator and it works good.
Sixfooter-On the 2 I had trouble with it only took a couple of minutes for the tank to start to expand-you could hear it.On the 69 it didnt have a vapor canister and I put a small piece of clear tubing on the return and I could see bubbles so I knew air was getting in somewhere.On the line coming from the tank where the rubber hose starts I could see a little fuel there inbetween the hose and line.When you would start the engine it would suck that fuel in for a minute then it would get wet again (after the pressure in the tank would build) leak a little and then stop.I tightened that clamp up and the bubbles went away so did the pressure,I loosened it and the bubbles came back.The other car(dont remember the year)acted the same but had a blocked off canister and the fuel pump was leaking slightly around the weep hole on the pump so I just replaced it thinking maybe it had a small hole in the diaphrapm causing the injection of air.
mrvette-when you used the plaster did you also use the little metal blockoffs in the gasket also?
noonie-Yes I have used the T return also instead of a regulator and it works good.
Sixfooter-On the 2 I had trouble with it only took a couple of minutes for the tank to start to expand-you could hear it.On the 69 it didnt have a vapor canister and I put a small piece of clear tubing on the return and I could see bubbles so I knew air was getting in somewhere.On the line coming from the tank where the rubber hose starts I could see a little fuel there inbetween the hose and line.When you would start the engine it would suck that fuel in for a minute then it would get wet again (after the pressure in the tank would build) leak a little and then stop.I tightened that clamp up and the bubbles went away so did the pressure,I loosened it and the bubbles came back.The other car(dont remember the year)acted the same but had a blocked off canister and the fuel pump was leaking slightly around the weep hole on the pump so I just replaced it thinking maybe it had a small hole in the diaphrapm causing the injection of air.
06-12-2007, 03:58 PM
#12
Race Director
Thread Starter
Thanks
06-12-2007, 04:06 PM
#13
Team Owner
I have found the plates in the gaskets that try to block off the heat riser cross over soon burn through, and the problem is back before end of summer.....got to the point I never even looked at them....just use the plaster, let it harden overnight, install in the Am....and that stuff get hard as a teen age dick.....hummm wonder if that's the reason they call it 'plaster of PARIS??".....
nevermind....
nevermind....
06-13-2007, 03:24 AM
#15
Team Owner
My experiences with SBC are quite limited, my first car was a '60 vette, but that's a LONG tyme ago...40 years+.....I had a chebby 350 n a 3/4 ton van...but don't remember the Qjet being trouble other than a rebuild once in 100k+ miles....then this '72 but I swapped to TPI in 6 months or less......
so I guess the answer is NO.....not that I recall....
I know what is was, a 318 Dodge van....man, you making me think now....
