Jay NE Ohio Posted November 10, 2007 Share Posted November 10, 2007 I bought a 88 Mack triaxle at auction and drove it home yesterday. It has a ~50 gallon fuel tank on each side. The fuel gauge is not working, so I checked each tank before I left and they both had about 9 inches. I went about 10 miles and ran out of fuel. At this point I went to unscrew the caps and found they were under considerable pressure (maybe 15 psi). The driver side tank was empty and the passenger tank now has about 12 inches of fuel. Of course it draws out of the drivers side and that is where the return line goes too.What would cause this?I put new fuel filters on and put some fuel in the drivers side to make it to the fuel station. The station was only a mile away and the tanks seemed to build up pressure again. I filled the driver side tank up to the top and drove it the rest of the way home with no problems. It was dark when I got home so I did not stick the tanks to see how much was left, but they were pressurized again. I looked in the tanks this morning and the drivers side is now half empty and the passenger side is full to the top. I did not put any fuel in the passenger side yesterday. The vent is on the passenger side. There is no vent on the drivers side. Just a suction line and a return line. The two tanks are plumbed together on the bottom. This thing really has me baffled. I thought that the tanks would have equalized overnight????? Quote Link to comment https://www.bigmacktrucks.com/topic/3069-fuel-tanks-pressurizing/ Share on other sites More sharing options...
MACKS Posted November 10, 2007 Share Posted November 10, 2007 Sounds like the air vents on the fuel tanks are pluged up, I would say the driver side is definetly pluged and it's forcing the fuel to the other tank. 1 Quote Link to comment https://www.bigmacktrucks.com/topic/3069-fuel-tanks-pressurizing/#findComment-14974 Share on other sites More sharing options...
skip Posted November 10, 2007 Share Posted November 10, 2007 I bought a 88 Mack triaxle at auction and drove it home yesterday. It has a ~50 gallon fuel tank on each side. The fuel gauge is not working, so I checked each tank before I left and they both had about 9 inches. I went about 10 miles and ran out of fuel. At this point I went to unscrew the caps and found they were under considerable pressure (maybe 15 psi). The driver side tank was empty and the passenger tank now has about 12 inches of fuel. Of course it draws out of the drivers side and that is where the return line goes too.What would cause this?I put new fuel filters on and put some fuel in the drivers side to make it to the fuel station. The station was only a mile away and the tanks seemed to build up pressure again. I filled the driver side tank up to the top and drove it the rest of the way home with no problems. It was dark when I got home so I did not stick the tanks to see how much was left, but they were pressurized again. I looked in the tanks this morning and the drivers side is now half empty and the passenger side is full to the top. I did not put any fuel in the passenger side yesterday. The vent is on the passenger side. There is no vent on the drivers side. Just a suction line and a return line. The two tanks are plumbed together on the bottom. This thing really has me baffled. I thought that the tanks would have equalized overnight?????Interesting mate. I wouldve thought there should be a breather on the drivers side as well. My only theory is that the build up in pressure has caused the push of fuel to the passenger side, because its vented. Sorry if this seems blatantly obvious. If its only returning to one side and its not vented then I dont think it has any option but to pressurise. If anyone can shed some light on this it would be great. Cheers. Quote Link to comment https://www.bigmacktrucks.com/topic/3069-fuel-tanks-pressurizing/#findComment-14975 Share on other sites More sharing options...
MACKS Posted November 10, 2007 Share Posted November 10, 2007 I'm sorry I see now that you said there is no vent on the driver side tank, the fuel cap should be vented so check the cap to see if its working. Quote Link to comment https://www.bigmacktrucks.com/topic/3069-fuel-tanks-pressurizing/#findComment-14976 Share on other sites More sharing options...
Jay NE Ohio Posted November 10, 2007 Author Share Posted November 10, 2007 I still don't understand why it would build pressure. The engine is using fuel, which would create vaccum on the tanks over a period of time. Please help me understand. Here is the chain of events that I would think are logical: The pump draws fuel from the drivers side. This should create a vaccum on the drivers side. Gravity and vaccum should force fuel from the passenger side to the drivers side. Air should enter the vent on the passenger side as fuel level goes down to compensate.If the drivers side were to be vented, then no vaccum would be created by the use of fuel. Gravity should still level the tanks.The only explanation that I can think of is that the return fuel is heated as it is pumped and therefore takes up more space than the fuel that is being used. But this does not really make much sense as this would mean that the tank level would never go down as fuel is used???Could air be getting into the system and being pumped back by the return line?? This would not make sense in a pressurized system though???Or is my original logic flawed? I'm sorry I see now that you said there is no vent on the driver side tank, the fuel cap should be vented so check the cap to see if its working. Quote Link to comment https://www.bigmacktrucks.com/topic/3069-fuel-tanks-pressurizing/#findComment-14978 Share on other sites More sharing options...
skip Posted November 10, 2007 Share Posted November 10, 2007 I still don't understand why it would build pressure. The engine is using fuel, which would create vaccum on the tanks over a period of time. Please help me understand. Here is the chain of events that I would think are logical: The pump draws fuel from the drivers side. This should create a vaccum on the drivers side. Gravity and vaccum should force fuel from the passenger side to the drivers side. Air should enter the vent on the passenger side as fuel level goes down to compensate.If the drivers side were to be vented, then no vaccum would be created by the use of fuel. Gravity should still level the tanks.The only explanation that I can think of is that the return fuel is heated as it is pumped and therefore takes up more space than the fuel that is being used. But this does not really make much sense as this would mean that the tank level would never go down as fuel is used???Could air be getting into the system and being pumped back by the return line?? This would not make sense in a pressurized system though???Or is my original logic flawed?It will expand and contract to a certain extent with the difference in temperature of the fuel coming back from the engine. The tank is also a cooling vessel as well as storage Quote Link to comment https://www.bigmacktrucks.com/topic/3069-fuel-tanks-pressurizing/#findComment-14979 Share on other sites More sharing options...
theakerstwo Posted November 10, 2007 Share Posted November 10, 2007 I bought a 88 Mack triaxle at auction and drove it home yesterday. It has a ~50 gallon fuel tank on each side. The fuel gauge is not working, so I checked each tank before I left and they both had about 9 inches. I went about 10 miles and ran out of fuel. At this point I went to unscrew the caps and found they were under considerable pressure (maybe 15 psi). The driver side tank was empty and the passenger tank now has about 12 inches of fuel. Of course it draws out of the drivers side and that is where the return line goes too.What would cause this?I put new fuel filters on and put some fuel in the drivers side to make it to the fuel station. The station was only a mile away and the tanks seemed to build up pressure again. I filled the driver side tank up to the top and drove it the rest of the way home with no problems. It was dark when I got home so I did not stick the tanks to see how much was left, but they were pressurized again. I looked in the tanks this morning and the drivers side is now half empty and the passenger side is full to the top. I did not put any fuel in the passenger side yesterday. The vent is on the passenger side. There is no vent on the drivers side. Just a suction line and a return line. The two tanks are plumbed together on the bottom. This thing really has me baffled. I thought that the tanks would have equalized overnight????? If you have a injector that is open or bad it could push air back into the injection pump into the return and put exhaust or compression into the tank but then you should have a noticeable miss when runing. glenn Quote glenn akers Link to comment https://www.bigmacktrucks.com/topic/3069-fuel-tanks-pressurizing/#findComment-14980 Share on other sites More sharing options...
Jay NE Ohio Posted November 10, 2007 Author Share Posted November 10, 2007 I think you might be onto something 'theakerstwo'. I have never drove a Mack before, so this is all new to me. It does seem to be a little low on power, as I had a hard time staying at 55mph empty up some hills in 5th gear high range. It is also idling slow (500 rpm). Thanks for the idea!If you have a injector that is open or bad it could push air back into the injection pump into the return and put exhaust or compression into the tank but then you should have a noticeable miss when runing. glenn Quote Link to comment https://www.bigmacktrucks.com/topic/3069-fuel-tanks-pressurizing/#findComment-14982 Share on other sites More sharing options...
MACKS Posted November 10, 2007 Share Posted November 10, 2007 The reason why it will build pressure if the vents are pluged is that the pump draws more fuel than the engin can use, just so you can understand what im saying, lets say for every gallon of fuel the pump draws from the tank the engin only uses one quart and all the rest is returned to the tank. Quote Link to comment https://www.bigmacktrucks.com/topic/3069-fuel-tanks-pressurizing/#findComment-14986 Share on other sites More sharing options...
twostickmutt Posted November 10, 2007 Share Posted November 10, 2007 The reason why it will build pressure if the vents are pluged is that the pump draws more fuel than the engin can use, just so you can understand what im saying, lets say for every gallon of fuel the pump draws from the tank the engin only uses one quart and all the rest is returned to the tank.take off (or loosen) drivers side cap and see if they equalize sitting there not running. Quote and remember "PREFORMANCE COUNTS" Link to comment https://www.bigmacktrucks.com/topic/3069-fuel-tanks-pressurizing/#findComment-14987 Share on other sites More sharing options...
HK Trucking Posted November 10, 2007 Share Posted November 10, 2007 If you have a injector that is open or bad it could push air back into the injection pump into the return and put exhaust or compression into the tank but then you should have a noticeable miss when runing. glennGlenn, I think your theory makes the most sense.The only thing I'm wondering is why, with that much pressure on return side of the system, those plastic return lines between the injectors and from the injectors to that return fitting on the pump haven't blown off? They're only a "slip on" fit.I had one of those pop off on one of my trucks once, and every drop of fuel that should have been returning to the tank was being pumped out onto the ground. What a mess! Quote "If You Can't Shift It Smoothly, You Shouldn't Be Driving It" Link to comment https://www.bigmacktrucks.com/topic/3069-fuel-tanks-pressurizing/#findComment-14991 Share on other sites More sharing options...
HK Trucking Posted November 10, 2007 Share Posted November 10, 2007 Now I'm thinking again.In 1988, didn't Mack use a vented cap on their fuel tanks?Did someone replace the cap on the driver's side tank with a non vented cap? Quote "If You Can't Shift It Smoothly, You Shouldn't Be Driving It" Link to comment https://www.bigmacktrucks.com/topic/3069-fuel-tanks-pressurizing/#findComment-14992 Share on other sites More sharing options...
theakerstwo Posted November 10, 2007 Share Posted November 10, 2007 If you have a injector that is open or bad it could push air back into the injection pump into the return and put exhaust or compression into the tank but then you should have a noticeable miss when runing. glennI can remember seeing only one mack engine do that but trhe way the compression gets to the return is licking back thru the injector line and back thru the delievery vavle in the injectiom pump and down into the barrel and plunger and back thru the return from the pump to the tank. Quote glenn akers Link to comment https://www.bigmacktrucks.com/topic/3069-fuel-tanks-pressurizing/#findComment-14993 Share on other sites More sharing options...
MACKS Posted November 11, 2007 Share Posted November 11, 2007 If the truck sits overnight and the fuel tanks do not level themselves out and the cross over line is clear witch we know it is than the only thing it could be is the vents on the tank or the vented fuel cap which ever one it has,like twostick said loosen the fuel cap and see what happens. Quote Link to comment https://www.bigmacktrucks.com/topic/3069-fuel-tanks-pressurizing/#findComment-14996 Share on other sites More sharing options...
rodperformance Posted November 11, 2007 Share Posted November 11, 2007 change position of the caps invert them while on it check the spring on the return valve on main pump Quote Link to comment https://www.bigmacktrucks.com/topic/3069-fuel-tanks-pressurizing/#findComment-15001 Share on other sites More sharing options...
David Posted November 12, 2007 Share Posted November 12, 2007 We had this happen once on a 237. The truck was running fine, ran out of fuel, one tank empty and one tank half full. Filled it up and ran about 50 miles and the engine started knocking. One of the injectors was loose and started coming apart. Tightened the pieces of the injector back up, installed it, and made it home without amy futher problems. Quote Link to comment https://www.bigmacktrucks.com/topic/3069-fuel-tanks-pressurizing/#findComment-15032 Share on other sites More sharing options...
Noid93 Posted November 13, 2007 Share Posted November 13, 2007 I rebuilt my injectors on 97 ch and the compression washers that set betweens the injector tip and head that were given to me were old stock and had been updated to a narrower washer for a better seal. Anyway, i put the injectors in and there was a little carbon in the holes, and when i started the SOB, and sure enough, there was compression leaking into the drivers tank, the tank that draws and has the return line in it. I ended up getting the new style washers and cleaning the holes real good. Sounds just like my issue. IF THERE IS A FOG OR HAZE IN THE TANK, THERE IS COMPRESSION IN THERE. Once i fixed the problem, the haze went away.Or take off the retun line and see if there is only fuel coming back or if there is air in there too.. As you stated, mine built ALOT of pressure in the tanks...I have seen some tank setups where there is no breather on the draw tank so it uses fuel out of the secondary tank first and then the primary tank. If compression is leaking back it must go out the secondary tank to escape out the breather, after it pushes the fuel out of its way. Quote There Is No Replacement, For Displacement!!!!! Link to comment https://www.bigmacktrucks.com/topic/3069-fuel-tanks-pressurizing/#findComment-15049 Share on other sites More sharing options...
HeyCharger Posted November 13, 2007 Share Posted November 13, 2007 The reason why it will build pressure if the vents are pluged is that the pump draws more fuel than the engin can use, just so you can understand what im saying, lets say for every gallon of fuel the pump draws from the tank the engin only uses one quart and all the rest is returned to the tank.Correct, MACKS. Jay have you physically traced the return line from the injector pump? You stated that it returns to the drivers side tank. I would not mind betting that it has been rerouted to the passengers side tank. That is why all the excess fuel is ending up there. You normally have to draw from the tank where the return goes to first. I learnt the hard way, as I dumped quite a few litres overboard. Why they pressurize.....hhhmmm.Rod. Quote Proud owner of;1961 Mack B61 prime mover.1981 International ACCO 1810C DualCab Fire Truck Link to comment https://www.bigmacktrucks.com/topic/3069-fuel-tanks-pressurizing/#findComment-15053 Share on other sites More sharing options...
jaked92 Posted November 19, 2007 Share Posted November 19, 2007 I bought a 88 Mack triaxle at auction and drove it home yesterday. It has a ~50 gallon fuel tank on each side. The fuel gauge is not working, so I checked each tank before I left and they both had about 9 inches. I went about 10 miles and ran out of fuel. At this point I went to unscrew the caps and found they were under considerable pressure (maybe 15 psi). The driver side tank was empty and the passenger tank now has about 12 inches of fuel. Of course it draws out of the drivers side and that is where the return line goes too.What would cause this?I put new fuel filters on and put some fuel in the drivers side to make it to the fuel station. The station was only a mile away and the tanks seemed to build up pressure again. I filled the driver side tank up to the top and drove it the rest of the way home with no problems. It was dark when I got home so I did not stick the tanks to see how much was left, but they were pressurized again. I looked in the tanks this morning and the drivers side is now half empty and the passenger side is full to the top. I did not put any fuel in the passenger side yesterday. The vent is on the passenger side. There is no vent on the drivers side. Just a suction line and a return line. The two tanks are plumbed together on the bottom. This thing really has me baffled. I thought that the tanks would have equalized overnight????? Quote Link to comment https://www.bigmacktrucks.com/topic/3069-fuel-tanks-pressurizing/#findComment-15164 Share on other sites More sharing options...
Dutchie2007 Posted August 30, 2015 Share Posted August 30, 2015 I have a 98 CH 613 n it keeps running out of fuel with the other tank half full or sometimes full. Funny thing is that either side will do it. We have cleaned tank caps nwhen it happens there seems to be pressure on the cap with empty tank when we open it. 1 Quote Link to comment https://www.bigmacktrucks.com/topic/3069-fuel-tanks-pressurizing/#findComment-302025 Share on other sites More sharing options...
gearhead204 Posted August 30, 2015 Share Posted August 30, 2015 my superliner is a 89 and has check ball style vents and breather hose on each tank. Cutter bees had plugged the vent hoses and caused me to run out of fuel as well. after cleaning the hoses and valves no more problems. I would say that if the tanks are holding pressure overnight you have a vent issue! but as mentioned in other posts it may not be the only issue you have. Quote Link to comment https://www.bigmacktrucks.com/topic/3069-fuel-tanks-pressurizing/#findComment-302046 Share on other sites More sharing options...
Underdog Posted August 30, 2015 Share Posted August 30, 2015 I have a 98 CH 613 n it keeps running out of fuel with the other tank half full or sometimes full. Funny thing is that either side will do it. We have cleaned tank caps nwhen it happens there seems to be pressure on the cap with empty tank when we open it.On the newer trucks they have done away with the crossover line between the tanks. Top draw and return to both tanks with a tee fitting mounted above the transmission. I had the same issue with my 2000 CX until I installed a crossover line at the bottom of the tanks. When I needed new fuel lines, I did away with the tee fitting and plumbed it old school single draw/return on the driver side. Worked fine for many years.GreggSent from my XT1254 using Tapatalk Quote Link to comment https://www.bigmacktrucks.com/topic/3069-fuel-tanks-pressurizing/#findComment-302058 Share on other sites More sharing options...
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