dogg rescue Posted November 25, 2015 Share Posted November 25, 2015 3 of our ch's fuel systems leak down overnight a '94, '96, and a '97. The suction lines look good, so I'm wondering is there a check valve that is bad? Thanks Quote Link to comment https://www.bigmacktrucks.com/topic/42811-e-7-losing-prime-overnight/ Share on other sites More sharing options...
mrsmackpaul Posted November 25, 2015 Share Posted November 25, 2015 I was told by some one much smarter than me that when this happens its the return line to the tank as there is no check valve in system and it siphons back around till the pressure is equal when its not running if this makes any sense Paul Quote Link to comment https://www.bigmacktrucks.com/topic/42811-e-7-losing-prime-overnight/#findComment-312880 Share on other sites More sharing options...
Baxter Posted November 25, 2015 Share Posted November 25, 2015 Our 96 RD was doing this until we replaced all the fuel lines with new ones. Since then no leaks or hard starts. 1 Quote Link to comment https://www.bigmacktrucks.com/topic/42811-e-7-losing-prime-overnight/#findComment-312884 Share on other sites More sharing options...
cstaples Posted November 27, 2015 Share Posted November 27, 2015 I'd bet its the little line from the front head down to the back of the pump or the line between the 2 heads. Check the slip on connections they get rusty and leak. I've had the same problem on 2 early E7s. Start there its cheap and easy to check. 1 Quote Link to comment https://www.bigmacktrucks.com/topic/42811-e-7-losing-prime-overnight/#findComment-313186 Share on other sites More sharing options...
blackdog2 Posted November 27, 2015 Share Posted November 27, 2015 There is check valves in the transfer pump or vent on fuel tank or fuel cap not working well. Quote Ed Link to comment https://www.bigmacktrucks.com/topic/42811-e-7-losing-prime-overnight/#findComment-313191 Share on other sites More sharing options...
ranchhopper Posted November 27, 2015 Share Posted November 27, 2015 I drove an RB that had a leak down problem hard starting every morning until I noticed there was a tiny wet spot on the return line that goes around the front head after replacing it no more problems. 1 Quote Link to comment https://www.bigmacktrucks.com/topic/42811-e-7-losing-prime-overnight/#findComment-313198 Share on other sites More sharing options...
bruiser Posted November 29, 2015 Share Posted November 29, 2015 Try taking fuel cap off when you park next. If this solves problem then you have fuel draining back to tank could be as simple as clogged vent. Quote Link to comment https://www.bigmacktrucks.com/topic/42811-e-7-losing-prime-overnight/#findComment-313481 Share on other sites More sharing options...
mrsmackpaul Posted November 29, 2015 Share Posted November 29, 2015 Fuel is meant to drain back to the tank its the way it is designed to work the fuel gets siphoned back thru the return line when the truck isnt running hence the fact it looses prime The so called check valves in in the transfer pump arent to hold the fuel up to the motor they are just to supply fuel when running Paul Quote Link to comment https://www.bigmacktrucks.com/topic/42811-e-7-losing-prime-overnight/#findComment-313507 Share on other sites More sharing options...
dogg rescue Posted November 29, 2015 Author Share Posted November 29, 2015 Thanks guys all good things to consider. I looked at the '94 and saw that the plastic return lines were crusty so I will be replacing them. Quote Link to comment https://www.bigmacktrucks.com/topic/42811-e-7-losing-prime-overnight/#findComment-313526 Share on other sites More sharing options...
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