Leaky fuel cap

PJambo

Well-known member
My tank made a sucking sound every time I opened the cap. I disassembled and cleaned gunk out of my vented cap and now the vacuum issue is fixed.

Now it's created a new issue where it seems it's venting 'too' well. The area around the cap is always moist, regardless of tank level. Is this a potential issue with a tank breather?
 

SARTech

Well-known member
Noticed any power loss?

Possibly, the boost diaphragm in your injection pump has a hole in it.
Turbo pressure is applied to the diaphragm to insure proper fueling at time of higher boost. If the diaphragm has a hole boost pressure will pass onto the other side of the diaphragm which is part of the fuel return system and it will pressurize the return side and therefor pressures the fuel tank causing fuel to vent out of the cap.

That is what was going on with my 1989 D90 19J. Replaced the boost diaphragm which had a large hole in it. Fixed the fuel tank venting prob and gained quite a bit of power.
 

chris snell

Administrator
Callsign: NW5W
Staff member
This happens to me regularly when the truck has a full tank. It's just diesel sloshing out of the cap. I could probably replace the cap but it doesn't seem to cause any problems besides an occasional oily spot underneath the filler.
 

crown14

Active member
This happens to me regularly when the truck has a full tank. It's just diesel sloshing out of the cap. I could probably replace the cap but it doesn't seem to cause any problems besides an occasional oily spot underneath the filler.

In years past, this was a badge of honor, the oil burning trucks spraying their musk like a tomcat.
 

jafir

Founding Member
Some people must be able to smell the musk better than others. I opened my fuel filler to show someone that little extension that pops out to make filling from a jerry can easier, and 15 minutes later someone came by and asked me if I had a fuel leak. I couldn't smell anything.
 

PJambo

Well-known member
Noticed any power loss?

Possibly, the boost diaphragm in your injection pump has a hole in it.
Turbo pressure is applied to the diaphragm to insure proper fueling at time of higher boost. If the diaphragm has a hole boost pressure will pass onto the other side of the diaphragm which is part of the fuel return system and it will pressurize the return side and therefor pressures the fuel tank causing fuel to vent out of the cap.

That is what was going on with my 1989 D90 19J. Replaced the boost diaphragm which had a large hole in it. Fixed the fuel tank venting prob and gained quite a bit of power.

Not really any noticeable loss in the six months I've had the truck, but that doesn't mean it's not there. I can get to 70-75 without too much trouble, which I read is appropriate for the 19J.

Would a hole in the diaphragm usually be accompanied by a boost leak squeal? I don't hear anything abnormal from the boost currently.
 

SARTech

Well-known member
Not really any noticeable loss in the six months I've had the truck, but that doesn't mean it's not there. I can get to 70-75 without too much trouble, which I read is appropriate for the 19J.

Would a hole in the diaphragm usually be accompanied by a boost leak squeal? I don't hear anything abnormal from the boost currently.

If your getting up to 70-75 MPH I don't the boost diaphragm is the problem. When mine was bad i could barely get up to 50 on the flats.
Just might be old seals in your cap.
Hope you get it figured out, fuel is expensive.
 
Top