The cap is the venting system. I drill a tiny hole in the cap to bypass the valve and never worry about it again. Hole is on the inside and does not exit the exterior part.
I had a stuck cap collapse the fuel tank, on vacation of course.

 

That is what I did too. No more worries.:bounce:

 

There is a small rubber valve on the underside in the cap, just remove it.
I did that on all my trucks.
The black top of the cap snaps over the white insert and can be separated by prying the tabs in the cap away to the outside one at the time and lifting up on the insert.

 

Does it matter exactly where to drill hole or anywhere on the inside part?

 

This works. Hole could be smaller, that bit was already in the drill.

 

Thanks exactly what I wanted!

 

Why does everyone care about a little noise enough to risk fuel contamination??? It's not like the fuel is so full when you take the cap off that it could spray out - the tank is nearly empty, and it vents while you're unscrewing the cap.

 

See glagulators post #2. It also allows the fuel pumps to function more efficiently.

 

I thought the issue was a little PRESSURE in the tank. If you have a cap that doesn't vent properly, replace it.

 

A little pressure helps push fuel to the IP which is in principle not bad.

No need to buy a new cap, just take the cap apart and clean it, usually lots of dirt buildup inside prevents venting

 

Yeah, I meant not venting to let air IN as the fuel goes out, which would cause the vacuum you said was the issue. In that case, I'd just replace the cap - they're not expensive, even for the locking one I had.

I initially though pressure in the tank was the OP's complaint. That's why my first response was "who cares?"

 

Why buy a new cap, might as well throw your money into the river, same thing.

The biggest issue is, most caps are for gassers and even the diesel ones are the wrong ones, which you have to modify anyway, so now your spending money to mod a cap, :confuzeld

 

Thanks for all the replies folks. Brand new diesel cap wasn't venting so I drilled a hole in it and all is well. No more issues.


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:thumb:

 

mine does the same you dont drill thru the cap or just in the bottom part thx

 

The inside part of the cap, as long as it is a diesel cap.

 

I have to chime in. I just had a sputtering issue a few days ago. I unscrewed my "diesel" fuel cap and didnt hear the vacuum release. This helped me easily diagnose my pre lp filter was plugged (no tank sock) causing too much restriction for the lp to pull the fuel. So this actually helped me diagnose the run issue. I suppose if my return was plugged this could have caused the same result but doubtful. Still I was able to open my t valve and see hardly any flow to tip me off. I say get the correct cap with no extra venting. I have 5 diesel vehicles all with no mods to the caps and no problems. The stress caused to the lp is debatable.

 

yes i bought a new cap n made sure to tell them my truck was diesel n how big like a 1\8 or smaller bit

 

DS4 manual clearly states the factory cap is designed to maintain between 1in/HG and 2PSI in the tank and this is critical to fuel system performance. My replacement "diesel" cap has been in the garbage for awhile.

I'll let you make adult decisions, I'm just here to spread the correct info.

 

Bump,

Do all caps say "DIESEL"? I purchased a 1993 with a BLACK cap, doesn't say diesel (maybe it rubbed off), but the threads are yellow. Is this the factory setup?

Thanks,

I hear a slight whoosh (sucking into the tank) when removing the cap but pretty minimal, I'll take a video clip after. Any dead giveaways between caps? I refuse to drill a hole as there does need to be some regulation inside the tank.

 

The GM ones don't say anything and are black.

You don't need any pressure or vacuum in a diesel tank.

 

ACDELCO GT207

 

Don't always trust rockauto, this part shows it fits a lot of GAS motor cars, I wouldn't use this one, there are others listed that are specific to diesel's listed in the same column ...

 

I have heard the advice about drilling the cap. I drilled a hole in my cap when I had some issues. I have since fixed the issues and I replaced the cap with a new diesel cap. I replaced my cap with Amazon.com: Stant 10819D Diesel Fuel Cap: Automotive

This is the part I do not get, the system is for the most part a closed loop system. The purpose of the cap is to relieve the loss of volume (diesel) due to it being injected into the cylinder. Diesel fuel caps are designed to hold a little vacuum ~1 PSI.

If the system is negative 1 psi it is operating at negative 1 psi across the entire system, tank to injector pump back to tank. The net effect is it cannot put more strain on the lift pump. Example:

With lift pump off the system will equalize. You will have -1 psi before and after the lift pump. When you power the lift pump you will get a pressure differential pre/post lift pump, pre/post filters due to restrictions and the movement diesel. If your lift pump was capable of generating 9 psi pressure difference it will still generate 9 psi while having the entire system still at -1 psi. The lift pump will not know it is operating under a vacuum since the entire system is under the same vacuum.

In theory you might measure a lift pump pressure output loss of 1 psi. The loss of 1 psi is due to the entire system having a vacuum of 1 psi. If you adjust the measured value to account for the system vacuum you still have the 9 psi output of the lift pump.

I agree that if you have a small leak allowing air into your fuel system drilling the cap to relieve the system vacuum could help prevent air getting into the system.

If you have a malfunctioning fuel cap I can see collapsing the fuel tank as you drive.

I guess my question is, did GM have a reason to operate the entire system at negative 1 psi? Could it be that the system vacuum helps with cavitation, to evacuate air bubbles from fuel or to help keep the system clean?

Any thoughts or am I crazy?

Brian

 

The slight suction probably only affects the return side (which wouldn't hurt)...1in/hg at the fuel cap wouldn't have much effect on the supply side.

 

So, drill a hole or don't!?!? There's no straight answer! Do our diesels need any pressure at all or is a "slight whoosh" OK??? I have a decent whoosh sound and I'm debating on drilling a hole, but don't want to screw anything up. Will I contaminate my fuel or am I going to eventually collapse my tank!?!? What to do!?

 

I have been wondering about this.. Even though I bought a new cap for my old truck, and checked the vent for operation I still had the same problem... Now it will be solved... Thanks a ton guys

 

Bought this for my 6.5 screws in the hole every single time:HiHi:

 

All I can say is all the diesels I work on have non vented caps, and a vent hose on the tank.... I have checked my vent for blockage, and bought a new cap. Still have a vacuum on the system thats way noticeable at even half a tank... I will drill a hole in mine now...

 

So, will drilling a hole really let a lot of contaminates/debris in? I want to drill a hole, but I'm hesitant to do so if it's going to foul my fuel.

Anyone?

 

Ive never had a problem with contaminated fuel and I have a hole drilled on both my trucks. Also, if you have good filters, it wont matter. Theyll clean it up for you. I do reccomend installing a pre-lift pump filter regardless of whether you drill a hole or not.

 

I do reccomend installing a pre-lift pump filter regardless of whether you drill a hole or not.

I've been researching this, well...the FFM/fuel sock delete with pre/post filters and a Walbro FRC-10. I will probably eventually do this, but all of my components are working well at the moment, so I'm sticking with it until I can piece the FFM delete together. Also, I've heard adding a pre-lift filter on the suction side of a stock lift pump will kill it prematurely.

The hole will do exactly what the vent valve does, let air in to replace volume created by used up fuel. It will just no longer keep a slight negative pressure and the cap can no longer malfunction. I have to guess that the slight negative pressure is to keep hydrocarbons from escaping into the atmosphere.

I appreciate it guys!

 

The hole will do exactly what the vent valve does, let air in to replace volume created by used up fuel. It will just no longer keep a slight negative pressure and the cap can no longer malfunction. I have to guess that the slight negative pressure is to keep hydrocarbons from escaping into the atmosphere.

 

I have to guess that the slight negative pressure is to keep hydrocarbons from escaping into the atmosphere.

Thats the best guess I have too.

 

[quote I have to guess that the slight negative pressure is to keep hydrocarbons from escaping into the atmosphere.[/quote]

That would make sense in this era of emission focus. Satisfy whoever.
Fwiw , a reason given on here many years ago for the negative pressure feature was to eliminate or reduce foaming.