Vacuum Question Related to Lift Pump Failure?

[gmenor]

Previous post I had a no start on the beast. Got some advice and went out and got new OEM lift pump and OPS. Hadn't installed them yet. Went to fill up tonight and did not get the usual vacuum sound when taking off the gas cap. I did notice this on the previous fill up. Could this be an indicator of possible lift pump, gas cap or IP failure? Any thoughts? I plan to get the lift pump and OPS change out tomorrow - another $200.00 for 50k of driving.

[DickWells]

Theoretically, you shouldn't get a vacuum hiss when you open your fuel cap. I always used to, until I started running with the cap loosened. Thought it had let me starve for fuel a couple of times, so I just tighten it till it clicks and then back it off till it's a little loose. I have taken the cap appart a couple of times, and it checks out, OK. But, I always got vacuum in there, regardless. Someday, I'll pick up a new cap and see if it still gets vacuum.
Did you check for lift pump failure by opening the petcock at the drain hose in the front and letting it run a stream, while idling the engine? If the engine stalls, then the lift pump isn't giving it enough fuel, or the OPS isn't running the lift pump at all. To check for lift pump, alone, I always just take out the relay from the power center on the driver's side, under the hood, and jumper terminals 87 and 30. That'll run the pump, even with the switch off. There are other ways, but I find this easiest on my Suburban.
A lot of us run a seperate relay on the firewall. That takes the load off the OPS. If you're interested, let us know, and one or more of us will post in here, how it's done. Keeps the little contacts in the OPS from burning out.
Good luck.
Dick

[catmandoo]

you shouldn't get a vacuum hiss at all it should be pressure.

[gmenor]

Forgot to add that I did change the fuel filter just as a precaution. Cleaned out the fuel filter bowl - a lot of crud on the bottom. Bled the air out of the lines. The engine did not skip a beat when I bled it at the fuel filter and the drain at the front. Ever since I've had the beast I had always had a semi-strong vacuum.

I have tried to recreate the no start scenario yesterday to no avail (I guess I have to be in a hurry or away from the house). I can hear the lift pump after I turn off the truck (less than 5 seconds running time).

There are no Codes set. Whether it's pressure or vacuum (still believe it is vacuum) it is absent. This morning I will go out and buy a new gas cap for peace of mind. Am I off track here whether we call it vacuum or pressure - that the absent of either one can be related to lift pump, IP or gas cap failure?

[DickWells]

Oh, it's probably vacuum. I've put soap suds around mine, back before I started leaving it loose, and I never got it to blow out, just suck in.
I have a 42 gallon tank in mine, and I feel lucky that I didn't colapse it, especially when I've towed for 500 miles, or better, and it took in the higher 30's to fill it. I guess a weak lift pump can be of some benefit. A strong pump would suck a tank down to nothing. Happened to me on a Fiat once. Yup, probably the only other vehicle on the road that could be more exasperating than an electronic 6.5. But I'd still rather push my Chevy than drive a ( ).
Good luck, Dick.

[jlog]

If you are getting vaccuum you should replace the fuel cap. it is designed to allow air in not out. The hiss is air escaping not entering the tank. sometimes if I remove the cap quickly it will almost be blown out of my hand.

John

[catmandoo]

if you don't have very far to go to get to the station the tank doesn't have much time to build up pressure,but if you drive for 6-7 hrs like i do you better be pulling the cap off slow or it's gonna hurt ya,driving the return fuel is heated from running thru the pump thus creating pressure,and when the tank gets low there is more volume in the tank for the pressure to build,sometimes i can crack the cap loose and it will blow pressure out for 4-5 seconds.the pressure also helps the lift pump,if it's a vacuum it's just hurting the pump,put a new cap on it.

[norm]

Because diesel fuel is not as volatile as gas, you won't always have pressure at the cap. The only way it seems to me that you are going to get pressure is the heating of the fuel/tank while driving/hot day, etc. When my tank has cooled, I almost always have a little bit of vacuum at the cap. This is a brand new cap from the dealer. It is designed to let air in, but after a certain amount of vac. builds up. (I am not saying this is good, but this is the way these caps seem to be). It is weird that we seem to have different experiences with this - is there a cap out there for sure that allows no neg. pressure??

[DickWells]

I would have been glad to get pressure. It was ALWAYS vacuum. Lots of it. To this day, take the cap off and suck on the little hole in the center and it will let air in. This says that the cap is good. Yet, put it on tight and drive for a few hours and it makes vacuum. Someday, I'll remember to buy a new cap and try it. But, until then, I'll just keep the cap loose.
I never had a problem with it on short hauls, but after towing for several hundred miles, I'd get some missing going on in the engine, if I had a weak lift pump. Jump out and let the air into the tank, and you were OK for a while. That's when I started leaving the cap loose. That was two years and close to 50K miles ago! Should be shot for not getting a new cap by now. Been too busy either traveling, or putting in the marine DB2, etc.
Dick Wells

[JohnC]

I've always argued that these tanks can hold pressure but not vacuum, at least when everything is working correctly. However, the inlet check valve in the cap does have a very light spring on it, so, theoretically, it can hold a very slight vacuum, probably significantly less than an inch. That said, it doesn't take much vacuum in a 42 gallon tank to make a lot of noise. The volume of air needed to relieve the vacuum is high when the fuel level is low, and it's getting sucked in past the threads of the cap.

None of this should have any effect on performance, at least as long as the lift pump is running. If tank vacuum is causing you problems then I'd look first at the lift pump. You can pop the cap apart and look at the check valve if the lift pump is not the problem.