Vapor lock or vent problem??-gctid387926

[Guest]

Hi,

I am having a fuel problem with my 1993 Ciera 2655 5.0L LX. It starts right up and runs perfectly for about 7 or 8 min and then starts to miss and drop in rpm until I eventually have to shut it down. I thought it may be water in the filter so I took the filter off and drained it. The engine starts right up and works perfectly for another 7 or 8 min and then starts to miss again. I noticed that as I slackened the filter I heard a suction sound. After noticing that I then only had to slacken the filter and not drain it. Once I tightened the filter back up the ran perfectly for a while and then repeat problem. I am wondering if this is vapor lock or is the fuel tank vent plugged or blocked off. Tomorrow I will run it with the fuel filler stopper off and see what happens. Anyone else have this problem. Any suggestions?? Thanks.

[Guest]

Sounds like a blocked fuel tank vent to me.

[yachtman]

To check for blocked fuel vent take the gas cap off and run it a while if no problems... problem diagnost.

[MidnightSun]

bfancy wrote:
Hi,

I am having a fuel problem with my 1993 Ciera 2655 5.0L LX. It starts right up and runs perfectly for about 7 or 8 min and then starts to miss and drop in rpm until I eventually have to shut it down. I thought it may be water in the filter so I took the filter off and drained it. The engine starts right up and works perfectly for another 7 or 8 min and then starts to miss again. I noticed that as I slackened the filter I heard a suction sound. After noticing that I then only had to slacken the filter and not drain it. Once I tightened the filter back up the ran perfectly for a while and then repeat problem. I am wondering if this is vapor lock or is the fuel tank vent plugged or blocked off. Tomorrow I will run it with the fuel filler stopper off and see what happens. Anyone else have this problem. Any suggestions?? Thanks.

Does sound like a blocked vent but you mention a suction sound? You may have another unrelated issue as there is an anti siphon valve at the tank which should not allow this from the filter. If the vent is good after your test, I would suspect the gas pickup screen in the tank may be obstructed. This happened to me many years ago an my dads Sea Ray while vacationing on the boat, it got so bad I had to paddle to shore and get somebody to tow me to a marina where they trouble shot and repaired it. The intake screen opening was just enough to let fuel in and run but eventually the crud got sucked back once again blocking it. When the engine was shut down the enough crud fell from the screen allowing the engine to fire up once more only to repeat the shutdown when the crud got sucked into the pick up screen again. Long shot, but that was my case.

[2850Bounty]

If a fuel tank vent was plugged, there should be enough air volume (above the fuel) that it would require some run time in order to create enough Neg Pressure to cause a problem.

Of course a full fuel tank would require less time, than if a half full or even near empty tank would.

But you can see the dynamics here.

I think Hans is onto something.

Do the test that Dave suggested. This will indicate or eliminate.

I'd still pull the fuel tank dip tube/pick up tube, and examine the screen, and I'd still examine the anti-siphon valve, or replace it.

By all rights, when removing a fuel filter cartridge, we should NOT be hearing pressure equalizing..... either Pos or Neg.

If you are hearing equalization (a hiss), something is not quite right!

.

[Guest]

tried it this morning with the gas filler stopper off and very little difference if any. Guess I am going to have to examine the fuel line inside the tank and the fuel pickup assembly. Thanks for your replies and I will keep you posted.

[Guest]

Checked things out today. First I pulled the fuel pickup tube out of the tank. It is just a straight plastic tube with an internal mesh ...no sock or check valve. Everything was very clean and unobstructed. I checked the tank vent and that is clear. The little filter where the fuel line enters the carb was pretty dirty with a fine reddish brown dust on the inside of the screen mesh. I removed the filter and will replace it later. I am hoping that is the problem. I didn't get chance to test run it as it much too windy so I won't know if I have the problem solved until later. What do you guys think? Thanks.

[Guest]

Problem not solved . Tried it today and same as before. Don't know what to try next.

[Guest]

As others have mentioned did you check the anti siphon valve? The little ball and spring inside the fitting could be stuck in a near closed position and prevent fuel from moving out of the tank. It should be located just down stream of the pick-up on top of the fuel tank.

[Guest]

Is this the piece that screws into the elbow at the top of the tank? I had that piece off but didn't notice a check valve. I thought it was straight through. I will take it off again and check it out. Thanks for the info.

[Guest]

bfancy wrote:
Is this the piece that screws into the elbow at the top of the tank? I had that piece off but didn't notice a check valve. I thought it was straight through. I will take it off again and check it out. Thanks for the info.

Yes, it should have a check valve inside the fitting. Mine has a spring loaded ball.

[Guest]

Just throwing this out there, I thought I had a fuel problem last season, I screwed with the fuel filter and it ran fine for a few minutes, then died, swore it was fuel related, dropped it off at the marina. 5 hours later they called and said the cap was shot (new / defective) and causing the problem. For piece of mind, change the cap / rotor.

[2850Bounty]

2859er wrote:
As others have mentioned did you check the anti siphon valve? The little ball and spring inside the fitting could be stuck in a near closed position and prevent fuel from moving out of the tank. It should be located just down stream of the pick-up on top of the fuel tank.

Again, didn't you describe that you heard a hissing sound when you removed the filter cartridge, as though suction was being trapped and held between the fuel tank and the fuel pump??????

I'm thinking that a restriction "UP-Stream" of the filter unit may cause this suction "hiss" when the cartridge is removed!

Since you removed the fuel tank fill cap for a test, it's obviously not the cap nor the fuel tank vent..........., so that leaves the the pick-up tube and/or the anti-siphon vavle.

.

[Guest]

There is no anti -syphon valve in the piece that's on the top of the tank. It is just a straight through fitting.

[welshswampy]

Try running it with a portable type tank with new fuel and new lines, if nothing else it will eliminate the possibility of blockage of your existing lines etc