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post #1 of (permalink) Old 07-01-2002, 04:30 PM Thread Starter
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help diagnose engine performance issues

Well bare with me. Here's a list of things I have. 304 headers, tr upgrade, howell efi, msd 6a. 22gal steel tank with no vent. I went wheeling yesterday. The first part of the day the truck ran great, I was going through various water holes some deep some not so deep. The truck ran great. After about an hour of driving trails and washouts (all dry dusty dirt) it would bog down under any throttle. Almost like I was out of gas(which I wasn't). If I turned the truck off for 5 minutes, it would start right up and run great until it heated up again. My check engine light came on a couple times when it was doing this, but would run good with the light on. As soon as the light went off, it would run like crap again. And it didn't do it on the highway at the end of the day. The reason I mentioned the no vented gas tank, is a couple of times during the day, I would hear my tank buckle in from pressure. Any help would be greatly appreciated. ty
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post #2 of (permalink) Old 07-01-2002, 04:53 PM
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Re: help diagnose engine performance issues

I had that problem with my pro-jection. Holley tech helped me figure out it was the tank not being vented. The vacuum gets to be too much for the pump to pull fuel. Either use a vented cap, run a vent to a charcoal canister, or stop every once in a while and take the cap off and put it back on to relieve the vacuum.
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post #3 of (permalink) Old 07-01-2002, 05:03 PM
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Re: help diagnose engine performance issues

What did the computer say? Did you run it in road test mode?
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post #4 of (permalink) Old 07-01-2002, 05:28 PM Thread Starter
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Re: help diagnose engine performance issues

No, i didn't run it in test mode. And now the light doesn't come on.
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post #5 of (permalink) Old 07-01-2002, 06:14 PM
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Re: help diagnose engine performance issues

Humm... no lighted SES lamp? Then your computer ( or something ) has lost it's 12 volts... memory or main power line... I'm not sure, but I'd start there. It'd be thinking the system's brain dead. Check the fuses and the voltage to the Howell unit.
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post #6 of (permalink) Old 07-01-2002, 06:17 PM Thread Starter
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Re: help diagnose engine performance issues

What i meant to say is that the light doesn't come on when the engine is running. If i put it test mode when the check engine light isn't on, will still give me the problem? Or do I have to wait until the light comes back on? Now I know why they call them idiot lights haha
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post #7 of (permalink) Old 07-01-2002, 06:24 PM
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Re: help diagnose engine performance issues

All test mode does is ground one pin and put the computer in road test mode. Then you can run down the road and see the SES flash fast or slow, signifing rich or lean mixture. Under normal conditions, If everything is running fine, the SES lamp will not display. It signifies a problem or is used as crude a diagnostic tool.

If you turn the ignition to "Run" the SES lamp should come on and not turn off untill the engine starts. If the SES lamp is not glowing nice and bright under this condition... I'd be getting concerned and look again at the comptuer input memory and power voltages.
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post #8 of (permalink) Old 07-01-2002, 06:28 PM Thread Starter
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Re: help diagnose engine performance issues

it does come on with the ignition and off with the motor running. i'll try and pull some numbers from it tomorrow. Thanks for the input
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post #9 of (permalink) Old 07-01-2002, 07:14 PM
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Re: help diagnose engine performance issues

Keith it sure sounds like you've already answered your own question.
The non vented tank with a non vented cap causes the tank to evacuate as you use gas, eventually it gets to where the pump can't pull enough out of it.
Sitting with engine off 5 minutes, or releasing the vacuum by taking the cap off breaks the vacuum.
Hearing it collapsing is a huge clue!
Symptom is just like running out of gas or running very lean, then dying.

When you ran it on the highway it may have stopped sealing and the problem went away.

Simple test - try it without the cap - see if it helps.

Even most "non vented" caps are still vented. There's a check valve that lets air in, but not out. Dirt, mud etc gets in and plugs it. Sometimes it's not really a valve, but the gasket's designed to function that way. When the gasket gets hard it stops letting air in to replace the used up gas.

The check engine light can come on as the mixture goes too lean, O2 codes, and the map sensor sees low vacuum when you try to keep it running.
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