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| Fuel line knock https://mail.slantsix.org/forum/viewtopic.php?t=68423 |
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| Author: | Dropin26 [ Tue Nov 11, 2025 11:30 am ] |
| Post subject: | Fuel line knock |
One thing I do love about owning a 'classic' car is they will provide hours of hobby time. Few daze ago I heard a knocking sound from underneath the passengers seat. It was intermittent and matched the engine rpm not tire rotation. Quick investigation, the fuel line from tank to pump was pulsating and hitting front frame rail. Has anyone else had a knocking fuel line? Thinking fuel pump. I replaced it years ago to get it started/home when first bought car. Clogged fuel filter causing back pressure? A loose frame/fuel line clamp? They seemed tight. Should I replace the whole line set-up? It's original. Would up-sizing to 3/8 be advantageous? Running a Holley 390. It does have newer sending unit, what sends incorrect fuel levels. Would dropping the tank and adjusting the float arm be reasonable fix for level readings. I know about the sender/gauge adjustment box but like trying free ways first. Car runs excellent beyond the tapping. Anyways I'm retired and have hobby time. Thank you |
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| Author: | SlantSixDan [ Tue Nov 11, 2025 12:35 pm ] |
| Post subject: | Re: Fuel line knock |
Quote: the fuel line from tank to pump was pulsating and hitting front frame rail. Adjust the position of the line slightly so it can't hit the frame there, securing it as needed, and/or put a slit length of fuel line hose over it to prevent metal-to-metal contact.Quote: Clogged fuel filter causing back pressure? Probably not. Quote: Should I replace the whole line set-up? I can't imagine why. Quote: Would up-sizing to 3/8 be advantageous? Not even slightly, no.Quote: It does have newer sending unit, what sends incorrect fuel levels. Would dropping the tank and adjusting the float arm be reasonable fix for level readings. Depends on what kind of inaccuracy you're seeing. If the resistance curve is correct but shifted in one direction or the other, bending the arm couild fix it. If the resistance curve is wrong, then only a sender with the appropriate curve will fix it.
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| Author: | mcnoople [ Wed Nov 12, 2025 7:11 am ] |
| Post subject: | Re: Fuel line knock |
I have seen what you are describing one time. But it is certainly not a valid comparison. I saw an engine mounted direct injection HIGH pressure fuel pump cause the front to rear fuel feed line to vibrate loud enough to hear it in the car. You could watch that line dancing whenever the engine was on. New pump did get rid of the vibration. But the HIGH pressure pump I replaced operates at 1000+ psi at times so I don't think it is an apples to apples situation. Regardless of if it is caused by you fuel pump, you might end up having a hard time getting a good slant fuel pump from the parts store. If you run correctly on the current pump I would try to better isolate/mount the hard line to prevent movement. Another option that isn't NHRA approved would be to get an extra long section of fuel hose (quality fuel hose) and run a circular loop of it from the fuel pump to the metal line. This loop would need to be properly secured but should absorb the pulses. If you go this route it needs good mounting/routing to protect the hose, fuel is not something you want leaking. |
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| Author: | Dropin26 [ Fri Nov 14, 2025 12:57 pm ] |
| Post subject: | Re: Fuel line knock |
Thank y'all for your replies. Knowledge is King. So with the engine idling, I listened to the fuel pump with my dowl-o-scope. It has a intermittent 'slipped spring' noise. Sounds like when your installing drum brakes and the pad spring slips while being attached. Thong! Checked my records, I did put a new one on 6 years ago. It's a Delphi. Ordered a Carter off RA. Checked the hard lines front to back. All looked right and tight. No tap/rub marks anywhere. As far as the sender/gauge nuisance. The original owners records show it being replaced in 2006. No brand given. Anyways gonna try a few things. Bends. rod lengthening, re-wrap windings idk. There are plenty of other gauge postings so I'll leave it for another day. |
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| Author: | SlantSixDan [ Sat Nov 15, 2025 10:00 am ] |
| Post subject: | Re: Fuel line knock |
Quote:
I did put a new one on 6 years ago. It's a Delphi. Ordered a Carter off RA You're likely setting yourself up to go from harmlessly-noisy to worse. New \6 fuel pumps tend to be much poorer quality than we used to be able to get (brands like "Carter mean nothing any more; they're names slapped on Chinese-made trinkets) and they've been coming through with much-too-strong diaphragm springs, giving 8psi fuel pressure — about double what it should be, which makes for all kinds of flooding problems. If you go too far the other direction, old fuel pumps pose risks as well—rubber doesn't react well to age, and some of it doesn't react well to alcohol in the fuel. There is no guarantee, but I had consistently fine results buying and using NOS AC № 6972 fuel pumps, even with ethanol in the gasoline. Cheap and easy on eBay, such as this .
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