fuel line hammer

[doggiedave]

Good afternoon, I've been thru the search. and the subject has been hopped around I have a knocking noise in front of engine, I beleive to be fuel line hammer. I have an oil dampened fuel pressure guage mount ed on my fuel rail. the needle bounces like crazy at idle then steadies at above idle I installed an adjustable fuel pressure regulator. I'm not sure if I adjusted it correctly will check. my understanding is that to set the pressure, the vacuum line has to be disconnected, not sure if I did that will try today. was fuel line hammer an issue on 90' models? any other suggestions? thanks

 

[Hib Halverson]

Fuel line hammer was an issue with many C4s prior to the 93 or 94 model year, however, a noise heard at the front of the engine is not usually fuel line hammer. Typically, you notice it when you're inside the car because it's the fuel lines running down the right frame rail which emit the most noise.

On a 90, you do not need an AFPR unless the engine is modified quite a bit.

Arbritrarily raising fuel pressure does not noticeably increase performance on a stock or near stock engine unless there is some other problem which decreases it, ie: weak pump, restricted lines, plugged fuel filter and so forth.

 

[Tuna]

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What he said. I know fuel line hammer was an issue with 92 and early 93 models. I had that issue with my 92 but it only occured on cold start, cold days and disappeared after a couple of minutes.

 

[doggiedave]

I have a 383 stroker engine in it with ported heads 6" rods, crane cam ported plenum, accel 26lb injectors Flowmaster exhaust with high flo cat hence the afpr, also have Hypertech thermomaster chip which I think is bunk. will go custom chip route soon. as to the fuel pressure, is the needle supposed to fluctuate wildly at idle? It bounces like crazy between 30psi and 50psi. then almost is almost steady at higher engine rpm, the Knocking is on the top side of the front of the engine seems to be under the plenum

 

[Tuna]

Not fuel line hammer in that location - at least not what Hib and I have talked about.

Definitely get a custom chip for that motor. I've had good results using fastchips.com (Ed Wright Automotive) in the Tulsa area - just called them on the phone, explained what work was done, sent them a check and got my chip in the mail. Plugged and played on the old OBDI computers.

 

[Dcalebaugh]

Well for what it's worth, I have an 86 that started having issues that I was pretty sure was fuel related. I had the fuel pump replaced and also the filters. I still had a problem, which I found out was actually two different things. My car had always rattled when I first started it up in the morning. It sounded like it was under the passenger side floor and sounded like something hitting metal to metal. It would seem to go away after awhile but this year things turned worse. After filling up my tank the car would run fine for a short time then would fall on it's face as if it was out of fuel. The rapping sound got really bad when this would happen and the car would buck really bad unless you feathered the gas and brought the fuel level down to about a half tank. I got it home and hooked up a fuel pressure gauge and it too was fluctuating quite abit and would not hold pressure after the ignition was off. Turns out that the pulsator that was replaced when I had the pump changed was leaking by when the pressure built up. Throw it away if you still have one. The rapping you describe sounds like what mine was doing. I thought for sure it was in the fuel regulator because it sure sounded like it was coming from that area, not sure why. As for the full tank part of the story seems the cap that covers the vent for the charcoal line out of the tank had come off and I guess was sucking gas up inside. Once I replaced that all was well again.

Dave

 

[ZumZum]

Just adding a possibility. On my '88 I had a knocking noise from the front of the motor. I traced it to the diverter valve for the AIR pump. Installed the AIR pump delete pulley from TPIS and removed the diverter valve and related hoses. Cured the knock.

Removing that equipment may not be an option depending on your local smog rules.

 

[doggiedave]

Thanks guys for the tips. will check both mentioned items. will give you update soonest

 

[JimBobC4]

Thanks guys for the tips. will check both mentioned items. will give you update soonest

To answer your question about the fuel pressure at idle. My '95 LT1 is dead steady at 41 PSI at idle. No bouncing no fluctuation.

 

[Hib Halverson]

Fuel line hammer sounds like someone lightly "thunking" the bottom of a small tin can. It is more obvious from inside the car, especially on the passenger side, than it is on the outside.

The noise typically would not go away after warm-up though it might change a bit.

The noise comes from the abrupt pressure fluctuations inside the fuel system caused by the injectors opening and closing. In MY93-94, GM made some changes to the fuel system and fuel line mountings to mitigate the problem.