When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
I've been messing with this issue off and on for two seasons now. I finally got to drive the vette this year just this week. So I have not fully eliminated all causes of my low fuel pressure problem.
What it is doing is running out of fuel at high RPM's due to fuel starvation. Let off the gas and it is fine again. I have an in line gauge that indicates 1.5 to 4 psi at random various times at idle. So it's not consistently too low for performance according to the gauge at idle. But is lower that it should be even at idle. I should be getting 5.5 to 7 if I want it. However I cannot observe the fuel pressure while driving it's under the hood. I don't really need to, I know what is happening.
The history.
So far I am on my third fuel pump. 4th if you count the one I originally started with. It was an airtex. Second one was an AC Delco. Both of those were stock style pumps. The AC was low pressure immediately out of the box. I decided to re-vamp my fuel system and go with the Holley style fuel pump. So far on my second one and still have low fuel pressure.
I do have a pressure regulator on the system feeding a Q-jet carb. Initially I took the regulator out to see if that was the problem. No change.
While I had it out I took it apart and there are no issues with it whatsoever.
Next was to drop the tank and replace the sock. There was nothing wrong with the sock in the tank, but I had a new one so it went on. The tank has a rubber liner and it was in good condition and no debris in the bottom of the tank. Rubber line from the tank to the steel line was in good condition still soft and pliable. This tank has been replaced I can tell as well as the associated rubber lines.
While I had the tank off and the fuel pump off I blew air through the line and it blew clear. I also ran some wire up the steel line as far as I could, maybe 2 ft or so to see if any rust was dislodged. none came out. So it's ok? maybe.
I am using steel braided fuel line from the pump to the regulator and same to the carb. No kinking of any sort. I bent some steel line on the suction side to connect to the steel supply line only very short 2" and 3" rubber connections. Not possible to kink or collapse.
The fuel filter inside the carb has been replaced. Upon removing the old one it was clean. No debris at all in it.
I set the float level a little higher. Just took marginally longer to run out of fuel.
I opened up the fuel inlet from .144" to .147" to see if that would help, a little, not a fix though.
There is only one thing I have not done as far as the stock system goes. Replacing the original suction side fuel line from the tank connection to the pump. I'm not totally convinced that is the problem, but what else is there? Am I missing something?
Also toyed with the idea of putting a positive pressure electric pump in on the tank side to push it to the manual pump. But they are noisy, require more wiring and if I'm going to do that why not just eliminate the manual pump all together.
It runs fine unless I want to get on it, which I do, then is starves for fuel about the top of second gear, might make it to third. 6200ish RPM shift.
This is for a modified 350 producing about 440 HP with a TH350 in it.
The problem started suddenly two seasons ago and I just figured it was the fuel pump. The only thing I can figure now is that something is in the original suction feed line. Rust perhaps.
Did everything you did here, including fuel pump fiasco here and blowing lines, tank, sock, filters..got great help from folks to finally a fix! https://www.corvetteforum.com/forums...on-road-6.html
- verify pressure AND volume at carb..in manual is x amount over 30 seconds cranking.
- mine was sucking air from rusted lines..that’s the theory because noissues still!
i know many say lower pressure best for quadrajet (before lars rebuilt and after ) but mine is just under 8.5 psi ( manual is 7-8.5 max at idle!) and have no issues. I would remove regulator.
Forgot to mention the volume test. Pump is rated at 80 GPH. In 20 seconds it pumped 16 ounces. If I got the math right that's about only 22.5 GPH.
1.5 qts/min X 60 mins = 90 qts/hr. 90/4 = 22.5 GPH.
Just did this yesterday, there is an issue there.
could the pump rod on cam ramp be worn or the pump rod be short?
I considered that as well. I measured the pump rod and it was same length as it was new. Also had no obvious signs of wear. It's a Howard's polymer rod.
I considered that as well. I measured the pump rod and it was same length as it was new. Also had no obvious signs of wear. It's a Howard's polymer rod.
what about the total travel of rod running on the offset lobe bolted to crank?
maybe at higher rpm, it sucks a little air aerating the fuel and makes it tremendously less efficient?
I love the idea of a rubber hose right to the tank, but then, again, if their original lines, I think it’s only a matter of time to replace them
what about the total travel of rod running on the offset lobe bolted to crank?
That I don't know, and don't want to go there. If it is the lobe on the cam I'll get an electric pump before I change the cam for a fuel pump lobe.
There is no reason that would be the case. The lobe is cast steel, the pump rod is polymer.
maybe at higher rpm, it sucks a little air air raids the fuel and makes it tremendously less efficient?
I love the idea of a rubber hose right to the tank, but then, again, if their original lines, I think it’s only a matter of time to replace them
Near as I can figure 45 GPH should be suffecient. The ratings on these pumps is unrestricted. So I figured 80 GPH unrestricted should net me at least 45 GPH @ 5 psi .
The fuel line to the pump is my next plan of action.
I have in my head I should take the pump off, Again! and measure the throw of the lobe on the cam just to eliminate that as a possibility.
agreed, but so many pumps not pumping is strange and isn't that steel offset cam pinned and bolted?
I don't know how much it moves anyway.
The cam on the pump? The lobe on the cam is cast in.
Here is a thought. If it were the cam lobe or the pushrod wouldn't you expect to see a consistent loss of pressure and not variable fuel pressures? I see anywhere from 1.5 to 4 psi. It's not consistent. None of the other pumps were either. I even watched one drop to zero as I watched the gauge. The opening and closing of the needle valve may account for slight fluctuations, just not as big as I'm seeing. And never 7 psi out of any of the pumps despite their PSI ratings.
Sounds like you have checked everything. I posted a thread a few years ago about mechanical fuel pumps.....and the need for proper fuel flow running a bigger engine, etc. There was lots of contributions from forum members, including Leigh, and he took his typical "scientific" approach being a Physics teacher (now retired)....and came up with some interesting information. I will try to find the thread and add the link.
But....I ended up buying a Robb MC pump out of California. They produce mechanical style pumps with much larger fuel flow capabilities, are very high quality, and USA Made. They are expensive. But, unlike Edelbrock and Holley mechanical pumps, not only do they produce higher fuel flow capability, they also have a return line fitting to bypass fuel back to the tank as Chevy designed the Corvette. It might be an option for you....although I don't know why the Holley didn't perform.....seems like it should have.
Thanks corvettepasion, one thing I notice on their pumps is the NPT ports are bigger 3/8" NPT and 1/2" NPT and they reccomend a minimum or 3/8" or an6 line for 450 HP. So I'm on the ragged edge there.
Holley pump is 1/4" NPT ports. So if 3/8" is the minimum ID of the line then the barb for the 3/8" hose has a significantly smaller diameter. Maybe this is where the problem lies. I'm not using AN6 connectors just 3/8" barb connectors. But part of the system has been that way for a long time and it wasn't an issue before. Maybe the combination of more of those barbs has created too much of a restriction?
That could explain the flow, but not the lack of pressure. Regardless of the flow, at idle I should see the full pressure...I think.
Once you jump up to 110 GPH the ports go up to 3/8" NPT on the holley pumps.
Discovered that the inside diameter of 3/8" pipe is .295" I did have drill to one connector to that size. It made no difference of course.
Thinking I'm gonna have to pull the pump again to see what the throw is on the cam lobe. Don't know what it's supposed to be, thinking a good 1/2" or so anyhow?
Discovered that the inside diameter of 3/8" pipe is .295" I did have drill to one connector to that size. It made no difference of course.
Thinking I'm gonna have to pull the pump again to see what the throw is on the cam lobe. Don't know what it's supposed to be, thinking a good 1/2" or so anyhow?
about .394 from an article I just read, be curious what you measure
about .394 from an article I just read, be curious what you measure
.375 is 3/8" so the inside diameter has to be less than.375".
Edit: sorry you were referring to the fuel pump lobe. I did have my stock cam and I measured .350" lobe on that one. So it should be similar on this cam.
07-16-2023, 10:01 AM
Low fuel pressure
