low fuel pressure, HELP!!!

[1972BlueC20]

New engine installed. I've tried 2 different mechanical fuel pumps and 4 different fuel filters and still getting 2.5-3 psi after truck is at full operating temperature. It starts out at 6 psi at cold start. Trucks running ok even after pressure drops, but I don't think it's right. Also getting air bubbles in the filter after pressure drops.

My next move is to check sock on sending unit, then replace fuel line from tank to pump.

Is there anything else that could cause low pressure??

[1972BlueC20]

Any ideas guys?

[MARKDTN]

3PSI is OK for a carburetor on a street driven, relatively low RPM application. Probably bottom end, but OK.

Replacing the sock is a good idea, it won't hurt. You can blow out the lines and see how much crud is in them.

[1972BlueC20]

Well the engine came with a Holley carb, so I put a Holley mechanical pump on first, then I wasn't happy with the Holley carb so I put my old Edelbrock back on and I went and got the Edelbrock 1721 fuel pump so it would match the carb. Same problems and same pressures with both pumps.

The gauge is an aeromotive brand liquid filled gauge that is mounted on the fuel line going into the carb after the filter.

The filter is after the pump just before it goes into the carb.

As for the rubber line, I'm probably going to replace it with stainless lines very soon, but assuming the lines are clear and not clogged in any way.....could just being rubber cause low pressure?

[Lee H]

Air bubbles in the filter are normal. Which pumps, stock replacements? Which pressure gage, how is it installed? Is the filter before or after the pump? I still think it's the 6 or so feet of rubber hose between the tank and the pump.

[davepl]

Originally Posted by Lee H

Air bubbles in the filter are normal. Which pumps, stock replacements? Which pressure gage, how is it installed? Is the filter before or after the pump? I still think it's the 6 or so feet of rubber hose between the tank and the pump.

Better not be. Long runs of rubber fuel hose under a vehicle (or anywhere) is just asking for immolation. Yeah, it's easier. Don't. That should be and needs to be steel line, and then Lee's concern with the rubber line goes away.

[slomotion]

Before I started chasing ghosts all the way back to the fuel tank, I'd do a couple of simple tests. Are you sure the gauge is accurate? What are the specs for your pump, and what is normal? Are you sure it's "broken"? What engine are we talking about? How close does your fuel line get to the engine...is it picking up a lot of heat from the intake manifold? I'm assuming this is a carb'd engine. Is it possible the bubbles you're seeing are from the fuel getting over heated? The first thing to do would be check the fuel pressure at the pump discharge. That should remain pretty constant, if the fuel lines are too close to the engine, move them out a little. If it's running OK, hot and cold, take the truck out to a safe place and see if it stumbles under load at highway speeds.
These are just a couple of ideas. If you give us a little more info, we may be able to help more.....

[1972BlueC20]

Originally Posted by slomotion

Before I started chasing ghosts all the way back to the fuel tank, I'd do a couple of simple tests. Are you sure the gauge is accurate? What are the specs for your pump, and what is normal? Are you sure it's "broken"? What engine are we talking about? How close does your fuel line get to the engine...is it picking up a lot of heat from the intake manifold? I'm assuming this is a carb'd engine. Is it possible the bubbles you're seeing are from the fuel getting over heated? The first thing to do would be check the fuel pressure at the pump discharge. That should remain pretty constant, if the fuel lines are too close to the engine, move them out a little. If it's running OK, hot and cold, take the truck out to a safe place and see if it stumbles under load at highway speeds.
These are just a couple of ideas. If you give us a little more info, we may be able to help more.....

I'm not sure the gauge is accurate, that's on my list to check sooner rather than later. It's a brand new gauge, but it could be defective.

As for the specs of the pump, I haven't been able to turn up much from a google search as to whats normal pressure. All I know is that the Edelbrock 1721 has a max pressure output of 6 psi. It's starts at 6 psi when cold, but then drops to 3 psi.

The lines and filter are running about 4-5 inches away from the exhaust manifold and engine itself. I don't think it's a heat issue. The lines and filter are cool to touch even after the truck is fully heated up.

The truck runs good, no stumbles or significant hesitations.

I'm having some other issues with my kickdown cable not working right now, and I think it could use some carb adjusting and some timing tweaking to get the performance up to par, but as for just normal driving it runs good and has plenty of daily driver power.

The engine is GM 350 part # 1921008. 330 Hp Crate Engine

http://www.summitracing.com/parts/NAL-19210008/

[geezer#99]

If you take it out and take it to max rpm in each gear and it doesn't quit or bog, then you have a vision problem.
Simply remove those things your eyes see but your brain can't process!
Remove the gauge and plug the hole.
Put on a filter you can't see into.
Done!!

[1972BlueC20]

Here is a video I made of the issue from the other day.

I know the filter appears too close to the exhaust manifold in this video, however it's really not as close as it looks.

I've since moved the filter about 4-5 inches away and up from the exhaust and engine block.

The filter and fuel lines are cool to the touch with the engine at full operating temperature.

I've also since replaced the filter with a metal filter that is not see through. Still having low pressure.


https://www.youtube.com/watch?v=OQ1J...ature=youtu.be

[cleszkie]

Is your fuel tank vented properly? Sounds like you are generating some vacuum with the fuel pump that is preventing full flow (and pressure) potential. On cold start, there is no vacuum built up, so the pump delivers full flow and pressure. As pump continues to pull fuel, a vacuum begins to build up in tank, which impedes fuel flow and therefore reduces fuel pressure. Fortunately for you, it sound like the vacuum is being slowly released such that the pump is able to maintain 3 psi and keep you engine going. If the vacuum was not slowly bleeding off, it bwould build to the point where the pump would not be able to pull any fuel.

[1972BlueC20]

Originally Posted by cleszkie

Is your fuel tank vented properly? Sounds like you are generating some vacuum with the fuel pump that is preventing full flow (and pressure) potential. On cold start, there is no vacuum built up, so the pump delivers full flow and pressure. As pump continues to pull fuel, a vacuum begins to build up in tank, which impedes fuel flow and therefore reduces fuel pressure. Fortunately for you, it sound like the vacuum is being slowly released such that the pump is able to maintain 3 psi and keep you engine going. If the vacuum was not slowly bleeding off, it bwould build to the point where the pump would not be able to pull any fuel.

Yeah I'm really thinking it's a tank issue the more I think about it.

My tank isn't vented since it's a C20, it did originally have 1 main feed line and 1 return line. Since I'm using new pumps with only 1 in and 1 out, I've capped the return line at the tank and just using the main feed line. When I remove the gas cap, there is never any pressure or release.

I tried another cap recently, and it was causing major pressure build up. It was sealing too well. After the truck ran for a while and the pressure would drop, I'd take the cap off and it would blow air out (release pressure)

The truck also wouldn't restart if killed and you tried to restart unless you removed the cap and released the pressure first.

Because of this, I switched back to the original cap.

[davepl]

Is it worse with a full tank than an empty tank? If so, that would back the vent theory. If the tank is empty there's more air space for it to evacuate, but fuel doesn't expand, so if the tank is closer to full, it should create the vacuum issue sooner. Just a thought.

[1972BlueC20]

I have the same problem regardless of how full the tank is.

Where is a good place to find steel fuel lines, perhaps locally....do auto parts houses have steel lines?

[MARKDTN]

Didn't you pay big bucks for a shop to install this engine for you? Do they have any thoughts? If you capped the return line and have a non-vented tank, as mentioned above that could well be it. Take the fuel cap off and see if pressure improves. I also agree that air bubbles are not good. If you replace your lines, don't just buy steel lines without ends-buy premade ones that have the correct barbs at the ends that hold the hoses on (keep the clamps from sliding off).

[1972BlueC20]

Originally Posted by MARKDTN

Didn't you pay big bucks for a shop to install this engine for you? Do they have any thoughts? If you capped the return line and have a non-vented tank, as mentioned above that could well be it. Take the fuel cap off and see if pressure improves. I also agree that air bubbles are not good. If you replace your lines, don't just buy steel lines without ends-buy premade ones that have the correct barbs at the ends that hold the hoses on (keep the clamps from sliding off).

yeah i'm a bit pissed off at the shop....they screwed up a few other things...I had to fix some vacuum lines that they had ran wrong....lets just say I won't be using them again...also as for this problem....I took the truck back to them and they said the pressure was good with no bubbles the whole time they had it....and of course, as soon as I get home after picking it up from them, low pressure and bubbles....I'm done with them!!

I just want to fix this myself. I prefer never to deal with them again.

As far as getting original style steel lines......because I'm not using the original style pump, I don't think the fuel line will line up with the inlet on my Edelbrock pump. Would it be ok to use a small 2-3" section of rubber line to connect the steel line to the fitting on the pump?

Or if not, what can I do about this?

[MARKDTN]

Yes, a few inches of rubber line is OK.

Lets slow down and take a big-picture look.

-new engine with Holley carb and Holley mechanical pump (did that come with the engine? What didn't you like about the Holley?)
-changed to Edelbrock carb and Edelbrock mechanical pump
-blocked the return line to the tank
-changed to non-vented fuel cap
-various filters and lines and hoses and routings from pump to carb

I would get a 1 gallon gas can and drop the suction line into it. See if pressure improves. If not, then you have pump or lines to carb issues. If it does, then fuel tank/line/cap/hose issue(s).

Might consider going back to a 3-line stock mechanical pump and using the factory return line. Unless you are racing, that should be adequate flow for you. I'm betting on fuel cap issue and/or an issue with blocking the return line.

[davepl]

Originally Posted by 1972BlueC20

Would it be ok to use a small 2-3" section of rubber line to connect the steel line to the fitting on the pump?

Yes, and there's normally a short section like that that connects the tank line coming out of the bottom of the cab to the steel line on the frame rail. You need those two sections just for flex between engine/cab/frame.

[brown7373]

I've been reading this thread, and have a question. Does a mechanical pump produce more pressure with higher rpms, like an oil pump, or is it constant? Logically, it seems like it ought to produce more pressure at higher rpms.

[ssxty8]

I imagine they weren't trying to see the secondaries opening with the engine running and the car standing still. They "normally" just work with the car running/moving, due to vacuum generation, unless somebody mess with the linkage. In the Holley web page used to be a section regarding vacuum secondaries testing, which involved using a paper clip on the vacuum canister rod, and running the car. I did it in my 600 vacuum secondaries carb, and worked right. Then I installed a kit, which comes with different springs/colored for the diaphragm, to make them open faster, "but with the car running", not standing still. Hope I made myself understand. Again, check by lifting the rubber plug at the gauge with your nail, making sure it is positioned upwards, or the oil inside the gauge will came out.

[1972BlueC20]

Originally Posted by MARKDTN

Still my recommendation after seeing the big picture.

Originally Posted by ssxty8

Again, check by lifting the rubber plug at the gauge with your nail, making sure it is positioned upwards, or the oil inside the gauge will came out.

I think on my way home today i'm going to buy another fuel gauge and try that first, if it's not a defective or faulty gauge then I will get a gas can and try putting the suction line directly in the can.

I'm going to run new lines anyways regardless so I'll take the seat out and check the sending unit sock too while I'm back there.

[ssxty8]

New gauge? This time try a dry one. Not oil filled.

[1972BlueC20]

Originally Posted by ssxty8

New gauge? This time try a dry one. Not oil filled.

yeah i was going to try that....they are cheaper anyways!

[imjeff]

Gauges are liquid filled to keep the needle from bouncing. The altitude it was filled has absolutely nothing to do with the pressure it reads. The air bubbles, especially when warmed up could be the start of a boil...beginning of vapor lock. Double check the proximity of your fuel line with exhaust and motor.

Jeff

[Scott Liggett]

Fuel pressure gauges under hood are notorius for dropping their readings as they get heated by the engine temp. Even the pricey ones. Many manufacturers of the gauges now say to only use them for setting regulator pressure and testing, then remove them. I have had many on 65 Impala over the years, several pumps, regulators, etc; every one dropped to near zero on hot days. Not once 18 years has it ever ran out of gas at 6000 rpm WOT. Gauge readings are not always gospel for accuracy.