Inconsistent fuel pressure

[41Draggin]

I recently swapped the straight six and three on the tree for a sbc and turbo 350 in my 1980 C10. Everything went well except the fuel pressure is inconsistent. It's a Goodwrench 350 thats looking pretty clean on the inside. Stock manual fuel pump to a holley fuel regulator to an edelbrock 650cfm carb. Went with a single point GM distributor. The issue i'm having is with the regulator set at 6psi it runs nice and smooth, then it starts to lose pressure and runs sporadic with the pressure dancing between 0-3 psi. Eventually it will smooth out again for a while then back to falling on it's face. It's noticeable while driving like all the sudden its running out of gas then it picks back up. I've tried a different fuel pump, replaced the fuel push rod, bypassed the filter and even replaced the condenser (just in case)and nothing has changed. Also worth noting the is an intermittent clacking sound like a stuck lifter but all the plugs look great and all rockers and push rods look all clean normal and well oiled. I could really use some help with this one I'm running out of ideas. Anyone seen this before? Thanks!

 

[Camar068]

try bypassing the regulator.

 

[nvrenuf]

I don’t know about the noise but what regulator are you using? Some of the cheaper designs can be inconsistent. Do you run a return line?

 

[75gmck25]

I also recommend you find a good new or used large cap HEI distributor (coil is in the cap) and make the swap. There is no reason to run (or try to fix) points ignition on an SBC, since the HEI has been standard for Squarebody trucks since 1975.

 

[PrairieDrifter]

I recently swapped the straight six and three on the tree for a sbc and turbo 350 in my 1980 C10. Everything went well except the fuel pressure is inconsistent. It's a Goodwrench 350 thats looking pretty clean on the inside. Stock manual fuel pump to a holley fuel regulator to an edelbrock 650cfm carb. Went with a single point GM distributor. The issue i'm having is with the regulator set at 6psi it runs nice and smooth, then it starts to lose pressure and runs sporadic with the pressure dancing between 0-3 psi. Eventually it will smooth out again for a while then back to falling on it's face. It's noticeable while driving like all the sudden its running out of gas then it picks back up. I've tried a different fuel pump, replaced the fuel push rod, bypassed the filter and even replaced the condenser (just in case)and nothing has changed. Also worth noting the is an intermittent clacking sound like a stuck lifter but all the plugs look great and all rockers and push rods look all clean normal and well oiled. I could really use some help with this one I'm running out of ideas. Anyone seen this before? Thanks!

What brand fuel pump? I've got nothing but junk ones. I've heard squeaking, I've heard rattling, I've heard clunking. All of it stops when the fuel pump dies. Can't get a good parts store pump. Had one clattering fresh out of the box, my cam lobe may be ruined now, don't know yet. I just got an expensive pump and I'll be fixing it in a couple days here. On like my 6th pump in five years now and worried they have ruined the cam lobe. The last one literally pounded the pin through the actuating arm, both sides, just looks like it was milled out lolol

 

[squaredeal91]

Throwing this out there. I've had lots of problems like this on different rigs. And it's usually that the fuel pump sucks air before the pump. Not leaking fuel visibly but causing inconsistent fuel pressure, little to no fuel pressure etc. If all the lines are metal then likely it's the short rubber hose on top of the tank or if you have or (had) dual tanks then it could be the junction at the selector valve.

 

[Ricko1966]

Trash in the gas tank ,evap canister not hooked up correctly. Try running it with no gas cap for 1/2 an hour aee if your problem is gone.Did the truck run and drive okay before the swap

 

[41Draggin]

First off Thank you everyone for chiming in. Definitely needed a fresh set of eyes on this one.

Regulator is my trusted Holley 12-803, same one I've had on my Chevelle for years and it does it's job without incident. Fuel pump is stock carter for an early sbc, no return line. I went with the old school single point distributor cuz it's what I grew up with and if it fails it's an easy fix (this will be my daily driver).

Tanks are clean not seeing any debris, and outside of being underpowered, a three on the tree and a heavy exhaust leak, seemed to run fine.

Now @squaredeal91 and @Ricko1966 you both bring some good light to the problem. Yes I do have two tanks and sucking air through an old fuel line makes a lot of sense. Also there was an "evap canister" charcoal canister that I disconnected from the straight six that I did not reconnect to the V8. I didn't reconnect it because I really don't completely understand how it works (I'm so old school when it comes to wrenching). With that I have the fuel filter right before the fuel pump and it never seems to fill up. I'm thinking this could be the issue???

 

[Ricko1966]

First off Thank you everyone for chiming in. Definitely needed a fresh set of eyes on this one.

Regulator is my trusted Holley 12-803, same one I've had on my Chevelle for years and it does it's job without incident. Fuel pump is stock carter for an early sbc, no return line. I went with the old school single point distributor cuz it's what I grew up with and if it fails it's an easy fix (this will be my daily driver).

Tanks are clean not seeing any debris, and outside of being underpowered, a three on the tree and a heavy exhaust leak, seemed to run fine.

Now @squaredeal91 and @Ricko1966 you both bring some good light to the problem. Yes I do have two tanks and sucking air through an old fuel line makes a lot of sense. Also there was an "evap canister" charcoal canister that I disconnected from the straight six that I did not reconnect to the V8. I didn't reconnect it because I really don't completely understand how it works (I'm so old school when it comes to wrenching). With that I have the fuel filter right before the fuel pump and it never seems to fill up. I'm thinking this could be the issue???

The reason I said drive it 1/2 an hour with no cap and see if the problems gone,that's going to tell me you disconnected the evap and now the tanks not venting,it's actually developing a vacuum as fuel is pulled out and no air to displace it. If you still have the problem with the cap off venting isn't what's causing your issue. Drive it,no cap. If that fixes things we'll get your evap hooked up correctly. What year is your Chevelle . I'm building a 66

 

[Ricko1966]

First off Thank you everyone for chiming in. Definitely needed a fresh set of eyes on this one.

Regulator is my trusted Holley 12-803, same one I've had on my Chevelle for years and it does it's job without incident. Fuel pump is stock carter for an early sbc, no return line. I went with the old school single point distributor cuz it's what I grew up with and if it fails it's an easy fix (this will be my daily driver).

Tanks are clean not seeing any debris, and outside of being underpowered, a three on the tree and a heavy exhaust leak, seemed to run fine.

Now @squaredeal91 and @Ricko1966 you both bring some good light to the problem. Yes I do have two tanks and sucking air through an old fuel line makes a lot of sense. Also there was an "evap canister" charcoal canister that I disconnected from the straight six that I did not reconnect to the V8. I didn't reconnect it because I really don't completely understand how it works (I'm so old school when it comes to wrenching). With that I have the fuel filter right before the fuel pump and it never seems to fill up. I'm thinking this could be the issue???

Take the filter off from ahead of the pump,if it gets vapor it's trapped and your fuel pump can't pick up vapor,it needs wet fuel to pump. You possibly are picking up an air pocket and dropping the siphon to your fuel pump,causing problems. You still need to verify you aren't creating a vaccum in the tank due to not having the evap canister functioning. You can track the vent line to the cannister, from the tank. Disconnect it and put a fuel filter in it there, going to the tank the other end open. This will let the tank vent,but not let,bugs,dirt,mud daubbers in the vent line. Use a clear plastic filter.

 

[41Draggin]

Ok quick update. I removed both gas caps off the truck and went for a 30 min cruise but only got about 4 blocks before the carb went completely dry, before the pressure would drop and the engine would stumble but it would pick back up and keep on running. not the case with the caps off. I had to prime the carb to get it running again.

Next I disconnected the vent tube from the tank to the charcoal evap canister. That seemed to help a bit but the pressure is still dropping intermittently, stumbling and then coming back. At this point the truck is drivable but if it's gunna be my daily i'd like to not have to worry about this issue.

Next I think I'll move the fuel filter. It's right before the pump and although I can see a small amount of fuel surging through the filter(wix clear plastic) It's nowhere near filling up.

Thanks again for all the help, I'm confident we will figure this out.

@Ricko1966 Congrats on your 66 that's one of my favorites! I've got a 68 I'm working on, it's a whole heap of fun.

 

[squaredeal91]

Ok quick update. I removed both gas caps off the truck and went for a 30 min cruise but only got about 4 blocks before the carb went completely dry, before the pressure would drop and the engine would stumble but it would pick back up and keep on running. not the case with the caps off. I had to prime the carb to get it running again.

Next I disconnected the vent tube from the tank to the charcoal evap canister. That seemed to help a bit but the pressure is still dropping intermittently, stumbling and then coming back. At this point the truck is drivable but if it's gunna be my daily i'd like to not have to worry about this issue.

Next I think I'll move the fuel filter. It's right before the pump and although I can see a small amount of fuel surging through the filter(wix clear plastic) It's nowhere near filling up.

Thanks again for all the help, I'm confident we will figure this out.

@Ricko1966 Congrats on your 66 that's one of my favorites! I've got a 68 I'm working on, it's a whole heap of fun.

Just a note on clear plastic filters. There hardly ever completely full. I know that's probably odd but that's how it is.

Maybe that's why the asin pickups had fuel filters with inlet and outlet at the top??

 

[Ricko1966]

Just a note on clear plastic filters. There hardly ever completely full. I know that's probably odd but that's how it is.

Maybe that's why the asin pickups had fuel filters with inlet and outlet at the top??

Yea, but my concern on his situation is that he's running it at the pump inlet,so if he hits a bump etc. he is possible sucking air,not fuel,for just a second. I'm not sure if that brief interruption is enough to lose prime on the fuel pump. If you position the fuel filter so the outlet is facing up it should fill completely,on its side air gets trapped because it rises to the highest point. Another little tidbit if you have a plastic and paper fuel filter,sideways and it's more than 1/2 full it means the paper on the bottom 1/2 is clogging and fuel is having to find a spot higher up to pass through the filter.

 

[Ricko1966]

Look at these pics. This is how GM dealt with fuel vapor. With the filter on end all air rises to the top and can bleed off and the filter will completely fill. Same principle as brake bleeders,they have to be the highest point of the caliper or wheel cylinder,or you won't get all the aire out.

 

[squaredeal91]

Yea, but my concern on his situation is that he's running it at the pump inlet,so if he hits a bump etc. he is possible sucking air,not fuel,for just a second. I'm not sure if that brief interruption is enough to lose prime on the fuel pump. If you position the fuel filter so the outlet is facing up it should fill completely,on its side air gets trapped because it rises to the highest point. Another little tidbit if you have a plastic and paper fuel filter,sideways and it's more than 1/2 full it means the paper on the bottom 1/2 is clogging and fuel is having to find a spot higher up to pass through the filter.

So a full one is clogged lol. Makes sense because I've seen vertically mounted ones even not fill up when new. Like on a 2n tractor I've seen one added too.

I like that Delco fuel filter setup. That's pretty cool.