Fuel pressure woes

[Jamieg285]

I've solved the leaky regulator problem, now I've got more trouble.

On first startup when the engine is cold the pressure is fine(5.5), however, after a 8 mile trip to work, and everthing warmed up, the pressure has dropped to just under 2, and drops slightly with revs.

I've double checked all the new plumbing for leaks and there are none.  

My suspicion is with the fuel pump, but that a total guess.  Anyone had any experience of this, or have any other ideas?

[55starchief]

What fuel pump is it? is it new or used?

[Jamieg285]

Quoting: 55starchief

What fuel pump is it? is it new or used?

It's what's always been there, apart from trying to change the oulet fitting, I've not touched it.

[Jamieg285]

BTW, I have no idea what it is, whether it's stock, original or replacement.  All I can tell you is it's a mechanical one.

[55starchief]

Ah ok you dont realy need a regulator with a mechanical pump mate, they are mainly for electric pumps where you need to control the high pressure of flow

[Jamieg285]

Quoting: 55starchief

Ah ok you dont realy need a regulator with a mechanical pump mate, they are mainly for electric pumps where you need to control the high pressure of flow

Yes and no.  The edelbrock carb is sensitive to pressure and needs to be no greater than 6psi.  Since I put it on the carb is running better (less bog)

Either way, it doesn't explain the pressure drop.

Is 1.5-2psi OK, or is it too low?

[55starchief]

Quoting: Jamieg285

Is 1.5-2psi OK, or is it too low?

i would say to low. You might find the pump is on its way out, th epressure would be right at low rpm but as the rpm increase the pump looks like its loosing efficency. if you have another pump i would put it on or get an electric and solve all the problems

[Jamieg285]

There's a spare on the 305, I'll see if I can swap that over before Sat (or at least take it with me to do on the day!)

[Jamieg285]

Electric is on the shopping list, but I've spent too much recently as it is, I can't justify a new one if there's a spare to be used.

[55starchief]

Problem is mate with low fuel pressure the motor is going to run lean which could cause bigger problems

[Jamieg285]

Quoting: 55starchief

Problem is mate with low fuel pressure the motor is going to run lean which could cause bigger problems

Well aware of that, which is why I'm trying to work out what the problem is. Definitely can't afford an engine rebuild.

[55starchief]

could find that the 305 pump is to small, i have a manual of the 400 not sure if it will fit but i can give it to ian sat morning

[Jamieg285]

Quoting: 55starchief

could find that the 305 pump is to small, i have a manual of the 400 not sure if it will fit but i can give it to ian sat morning

I'll look into it.  Will one from the poncho fit the chevy?

[55starchief]

Quoting: Jamieg285

Will one from the poncho fit the chevy?

all i can do is let ian have it

[Jamieg285]

It won't fit, and it looks like the 305 one will be OK.

[Jamieg285]

Fore-warned is fore-armed.

What's involved in swapping the pump?  I assume it's as simple as disconnect fuel hoses, unbolt and remove.  Check gasket, reinstall, bolt up and reconnect lines.

Bearing in mind my track record, what's going to go wrong???  

[sixpack2639]

Quoting: Jamieg285

What's involved in swapping the pump? I assume it's as simple as disconnect fuel hoses, unbolt and remove. Check gasket, reinstall, bolt up and reconnect lines.

There's also a push rod that bridges from the cam to the pump arm. They can be a PITA (wanting to slide down) while trying to install the new pump. I coat the push rod with some bearing grease (vaseline works well also) and shove it up in, the grease will hold it in place until you get the pump in.

[Jamieg285]

Quoting: sixpack2639

What's involved in swapping the pump? I assume it's as simple as disconnect fuel hoses, unbolt and remove. Check gasket, reinstall, bolt up and reconnect lines.

There's also a push rod that bridges from the cam to the pump arm. They can be a PITA (wanting to slide down) while trying to install the new pump. I coat the push rod with some bearing grease (vaseline works well also) and shove it up in, the grease will hold it in place until you get the pump in.

Is there likely to be a problem with that push rod?  Should I be looking to swap that as well?

[sixpack2639]

Quoting: Jamieg285

Is there likely to be a problem with that push rod? Should I be looking to swap that as well?

Shouldn't be a problem Jamie. Very unlikely you need to change it. Just wanted you to know it can slide out and be a PITA.

Cheers, Carl

[F Body]

This is the standard '79 350 pump :



Can't find any details for the larger engines

Edit : Dam can't link directly to the Classic Industries page, you'll have to go to the main page below and search 1979 fuel pumps :

http://www.parts123.com/parts123/yb.dll?userwatch~dynamicSearchPage~cadegghb\" target=\"_blank\" rel=\"nofollow\">http://www.parts123.com/parts123/yb.dll?userwatch~dynamicSearchPage~cadegghb

[Jamieg285]

Quoting: F Body

Edit : Dam can't link directly to the Classic Industries page, you'll have to go to the main page below and search 1979 fuel pumps :

Tis, OK - I've always got the full catalogue to hand - downloaded on work and home PCs and hardcopy in the garage

[Roadkill]

Quoting: F Body

This is the standard '79 350 pump :

Looks the same connection as the one off my (92) 305ci.

I'd guess it'd deliever fuel fine  - especially as a temporary measure . . .

Is the diaphram intact on the old pump . . . could be why the pressure is dropping ?

[Jamieg285]

It's pumping fuel OK, the engine wouldn't run otherwise.

Pressure on cold startup is fine, but drops when warm.

I've been advised to try running direct from a can, to verify that there isn't a problem pulling from the tank.

If the results are the same, I will try swapping the pump for the spare I have.

[Jamieg285]

Well that was pretty inconclusive.  Just spent most of my lunchtime in the car park, and I'm not really any wiser.  

Took the car out for a spin to warm her up and get the pressure drop.

pulled the supply pipe and stuck on a short length, dipped into a can. Restart and got full pressure!

Left it running for a while, as temps had dropped.  Pressure fluctuated a bit, but did drop to 4-4.5 as the 'constant' level.  As time ran out, I put the main line back on and restarted, and it sat at that same sort of level.

As there wasn't a big drop in pressure, I'm not sure if this test has proved much.  It might be that it needs to be driven to really show the problem, which is a problem with a can as the tank!

I guess all I can do now is change the pumps over and see if the problem goes away.

[55starchief]

Have you got a fuel filter between the tank and the pump? as it could be blocked which would show when full demand was needed by the pump