Correct fuel pressure at Cold Start Valve

[Travis71280]

Hi all,

Does anybody know what is the correct fuel pressure at the cold start valve in psi and whether it should be set with the engine running or not? The reason I'm asking is I'm installing an after-market fuel pump (TREPERFORMANCE 255lph In-line) tomorrow and I was wanting to know what to set the fuel pressure regulator at (after-market also). Also does it hurt if the pressure is set too high (I imagine it would, but I was just curious)?  Thanks again in advance for all the help.

Travis

[jacovdw]

Hello Travis,

There is a fuel by-pass valve on the fuel injection pump that opens at a nominal pressure of between 0.8 to 1.1 atu (about 11 to 16 psi).
The measuring point (for pressure) is just before the cold start valve.

The feed pressure of the fuel pump is thus the same as the pressure rating of the fuel by-pass valve.

Now, the BBB mentions another value with the measuring point the same as above and that is the final feed pressure.
That value is a minimum of 1.3 atmospheres (about 19 psi) when squeezing the fuel return line.

Lastly, the measurements are performed with the engine stopped and a minimum voltage of 11 V at the terminals of the fuel feed pump.

[Travis71280]

Thanks Jacovdw, went ahead and installed the fuel pump, and she is cranking out 26psi at the Cold Start Valve. Is that way too much or does it matter? What symptoms would the car experience from too much pressure?

[ja17]

Hello Travis,

Typically the engine would be running a bit richer with fuel pressure this high. First of all, I would take a fuel volume check also. If your volume is not adequate, you could still have running problems. I ran into this problem years ago. I tried to use a Holley electric fuel pump. The pressure was great, but volume was not sufficient.

Anyway, if your volume  is sufficient, you may try enlarging  the vent hole in the return fitting on the the injection pump.  This is what creates the back pressure in the system. It is located where rear rubber fuel line is connected to the injection pump. enlarging the hole should lower the pressure.

If your car has the fuel line by pass at the electric fuel pump, then you may only have a check valve (regulator) built in the return fitting.

Check your fuel volume, then remove your fitting to see what you have first.

Joe Alexander
Blacklick, Ohio

[Travis71280]

Thanks Joe, I went ahead and got the pump down to 19psi. If this is too high I can still turn it down, I found out I had the after-market fuel regulator on backwards. Its suppose to have about the same flow if not higher than the original pump (255lph).  Haven't checked the actual flow yet though, but the injector pump stays cool to the touch while the engine is running. Hadn't had any vapor lock issues like previously with the old pump (too low pressure), but the motor seems like its not running 100%. Pulled the spark plugs and they all look good except for the No. 3 cylinder which was black with oil. I'm suspecting a valve guide leak as the culprit, but could this be the issue or is it something more serious?

[ja17]

Hello,

It is a good idea to perform a volume check first chance. Reducing fuel pressure will reduce thefuel  volume so your factory specs on the after market pump are no longer correct.  19psi should be fine for fuel pressure.

You can remove the valve cover and view the valve seals and the top of the valve guides on your third cylinder. If oil is getting on your plugs (black crusty deposites), fuel or coolant could be a wet black deposit.  Use a flashlight to see if a seal has popped off from its guide.  With spark plug fouling look for problems with the intake guide or seal or piston ring problem.

Hope for an easy fix !

Joe

[ja17]

Hello,

Looks almost like the Bosch electric fuel pump used on later Mercedes cars. I figured these would work with a little tweeking to reduce pressure. Most likely a fraction of the cost of the original.

Remember you could always reduce fuel pressure by opening up the hole in the return fitting on the IP and or reducing the pressure by adding a "T" fitting right after the electric fuel pump and connecting it to the fuel return line at the tank.

Choking down the pressure of the pump with a regulator as you describe reduces fuel volume and also increases back pressure and strain on the electric fuel pump (possible shortening its life. Time will tell. Keep us up to date.

Joe

[Travis71280]

Thanks for all the help, I'm definitely looking into redoing the valve guides anyways so I'm hoping that will fix the problem. The setup I'm running actually has the return fitting completely drilled out (I thought it was overpowering the original pump, but come to find out it was the pump itself). Running as is it was only putting out 10psi, so I installed the pressure regulator on the return line by the fuel tank allowing me to adjust the pressure between 12-19psi (the 26psi reading was with the regulator accidentally installed backwards ). The regulator is a Mr. Gasket unit intended for carburetors. I'm going to check the flow first chance I get, but its running good so far. By the way, the pump is based off the Bosch 044 pump used on the 1981 300sl. I'm hoping this works out since it saves me $600 between buying a stock new unit. I'll definitely keep everybody posted.

[ja17]

Ok, installing the regulator on the return line should not cut down the fuel volume.