Stock fuel pump pressure.

[Clone TIE Pilot]

Anyone know what the pressure should be for stock L69 mechanical fuel pumps? My old A/C Delco pump was putting out at least 10 psi as it maxed out my gauge. It seemed too high so I got a replacement pump from Delphi and its still seems too high at 9 psi. GM no longer makes replacement fuel pumps for L69s and I can't find the specs.

 

[pontiacgp]

Anyone know what the pressure should be for stock L69 mechanical fuel pumps? My old A/C Delco pump was putting out at least 10 psi as it maxed out my gauge. It seemed too high so I got a replacement pump from Delphi and its still seems too high at 9 psi. GM no longer makes replacement fuel pumps for L69s and I can't find the specs.

do you have another gauge to test the pressure?

 

[fleming442]

Q-jets only need 4.5-5psi

 

[Clone TIE Pilot]

do you have another gauge to test the pressure?

I don't have another gauge.

Something weird is going on. Looking at the specs for replacement fuel pumps for MCSS, they are all rated about 9 psi. However, fuel pumps for LG4 305s, Olds 307s, and Buick 231s are rated 6 to 7 psi. I don't see why the L69 pumps would be so much higher despite using the same basic carb.

The GM service manual pdf I have states the fuel pressure shoud be 5.5 to 6.5 psi while my Haynes manual states 7.5 to 9 psi, not sure which to believe. I was hoping somebody here would know. Guess some of the original specs for these cars are becoming lost in time.

 

[pontiacgp]

I don't have another gauge.

Something weird is going on. Looking at the specs for replacement fuel pumps for MCSS, they are all rated about 9 psi. However, fuel pumps for LG4 305s, Olds 307s, and Buick 231s are rated 6 to 7 psi. I don't see why the L69 pumps would be so much higher despite using the same basic carb.

The GM service manual pdf I have states the fuel pressure shoud be 5.5 to 6.5 psi while my Haynes manual states 7.5 to 9 psi, not sure which to believe. I was hoping somebody here would know. Guess some of the original specs for these cars are becoming lost in time.

I have read that for L69 they even added a pump in the tank to help the mechanical pump. It was to help to increase the fuel pressure to help prevent fuel vapor lock....I read that on forums on the internet so you know it has to be true....

 

[Clone TIE Pilot]

I have read that for L69 they even added a pump in the tank to help the mechanical pump. It was to help to increase the fuel pressure to help prevent fuel vapor lock....I read that on forums on the internet so you know it has to be true....

They did, but only for F bodies. It was a low psi in tank electric fuel pump to pressurize the long suction side of the main mechanical fuel pump to prevent vapor lock. They weren't high pressure like EFI electric fuel pumps and are long discontinued. However, they could have used higher psi mechanical pumps than G bodies and part manufacturers consolidated the pumps? Just can't find the original specs to know for sure.

 

[bracketchev1221]

Does the car run ok with the new pump?

 

[Clone TIE Pilot]

Does the car run ok with the new pump?

Still runs rich and idles rough. I still have the idle air bleed valve screwed nearly all the way down and I have no control over the fuel mixturd dwell which is stuck at 45 degrees which is a full lean command. I pulled a vacuum hose to cause a leak and the car ran better and the dwell dropped some.

With all the other engine options using 6 to 7 psi fuel pumps, it seems odd that the HO 305s used 9 psi fuel pumps. Only other thing I can think of is if stock L69 Qjets used small needle valve seats with high fuel pressure but I just don't know. I suspect more that its the aftermarket F-ing up again. Perhaps Camaro L69s used different fuel pumps from the MCSS L69s, but the manufacturers only make the Camaro ones and lie and say they work for MCSSs too?

 

[Clone TIE Pilot]

Looking through old postings from various sources, there was a lot of conflicting intormation even years ago. Some posters even got in fights over what the correct fuel pressure specs are. Moreover, I believe that 9 psi is too high for a Qjet. On Ciff's forum, he states 7.5 psi is generally as high as you want to go with Qjets.

On another forum, a poster suggested to me to use a Carter M6101 as its supposed to be a correct replacement pump. However, there is again conflicting info regarding this pump. Summit and Jegs state the Carter M6101 produces 6.5 psi while Rockauto and Carter's own website state it produces 9 psi. Not sure who to believe.

Guess my other option is install a dead head fuel regulator between the pump and Qjet. Through it would be a mess to run a new pump to carb line. Not sure if I should give the Carter pump a try or just get a regulator.

 

[maxi426]

Does this car still have the EVAP system hooked up? I remember way back when diagnosing over rich condition on OBD1 equipped Chevy Qjet vehicles that we would often find a defective purge valve. This is is a black plastic item tword the front of the intake manifold with 3 or 4 vacuum lines with one of them being 5/16 or 3/8 in diameter, and connected to the carbs float bowl vent. A ruptured diaphragm would allow fuel to be drawn into the the intake, preventing the ECM from controlling mixture. The first time I found this, it really kicked my a$$. After that we replaced dozens of them.