pcard

I am hoping someone can explain a few things to me about the procedure for testing the fuel pump pressure.

First of all, I have seen on youtube two ways to attach the gauge: the first is to disconnect the line to the carb and attach the gauge to the end of the fuel line, running the engine on the fuel remaining in the carb. The second is to install a "T" connection and have the gauge "in parrallel" while the line to the carb remains.
Wouldn't these two methods give different pressure readings for the same pump operation?

Secondly, the Service Manual says to block off the fuel return line if the pump has one.
Wouldn't doing that alter the pressure reading from that which the carb is experiencing in normal operations?
What scale of pressure drop does a properly operating return system induce?

Thanks, Peter

Bronco Fan

Yes, but the difference would be minor and insignificant from a diagnostic standpoint.

Blocking off the return line (if it has one) will show what maximum pressure the pump is capable of. It may or may not alter the fuel pressure to the carburetor depending on other variables.

I don't know if there is a factory spec on this.

When testing a mechanical fuel pump I have found that pressure is seldom an issue. A normal pressure is somewhere around 6psi. which is pretty easy to achieve. What is of larger concern is how much volume it can produce within a given amount of time. As long as it can keep the float bowl filled under any amount of fuel demand from the engine it should be adequate. A good factory pump can push a pint of fuel out after just a few seconds of cranking the engine. I could probably count on one hand the number of pumps I've even had to test for pressure because when they are bad they either leak fuel or fail a volume test right off the bat. If it doen't fail for leaks or poor volume, it is probably good. If I still suspected a pressure problem, then I would check the pressure with the return line blocked and unblocked to see if there is significant difference between the two readings.

You never did say what kind of problems you are having that has led to the need for testing the fuel pump in the first place.

pcard

Thanks for the reply!
Sorry for the lack of reference. I have a qjet that is getting soaked with fuel,
and one of the suggested causes is too high a pressure from the fuel pump. Please see,
https://classicoldsmobile.com/forums...970-455-a.html

Cheers, Peter

Bronco Fan

Ok. My mind went to low pressure instead of high pressure. It seems more likely that there is a problem with the carb itself, but it never hurts to rule everything else out. If I had to guess, it sounds like a heavy or misadjusted float or something stuck in the needle and seat. After reading the other thread I would agre with the others who said DO NOT overtighten the bolts on the carb. If those gaskets are leaking then the carb should be rebuilt. Let us know what you find. Good luck!

pcard

Thanks for the advice. I had thought to test the fuel pressure first, but then I remembered that when I replaced the accelerator pump, I had to take the carb off the intake in order to remove the fuel line because it was hitting the thermostat housing. So, since I have to take the carb off anyway, I will now check the float and needle first.