Best of Deals Car Reviews Repair Shops Cars A-Z Radio Show

Fuel system issues, old mechanical type and Qjet

Maybe this is the problem? I haven’t seen this before. I think there was a little streak around the rim once I thought was oil seap from somewhere but not showing like this.
Trying a new pump first thing.

Since you’ve beefed up the engine to make a little more horsepower, I think your stock pump just can’t quite deliver enough volume. I’d just replace that thing with a higher-volume electric pump, and replace the dinky filter in the front of your Q-jet with an inline one, as someone else mentioned for beginners.

When the engine is being taxed to the max, anything is possible.
Let us know how the new pump works. We do care.

As a matter of fact, I’ve been pondering the problem these past days and keep wondering about the ignition system and whether the coil could be breaking down. Like all the other possibilities, I’d expect it to not recover just from backing off the power and reflooring it, but I’d feel remiss if I failed to mention it.

Its probably upshifting and that cuts the fuel flow a little until the engine winds up again. Thats what happened with mine and it looks like history repeating itself again.

I will see this weekend maybe. Wife has a lot on our plate but it doesn’t take long to change one out.

I changed the fuel pump out and it seems to have helped this issue. I went from an unknown Gallon per hour rate to a known 70 GPH rate pump. The overall track ET went down due to traction in the colder temps but it doesn’t fall over now and the trap speed is up a couple more MPH. So chalk this one up to a bad pump.

Sincere thanks for the followup. Happy to hear you got the problem fixed.

TSM, I know a lot of the time we on this board do not hear the outcome that fixes it. I do want to show you guys I appreciate the help running through the possibilities of what it could be. I’m an engineer and sometimes get tunnel vision on what a problem is and it helps to bounce it off others.

So to all that posted, thanks for the suggestions

I am new the Car Talk community and Have a 69 442 with a 455. From reading this topic I am having the same problems. I have tried two fuel pumps and have seen Idle pressures from both around 6 to 7 psi. Under full throttle after a few seconds the pressure will drop to 2 to 3 lbs of pressure and you can feel the car flatten out. Starve for fuel. What pump finally fixed the issue???. Thanks in advance for any advice. The tank sock and pickup are new.The fuel lines front to rear are new.The Quadrajet has been gone over by Cliff s High performance. Very frustrated. A pusher pump has been rec’d but I dont want to do a electric pump if I dont have to.

Back in the day, it was common to put an electric pump back at the tank to feed the mechanical pump on the block. The electrics were much lower pressure then than they are today. This kept the fuel supply to the mechanical pump more consistent. The mechanical pump controlled to flow to the carburetor better so that gas wasn’t pushed past the needle and seat and dump unmetered fuel into the engine.

Edit: make sure you aren’t using an unvented gas cap. That will drop your fuel pressure under heavy flow conditions. A new vented gas cap might help.

1 Like

Just some food for thought. Doesn’t this pump have 3 fittings? Feed, return, and vacuum line?

Any chance the vacuum source is incorrect? At wide open throttle manifold vacuum drops to zero which could be affecting the pump.

Maybe determining which vacuum source it has (manifold vacuum or ported vacuum) and reversing the vacuum hose would help.

Never seen a GM mechanical fuel pump with a vacuum line. Just two fuel lines, fuel in from tank and out to carb. The pump lever rides on a lobe of the crankshaft via a pushrod. What purpose would the vacuum line serve?

First time I’ve heard of a mechanical fuel pump not operated by a lobe on the camshaft.

cap the return line at the fuel pump and road test the car. That’s just such an easy and cheap test for a common problem why haven’t you tried it yet?

Oops, right, my typo.

@2170mark … have you done the fuel pump volume flow experiment during cranking yet? If not, that’s the place to start. You’ll have to look up the specs for your 442. The way I’ve done it on my 1970’s Ford 302, I disconnect the fuel inlet to the carb and put the hose into a small measuring container, then crank the engine for 10 seconds. Measure the amount of fuel pumped into the container in 10 seconds and calculate the flow rate. If it is too low, no sense worrying about anything after the pump (carb and engine) until the flow rate problem is fixed. Standard shop precautions apply when working with gasoline of course.

The OP’s car has a fuel pressure regulator at the gas tank and the fuel pump has a release port that feeds back to that regulator which dumps fuel back in the tank when the pressure exceeds the rating on the valve. But if the pressure relief valve leaks pressure it can cause the exact problem that is being discussed here. Allowing the pump to operate free flow will give an inaccurate reading if the problem is in that valve. And if the valve is the problem the best solution is installing an earlier model fuel pump without the bypass port.

I just now scrolled back to find that this is a resurrected thread from 2012 that has been brought back to life and is now partying hard. Oh well, I’m glad there was some success from those involved.

Thanks for all the action on this topic. But I think we’re getting off track. I mainly wanted to know what fuel pump W30post ended up with that solved his issue. But thanks again for good ideas.

As mentioned, maybe a pump without the vacuum line although if it were my car I’d want it stock. Then again, I’m kind of a purist with the old stuff.

Without knowing if you have manifold or ported vacuum to the pump I can’t tell you what to do except reverse the vacuum source and see what happens.

American Motors had a combination fuel pump/vacuum pump but Oldsmobile? GM used electric wipers at that time.