Hello Everyone!
I have a weird one on my hands and I'd rip my hair out if I had any! Here's the situation... Mildly built 400, roller cam, .030 over. The car has ran perfect until recent. Car runs good and for no reason starts to bubble and then starve for fuel. Not a vapor lock problem. I have rebuilt the carb, installed a see through fuel filter between the pump and carb. (thus how I can see it fail) All the hard lines and fittings are 3/8. Dropped the tank, blew out all the lines, put in a new fuel sock, all brand new fuel lines and tightened all clamps. I installed an electric "prime pump" that shares the mechanical path. (All this started before installed it so I don't think its causing problems.) Any idea? I'm at a loss! Thank you in advance! -Craig

 

Hi, Craig.

See-through fuel filters aren't a reliable way to determine fuel flow or pressure. Have you tested the actual fuel pressure?

 

It's a quick way to see what's going on with it. I haven't taken a pressure test. The only think I can think of is maybe there is a hairline crack in the hard line. i think I am
going to buy a long piece of hose any bypass it to see if that fixes it. Cant think of anything else causing it

 

Okay. I just like to get accurate readings and measurements of things before I go throwing too much money or work at the problem, is all.

BTW, don't leave that rubber hose on there too long.

 

Craig, go get it to do it, then shut it off, hie thee around to the stern, and pull the gas cap. If you get a whoosh of air rushing into the tank when you pull the cap, you have a vent problem. Further confirm while driving sedately around with no cap.

Your favorite helper holding a red can with a line out the window going into the fuel pump will confirm if the problem is from there back to the tank. Same helper, line running up under hood cowl from the carb inlet will confirm if problem is not carb related, i.e, is fuel related.

You can also check the fuel pump real hood style by the three red can method. One can to pull from, one to feed to, and one to gravity feed the carb. While the engine is running, the fuel pump should reliably and constantly fill the can.

Unless, and this happened to me, I ran my 442 off a can and it drank that sucker dry in ten min, because the return line went to the tank.

 

What carb are you running?

 

Isn't that why we are in the car hobby? Throwing money at it??? I will just use the hose to see if its a hard line problem.

 

So, here's a thought I wondered about. I am not running the return (vent) on it, in fact when I put it all back together this time I caped off the 5/16 vent line off the fuel pickup. The 68 has another vent on the the tank that runs to a plastic collector. I figured it was enough to properly vent the tank. The car has run for years with the vent line(5/16) hooked up but capped off just past the firewall. I will try taking the gas cap off when it acts up. Thanks for the ideas! This is so weird that it happens when it idols after awhile. I could see where it might happen while driving at a higher RPM... Strange!

 

Stock Q-Jet

 

Measure the pump outlet PSI before and when it acts up to eliminate pressure from the list. Do it with the pusher pump out of the loop.
That pusher primer pump isnt necessary if the mechanical pump is out putting the correct ~5 PSI. If you insist on leaving it there make sure its wired to an oil PSI cut-off switch.

Look down the air horn when its acting up. Do you see and hear fuel gurgling?

Is the inlet filter present in the Qjet ?
AC or no AC car?
Is the engine hot when it acts up?
Is it hard starting cold or hot?

 

Going to have to find something to measure output. I replaced the pump, same one I have ran for the past few years without problem. The pusher pump is only for
priming the pump after sitting for long periods. It takes forever to get fuel to the bowls. It's on a toggle switch and is shut off after it is primed.

I don't think it is gurgling, are you thinking vapor lock?

I removed the inlet filter when I installed the filter between the pump and carb.

Engine temp is good when it acts up, doesn't seem to be a heat issue.

It is an AC car but currently now hooked up.

It is a little cold hard starting. I believe it is a choke adjustment as it has a aluminum intake that doesn't support the heat choke (divorced set up, I believe)

Been thinking about the venting in it. Do you think blocking off the 5/16 vent (return line) is causing problems? As mentioned before I figured the tank vent hoses that is on the 68 is enough. Would you agree?

Thank you!

 

Have you considered ignition system breakdown? Coil?

Yes Id agree the vent at the tank is enough and as designed.
68s have the non vented gas cap as part of the system.

Are you using the double nipple A/C fuel pump? That second 1/4 nipple is the return line to the tank not a vent.
The A/C cars have this to help regulate fuel temp from higher under hood temps when the A/C is blasting on a 100° day by returning a small amount of hot fuel back to the tank. Keeping the pump itself cooler.
Not a bad idea to have it hooked up even if the A/C isnt hooked up or working. Not likely your problem though.

You describe the cars behavior as burbling starved for fuel... define that. Hesitates? Stumbles at throttle tip in? Rough curb idle?
If you pull the gas cap when this happens does it let vapor pressure out, suck air in, or neutral?

What is the manifold vacuum signal at warm curb idle?
How do your spark plugs read? Lean, rich or just right?

Before jumping to the ignition system lets run this fuel trouble shooting to its end by reporting the fuel pressure reading and spark plug observations.

I have one of these dual purpose gauges in my tool box. Read the vacuum signal first. Then Plumb a T into the rubber line you have between the carb & pump to read fuel PSI and report back.

 

I made a video of the problem. You can see what is going on as well as my set up. Thank you for your input!

 

After it starves and quits, does it start right up again, or do you have to crank to fill the carb?
Is the carb float bowl empty of gas when it stalls?
A few questions to come my mind?
Is the carb float bowl empty because gas delivery to it?
If the gas delivery is ok to bowl, is there an issue with the float needle and seat, float height adjustment , crack or leak in the bowl leaking (worse when hot because of expansion)
Throwing different ideas out.
Good luck.

 

Have to crank it, choke it etc. Seems to be bone dry in the bowl. It is a delivery problem in my opinion.

Thank you!

 

Like another poster said, I would let the fuel
pump drink directly from a gas can with a new a new hose hooked to the inlet of the fuel pump. Try it to see if it fails
you may have already tried this.
By pass all the fuel lines and tank.
start to narrow down the problem area
check your fuel pressure
something weird going on.
- it runs out of gas on idle around 10 mins.
- does it run out of gas quicker on higher Rpm’s ?

 

My money is on the pump is failing.
Can you flip the switch on the primer pump when it does this?

 

-Doesn't have an impact when revving.
-The pump is brand new and the old pump did the same thing.
-Turning on the prime pump doesn't help much. When trying to restart it needs the it to start again (i believe because the bowls are dry)
Notice the bubbling in the filter when it starts to fail, does this indicate air in the line?
I'm also low in fuel (don't want to fill it yet if I have to droop the tang again.)
My next step is to run a line out of a gas can as suggested.
This is so maddening! Thank you all for your input... A real puzzler!!!

 

Lets get that pump pressure reading.
You could make the hose long enough to tape it to the windshield to observe the pressure during the road test.

 

Will do!

 

UPDATE
-Still need to do a fuel pressure test (I need to get a gauge as the one I have is for fuel injection... off to Harbor Freight!)
- I hooked it to a gas can directly to the pump. It did the same thing so doesn't seem to be a fuel line problem.
Runs fine until it heats up. (normal operating temp) If you shut it off and then restart it begins to lose pressure and surges in the fuel filter. Still ran
but bogged a bit hitting the throttle.
So...
Pump problem? This the second pump, did the same thing with the first pump. Possible that both pumps are bad? Unlikely
Could it be the pump mechanism riding on the cam?
What am I missing? Driving me crazy!!!! Thanks guys!

Craig



This is the pump that is in it

 

Worn fuel lobe on the cam ? Possible?
Another bad fuel pump? Possible?
Fuel line issue from the pump to the carb? Possible
carb issue? Possible
need to check fuel pump delivery constant pressure and go from there
you are getting it narrowed down slowly
tank and lines are ruled out now to the pump.

 

Will take a reading tomorrow!

Thanks!

 

Craig, can you rig a tank on gravity feed to the carb and put the pump outlet into a 5 gallon can? Warm the car up and check for a few things:

1, Is the problem with the car running gone?
2. If, when the problem should occur, does the fuel pump crap out (get familiar with how it pumps, it pulses.)

This assumes you can get it to misbehave in the garage.

It may be time for the notorious clothespins on the fuel line trick.

 

That sounds like the next step! I'll do that tomorrow
This all takes place in the garage and driveway... too afraid to take it too far and have to call Hagerty!
What is the close pins on the fuel line trick?

Thanks for the input!

Craig

 

Putting a bunch of wooden clothes pins on a metal, or even rubber if it doesn't pinch it, fuel line when near the engine acts like a very crude heat sink and keeps that line cooler. If it's vapor locking there, it will cause enough difference to be noticeable, then you fix it in a better way.

 

I am curious about the fuel pump. I am recalling a certain brand of fuel pump (Holley ?) having problems with cracking. Since it appears you bought a foreign "knock off" of the Holley, you might have to suspect that.
I recall Airtex and another brand as being preferred.

 

Wouldn't the fact that he's run an electric in line with the mechanical kind of rule out the fuel pump?

I would still be curious to know the fuel PSI.

 

No because I believe he just primes with it.

Personally, I use the seven seconds they take to launch from weeks sitting to make sure it and I are right with the Lord before combustion begins.

 

Ok, here's the update to my headache!
Hooked up a gravity fuel feed to the car and it ran forever, perfect! So guess it has to do with the pump. I bought a pressure tester at Harbor Freight (cheesy!) but it didn't have the proper fittings
so ordered from amazon, should be here tomorrow. I will take a fuel pressure reading tomorrow.
So what could it be?
-Is it possible to be vapor locking in the pump? It does have a thin gasket where it attaches to the block? Would a thicker gasket or an insulator work?
-Problem with the actuator rod in the pump or the engine thingamagingy the runs the rod in the engine?
-As i said before this is the second pump and both do the same thing. The first pump has been in the car for over 10 years. A quick college deduction indicates it probably isn't the pump.
What do all you Olds gurus think?

Thank you!

Craig

 

You are getting it narrowed down.
- fuel pump issue
​​​​- fuel pump lobe on cam wearing/ defective
- outlet line to the carb collapsing, partially blocked, Vapor locking.
has to be one of those three items.

verify correct fuel pump
run a new output line from the pump to the carb
new fuel fuel pump
If that doesn’t work, has to be the cam. Unlikely.

At least you know it’s NOT the fuel tank, feed lines to the pump or carb. Ruled them out.

 

It has a new line to the carb (added glass fuel filter to it)
I'm thinking it is vapor locking at the pump. After running the gravity test, I hooked the fuel pump to the carb back up and noticed it bubbling.
(It was showing some fuel in the glass.) Could just be it was draining back in to the carb line. When the problem occurs the fuel slowly drains out of the
fuel filter, bubbling, and starves. Vapor lock? Adding a thicker pump to block gasket to isolate?
i will pull the pump tomorrow and inspect it.

Thank you for sharing my headache!!!

Craig

 

It sounds like the same problem that you had when you started this thread.
You haven't pressure tested the fuel pump flow. The engine runs fine when gravity fed fuel from a can. You ran fuel from a can to the carb and it had the same problem.

 

^^^ !!!

 

Going to do a pressure test tomorrow.
Will report back then.

Thanks!

Craig

 

Get rid of the glass filter!!

You may have a fuel supply issue. If there is any holes or flaws in the fuel line, the pump will draw air instead of fuel. Ever tried to drink with a split in the straw? Same principle.

Before doing anything else, carefully inspect the fuel line to the tank. Look closely and bends, and especially around the clamps. Look for corroded, damaged, kinked or crushed lines. You may not see fuel leaking, but you might see the evidence. Look for discolored or stained areas around the clamps.

 

Nonsense, a failed pump and a pump of Chinese junk can absolutely both be bad.

Vapor lock comes from heat, not a pump not having any *****. If you can run it off a gravity feed can like you did, and you want to narrow down further, run a different rubber hose, no filters or anything, from the pump outlet into a fuel can. That pump will pump fuel while the car is running. Observe how it pumps. Does it choke off?

If your hoses are fine, and your carb is fine, the pump is the next probability. Wiping out the cam eccentric (it's not a lobe) is a rare thing and you'd have crap in the oil pan.

 

Ok, so here is the video of the pressure test. Runs at 6.5 psi. I ran the electric and mechanical at the same time. when I turned off the electric pump it began to fail.
It's obviously the pump. The engine is somewhat new. about 10,000 miles on a rebuild. I will check the cam eccentric when I pull it out. Still weird that 2 pumps are doing the same thing. First ran in the car for the past 8 years.
So...
What pump should I run? The mechanical pump is a high flow 80 GPH. The engine is hotter than stock and this is the pump (the first one) that the builder installed. What would you guys recommenced replacing with?
Should I just go electric?

Thanks all!

Craig

 

I bypassed the lines and ran a gas can directly to the pump inlet and it still did the same thing.

 

Excellent, get a new pump of a different brand. NOT CHROME.