Fuel Pressure Measuring?

[doubleyellow]

Been having trouble with the engine. Runs great at idle, but as I accelerate from about 2500 RPM and above, the engine bogs down badly and loses all power. But then back to smooth running at idle.

Please note: in the driveway, when I pull the throttle and get above 3000 RPM I do not hear or notice any obvious power loss

I’ve changed out many obvious things (carb, distributor, coil, plugs, plug wires, checked main grounds) which I may need to explain later, but thinking it may be a fuel starvation issue, I’ve been trying to measure the fuel pressure.

I first borrowed a fuel pressure gauge from the local O’Reillys. Think I hooked it up properly just outside the card (AVS2). The gauge really was more for fuel injection so the range on the dial was up to 100psi. Since I’m trying to read mine to be about 6 psi, the small dial was hard to read. Anyway, fired it up and the dial read about 9psi steady at idle.

9 psi seemed strange. So I dug up my Vacuum Gauge which is also a fuel pressure gauge. Hooked it up in a T-configuration.

Started the engine. At idle, the dial needle started jumping around between about 2 and 9 psi. As I increased the RPM the needle started to stabilize at around 4.5 psi plus or minus 1 or 2 psi. This seems low, but the needle was very erratic and not stable, so hard to tell what the average pressure was.

Shut it off and said have to ask the experts:

- Am I measuring the psi correctly? Correct gauge connections? Could both gauges be bad? What RPM’s do you check the pressure? Is pressure supposed to be stable at all RPM’s?
- Do I actually have a pressure problem? Bad fuel pump? Faulty vapor separator (its my original 1970 unit)? Something in the feed or return lines? Plugged sending unit (it’s new)?

Thanks for the feedback

 

[doubleyellow]

OK, I just realized the hose to the gauge is too long and filled with air. The air pocket is like a spring and causing the dial bouncing. I’ll try again with the shortest length possible.

In the meantime any advice on the fuel pressure is highly welcome, Thanks

 

[doubleyellow]

OK, so I shortened the tube going to the gauge. Ran it again. At idle the needle still bouncing a bit, but averaging about 4 psi. As I increase the RPM the pressure starts to drop below 4 and head toward 3 psi.

Is this running too low? Maybe I still am not measuring the pressure properly with my setup?

Would lower pressure cause the loss of power I am experiencing during acceleration?

 

[Xcudame]

Try this. I used this setup to fire up my 64 Imperial I bought the January that was sitting for 12+ years without running as I didn't want to pump any goo from the tank into the carb. Take a cheapo electric fuel pump and a gallon gas can and hook them up to the carburator fuel inlet. You should be able to cram the gas can somewhere under the hood and secure it. Hook the electric fuel pump to a 12 volt source, stick a rubber fuel line in the can hooked to the pump and take it for a test drive (don't go far or get too crazy!). If my suspicion is correct, you have either a bad mechanical pump or fuel pump rod wearing out. You're running a new metal canister fuel filter, right? What ignition are you running (i.e. electronic or points)? If electronic, what is your control unit? Could also be an ignition problem at higher rpm.

 

[doubleyellow]

I may not try the gas can method so I don’t fry the Chally, but I did order a new Carter mechanical pump to try.

My original vapor separator may also be the culprit, so I will try to bypass it and go direct from the pump to the carb.

I did switch between a mechanical points distributor and an orange box electronic. The higher RPM power loss was still happening, but both distributors could be off. I do have a new electronic module I could try.

As far as the fuel pump rod. Can I pull that out with a magnetic probe easily for inspection?

Still looking for if 4.5 psi at idle, with the fuel pressure dropping as RPM goes up, is not what should happen on a stock 440 set up?

 

[Xcudame]

I understand about the gas can method. I've always been a little crazy! With the fuel pump off, there's a pipe plug just underneath the opening for the fuel pump arm. Take it out and then you can get the rod out. Like you said, you'll probably need a magnet to get it out, but the dang thing will naturally slide out with gravity when you put it all together. Rod should measure 3.220". Pressure should go up some with higher rpm, yes!

 

[doubleyellow]

I understand about the gas can method. I've always been a little crazy! With the fuel pump off, there's a pipe plug just underneath the opening for the fuel pump arm. Take it out and then you can get the rod out. Like you said, you'll probably need a magnet to get it out, but the dang thing will naturally slide out with gravity when you put it all together. Rod should measure 3.220". Pressure should go up some with higher rpm, yes!

Thanks X

 

[540HemiCuda]

Is that an old orange ECU mounted on the engine? First, try a different ECU if you haven't already. Maybe more than one. There have been a TON of crappy ones over the years and they have been known to cause the very problem you are experiencing. Second, get it off the engine. Too much vibration.

The fuel pump push rod has always been a problem area so checking it's condition is not a bad idea.. If there is a problem with it, you will know immediately; one end will be much rougher than the other. But the carb has fuel bowls and if the floats are set right, they don't empty and refill as quick as the description of your problem would indicate. A short rod generally starts to introduce problems at continued load. Otherwise, it runs fine. So, I don't think this is your problem.

For light fuel demand such as cruise and light acceleration, it doesn't take much fuel pressure at all.......if there are no other problems.

 

[Challenger RTA]

I understand about the gas can method. I've always been a little crazy!

Great mind think alike! Been there done that. I trust in the theory gasoline doesn't burn or light, it's the fumes.

You can throw a match into gasoline and it goes out. But don't use a match to see how much is in the tank!
Don't try this at home. Go to the North 40.

 

[doubleyellow]

Yes it’s on the engine because I’m going for OEM look and was not going to drill holes on the firewall.

I do have another box I could try. BTW recommendations on best quality box currently on the market?

Eventually I could hide it under the battery tray? Or is that also a lousy spot due to dirt and moisture?

Is there a quality all-in-one distributor that doesn’t use an external box?

I did put a mechanical points distributor on and it still had the power loss (that distributor is a crappy asian one so could also be bad), but let me try the electronic box change out first.

I will have a look at the pump rod

Finally…..I’m still running my original vapor separator…..could the separator or the fuel return line cause power loss? I’ll bypass the separator today…. easy enough to try

Thanks for all the input so far !

 

[Challenger RTA]

Is there a quality all-in-one distributor that doesn’t use an external box?

Yes it's Bluetooth and not really time tested or real cheap. I have to find it.

 

[Challenger RTA]

could the separator or the fuel return line cause power loss?

All the above info is good to check.
I would say yes under the right conditions. Are all the fuel lines stock size and routing? Remove and plug the separator lines. Then see if you have the problem.
I'm thinking it might not be your problem there probably would be gas leaking somewhere from the separator.

But any how I was waiting to see how it was going.

A long time ago I had a similar situation. Only experienced once and someone else also.

Runs great at idle, but as I accelerate from about 2500 RPM and above, the engine bogs down badly and loses all power. But then back to smooth running at idle.

Also it takes a bit to get up to speed, because of lack of power. I did a lot of checking and changing of parts.
So what it came down to the vacuum advance was not working correctly. Changed it and ran perfect!

 

[doubleyellow]

Thanks so I’ll check pump/separator and orange box and then vacuum advance.

 

[540HemiCuda]

I will say again; the carb has fuel bowls (basically reservoirs) of fuel in them. If I understand your problem correctly, they will not drain nor refill as quickly as you are describing your problems. At any rate, check your fuel pressure WHEN the problem is occurring. If you have much of any fuel pressure then, that is not your problem. If fuel delivery is your problem, the engine would need to consume fuel faster than it can be delivered. I have taped a fuel gauge to my windshield on many occasions chasing a problem. I would do that FIRST because it is easiest.

As to spark, an inductive timing light is a good diagnostic tool. Watch the spark when you experience the problem. If the spark is scattered, you can see that in the light. I used to have a very long plug wire that I would use so that I could see the timing light while I was driving.

I have no idea which ECU to use. The parts store ones have been JUNK forever. And the orange boxes have too. But here is an older thread whose info might still be valid: Mopar ECU box

Overall, the key is to do tests while the problem is occurring. Any way you can. Otherwise, you spend a lot of time and aggravation in the wrong places.

 

[Challenger RTA]

Yes it's Bluetooth and not really time tested or real cheap. I have to find it.

Chrysler

Fully electronic timing control. Tuned with your smartphone or tablet. Wireless Bluetooth connection

progressionignition.com

 

[Chryco Psycho]

4 psi is low side , should be 5-7 psi .
I would first take the top off the carb & make sure the float level is set correctly , none are from the factory ,
The spring on the secondary door could be set too light & opening too soon causing power loss with too much air &*no added fuel

 

[Bret Schneider]

Check for a restricted exhaust.
Does the engine have a hear riser valve? If so, make sure that it's operating like it's supposed to.

 

[doubleyellow]

Guys thanks for all the tips. Will check all that out.

I put a new Carter fuel pump in. bypassed the vapor separator.

I did not yet check the fuel pressure with the new pump (i just ordered a new pressure gauge in case my old one is off)

Took the car around the block and it’s still bogging down. It’s not really hesitating/breaking up, it’s more bogging down/power loss when i shift into second and kick in the secondaries.

I checked the floats, but will check again. will check the secondary flap. It’s a AVS2 manual choke (i did wire open the choke flap until i get the proper choke coil installed).

Could a vacuum leak cause this bogging under acceleration? it runs great at idle with pretty steady 15.5 vacuum. i checked all possible leak points but will check again.

Again thanks and will keep trying to solve this