Stumbling on Highway

[nutz4spd]

My car either has two left feet or a fuel supply issue. Cruising down the highway at 65mph or so she'll start bucking and stumbling like it's starving for fuel. Here's the setup. It's a 413 with a 650 Holley single feed. A stock mechanical fuel pump with an optional inline electric fuel pump at the tank feeds it. After the first couple of times it died on the highway, I noticed there wasn't any fuel in the filter. After turning it over a few times it would start and run fine. Now when she starts stumbling I switch the electric pump on for a few seconds and it smooths right out. I've noticed the fuel pressure fluctuates on the mechanical pump and ranges from 3psi to just under 5. Usually staying around 4. Using the electric pump it's 8.5 psi. Could the mechanical pump be too weak to supply enough fuel at cruising speeds? It does has a little hesitation at take off as well, but that could just a carb tuning issue.

 

[pnora]

Probably crap in the tank. Does it have a filter sock on the sender?

 

[nutz4spd]

Probably crap in the tank. Does it have a filter sock on the sender?

Yep. The tank was removed and cleaned out about two months ago. Along with the sending unit.

 

[twecomm]

Have you checked the length of the fuel pump push rod ?

 

[nutz4spd]

Have you checked the length of the fuel pump push rod ?

I have not, no. What should the correct length be?

 

[Jerry Hall]

The most obvious culprit is a bad fuel pump. I had the same issue recently on my Superbird. Initially I thought it was a vapor lock issue, then the vapor separator/filter. Checked the fuel pump push rod, which is the original to the engine and it was perfect. The only reason I even though to do this is all the recent drama about them. Then after 2 mail order fuel pumps which were incorrect and 2 parts store pumps, also incorrect, the parts store got out their old paper parts book and was able to get me the correct part. Now the fuel pressure is a steady 7 psi. The pump I pulled off could only generate 2 psi before being removed. It also indicated a internal leak which necessitated a oil and filter change. Now 3 weeks later she is purring again after what should have been a 1 hour job from opening the hood until closing it.

 

[twecomm]

New should be 3.25 in. They do wear..

 

[Jerry Hall]

New should be 3.25 in. They do wear..

Nothing like original parts. It's the quality of today's parts.

 

[turbine68rt]

I have not, no. What should the correct length be?

Technically speaking, 3.22".

 

[Geoff 2]

I would lose one of the pumps. Use one or the other. Neither was designed to work the way you have them configured.

 

[BSB67]

Can you read the guage while driving, or were the pressures posted at an idle? Being able to monitor fuel pressure while driving will provide really good information.

If your mechanical pump is supplying 3-5 psi while cruising, it is not the pump, or the pump rod or pluggage.

Does it lay down or die under full throttle. If WOT is fine, the problem is not the pump, the pump push rod, or pluggage.

Does it do it when the motor and air temperature are cooler? I could be vapor lock

 

[nutz4spd]

Can you read the guage while driving, or were the pressures posted at an idle? Being able to monitor fuel pressure while driving will provide really good information.

If your mechanical pump is supplying 3-5 psi while cruising, it is not the pump, or the pump rod or pluggage.

Does it lay down or die under full throttle. If WOT is fine, the problem is not the pump, the pump push rod, or pluggage.

Does it do it when the motor and air temperature are cooler? I could be vapor lock

The gauge is under the hood at the carb. I guess I could re-route it to the outside temporarily and see what it reads at speed.

Under full throttle it hauls tail. It's only at cruising on the highway/interstate is when it'll start to stumble and buck.

No, it does it regardless of the temperature.

I would lose one of the pumps. Use one or the other. Neither was designed to work the way you have them configured.

Both pumps don't run simultaneously. The electric pump is on a switch. I run the mechanical one full time.

 

[BSB67]

Under full throttle it hauls tail. It's only at cruising on the highway/interstate is when it'll start to stumble and buck.

That tells you something.

 

[nutz4spd]

That tells you something.

Which is??

 

[hunt2elk]

Sounds like to lean at cruise?

 

[dadsbee]

Sounds like my Swinger at 3200rpm. M1 manifold just doesn't like that cruise spot.

If it goes like a scalded cat at WOT there is nothing wrong with the fuel pump.

 

[Geoff 2]

The elec pump is in the fuel line somewhere the p/up in the tank....& the inlet of the mech pump. It will cause some restriction to flow.
The mech pump will also be playing mind games when the elec pump is on.

 

[dart14]

My car either has two left feet or a fuel supply issue. Cruising down the highway at 65mph or so she'll start bucking and stumbling like it's starving for fuel. Here's the setup. It's a 413 with a 650 Holley single feed. A stock mechanical fuel pump with an optional inline electric fuel pump at the tank feeds it. After the first couple of times it died on the highway, I noticed there wasn't any fuel in the filter. After turning it over a few times it would start and run fine. Now when she starts stumbling I switch the electric pump on for a few seconds and it smooths right out. I've noticed the fuel pressure fluctuates on the mechanical pump and ranges from 3psi to just under 5. Usually staying around 4. Using the electric pump it's 8.5 psi. Could the mechanical pump be too weak to supply enough fuel at cruising speeds? It does has a little hesitation at take off as well, but that could just a carb tuning issue.

I had the same issue with stumbling at highways speeds. The car eventually died and would not start. After a tow home, I checked the fuel pump and the two attaching bolts had become loose enough so the pump rod was not engaging.

 

[WileERobby]

Technically speaking, 3.22".

Thank you, Mr. Spock.

 

[nutz4spd]

Update: I bypassed the mechanical pump and started running just the electric pump with a regulator set at 6psi. It doesn't stumble or die anymore on the highway. That tells me the mechanical pump is too weak to keep up. Guess I need to relocate the pump to the rear of the car though. It's pretty noisy being right under the front of the trunk pan.