Fuel pressure drops once engine warms up.

Hey pals, maybe this is an easy one that you can help with.
Trying to get the car ready for the trip to So Cal. The Charger runs fine but the fuel pressure reading is confusing me. When the car is cold and first started, the guage on the dual feed line reads a solid 7 lbs at idle. The clear filter is full and things all seem good. Once the engine warms up, the fuel pressure goes down. This has happened in the past, noteably during camshaft break in where the engine began to stumble. Once the pressure drops, the fuel filter also shows it: Fuel no longer fills it completely, it sometimes just pulses and bubbles in there. The pressure drops to zero sometimes.

The engine is a 440/493, MP 528 cam, 2" headers, 3" exhaust, Holley mechanical pump with less than 2500 miles on it. I have the A/F tuned with a wideband guage. ALL fuel line connections are secure. Gas tank and sender was new in 2003. The car runs fine when cruising but will stumble sometimes at WOT. I had a Carter pump on it once but it read 11 lbs at idle. I figured I'd try it sometime later when I had the time to integrate a regulator and a return line. I didn't have the Carter pump on the car long enough to see how it was with the car in use.

I don't know if this could be considered vaopr lock. The car runs a consistent 200 degrees. This happened before with another Holley pump but I didn't want to bash the brand. I'm probably going to try the Carter pump again just to see what happens with the engine up to temp.

Any suggestions on a decent adjustable fuel regulator with a return port? I don't know much about these setups but I was planning on using the 1/4" vent line for the return line. The fuel line is a 3/8". The 70 Charger has the 2 nipples on the fuel sender as well as 5 hoses that come off of the top of the tank, routed to an evap tube mounted to the RH side of the trunk floor near the wheel tub.

I think the first thing to do is check how hot the fuel lines are getting. Maybe running to close to the exhaust or against the block somewhere.

What Stump said, check for a vapor lock (heat) prob then plumb the pump inlet to a can of gas to elim or confirm a prob from there rearward (line dia too small/pinhole in line/pickup and sock restriction) then what is left is pump/line capacity up front. I like to T at the carb inlet & run it to a reg & then to a return line to the tank & use the reg to modulate the amt of return flow to the tank to control the psi and to keep a constant flow of fuel

It sure sounds like a heat issue as all fuel lines have been checked/replaced. Insulating or re-routing hot areas may help. Also, no harm in trying the Holly pump but I don't guess that is the issue. Also, per my understanding, a Holly type carb is happy up to 9 psi. 11 psi is really pushing it and may overpower the float and needle valve and make bigger problems. Carter/Edelbrock carbs do not like more than 5 psi.

The two robust fixes I know of are a return style fuel system or an electric pump mounted on the frame rail where it is cool. I have used a booster electric pump to solve this problem. There are several threads on this, many approaches and many opinions.

Thanks guys.
Looking at the car again, the fuel line from the pump to the carb could be a contributing factor. It runs up from the pump, under the alternator, (touching the alternator bracket) then rests against one of the the heater hoses. Where it snakes around the distributor cap, the outer edge touches the upper radiator hose. I think the contact points are probably raising the temperature of the fuel line and the gas in it. I have a Harbor Freight thermometer gun. I'll probably take a few readings just for the info, then run a new fuel line. Space is tight in there. It is hard to stay clear of heat sources but my guess is that I am better off with at least some clearance away from things rather than resting against them.
I do want to run the Carter pump with the regulator and return line. I like the idea of having the fuel running in a loop.

Fuel tank not venting right? When it happens pop the gas cap and listen for a sucking sound. Fuel pressure will rise again too.

Saw a 97 Chevy van actually suck the tank in a couple inches due to a plugged vent.

I had the same thing happen on two of my hot rods...the Steel braided fuel line from the fuel pump back to the tank would collapse from the pump pulling a vacuum. On the other rod the pump was mechanical on the engine, and the vacuum pulled by the pump would eventually collapse the liner in the hose.

I replaced it with steel braided Teflon (PTFE) hose and fittings, and never had the problem again.

Also, fluid filled pressure gauges mounted in the engine compartment will also change readings as they heat up. Not in your case since the engine gets starved for gas.

BR, Mark

I could not make a Carter High Volume mechanical fuel pump work for the life of me. As soon as it heated up any it would start dropping in flow.

I don't give up easy but ended up dumping the mechanical fuel pump and went electric.

Talked to a lot of people about it including the manufacturer reps and most agreed its a common problem on high volume pumps.

Is this a high volume?

Yeah, I don't recall the specifics but it was rated higher than stock.

you didn't say which heads / intake you are running. I have found on several vehicles that the exhaust crossover on stock heads is a problem area for vapor locking with the newer fuels. Especially if the fuel line is close to it. Re routing the line or fabricating a heat shield has worked in those situations.

The engine has Edelbrock heads with no crossover. We do have ethanol in the fuel, the pump sticker states "Up to 10% Ethanol."


I used the infrared thermometer to see what temps I was getting.
Just idling in my 75 degree shop, engine at 195-200, the carb primary bowl was at 135, the fuel line was between 110 to 140. Fuel pressure started to drop to 4, the filter started to empty out a little. I went for a drive, 20-30 minutes or so. When I got on the throttle, it ran great then started to drop off as I went to 2nd gear. This is probably where I am running out of fuel. Back at home, I checked temps again. The fuel line had jumped to the mid 160s, the carb primary bowl was at 135-140. The filter was at 125.
I prefer to know why something is failing rather than to just replace it. I'm sure an electric pump, regulator and return line would solve this.

Another option, junk in the tank plugging the pickup tube?

Had that happen to my Jeep.

Might be hard to separate run time from heat soak, but might be a good test. If you have any braided hose, could even be the inner liner separating and creating a "flap" that impedes flow.
Once had a similar problem and it turned out to be a rag that some joker put in the gas tank; it would get sucked up against the sock.
I believe your car is modified, not an OEM resto, right? In that case, I'd put some fire-sleeve/heat-sleeve on the lines/hoses in the engine compartment. I've had good luck with that stuff, even running it for the whole length of fuel line under a car in some cases.

Temps on the carb bowl sound reasonable. The fuel line at 160 sounds kinda hot. Was that on the suction or pressure side of the pump? The suction side is most sensitive and what usually causes vapor lock.

The 160 reading was on the pressure side between the pump and the carburetor.

check the fuel pump push rod, it could be worn down, seems to be a common problem.

Thanks. I did check the pushrod for the pump when I swapped camshafts last May. This issue has happened before and is worse with warmer weather when I take the throttle beyond a casual type drive.

I am now looking at revamping the fuel system with a bypass type regulator. This Holley unit is preset to 7 psi, right where I should be.


http://www.summitracing.com/parts/hly-12-803bp/overview/

I just checked the google search function here to see how others plumbed their setups. I am thinking of running the Carter pump with the next setup. Check out how Mallory shows the routing of their bypass regulator:



http://www.jegs.com/InstallationInstructions/600/650/650-4309.pdf

In 3A. I could simply attach the regulator behind the feed line to the secondary carburetor bowl and plumb the return line toward the firewall and down.
In 3B, I could totally reroute the output/pressure side of the fuel line and run everything back toward the firewall away from engine heat and obstacles.
My next question is this: Since I already plan to integrate a means to keep the fuel in a constant loop by way of the return line, would that be enough to reduce the fuel temps? I'm unsure of what temperature the fuel starts to boil but it seems that the return type setup should make some difference. I ask because the 3A method would be the easiest. I don't want to spend the time and then miss out on the benefits.

Red neck vapor lock check. Split a 3 or 4 inch length of fuel line and put it over the line to the carb before the fuel pressure gauge. Tie it on with a couple of zip ties and see if it helps the problem. If it does then you know the problem is fuel heat.

Hey! Red neck or not that is worth a try ! Thanks !

this is the one I used, it is for a mechanical fuel pump, it has a jet bleed, I have a 3/8 supply & 3/8 return, I also wrapped all my lines. With the ac running & 107 outside temp I no longer have any heat related fuel problems.

link to bypass regulator

the Holley reg that you posted says this in the part description
Notes: Cannot be used with mechanical fuel pumps.

Dang. I ordered that Holley regulator and thought it would be okay. Nothing I saw in the description gave me warning. Maybe I missed it ? The instructions in the box state just what you wrote: Not for use with mechanical pumps.
I'll look at the one in that link you listed. Thank you.

Is the hose from the sending unit to the steel line sucking shut?

I didn't look for that. I will though, good suggestion and thank you.

I ordered, received and installed this regulator yesterday.
I mounted it to the right frame rail by the pump. The outlet of the pump enters the regulator then exits to a new steel line that runs back up to the filter and dual feed line to the carburetor. I thought of mounting it behind the carburetor but this looks cleaner and is mostly out of sight. I also ran the return line to the stock vent line. The fuel temps did go down.
Primary Carburetor bowl Before: 135+ After; 127-130
Fuel line from regulator to filter Before: 160-165. After: 145-148.
Odd thing though: The Carter pump I also installed used to register an 11 lb reading before. This regulator is supposed to be preset to 6.5 lbs. It now idles at 3 lbs but jumps to 6 or so when the engine revs up. I tried adjusting the locknut as per the instructions but it made zero difference to the pressure reading. Their tech line is closed on weekends, of course.
The filter never emptied out though. It almost always did before. The pressure never dropped to zero like it did before.
I thought that I may have the regulator plumbed backwards. The instruction sheet is very basic, the regulator is not marked INLET or OUTLET, nor does it have arrows to indicate the intended direction of fuel flow.
If nothing else, it does not run dry. That is an improvement.

the opening opposite of the adjuster is the return .

3 psi at idle is fine, to raise the idle pressure you have to change the jet to a smaller one.

I do have it plumbed the way you wrote. The instructions did state that, they just didn't state which end was an inlet. Maybe the unit is designed to be used either way?

It does not matter in or out, also mounted down low & that far from the carb the idle pressure will have a slight drop . mine is mounted next to the carb to get rid of as much hot fuel as possible,,, it gets hot in south Texas!!

Hmmm...I mounted mine down low thinking that the plumbing would be simpler and that it might run cooler down there. It did drop the temps in the fuel line by almost 20 degrees. I still do not know at what temp that ethanol laced gas boils.

You should be ok, here is mine, the regulator is wrapped next to the ac compressor.