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Fuel pressure drops once engine warms up. #2055824
04/16/16 11:43 PM
04/16/16 11:43 PM
Joined: Feb 2010
Posts: 18,493
Granite Bay CA
Kern Dog Offline OP
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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.

Last edited by Frankenduster; 04/16/16 11:44 PM.
Re: Fuel pressure drops once engine warms up. [Re: Kern Dog] #2055864
04/17/16 12:34 AM
04/17/16 12:34 AM
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Grand Prairie,Texas
stumpy Offline
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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.

Re: Fuel pressure drops once engine warms up. [Re: stumpy] #2055872
04/17/16 12:49 AM
04/17/16 12:49 AM
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Lincoln Nebraska
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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


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Re: Fuel pressure drops once engine warms up. [Re: stumpy] #2055873
04/17/16 12:49 AM
04/17/16 12:49 AM
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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.

Re: Fuel pressure drops once engine warms up. [Re: Kern Dog] #2055912
04/17/16 01:20 AM
04/17/16 01:20 AM
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Granite Bay CA
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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.

Re: Fuel pressure drops once engine warms up. [Re: Kern Dog] #2055994
04/17/16 08:27 AM
04/17/16 08:27 AM
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ruderunner Offline
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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.


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Re: Fuel pressure drops once engine warms up. [Re: Kern Dog] #2056093
04/17/16 01:17 PM
04/17/16 01:17 PM
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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

Re: Fuel pressure drops once engine warms up. [Re: Kern Dog] #2056189
04/17/16 03:35 PM
04/17/16 03:35 PM
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Morrow, OH
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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?


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Re: Fuel pressure drops once engine warms up. [Re: markz528] #2056190
04/17/16 03:36 PM
04/17/16 03:36 PM
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Granite Bay CA
Kern Dog Offline OP
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Yeah, I don't recall the specifics but it was rated higher than stock.

Re: Fuel pressure drops once engine warms up. [Re: Kern Dog] #2056265
04/17/16 05:46 PM
04/17/16 05:46 PM
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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. beer

Re: Fuel pressure drops once engine warms up. [Re: TJP] #2056326
04/17/16 07:32 PM
04/17/16 07:32 PM
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Granite Bay CA
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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.

Re: Fuel pressure drops once engine warms up. [Re: Kern Dog] #2056373
04/17/16 08:54 PM
04/17/16 08:54 PM
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ruderunner Offline
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Another option, junk in the tank plugging the pickup tube?

Had that happen to my Jeep.


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Re: Fuel pressure drops once engine warms up. [Re: Kern Dog] #2056430
04/17/16 10:46 PM
04/17/16 10:46 PM
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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.

Re: Fuel pressure drops once engine warms up. [Re: topside] #2056438
04/17/16 11:18 PM
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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.

Re: Fuel pressure drops once engine warms up. [Re: ahy] #2056553
04/18/16 01:43 AM
04/18/16 01:43 AM
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Granite Bay CA
Kern Dog Offline OP
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The 160 reading was on the pressure side between the pump and the carburetor.

Re: Fuel pressure drops once engine warms up. [Re: Kern Dog] #2056578
04/18/16 02:49 AM
04/18/16 02:49 AM
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CSK Offline
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check the fuel pump push rod, it could be worn down, seems to be a common problem.


1968 Charger COLD A/C Hilborn EFI
512ci 9.7 compression, Stealth heads, 4.10 gear A518 ODtrans 4100lb,10.93 full street car trim
2020 T/A 392 Stock 11.79 @ 114.5

Re: Fuel pressure drops once engine warms up. [Re: Kern Dog] #2057220
04/18/16 10:47 PM
04/18/16 10:47 PM
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Kern Dog Offline OP
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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.

Last edited by Frankenduster; 04/18/16 11:10 PM.
Re: Fuel pressure drops once engine warms up. [Re: Kern Dog] #2057374
04/19/16 01:27 AM
04/19/16 01:27 AM
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Grand Prairie,Texas
stumpy Offline
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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.

Re: Fuel pressure drops once engine warms up. [Re: stumpy] #2057375
04/19/16 01:31 AM
04/19/16 01:31 AM
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Granite Bay CA
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Hey! Red neck or not that is worth a try ! Thanks !

Re: Fuel pressure drops once engine warms up. [Re: Kern Dog] #2057400
04/19/16 02:24 AM
04/19/16 02:24 AM
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Pattison Texas
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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.

Last edited by csk; 04/19/16 04:30 AM.

1968 Charger COLD A/C Hilborn EFI
512ci 9.7 compression, Stealth heads, 4.10 gear A518 ODtrans 4100lb,10.93 full street car trim
2020 T/A 392 Stock 11.79 @ 114.5

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