Fuel pressure drops when motor hot.

[john.thompson068]

I just cleaned out my tank and replaced the filter because my filter was all clogged up from foam bits and crud floating in my fuel cell. I start the car up and suddenly it has 8 psi fuel pressure. So I turn the regulator out and bring the pressure back to 6.5 psi. The gauge is very responsive to adjustments. After driving the car for a while with the engine temperature getting hot, I check the fuel pressure and the guage is only reading 3 psi. So I turn the pressure regulator in all the way and the pressure still only reads 4. The guage is also not very responsive to the adjustments. It seems when the car gets hot the fuel pressure drops and is erratic at times. What could be happening?

The car has a fuel cell with 8 AN line, fuel filter, 1/2 aluminum line to the Holley mechanical fuel pump, some type of regulator with a Auto Meter guage, and 1/2 line all the way to the carb.

 

[john.thompson068]

That pump is also brand new. It is part number 12-440-11 with a free flow rate of 110 gph and maximum pressure (psi) of 6.5 to 8 psi.

 

[67 B-body]

It might be the under hood temps changing the guage readings. I know when I look at my guage readings on a cold day and compare it to a warm day and higher engine compartment temps, they very from eachother. I believe the starting readings without fuel pump running is also different by aprox the same amount....
So, check the guage before starting the car and see if it has a reading other than 0, record this reading, start the car and record that reading. Drive it untill your pressure drops and record the lower pressure. Shut things down, and record the pressure now.... See if they both have close to the same range......

 

[john.thompson068]

Forgot to look at fuel pressure before starting motor. Started motor and car had 8 psi because the regulator was screwed almost all the way in from tinkering last night. Just as pump descriptions stated, it generates a maximum of 8 psi. I turned the regulator out till pressure is at 7. Then I drive the car for about 20 minutes, and when I come back and open the hood with the engine still running. Fuel pressure was less than 3. Then while I am staring at it with disbelief, the pressure slowly comes up to 5 and leveled off. Is this suppossed to be happening?

 

[midnightrider1818]

hhmmm did you change the fuel pump pushrod. is this a mechanical pump. maybe when the oil is cold it is thicker and pushing the rod out farther and once the oil gets hot it gets thinner and therefor the pushrod is not going out as far. ???

 

[john.thompson068]

hhmmm did you change the fuel pump pushrod. is this a mechanical pump. maybe when the oil is cold it is thicker and pushing the rod out farther and once the oil gets hot it gets thinner and therefor the pushrod is not going out as far. ???

Fuel pump push rod has not been changed. Yes it is a mechanical Holley fuel pump.

 

[bigmanjbmopar]

Just curious but why are you running a regulator with a mechanical pump? My fuel pressure gauge bounces around a bit hot or cold with my eddy mech pump.

Did the car die when it got hot? what kind of gas are you running? what carb? do you have a spacer under the carb? need a bit more info to help you.

 

[john.thompson068]

Just curious but why are you running a regulator with a mechanical pump? My fuel pressure gauge bounces around a bit hot or cold with my eddy mech pump.

Did the car die when it got hot? what kind of gas are you running? what carb? do you have a spacer under the carb? need a bit more info to help you.

I don't know why I am running a regulator with a mechanical pump. It came with the set up. It does push 8 psi though and the Holley 4150 works best with like 6.5-7 psi I think. So I try to set it at about 6.5. The car does not die when hot. It runs fine on 93 octane with 10.5:1 and aluminum heads. There is no spacer.

For all I know it has always done this. I just recently started paying more attention to the fuel pressure guage after a recent episode of having the fuel filter getting completely clogged up from foam pieces in the fuel cell and other gunk. I drained the tank and cleaned everything spotless with a new filter screen.

 

[bigmanjbmopar]

Lol with a fuel cell in the trunk that car must have seen some track time and probably had an electric fuel pump on it at one time. Just take the regulator off and go have fun don't worry about it the worse that could happen is it floods from too much fuel being pumped in but if that hasn't happened yet your good.

Your original question of the fuel pressure dropping when it gets hot is now more of the fuel gauge shows 3 psi at idle? could be caused by the regulator and the mechanical pump, imagine pumping action of the mechanical pump with the engine rpm's? low idle rpm low pumping..get it?

When you throw in hot we automatically think of boil out or something like that with today's gas being so crappy it happens a lot. especially with 10.5:1 you are just around that mark were gas would be a problem IF you lived here in cal were all we can get at a normal station is 91.

I think your fine but you don't really need the reg, let some of the racers chime and see.

 

[Meep-Meep]

Hmmm, that's 20% variance. What is the published accuracy of the regulator? Seems it should be better than that. Is the regulator getting hot?

 

[493charger]

First, let me ask, is it a chrome 4.5-9 psi regulator (holley 12-803 or similar)?

If so, I actually have some experience with this issue. I used to run one also, but found the pressure to fluctuate too radically. Even from cold start to cold start. So, I ended up removing the regulator completely as my electric pump only put out 7 psi max anyway. After that my pressure was 6.5 psi all the time at the carb.

In your case, I would remove it and see what your fuel pressure is pump-only at the carb. If it is above 7 psi you stand a chance of fuel pressure pushing open the float valve and flooding the engine. If you must use a regulator, I would recommend one that has been cnc machined (i.e. billet aluminum or steel). I have had better results from them compared to the cast "pot metal" ones. They hold fuel pressure a lot more stable.

Edit: Also, the cheaper regulators tend to be a little more accurate when using the return port on them. Even though it says you can use them returnless.

 

[john.thompson068]

First, let me ask, is it a chrome 4.5-9 psi regulator (holley 12-803 or similar)?

If so, I actually have some experience with this issue. I used to run one also, but found the pressure to fluctuate too radically. Even from cold start to cold start. So, I ended up removing the regulator completely as my electric pump only put out 7 psi max anyway. After that my pressure was 6.5 psi all the time.

If you must use a regulator, I would recommend one that has been cnc machined (i.e. billet aluminum or steel). I have had better results from them compared to the cast "pot metal" ones.

Meep Meep and 493:

I don't know what the heck it is. There are no visible part numbers on it. Take a look at it in the pictures. Meep Meep, I suppose it heats up a little bit, but what would that have to do with the pressure. This is weird. Maybe there is a part number on the bottom and I will have to remove it to take a look. I just realized why the pressure went from 3 to 5 as I was staring at it. When I opened the hood a lot of heat escaped and so the pressure started reading higher on the guage. I don't get it. I'm not sure what Auto Meter gauge that is either. If you notice that the adjuster is all the way in, it is because I was figuring out that my filter was clogged and messing with the regulator was not going to bring back my pressure at the time I took these pictures.

 

[john.thompson068]

So far it looks like an Edelbrock 8190, Mr Gasket 2015, Proform 67383, or maybe even a Jegs 15912 or Holley 12-803. They all look exactly the same as mine except mine is not chromed. They all cost about 60$ with the exception of the Holley and Jegs which are half that cost.

 

[bigmanjbmopar]

Hey since your taking pics can you snap a few of that fan shroud? Need some ideas on a custom one. :headbang:

 

[lewtot184]

its not abnormal for a mechanical pump to vary a little between hot and cold, but not that much. my 2cents; the diaphragm in the pump has issues. try another pump and see what it does. another issue is those little regulators. they only have .220 feed orifice to the carb. thats less than a 1/4", smaller than the i.d. of 5/16" tubing. if it were me i'd ditch the regulator and try a different pump.

 

[john.thompson068]

Hey since your taking pics can you snap a few of that fan shroud? Need some ideas on a custom one. :headbang:

Yeah man, I will post some picks and tell you all about it tomorrow or maybe even tonight.

 

[493charger]

Since the pump may be suspect, may I suggest an Edelbrock 1723 fuel pump. Then you can do away with the regulator all together.
I have been using one on our 493 and it has worked great. It's over 600HP and has never starved for fuel. Just another option/opinion.

 

[john.thompson068]

I can't afford to replace anything as long as it gets the job done. Once I get the rollover valve and some teflon washers installed in the cell, I will be able to put the foam in the cell and then fill it up with gas. At that point I will flog the car and see if the fuel pump is providing the fuel I need. If so, I really don't care if I get a screwy fuel pressure reading. The pump is brand new and everything was working fine before my fuel filter got all clogged up. So I'll know by this weekend when I hit the freeway or industrial "test road". Currently I am driving without any foam in the cell and only about one or two inches of gas. I don't want to put much in there as I will be reaching my arm in there tomorrow. Occasionally after a turn or braking the gas will slosh away from the pickup and the car will stumble a little bit. I flogged it a few times up to about 70 mph and there was no starvation and throttle response was good. Right now I am believing that having fuel slosh out of the pick up is what caused my stumbles and lack of throttle response yesterday. I drove the car to the muffler shop today to have a tailpipe bracket welded back together and it performed good the whole way and the car had only 3 psi fuel pressure according to the guage.

 

[bigmanjbmopar]

Since the pump may be suspect, may I suggest an Edelbrock 1723 fuel pump. Then you can do away with the regulator all together.
I have been using one on our 493 and it has worked great. It's over 600HP and has never starved for fuel. Just another option/opinion.

x2 on the eddy 1723 I have one and t is working fine 493 also hmm 600hp huh interesting......

 

[493charger]

hmm 600hp huh interesting......

Les Terrel built the motor and we tuned it. The engine has an a "cousin" that was built a few weeks before ours for another customer. It made over 550hp on the dyno. Ours has a bigger cam, more compression and a lot more porting on the heads. So, perhaps 550+ would have been a more fair assumption. And, would you believe they're both using 452, cast iron heads. The engine is a beast. It's been a "challenge" keeping the valvetrain intact. It was eating rocker adjusters like candy. The strongest adjusters I have found yet are the Harland-Sharp with the ARP locks. Not broken them yet...