Hot Rod Forum : Hotrodders Bulletin Board

Hot Rod Forum : Hotrodders Bulletin Board (
-   Engine (
-   -   Why does fuel pressure drops when electric fan turns on (

Palmtrees57chevy 03-27-2005 10:13 PM

Why does fuel pressure drops when electric fan turns on
Hey guys, how's everyone doing? I am in need of some help here. It all started last June 2004. When my electric fan comes on, my fuel pressure begins to drop down. I have a very mild small block 350. It has an Edelbrock 1406 carb, mild cam, headers, high flow water pump, 180 thermostat, and a heavy duty radiator. I have installed a Carter electric fuel pump rated at 7 psi as well as a Holley mechanical fuel pump and pressure regulator.

I have flushed the radiator, adjusted the timing as well as readjusted the carb. I have also tried water wetter as well as the 40 below stuff. All is well as the motor begins to warm up until the electric fan comes on. The liquid filled pressure regulator shows 7 psi, then as the temp reaches 180 and the fan comes on, the pressure begins to drop down around 2-4 psi at idle. The pressure use to drop down to zero and it would bog down and die. The temperature stays constant between 180-200. The engine runs smooth and doesn't smoke, or burn any oil. The motor only has 498 miles on it. I haven't driven it since last June so I don't know how it will do while being driven. Can someone please help me out here. I would like to drive it before I am too old to see. Oh by the way it is a 57 Chevy Step-side. Thanks to all in advance.

NXS 03-27-2005 10:15 PM

You need a higher amp alternator and/or a higher amperage battery.

Palmtrees57chevy 03-27-2005 10:26 PM

Hey, I have a 100 amp alt. What I don't understand is that it did this prior to the installation of the electric fuel pump. At the time it only had a mechanical fuel pump. So what would having a higher amp alt fix? Isn't the electric fuel pump suppose to stay constant?

67 Deuce 4 Me 03-27-2005 10:47 PM

The load of the fan coming on my be causing your alt to pull the rpm of the engine down low enough to cause poor fuel pump performance. Thats the only way I can think of a mechanical fuel pump would lose pressure under the coditions you stated. :confused:

sbchevfreak 03-27-2005 11:14 PM

What's your idle set at? Could be like Deuce stated, and the engine is slowing down enough to lose fuel pressure. Just for fun, why did you install both an electric and a mechanical pump? I may have misunderstood you, but I'm reading that you have them both in the system at the same time.

Palmtrees57chevy 03-27-2005 11:39 PM

Hey guys, thanks for the replys, it seems to me that the rpm doesn't change when the fan comes on. So if it is slowing down I can't tell. The idle is set around 750. To answer chevfreaks question, I was told before to install an electric pump in the system to solve the problem, but it didnt stop it. So now I have the electric pump as well as the mechanical pump on the truck. So is the 100 amp alt too small? I have gone to numerous car shows and have looked at other guys set ups and mine looks just like most that I have seen, nothing special. So I don't understand why "I" am having this problem! I have braided lines running from the pressure regulator up to the pressure gauge. From the pressure gauge is the filter then a hard line to the carb. I have even tried putting on some insulation over the regulator up to the filter. It didnt stop the pressure drop. I am all out of ideas!

sbchevfreak 03-27-2005 11:50 PM

Does the fuel pressure return to normal when the fans shut off? Or does it just fall off when it is warm? I would suggest running a seperate electric pump and seperate fuel source (like a 5 gal. fuel jug) to the carb with temporary lines, and try that. If the thing runs like a top, your plumbing may be at fault. If not, I think we may need some more info.

Palmtrees57chevy 03-28-2005 12:06 AM

Once the fan comes on it stays on until the temp drops below 175. A little bit more info, I don't have a fuel return line, just the one line from the tank to the carb. After I had connected the electric fuel pump, and prior to reconnecting the mechanical pump, I had turned the key to see if the electric pump worked. It shot fuel out the line like you wouldn't believe. So the fuel goes through the line ok.

Hey just a quick thought, could the problem be from the tank not being vented properly? I have the vent line running from the top of the tank into the filler neck with a vented gas cap. The tank is a 20 gallon tank out of a 83-84 Ford Ranger.

sbchevfreak 03-28-2005 12:13 AM

This is going to sound really dumb, but have you checked your fuel filler cap? Is possible that it isn't venting and you are getting a "pressure differential lock" that coincidently happens about the time the fans come on. This is why I asked if the pressure comes back up. That mechanical pump, and most likely the electric one too, only pulls about 11"hg suction. If the cap isn't venting, you'll pull that vacuum inside the tank and no fuel will move. Or, if it is only partially venting, reduced fuel flow.

--------------------------------------------------------------------------You must have been reading my mind, you posted just as I wrote this.

Palmtrees57chevy 03-28-2005 12:17 AM

At one point I did remove the cap, but it still did it. I haven't tried it since I have installed the electric fuel pump. I'll try it in the morning, and let you guys know. I had thought about changing the tank all together. I had seen a 15 gallon tank in Summit that I thought about getting. What you are saying makes sense to me.

camaroman7d 03-28-2005 12:22 AM

What kind of fuel gauge are you using? If it is an electronic gauge that could be he source of your problem. Try connecting a mechanical gauge and see if the solves the problem (assuming you are running an electric sending unit).

Does your performance suffer from the fuel pressure drop? I don't see how an electric fan coming on would effect a mechanical pump. Do you have a Voltmeter? If not buy a cheap one and connect it for test purposes.

Is your electric fan wired through a relay? Is your fuel pump wired through a relay?


sbchevfreak 03-28-2005 12:23 AM

If it still does it with the cap off, I doubt it is a venting problem. Almost sounds like a vaporization issue. I'd try the external fuel source, both through temporary lines, and through the existing setup. This will eliminate either the tank, venting or the lines. I'll think on it overnight and thriugh work tomorrow, maybe have a look at a couple of Ranger tanks I've got kicking around the shop, and get back to you tomorrow evening, as it is nearlt 1:00AM here. Talk to you later,

Palmtrees57chevy 03-28-2005 12:30 AM

Thanks Chevfreak, talk to you later. Hey Camaroman, I have Dakota Digital Gauges, but I have a mechanical fuel pressure gauge mounted inline. The performance doesn't suffer because I haven't driven it since June 2004. Just been trying to fix if at idle. When it first did it, yes the performance suffered. It would spit and sputter and die. However now the pressure just drops without any noticable change in performance. I am afraid to drive it like this, don't want to get stranded away from the house. Yes both the fan and fuel pump are connected to relays.

camaroman7d 03-28-2005 12:36 AM

Very interesting. Are you 100% sure the problem occurs only when the fan comes on? You could have a fuel line collapsing. Do you have a long run of rubber fuel line? This problem usually would only occur if you had a vent problem, that doesn't seem to be the case if it still does it with the gas cap off. Have you tried to run it without the mechanical pump in the line? Try bypassing the mechanical pump, that seems to be the only common part in the system through all your problems. If for some reason it is restricting flow, adding the electric pump wouldn't fix the problem.


Palmtrees57chevy 03-28-2005 12:48 AM

I have about 16 inches of braid line running from the tank over to the electric fuel pump, then connected to the hard line up to the mechanical pump. At first I had a cheap $13 dollar mechanical pump on it. Then I installed the electric fuel pump, then last week I installed the new Holley mechanical pump. And yes the problem only occurs when the fan comes on. As long as the temp doesn't get to 180 the pressure stays at 7 psi. However once the fan kicks on at 180 and hot air begins to blow the psi drops down.

Even if the mechanical pump was the problem, wouldn't the electric pump still produce a constanct flow?

All times are GMT -6. The time now is 08:34 PM.

Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2014, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0 PL2
Copyright 1999 - 2012. All Rights Reserved.