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Topic Review (Newest First)
03-31-2005 11:47 PM
Dropping fuel pressure problem fixed!

Hey everyone. Here is the latest update on my problem. Today I disconnected the electric fuel pump, re-adjusted the carb and timing and changed the liquid filled fuel pressure gauge to a non-liquid gauge. Started it up and watched the temp reach 180. The fan came on, the fuel pressure stayed at 6 psi and the temp reached up to 220 degrees before I shut it off.

The pressure never dropped below 6 psi. I don't know what happened but it seems to be OK now. So I want to thank EACH and EVERY one of you for your help. I was about to give completely up. So again I thank all of you guys.

03-31-2005 10:49 AM
TurboS10 Wow, three relays for two are a circuit junky.

I agree on my fan situation, and I will likely switch them up when I get time. It was one of those last minute, to start the engine I need to do this kindof things. It is fused at 20A which should pop before the wire melts. I was surprised myself that it has not popped yet. I just goes to show you that engineering in saftey factors can become redundant since every engineered piece of any system as a 20-50% saftey factor. They all add up to many times what is actually needed in some cases. I think cooling fans might be one of these instances.


03-31-2005 09:40 AM
camaroman7d Chris,
I understand where you are coming from. If you are not going to wire the relay right it's not worth doing. I also agree with the calculating wire size and if you bump the size up enough that will help. The problem is your average Joe doesn't understand this. That still would prevent the problem with an automotive switch being able to carry the load either. Since you wired your directly to the fuse box there is no switch in the circuit to fail.

Personally I really think you are pushing it with your fans on a single 12 gauge. For short periods that should be fine but, if you ever get out and drive for any length of time you are running the risk of a slight melt down. I ran my duel fans off of three relays (I work on fault tolerant systems, so I am a little anal), that way any two of the relays could fail and I would still have my fans. Most fans need a fuse and relay rated at 30 amps, you are running two fans on a single fuse/wire. In a drag race only application you will probably never see a problem. I drive my cars long distances and they see a lot of street time, so they have to be rock soild and dead reliable.

If it works for you that's fine.

03-30-2005 10:30 PM
TurboS10 Royce, I thought that is what you are getting at. I am a sr. engineer for a controls company and I completely understand your point. While a relay is a great way to elimate problems, calculating voltage drop and load on the wire based on its rated drops will prove alot of times that a bump in wire size will work fine. That said, I do agree that the relay is the best way, but not the only. However, it is pivotal to understand why the relay is important and how to wire it. If the relay is poorly placed or wire size is not taken into account, you are still pissing in the wind.

I actually run my fans on my truck straight off the fuse box(because I was too lazy to install a relay and run the wire)and have the pump relayed. I have two fans on one 12 guage at about an 8 foot length wire with no issues at all.

03-30-2005 10:32 AM
camaroman7d Chris,
Electronics is my profession. The reason a relay is the "only" way to run any electric motor is simple. Running a long smaller gauge wire will result in a voltage drop the longer the wire and smaller the diameter the more the voltage drop. Using a relay when wired correctly will eliminate the drop. You also do not want to run long runs of "hot" wire in and out of the car if you can help it, that is just more potential for a short/fire. The last and more important note is there are very few automotive switches that can handle a constant high amperage draw. The switch will fail it is a matter of time. The is more to electricity that "voltage", current (amps) is the thing to be careful of. I am not sure what I said that "you had a problem/hard time with"? If he was running 2 gauge wire and a circuit breaker to the motors (electric) then there probably wouldn't be any worries. The thing is that is usually not the case. People tend to use the same size wire that is on whatever they are wiring. On the fuel pump it is probably 14-16 gauge, a run of wire that small to a constant draw from the front of a car to the rear will have a voltage drop and also get hot, if it gets hot enough the vinyl will melt and that's how fires start.

I still say 100% the problem has either been compounded or mis diagnosed. There is NO possible way the fans turning on would effect the mechanical fuel pump. I will agree it "could" effect the electric pump. With the pressure dropping to 2 psi, that tells me there is another problem and I think it's just a coincidence it happens about the time the fans turn on.

Another thing I would like to see tested is this (to eliminate the fans), start the car run the garden hose into the radiator, regulate water flow just enough to keep the engine cool without the fans. I am willing to bet one the engine gets to operating temperature or run whatever amount of time it takes for the fuel pressure to drop, it will happen again (even without the fans). If I'm wrong I will drive down to Bakersfield and fix it myself. The problem is not the fans and I doubt it is the fuel pump or pumps. It's either the pressure sender/gauge or some restriction in the fuel system, I agree it could be heat related.

One last point when did you set the fuel pressure? Did you do it cold? If so that could very well be your problem. You need to set the fuel pressure with the engine warm and running. If you set it cold or with the engine off, then the pressure drop is happening because once the fuel bowl/s are low the regulator is not letting enough fuel through to keep the pressure up. Once the fuel pump (electric)/and fuel heats up pressure will go down a bit. Cold fuel is dense, warm fuel is not, it is perfectly normal to see the fuel pressure go lower on hot days.

Another VERY important thing we haven't covered or mentioned. Where is this electric pump mounted? If it is above the fuel supply that can create a problem. Also you need to have a filter before the electric pump (meaning between the tank and the pump).

03-30-2005 07:25 AM
coldknock Lots of good theories so far but one very small but very, very significant one hasn't been mentioned.

The voltage sensing wire on the alternator. If it's connection is too close to the alternator the alternator may not increase output sufficiently to maintain voltage when the fan comes on. That would cause the pressure drop you're describing.

Do an amperage/voltage test to see if the alternator is bumping output to compensate for the increased load. If it isn't, the voltage sensing wire may be too short for the alternator to accurately determine the neccessary output and adjust accordingly, the diodes may be bad too. Install a longer wire to make the connection as far away from the alternator as possible.

A 100amp alternator should have no problem maintaining power. If you have a one wire alternator, you have an entirely different situation .

03-30-2005 06:42 AM
BstMech Just for HA HA's, take a set of jumper cables or wires and ground the firewall/chassis to the engine block and see if the problem changes. If the low pressure problem was present with the mechanical pump as well, I would have to lean towards a heat related problem (vapor lock etc.). Then again, this might be one of those "you had to be there" kinda things.

Maybe Doc can post a little tutorial with pics/figures on how to do a voltage drop test and resistance test. Very simple to perform with a common DMM/VOM. Or you could give the ground thing a shot for the heck of it. HTH
03-30-2005 06:28 AM
TurboS10 The fact that the pump is petering out as soon as the fans come on is a dead give away. That likely means that the fans are lowering voltage when they come on due to the increased amp draw. You need to measure voltage at the pump when the problem occurs. Untill you do that there is no more need to discuss anything really.

03-29-2005 10:13 PM
Palmtrees57chevy Hey turbo explain to me why you think it's partially a voltage problem?
03-29-2005 10:03 PM
TurboS10 From what you are describing, it is most certainly at least partially voltage related. You need to monitor that and tell us what it shows.

Royce, I am having a hard time with the relay statement. While running, system voltage should be constant unless there is a strange wiring situation where an overamperage is occuring.

03-29-2005 09:39 PM
Palmtrees57chevy Hello everyone. Thanks for responding. Hey Tech, I was wondering if the gauge was faulty. I do have another pressure gauge that is not liquid filled so maybe I'll try it. I agree with camaroman, I shouldn't need two fuel pumps, but I was told before that adding an electrical fuel pump would eliminate my vapor lock problem. If I were to by-pass the mechanical pump for test purposes, would I mess it up if I left it installed on my motor? As for having relays, yes I do have both the fan and electric fuel pump on relays.

As for it spitting and sputtering, it hasn't done that since June 2004. I haven't driven it since then so I don't know how far down it will actually go. As for now it will drop down to around 2-4 psi at idle after the engine temp reaches 200 degrees. I have a liquid filled pressure gauge mounted in-line between my regulator and fuel filter.

When the engine is warming up the pressure gauge reads 7 psi. Once the engine reaches 180 degrees and the electric fan comes on, the pressure gauge slowly starts to drop down to 2 psi. At this point the operating temperature shows 190 to 200 degrees. I have a liquid filled temperature gauge mounted in my intake. I must admit that I know nothing about electrical stuff. I had wired the entire truck with the E-Z wire harness. Everything was labeled so I wired it according to the instructions. I Think?

If I didn't mention this before, the truck used to run just fine. One day I was driving it when it started spitting and sputtering. I looked at my clear fuel filter and it was nearly empty at idle. after sitting for a while it started up but as soon as it warmed up again it would sputter and die. At that time it just had the mechanical pump and a smaller electric fan.

Now I have a bigger fan (3300 CFM) and electric pump and it still does it. i am about to lose my mind here.
03-29-2005 04:46 PM
camaroman7d There is no way the RPM level is dropping enough to make a mechanical pump lose pressure. Even an A/C compressor doesn't drag the RPM down enough the lose fuel pressure. We are all taking educated guesses because we can't see/hear/feel the problem. In a situation like this we need ALL the details possible. What has been tried, etc...

The problem is getting compounded by adding pumps, changing pumps, etc.. and not going back to the baseline. If you change something and it doesn't fix the problem you need to return back to where you started, that way you don't introduce "new" problems to the situation. Troubleshooting really isn't hard (I have done it for a living my whole life).

You should not "need" both a mechanical and electric fuel pump, pick one and stick with it. The mechanical pump should be more than enough on it's own as well. Running the mechanial pump, the fuel pressure drop shouldn't happen at all as long as the engine was still running (I don't care how low the RPMs drop).

Now by adding the electric pump, I can see how it would "restrict" flow if the power level drops (ie... from the fans), this could "possibly" limit the fuel pressure the mechanical pump could provide.

If it was my car I would, eliminate one pump or the other. Monitor the voltage and fuel pressure to see if the drop is related. You can buy a inexpensive volt meter and mechanical fuel gauge, wire and plumb them, tape them to the windshield and cruise around your neighborhood. this will give you a much better idea of what's going on. If you go with the mechanical pump there is no way the fans coming on is causing you pressure drop.

When you say the pressure drops how low is it dropping? Does it continue to drop slowly until it bottoms out? Does it drop instanly to a certain point and sit there? You need to eliminate the digital gauges for trouble shooting, the problem could very well be the gauge/s or sender.

You never said (or at least I didn't see) if you have relays running both the pump and the electric fans. If you don't the the fuel pressure drop is the least of your worries (fire being a bigger worry). Also neither eletric motor is getting full battery voltage if a relay is not used (and wired correctly).

Where are you taking the fuel pressure reading from? (regulator, carb, inline, etc..)

03-29-2005 04:09 PM
Eliminate the Power

Doc here,

One way to eliminate the Power, Is put a meter where you can monitor it, on the battery, should read 13.2 to 14.4 static....(not running)

Then, REMOVE the alternator belt, Run the engine (carefully monitoring the TEMP, Don't overheat it!) and watch the volt meter, and pressure regulator...When it comes up to temp and the fan cuts in look at the battery voltage, and regulator, do they both still drop?

If not, you have the fan circuit Taxing the system , imposing almost a "Rotor Lock " condition on the alternator....causing RPM drop, hence low pressure...

Next reinstall the belt, and pull the fan relay, or disable the test again...

If it still drops, most likely it's mechanical...look elsewhere.

I had to go back and re~read the thread, (unless I missed it, which is highly possible) you didn't say when this pressure drop occurs, did the RPM's drop...Like the Choke coming off the detent...(or possibly in this case, more drag on the engine from the alternator)

03-29-2005 10:39 AM
Tech @ BG In one of the later posts it reads as though the engine doesn't spit or sputter anymore now that is has the electric pump on, just that the pressure fluctuates.

Since the problem has occured with, and without an electric fuel pump, it is most likely not an electrical problem. Since it also occurs with the cap off, it is most likely not a venting problem. It could be a heat soak, or vapor situation. Try insulating the fuel lines, and possibly running a fan over everything when tuning to see if that makes a difference.
03-29-2005 07:51 AM
Originally Posted by Tech @ BG

It sounds like you've actually had one problem, and one ghost. The problem being your fuel pump was not keeping up with the demand of your engine. From what I've read now since you've gone to an electric fuel pump the performance does not change even when you see a change in fuel pressure. Now for your ghost, the fuel pressure is most likely not acutally changing!!! The liquid filled fuel pressure gauges are filled with glycerin. As Gylcerin gets hotter, it gets thinner. This is causing a false reading. If you try a non liquid filled gauge, your problem will probably go away.
I would agree except he claims the engine dies when pressure goes to zero on the gauge so it appears to be telling the truth. I would try using just the mechanical pump with no pressure regulator which would eliminate and faulty regulator the voltage drop problem that may be occurring with the electric pump. Running just the mechanical hi-po Holley mechanical pump you should be able to supply fuel to +10 w/ engines like yours at idle. If pressure then still falls, you have eliminated the pump as the problem. If that does isolate the problem as electrical, you have a severe voltage drop in your harness. My Willys did sort of the same thing - 100 amp alternator wouldn't keep up w/ electric fan and AC until I discovered I had hooked the alternator voltage sensing wire incorrectly. I had hooked it to the battery cable so the alternator thought voltage was fine. I should have hooked it to the fuse block main where the voltage drop was occurring @ high load so the alternator would up the output to keep up with high demand.
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