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MG MGB Technical - More Fuel Pressure Woes

My earlier thread was called "fuel pump Woes." The saga continues. I replaced the old pump with the new - a "Carter" from NAPA. Worked great at first. Took the car on the highwa and everything. Later in the da the same trouble as before appreared. I've found that the pump works for a bit (sometimes REALLY hard) then stops. It gets REALLY HOT and stops. Could there be a blockage in/from the tank to the pump?
CRH Hoebel

CRH,

1. What I would do is run a hose from the pump to a gas can and see if it flows ok.

If it does than there is no problem from the tank to pump.

2. Now connect it back up and disconnect to fuel line from the carburetor and again run a hose to a gas can.

Do you get a nice steady stream. Mine filled up a 5 gallon can in about 10-15 minutes. So it does pump fuel pretty fast.

If not than I would think you have blockage in your fuel line from the pump to the carburetor. Being a 1980 I would just replace the lines. They sell a complete kit that fits easily.

Try the listed above first and see what happens.

Ray

Let us know what happens....
Ray 1977mgb

Having read both threads, I would recommend you consider using a good quality fuel pressure regulator with your after market pump. I ended up with an Edelbrock unit and have had no more problems with any pump I have used, even if the pump claims 2-4 psi. The SU float valves seem, for whatever reason, to be more sensitive than most to modest pressure variances and a good regulator can really help even that out.

Also, check your fuel lines! As fuel lines age, many of them sprout lengthwise cracks which look like they are just on the outside but can actually go all the way inside. This can be especially true at the hose fitting areas when sharp bends are involved. If these cracks are bad enough, you get what I call the "broken straw syndrome". Just like sucking on a broken straw, a pump can suck more air than gas from the tank if there are cracks in the line or the hose clamps loose. New hoses, if necessary, can save you time and perhaps dealing with yet another pump. Good luck!
Bob Muenchausen

Thanks for the advice. I'm sure that the fuel lines could stand to be replaced. If "broken straw syndrome" is the case, would the pump appear to run normally, but the fuel at the carb be deminished? I'm just trying to figure out the overheated pump that just stops.
CRH Hoebel

Check that the vent line from the fuel tank to the charcoal cannister is open and the tank can vent properly. I have seen them get clogged, creating a vacuum in the tank that the pump cannot overcome. Quick check is to simply remove the filler cap and listen for the sound of air being drawn into the tank after the engine has been running for several minutes. Les
Les Bengtson

Hi CRH.

The fact that the pump works 'sometimes REALLY hard' does suggest that the pump may be pumping air as Bob suggests.
Maybe the pump builds up a reservoir of pressurised air that the pump 'interprets' as adequate fuel pressure, so it stops pumping.
I guess it is possible that the pump could actually seize due to thermal expansion of internal parts if it gets hot enough, and re-start on cooling.
It is certainly worth trying Les's quick and easy check whenever a fuel delivery prob appears.

Don
Don

Hi CRH,

I just went through this problem on my 1971 MGB. I was having fuel flow problems and replaced the old pump with a new original style SU. The new pump pumped like grazy when I first turned on the key, but on start up the car would run fine for awhile but then die. I spent two days of trouble shooting until the problem was found. It was the new pump! The pump would fill up the float bowls with fuel and then stop as expected. The problem was that the pump would not start up again as the bowls emptied, and the engine starved of fuel. I returned the original style pump and replaced it with the electronic version of SU and all works fine. I am not sure what a "Carter" pump is but could the same thing be happening?
To check this out do as Ray suggests with the hose from the pump to a gas can, but apply backpressure to the flow using your thumb or by pinching the hose. After the pump stops release the backpressure and the pump should start pumping again. Mine did not!

Good luck!
Tim
Tim Mayor

Thanks. And this might be a supid question, but what is the correct diameter for the rubber fuel hoses? I've repaced most of it with what already there - 1/4 diameter hose.
CRH Hoebel

The new pump runs, charges the line then quits, heats up, and won't restart. I tested it by pinching the line (just air running through it and with fuel, and not through all the lines. Just the in and out at the pump). Ugh.
CRH Hoebel

Normally when a pump stops and then heats up, it is because there is an obstruction on the inlet side (between the tank and the pump. I can't say for sure on the Carter pump that you are using, since I have not dealt with them, but the If an obstruction occures ont he inlet side with a SU pump, or one of the SU clones, the pump will stall in a current on condition, which will cause the pump to overheat.

You state "The new pump runs, charges the line then quits, heats up, and won't restart". When this happens, loosten one of the connections between the pump and the carburetors and see if the pump will start. I would suspect that it will not, but if it does, then you have a defective pump. If the pump does not have a bypass valve in it like the Facet pumps have (which is why they run all the time), then the pump will stop when the float bowl fills and shuts off further fuel flow, but this should stop the pump in a current off condition so the pump won't over heat. If the pump doesn't start with the an open connection betweenthe pump and the carburetors, then remove the inlet line to the pump. If the pump starts at this point, then there is an obstruction between the pickup in the fuel tank and the pump. If it still doesn't start, then the pump is bad.

Do check the tank vent as Les suggests, a bad vent will cause a vacuum in the tank to the point that the pump will stall in a current on condition (just as an obstruction in the inlet line to the pump will) and cause the pump to overheat. That said, a bad vent will usually bleed off the vacuum in a short period of time once the pump is turned off and will allow the cycle to start over again. Good luck - Dave
David DuBois

WOW This is Great!!!! I just worked on my 1968 MGB GT today for about 5 hrs trying to figure out why the heck I have 6 lbs of fuel presure when I start the car then 5 to 10 minutes after 0 lbs of presure????
I had a stock MG pump, the car started running funny so I put a fuel presure guage righ before the carb's and saw that my presure was about 1.5 to 2 lbs so I changed out the stock pump

my curent configuration:
(two weber 45 DOHC on an aluminum crossflow head)
pic can be seen here: http://home.comcast.net/~enomadra/family/mgbgt/mgbgt.htm
The new pump I put on, the carter street/strip electric pump.

I think my issue is simular to yours.

After putting on the new carter pump I found that I would get great fuel presure for about 5 to 10 minutes then it would drop to zero. So today I jacked up the back end of the car to get it ready so I could take off the pump and do some troubleshooting. But first I wanted to see it loose pressure again. So I started up the car and let it run, and run, and run, and run. To my suprise the fuel pressure never dropped. in fact when I first started the car the presure was at 12 lbs. I quickly adjusted the regulator to bring it back down to 6 lbs then it was running good. But I could not get it to drop the fuel presure. Until I decided that maybe for some weird reason the cars back end being way up in the air on the jack was keeping it going. And would'nt you know, as soon as I lowered the back end of the car to be level with the front the presure droped to zero. But wait this gets better. I then thought OK their is something happening in the tank with the pickup GREAT!!!. So I got a gas can full of 92 octain and put a hose on the inlet side of the fuel pump and put the other end in the gas can. I started the car and it would not run. still very low presure. I then took three antifreze bottles and put them right next to each other and put the gas can on top of them making the gas can level with the pump ( I was thinking the pump was having some issue pulling the gas up). The car ran great for a while then did the same thing zero lbs of presure. I then put my hand on the fuel pump and it was smoking hot. I thought OH maybe a blockage in the fuel line going to the carbs. I took compressed air and blew out the fuel line after disconnecting it from the carbs. nothing came out but air, and really fast (like nothing was blocking it)Oh their was some gas that came out too.

I hooked the pump back up to the gas tank and took off all excess fuel line in the route so it was the shortest possible route from the tank to the pump and to the carbs. I still got the same result. It runs good for 5 to 10 minutes then zero fuel presure.

Here is some additional info:

1. I have the pump wired up so when the cars key is on the pump is on (no switch to shut power off to the pump should the motor shut off and the key is still in the on position) but this should be fine for me to just test things out and get the car running right? right?

2. Thier is no return line going back to the gas tank. Just the one line from the gas tank that goes to a filter then the pump then the carbs. (this has always made me wounder, when the fuel bowls in the carbs are full is the float riseing up and stoping the flow of fuel)?

Anyway I hope some of the stuff I have gone over helps being that we have the same type of pump. If I find out how to solve it I will let you know.

Thanks,

Mike
Michael

Mike - Did you try removing the fuel filler cap on the car and try running the pump? If your tank vent is blocked, you will draw a vacuum and stop the fuel from being pumped out. Jacking the back or the car up may have gotten gravity working in your favor to overcome part of the vacuum and allowed the pump to continue working. The other possibility is that the Carter pump is meant to operate in a position that is lower than the tank. The stock SU pump is designed to be mounted anywhere from the lower level of the fuel in the tank to as much as 18 inches above that level. I don't know what that spec is for the Carter pump.

You say that with the stock SU pump, the engine started "running funny". Was this as soon as you installed the dual 45 DCOE carbs? if so, it is quite possible that the carbs need a greater flow than the stock pump can provide (about 2 pints per minute, or about 15 gallons per hour). If the engine ran ok after you installed the new carbs, but then after a period of time started "running funny", you may have a blocked vent or an obstruction in the inlet to the pump issue.

Do you have any specs on the pump that you can send me or is there a web site that I can go to that tells about the particular model of Carter pump you are using? I would feel a bit more comfortable talking about the Carter pump if I knew more about them. I can tell you that the 1.5 to 2 psi that you were getting from the stock pump was about right (maximum pressure for the AZX 1300 series pump is 2.7 psi and you will not get a real accurate reading on a closed system where the pump just clicks a couple of times and shuts off because the float bowl is full). Cheers - Dave
David DuBois

Dave and Mike,

There are two low pressure carter pumps. One is about 2.5 -4 psi and the the other is 6 - 8 psi. You should use a regulator with either pump.

As stated in my other post I had the weber set up and my car worked fine with the 2.5 - 4 psi pump. No regulator. Weber set up needs more pressure than 2 psi.

My pump is higher than the fuel tank. It is mounted in the same exact location as the SU pump. If you read the instructions it doesn't matter where you mount it this pump has enough power to pump a 55 gal drum placing the pump on top of the drum.

If the fuel lines are good and no blockage than check the vent tank.

Ray
Ray 1977 MGB

CRH,

Go to your other thread as I posted a answer that should help you out.

Ray
Ray 1977 MGB

Okay folks, here is where er're at. The CORRECT Carter fule pump (P60504) is in. It runs. It charges the line with fuel. While the car was running, I pulled off the tank outlet end. It stopped "pulling" fuel. As the car began to sputter, having used the fuel in the lines, it began "pulling" again. Is this is how it is supposed to work, or should it "pull" fuel constantly? Also, before I did this I checked the flow at the fron and it was steady and strong. The filler cap does not seem to be the problem too. Please let me know. Thanks!!!!!
CRH Hoebel

Oh, and it seem to get hot too, but nots as much I think.
CRH Hoebel

CRH,

You are getting us confused. In your other thread you said you don't have a P60504 now you do. So I assume you went an bought this one.

Again to clear everything up without going back to the other thread:

This is a Carter (Federal Mogul) P60504 Gerotor,low pressure, self-priming, positive displacement pump. It's a 12v, 2.5-4.5psi, 30 GPH (Gallons Per Hour) pump. It's small, quiet and will keep the entire fuel line to the carb pressurized to alleviate any potential "vapor Lock" problems. It draws 1 amp and runs on 7 volts while it also gives over 5000 maintenance free hours of life. This is a great pump that is much cheaper than the SU's and you can't hear it with the rubber mounting.

Now CRH, once your fuel line is pressurized and full of fuel, the pump will stop working because the pressure builds up and tells the pump to stop. Once your car uses some of that fuel the pump will start pumping again.

I am glad to hear that this pump is working, but as David mentioned and I will again, if you have gunk in the tank and with a 30 year old car, assuming it was never flushed, you will have this gunk on the bottom of it. I would bet without flushing the tank it is only a matter of time before the same overheating problem happens to this pump. Please take the time and do it right. Flush the tank just to be on the safe side and if there is no gunk at least you will have a good piece of mind knowing your tank is clean.

Ray


Ray 1977mgb

Ray,
Yes, I bought the correct carter pump. I plan to drain and flush the tank as soon as I can. Your info is helpful about the power needs for the pump, but just to be clear, all I need to do is connect the existing power line (and ground, of coarse)to it, correct? Also, you say that the pump should stop pumping once pressure is established. Will it continue to "whur" though, even when the pressure is good?
Thanks again,
Chris
CRH Hoebel

Chris,

Nice to finally now your name. If I recall you only have two wires. One Ground and the other power that is turned on by the ignition switch.

Once you clean out your tank, and if it has gunk in it I would put in a new fuel filter, and pump some gas thru the pump to clean it out also. Than after you install the tank hook everything up again.

Disconnect the front fuel line to the carburetor and run a small hose toa can so that id there is any gunk in your system after cleaning it won't go into your carburetor. Once you turn on your ignition switch the pump will start working and get your system pressurized. After you see that the gas is running smoothly and looks clean hook it back up to the carburetor. Than I think you are good to go.

I am not sure what "Whur" means, or if that means it is still pumping. Don't worry about it. Take the car for a test drive and see what happens. Than come back and feel if it is overheated like before. I would assume not.

When I put my pump on I never once checked if it overheated. Long as it pumps gas and your car is working there is no need to worry. I think you are very precautious which is good, but maybe a little to much. What you want is for your car to keep running and these Carter pumps will do just that. If it is mounted right you will not even hear it at all.

Ray
Ray 1977mgb

Gotcha. As for the "whur," I'm just trying to establish whether the pump should completely shut off while while the pressure is good, or does it keep running to maintain that pressure, then start actually pumping when it senses the pressure needs to come back up again. Yeah, I suppose I am being cautious. Don't want to blow another pump! Also, an earlier question I don't recall being answered - does the inlet end of the sending unit float up a little with the fuel level, or is it stationary?
CRH Hoebel

Chris,

To be honest I have never stayed under the car to see if it shuts of completely. It should until the pump realizes that the pressure went down and than starts pumping again.

By the inlet of sending unit I think your now talking about in the tank and the float. The sending unit goes in one way at least on my mgb. The float can't remain staionary. When the gas level goes up or down the float goes up or down. This is what makes the fuel gauge readings.

You are really trying to get down to beyond basics.

Ray

Ray 1977mgb

Okay. Back to work tomorow, so there might not be any news from this end for a couple of days.
CRH Hoebel

The sending unit is the electrical unit for the fuel gauge, the fuel pickup is for the gas line. Just to be clear.
Tom

Well, I've had the car out a few times and everything seems well. Thank you to everyone who advised me! This BBS is wonderfull!
CRH Hoebel

This thread was discussed between 20/08/2005 and 27/08/2005

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