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  1. #1

    Default Mechanical Fuel Pump low pressure

    Hi All,

    I searched thru this forum before posting to see if I could find any other situations similar to this one since I wasn't looking to waste anyone's time. I appreciate any insight or help anyone can provide...

    I have an 85 GT that I have owned since 89. It has gone thru many upgrades and maintenance and is in great condition since it has been garaged it entire life since I've owned it. Here is a quick low down of the mods:

    The engine has GT40X heads, E303, Performer RPM, 600 DP, Performance Dist, Ford Racing headers, 2.5" stainless exhaust with Dynomax VT's. The car is a 5 speed ad well with 3.55's. Holley mechanical fuel pump and under-drive pulleys. The AC has been upgraded to R134 as well and has the a throttle solenoid for idle... And a wide-band with gage.

    The issue that I am having is, during a trip with the wife the other day on our way to the Mt. Washington Hotel in NH, we were going up a steep hill, the notch, and the car was bogging and running out of gas. As soon as I let off a little, I was able to keep going. Unfortunately, the car was now running super lean (15-16) and not wanting to idle. Great way to arrive at the hotel for lunch. After letting the car sit for a couple hours, I turned out the idle screw to richen up the mixture. The car still didn't want to idle but a few miles down the road, the cruise was back to normal and the wide-band was reading in the 12's. We were able to make it back to Maine with no problem.

    We did fill up with fuel at the beginning of the trip and was wondering if there may have been water or something else in the fuel. The car does sit most of its life these days so who knows what kind of contaminates could be in the tank.

    The next day, I pulled the carb and went thru the whole thing an cleaned it out with some carb clean and compressed air. Oddly, nothing was discovered other that the carb studs maybe not torqued as much as they should be. I ran the car and noticed that the fuel pressure was at around 7 psi until it warms up. Then it goes down to 2.5-3 psi. The gage spikes once in a while. Certainly, this low pressure will not keep up with filling the carb bowls during hard acceleration.

    Has anyone seen this type of thing happen with the pressure? I am wondering if the rubber hose in the fuel system may be collapsing or the fuel sock is clogging? or the pump may be defective? I may try to test the fuel pump after warming up the engine by using a gas can as the supply and see if the pressure goes back to normal. Anything I may have missed? I also may replace the rubber hoses and drop the tank to clean it out and check the pick up sock.

    Thank you in advance for any in-sight or recommendations you can make. Cheers

  2. #2

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    Which 600 DP are we talking about? A 4776? Any changes to it?

    Is that mechanical fuel pump "dead heading"? One line from the tank into the pump, and one line out to the carburetor? Incorporating a small return line back to the gas tank, at/near the carburetor with a small restriction in it, can be a help fighting heat and corn liquor issues...

    The "bogging and running out of gas" sensation could have had to do with your carburetor studs not being snug, and/or could also be running too lean or rich on the primary main circuit (float level, jets, power valve channel restrictions). A vacuum leak in general with a carburetor will show up more-so at idle as a sensation of running out of fuel. At what speed, rpm, which gear (5th?), was this happening?... assuming you got into the throttle some more for the hill, the jetting or the float level up front being/getting low (or a too late opening power valve) could be the issue...

    Were you seeing 15-16 AFR (air:fuel ratio) when you let up a little while cruising up that hill, or later at idle? BTW, a cruise AFR of 15-16 is not "super lean" unless the vehicle is slightly surging or "trailer hitching", at cruise as well as with light part throttle acceleration that is still within and utilizing the primary idle/transition circuit (where the great majority of driving is done)... 16+ is possible and right where you want to be with a vehicle with a carburetor... AFR readings in the 12's for "cruise" is not normal, and is matter of fact quite a waste of fuel... mid-12's is for wide open throttle, maximum load and on up to maximum rpm with the main circuits, when the boosters are administering the fuel (jets and power valve channel restrictions)...

    The behavior and observed fuel pressure variations sounds like vapor-lock in the fuel line from pump to carburetor, and/or that and/or percolation in the float bowls and idle/main wells in the metering block. Unless a fuel pump is dead, there's not much affect on carburetor function... 4-5psi is better than 7, and will easily keep the float bowls full at wide open throttle with 0.110 needles and seats, but will especially not create any functional problems at idle or off-idle or part throttle or cruise.

    Ethanol, corn liquor's lower boiling point included in most fuel nowadays can cause havoc where it's allowed to get heated up too much, even with EFI lines or fuel rails etc... and so, see that the fuel lines are not near or touching sources of heat, or are insulated from getting heated up... probably especially the fuel line from the pump to the carburetor. It'd be good if you are able to fit one of those ~1/4 insulating carburetor base gaskets in there beneath the carburetor, or a thin phenolic spacer and gaskets, to combat too much heat getting to the fuel inside the carburetor and float bowls.
    Mike
    1986 Mustang convertible ---> BUILD THREAD
    Past Fox-chassis "four eyes":
    1983 Mercury Cougar LS
    1986 Ford Thunderbird ELAN
    1980 Capri RS Turbo

    Work in progress website ---> http://carb-rebuilds-plus.boards.net/

  3. #3

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    Hi Mike,

    Thank you for the reply. Obviously there are a lot a things to consider an I appreciate the help.

    The carb is a Holley 600 DP. It sits on a 3/4" phenolic spacer with 4 holes. I'm not sure if the fuel is boiling in the carb but it has been driven in 95 Deg heat with no issues like I have described earlier but I suppose it could be possible. The line from the pump is a braided line with 6 an fittings that runs in the stock location and its possible that it gets hot but has been in that location since 2000 and have had no issues. But the fuel is loaded with alcohol now unlike back then. The jets are 68's for the primary and 68 or 9 for the secondary. The power valve is a 9.5 and idle vacuum is at 13.5-14. Squirters, I will have to check my log when I get back home. I noticed at cruise, when I hit he gas a little its in the 12's but goes closer to 13-14 when cruising. The lean readings I saw were out of the ordinary, that's why I was alarmed. I was opening the secondaries while going up the hill I had mentioned. I had shifted from 5th to 4th at about 60-65 mph. That's when it was running out of fuel. It seemed like the supply was cut down but I cant be sure.

    Should I get some kind of heat shield wrap for the pump to carb line?

    What do you think about testing the pump after warming up the motor and seeing if it improves?

    Have you heard of the pickup sock getting clogged restricting the flow?

    Could the internal pump regulator be failing?

    Thanks again for your help

  4. #4
    FEP Senior Member BMW Rider's Avatar
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    I would run a vaccum test on the pump. Mechanical pumps for carbureted engines don't need to make a lot of pressure to supply adequate volume, but they must be able to draw good vaccum to pull the fuel from the tank all the way to the engine.

  5. #5
    FEP Super Member xctasy's Avatar
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    Mandatory service check of the fuel tank, its lines and the presence of water. I dropped mine on a soccer ball, and used the Ford Explorer gas tank line service tools to ensure the pickup points and fuel tank sensor weren't toast like most old tanks are.

















    The Mustang gas tank without an electric fuel pump is able to scavenge down to the last ounce of gas, but its a 35 year old car, so check the lines. Like Fords wires, they were on the low end of quality at birth.

  6. #6

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    First, the reason I asked which 600 double pumper is because I look at Holley's "Numerical Listing" specs when anything is in doubt, and for ANY 600 double pumper with 1-5/16" venturi & 1-9/16" secondary throttle bores (or vacuum secondary - they both have the same bores/venturi sizes), #68 or #69 secondary main jets out back are very much not big enough. The base LIST-4776 600 double pumper came with #69 primary jets and #71 secondary jets (those being so close in size, the base carburetor likely came with primary and secondary power valves, with their main circuit fuel augmentation through power valve channel restrictions). So unless you've indeed got a base 4776 with primary and secondary power valves... Later dash numbered (-1 & -2) LIST-4776 carburetors (no doubt without secondary power valve) came with #66 primary jets and #76 secondary jets, and later yet dash numbered (-3 to -7) LIST-4776 carburetors came with #66 primary jets and #73 secondary jets. I would suggest that at least #73 secondary jetting is required for any 1-5/16" venturi & 1-9/16" secondary throttle bores, that are not augmented with a power valve circuit, so as to not go lean like you saw when you also got into the secondaries going up that hill. The 9.5 power valve is probably fine... don't base your choice of power valve on idle vacuum, but on greater part throttle necessity over and above what the primary main jets provide at lesser part throttle amounts.

    Pickup filter socks get jammed up with 30+ years of gunk all the time. I'd remove the tank line from the pump and blow air back it and get somebody to listen for air/bubbles back in the tank with the gas cap off... doing that will also at least temporarily clear away any collected crap from the pickup if that was/is an issue... which can totally affect pump performance... as well, verify that the tank is vented... no venting equals a pump working it's guts out creating a vacuum in the fuel tank and a pretty quick eventual death for the pump. Running with the fill cap off temporarily, and if no issues then, would point to a blocked vent...

    Thanks for the further info. I guess unless the engine and under hood is getting way too hot, you shouldn't be having any vapor lock or percolation issues... popping out a sight plug like when you'd check/set the float levels from the float bowl when it's acting up will tell the tale if the fuel is boiling/bubbling up... or running low in the float bowl/s from a low pump volume, pickup/line blockage, tank non-venting issue...
    Last edited by Walking-Tall; 10-24-2019 at 03:29 PM.
    Mike
    1986 Mustang convertible ---> BUILD THREAD
    Past Fox-chassis "four eyes":
    1983 Mercury Cougar LS
    1986 Ford Thunderbird ELAN
    1980 Capri RS Turbo

    Work in progress website ---> http://carb-rebuilds-plus.boards.net/

  7. #7

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    Thank you all for your help. I appreciate the support.

    I tested the pump last night as follows:

    Started the car and the choke is on with the fast idle at about 1300 RPM. Let the car warm up for 5 minutes then rev'd the engine to disengage the choke. The pump pressure started out at 4 psi during fast idle then gradually rises to 7 psi after 5 minutes. The pressure maintains until the engine gets to running temp, which is about 1/4 of the gage (stock gage). Then the pressure starts to drop gradually while the engine is idling. It finally drops to 2.5-3 psi. I shut off the engine and disconnected the supply hose from the pump(Holley Mechanical). Then put the hose inside of a 2 gallon gas can for the supply. Then started the engine. The same pressure was observed even reving the engine and holding at 1300 rpm. I would say that it appears there is no restriction in the line from the tank. Seems to be that the pump is failing after warm up.

    I am considering changing back to a stock pump and using the return line. Should this be adequate for the operation of the 600 DP? How much pressure will it build?

    Thanks

  8. #8

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    Quote Originally Posted by Steve View Post
    I would say that it appears there is no restriction in the line from the tank. Seems to be that the pump is failing after warm up.

    I am considering changing back to a stock pump and using the return line. Should this be adequate for the operation of the 600 DP? How much pressure will it build?

    Thanks
    The test you conducted is good, but doesn't answer the tank sock/lines/venting questions, gauging the situation with an ailing pump... so while a replacement pump should be fine, work better, I'd still get solid answers to the questions remaining, so that another pump isn't potentially also killed after a time. You'd have to measure the pressure a stock replacement pump will generate, because they likely vary all over the place.
    Last edited by Walking-Tall; 10-25-2019 at 03:19 PM.
    Mike
    1986 Mustang convertible ---> BUILD THREAD
    Past Fox-chassis "four eyes":
    1983 Mercury Cougar LS
    1986 Ford Thunderbird ELAN
    1980 Capri RS Turbo

    Work in progress website ---> http://carb-rebuilds-plus.boards.net/

  9. #9
    FEP Senior Member liv2roc's Avatar
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    a few years ago I had the put a spacer under my carb because it would boil the fuel out making it hard to start when hot.

  10. #10

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    I was able to do a few checks and replacements over the weekend.

    I checked the evap system and it appears to be working. I also replaced the fuel cap and checked the charcoal canister. I removed the Holley mechanical pump and went back to the stock one with a return line and covered the fuel supply line with a piece of cool-it sleeve. The car seems to run great with the stock pump. The only thing that is weird is the pressure on the gage fluctuates back and forth from approx. 4-5 to 8 or 9 but it is a stock replacement so I would imagine it is working as designed. Does anyone have any experience with these stock pumps? The carb doesn't seem to run out of fuel now when going up steep hills. Hope this is a fix for now.

  11. #11

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    Quote Originally Posted by Steve View Post
    I was able to do a few checks and replacements over the weekend.

    I checked the evap system and it appears to be working. I also replaced the fuel cap and checked the charcoal canister. I removed the Holley mechanical pump and went back to the stock one with a return line and covered the fuel supply line with a piece of cool-it sleeve. The car seems to run great with the stock pump. The only thing that is weird is the pressure on the gage fluctuates back and forth from approx. 4-5 to 8 or 9 but it is a stock replacement so I would imagine it is working as designed. Does anyone have any experience with these stock pumps? The carb doesn't seem to run out of fuel now when going up steep hills. Hope this is a fix for now.
    I feel your pain.

    I've been through half a dozen stock fuel pumps in the past few weeks trying to get one with the correct pressure. Most of them put out in excess of 12psi. The Carter pump that many on here recommend on this forum put out 14 psi at idle! These all overpowered my needle & seat and made for big headaches. The 5th one was ordered for an earlier mustang without the return provision and puts out a solid 9 psi. That's the lowest pressure I've seen so far. Still too much.

    I've got my eye on the Scott Drake one that supposibly puts out 6.5 psi max - https://www.summitracing.com/parts/sdk-c5az-9350-b Not correct for the 4-eye return system, but at this point, I'll try anything that prevents me from having to plumb up a regulator or run an electric pump.

    Quality seems to be lacking across the board on everything the parts houses sell. Summit sells an Edelbrock pump that supposedly puts out 5 psi max. It's also $150.00! Good luck finding a good one that puts out the correct pressure.

    Please let me know if you get a good one. I'm still looking.
    84.5 GT Convertible Build Thread
    86 LX Coupe

  12. #12

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    I was using a Holley 12-833. When working properly, it held at 7 psi without a regulator or return line. Too bad it is failing and I thought that I could go to a stock one not to have any issues. I guess its not so easy.

  13. #13

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    Quote Originally Posted by Steve View Post
    I was using a Holley 12-833. When working properly, it held at 7 psi without a regulator or return line. Too bad it is failing and I thought that I could go to a stock one not to have any issues. I guess its not so easy.
    At least that's in the ballpark of what the carb can handle! How long did that Holley pump last? Summit only has a 90 day warranty on them. That's not encouraging.

  14. #14

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    Quote Originally Posted by graphicdesigner80 View Post
    At least that's in the ballpark of what the carb can handle! How long did that Holley pump last? Summit only has a 90 day warranty on them. That's not encouraging.
    I put about 5000 miles on it before it started to fail.

  15. #15
    FEP Power Member Jerry peachuer's Avatar
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    Quote Originally Posted by Steve View Post
    I was able to do a few checks and replacements over the weekend.

    I checked the evap system and it appears to be working. I also replaced the fuel cap and checked the charcoal canister. I removed the Holley mechanical pump and went back to the stock one with a return line and covered the fuel supply line with a piece of cool-it sleeve. The car seems to run great with the stock pump. The only thing that is weird is the pressure on the gage fluctuates back and forth from approx. 4-5 to 8 or 9 but it is a stock replacement so I would imagine it is working as designed. Does anyone have any experience with these stock pumps? The carb doesn't seem to run out of fuel now when going up steep hills. Hope this is a fix for now.
    It will pulse because it's a dowel pin that spins and hits the lever every revolution (mechanical)

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