Help! Bucking, hesitating under load
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Elder User
Joined: Apr 2015
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From: Boston, KY
Help! Bucking, hesitating under load
I recently replaced my distributor because the PIP sensor was bad. Now, my truck starts up great, idles smoothly, and revs like a champ when parked. However, when I try to drive it, I get hesitation, popping, and all sorts of trouble. If I floor it in first or second, I get through the roughness and get power at higher RPMs. As I shift to higher gears (more load?) the problem gets worse. I have checked the firing order (it's correct) and the timing (10*). All wire connections seem good. The cap seems a bit loose - it's a 2 part cap with adapter where the adapter clips to the distributor, and the cap clips to the adapter. Would a small amount of play in the cap make that much trouble? What else can I be checking for?
Freshman User
Joined: May 2012
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I recently replaced my distributor because the PIP sensor was bad. Now, my truck starts up great, idles smoothly, and revs like a champ when parked. However, when I try to drive it, I get hesitation, popping, and all sorts of trouble. If I floor it in first or second, I get through the roughness and get power at higher RPMs. As I shift to higher gears (more load?) the problem gets worse. I have checked the firing order (it's correct) and the timing (10*). All wire connections seem good. The cap seems a bit loose - it's a 2 part cap with adapter where the adapter clips to the distributor, and the cap clips to the adapter. Would a small amount of play in the cap make that much trouble? What else can I be checking for?
If could be not getting enough fuel. I had the same kind of problem. I installed a new fuel pump assembly
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Elder User
Joined: Apr 2015
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From: Boston, KY
I'll admit I've been working through fuel issues as well, but the change happened suddenly with the only difference being a new distributor. With the old distributor, there was no hesitation. I'm going to try duct taping the cap in place to see if that fixes it, but it seems to me that there is something else going on.
Fleet Owner
Joined: Oct 2005
Posts: 20,789
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From: Kentucky
Did you remove the SPOUT when setting the base timing?
For reference: Ford Fuel Injection » Setting the timing
For reference: Ford Fuel Injection » Setting the timing
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Elder User
Joined: Apr 2015
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From: Boston, KY
Yes, I removed the SPOUT. Also did a fuel flow test last night and it took 3.5 minutes to fill a 1 gallon jug. If my calculations are correct, that works out to be about 66 lph. Does that seem low? After closer inspection, what I thought was a frame-mounted filter was actually a small fuel pump, in addition to the one I have in my tank. I wonder if I could upgrade the pump to a higher rate/pressure. Then again, I'm still thinking this is ignition related.
Fleet Owner
Joined: Oct 2005
Posts: 20,789
Likes: 1,753
From: Kentucky
If the truck in question is the 1987 F250 then you have a low pressure pump in each tank, a dual function reservoir to switch between the two, and a frame mounted high pressure pump. Yes, 66lph is low but means nothing to me. Measure the fuel pressure at the test port on the fuel rail. Should be ~30-34 PSI with the engine running at idle/no load. Remove the vacuum hose from the Fuel Pressure Regulator, the pressure should jump to 40-45 PSI. The fuel pressure should remain in that range with the engine under load/WOT.
Many of those Dual Function Reservoirs have a filter in them.....
Many of those Dual Function Reservoirs have a filter in them.....
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Elder User
Joined: Apr 2015
Posts: 613
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From: Boston, KY
If the truck in question is the 1987 F250 then you have a low pressure pump in each tank, a dual function reservoir to switch between the two, and a frame mounted high pressure pump. Yes, 66lph is low but means nothing to me. Measure the fuel pressure at the test port on the fuel rail. Should be ~30-34 PSI with the engine running at idle/no load. Remove the vacuum hose from the Fuel Pressure Regulator, the pressure should jump to 40-45 PSI. The fuel pressure should remain in that range with the engine under load/WOT.
Many of those Dual Function Reservoirs have a filter in them.....
Many of those Dual Function Reservoirs have a filter in them.....
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Lead Driver
Joined: Feb 2002
Posts: 7,970
Likes: 595
From: Greater Austin, Texas
I suspect fuel starvation as well. 4psi at the carb in on the low side, should be about 6-7 IMO. Does your setup use the original high pressure fuel pump from EFI with a regulator to drop the pressure?
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Elder User
Joined: Apr 2015
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From: Boston, KY
I think Edelbrock recommends 5.5ish. It is an aftermarket pump and I think it's only putting out 4 PSI because my 4-9 psi regulator does not go any higher than 4 PSI even at the highest setting. My cheap fuel gauge broke the other day, otherwise I would measure at the pump.
UPDATE: After taping the distributor cap down to prevent it from wobbling I saw an improvement. Now, my truck only hesitates and backfires at very low RPMs or when in gear and decelerating with the engine, but has plenty of guts at full throttle. I would think if it were fuel starved it would be hesitating more at high RPMs/WOT. It's got me thinking the plug wires need to be replaced (there is a small cut in one of the wires from the aftermarket wire holder). That is tonight's job.
UPDATE: After taping the distributor cap down to prevent it from wobbling I saw an improvement. Now, my truck only hesitates and backfires at very low RPMs or when in gear and decelerating with the engine, but has plenty of guts at full throttle. I would think if it were fuel starved it would be hesitating more at high RPMs/WOT. It's got me thinking the plug wires need to be replaced (there is a small cut in one of the wires from the aftermarket wire holder). That is tonight's job.
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Elder User
Joined: Apr 2015
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From: Boston, KY
Sigh....well, the spark plug wires have been replaced, and the issue is still there. The wires were cracked and definitely needed to be replaced, but it wasn't the issue. I am at wits end, so here is a video showing what's going on. Maybe you guys will see something I'm missing.
Laughing Gas
Joined: Aug 2002
Posts: 1,069
Likes: 19
From: Central Wisconsin
I don't see any need for that 'high pressure' fuel pump or regulator. The Edelbrock should run fine 4 1/2-5 1/2 psi of pressure. The only time I've used a regulator was on vintage Stromberg 97 carbs on vintage hot rods or oval track engines.
I think the dizzy would prefer having the vacuum advance running at true manifold vacuum rather then drawing the vacuum from above the throttle plates as it seems you have it connected… others will chime in.
I'd scrap the in tank pumps, install a cheap, simple diaphragm fuel pump and call it good.
I think the dizzy would prefer having the vacuum advance running at true manifold vacuum rather then drawing the vacuum from above the throttle plates as it seems you have it connected… others will chime in.
I'd scrap the in tank pumps, install a cheap, simple diaphragm fuel pump and call it good.
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Elder User
Joined: Apr 2015
Posts: 613
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From: Boston, KY
I don't see any need for that 'high pressure' fuel pump or regulator. The Edelbrock should run fine 4 1/2-5 1/2 psi of pressure. The only time I've used a regulator was on vintage Stromberg 97 carbs on vintage hot rods or oval track engines.
I think the dizzy would prefer having the vacuum advance running at true manifold vacuum rather then drawing the vacuum from above the throttle plates as it seems you have it connected… others will chime in.
I'd scrap the in tank pumps, install a cheap, simple diaphragm fuel pump and call it good.
I think the dizzy would prefer having the vacuum advance running at true manifold vacuum rather then drawing the vacuum from above the throttle plates as it seems you have it connected… others will chime in.
I'd scrap the in tank pumps, install a cheap, simple diaphragm fuel pump and call it good.
Laughing Gas
Joined: Aug 2002
Posts: 1,069
Likes: 19
From: Central Wisconsin
I have seen a Mustang that was converted from EFI to carb. The owner had kept the original in tank pump then added an external inline pump much like yours. The car had several driveability problems one of which was cured by removing the aftermarket pump and just using the in tank pump. I know this sounds opposite of what I just told you but I was really trying to say you only need one pump.
Try it with just the in tank pump and see how it runs. Or you MAY be able to simply pull a fuse to stop the in-tank pump from running then just use the pump you installed. BTW those pumps PUSH much better than they pull… the pump you installed needs to be within a foot or so of the fuel tank.
I've run several cars with just those simple el cheapo pumps with good results in stock applications.
Try it with just the in tank pump and see how it runs. Or you MAY be able to simply pull a fuse to stop the in-tank pump from running then just use the pump you installed. BTW those pumps PUSH much better than they pull… the pump you installed needs to be within a foot or so of the fuel tank.
I've run several cars with just those simple el cheapo pumps with good results in stock applications.
Thread Starter
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Elder User
Joined: Apr 2015
Posts: 613
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From: Boston, KY
I have seen a Mustang that was converted from EFI to carb. The owner had kept the original in tank pump then added an external inline pump much like yours. The car had several driveability problems one of which was cured by removing the aftermarket pump and just using the in tank pump. I know this sounds opposite of what I just told you but I was really trying to say you only need one pump.
Try it with just the in tank pump and see how it runs. Or you MAY be able to simply pull a fuse to stop the in-tank pump from running then just use the pump you installed. BTW those pumps PUSH much better than they pull… the pump you installed needs to be within a foot or so of the fuel tank.
I've run several cars with just those simple el cheapo pumps with good results in stock applications.
Try it with just the in tank pump and see how it runs. Or you MAY be able to simply pull a fuse to stop the in-tank pump from running then just use the pump you installed. BTW those pumps PUSH much better than they pull… the pump you installed needs to be within a foot or so of the fuel tank.
I've run several cars with just those simple el cheapo pumps with good results in stock applications.
I tried disconnecting the aftermarket pump and realized that the in-tank pump is not pumping. If I am reading correctly, an in-tank pump that is not turned on will allow fuel to freely pass through it - right? If that's the case, do you think a higher flow/pressure aftermarket would fix the problem? Is it obviously an issue of being under-fueled?
And by the way, thanks to everybody who has given me input at this point. The help on this forum is worth more than any Haynes manual could ever provide!
Lead Driver
Joined: Feb 2002
Posts: 7,970
Likes: 595
From: Greater Austin, Texas
I think the dizzy would prefer having the vacuum advance running at true manifold vacuum rather then drawing the vacuum from above the throttle plates as it seems you have it connected… others will chime in.