EFI diagnosis/ engine stumbles at 1/2 tank .

[mbaird]

I have a 93 Mazda B2600i that stumbles when gas tank gets to 1/2 .
I have installed 2 new fuel pumps and socks , new fuel filter , checked fuel pressure after filter , loosened gas cap , checked for leaks etc .
Running time isn’t a factor. If I top off the tank it won’t do it again until the tank gets to 1/2-3/4 . This is a pre OBD system .
Possibly charcoal canister ?

What am I missing? Factory manual is no help .

 

[TT5.9mag]

Is the tank baffled? And where does the return fuel go back in to the tank?

 

[mbaird]

Is the tank baffled? And where does the return fuel go back in to the tank?

Not sure if the tank is baffled.
Return line is Siamesed to the pickup tube .

 

[TT5.9mag]

Not sure if the tank is baffled.
Return line is Siamesed to the pickup tube .

The whole situation seems odd to me and the only thing I can think of is you’re aerating the fuel for some reason and it’s affecting the pump at the 1/2 tank level. Then you said it happens at 3/4 full also so hell IDK.

 

[TT5.9mag]

Most modern in tank pumps have a bucket that the pump sits in and the return dumps right back in to the bucket. That keeps the air out of the fuel. The pump and pickup for that truck looks like an old school carbureted pick up with a pump on the end. Early adaptation to efi.

 

[TT5.9mag]

Are you sure the stumble is fuel related?
Reason for asking is the ecu might trigger an evap solenoid at a specific fuel level and that system may have an issue resulting in a vacuum leak or something like that.

 

[mbaird]

If this helps

 

[mbaird]

Most modern in tank pumps have a bucket that the pump sits in and the return dumps right back in to the bucket. That keeps the air out of the fuel. The pump and pickup for that truck looks like an old school carbureted pick up with a pump on the end. Early adaptation to efi.
View attachment 1716482056

That’s my assembly .

 

[mbaird]

Are you sure the stumble is fuel related?
Reason for asking is the ecu might trigger an evap solenoid at a specific fuel level and that system may have an issue resulting in a vacuum leak or something like that.

Could be …. All I know is if add fuel it runs fine until tank level gets down to 1/2-3/4 tank .
I replaced the pump again last week and drove it for a few days . It happened again but this time it did it at 3/4 tank instead of the usual 1/2 tank .

 

[TT5.9mag]

Any CEL?

 

[mbaird]

Any CEL?

Nope . But Im not sure the CEL is functional.

 

[mbaird]

You may be on to something with the signal to ECU . I noticed the fuel gauge didn’t go all the way to full last time I topped it off . Possible I bent the float arm installing fuel pump last week and now ecu thinks fuel is lower than it really is . Which would explain why it happened at 3/4 instead of 1/2 tank last time .

 

[dano]

Does it sit outside and/or for a long period of time? Did it do it over the summer? My only thought is condensation building in the tank.

 

[mbaird]

there is no line going from this tank to the intake manifold . Intake port is capped off . Possible issue …

 

[MadScientistMat]

Can you get a fuel pressure gauge on the motor and see if it drops when you have the problem? I doubt a truck that old would have the ECU monitoring the fuel level, but I could see a venting issue going on.

 

[mbaird]

Not in the cab while driving.
Its bizarre.

 

[jos51700]

Single stumble or persistent?
Steady throttle or on acceleration?
Under 1/2 tank, it stops stumbling?

 

[MadScientistMat]

Not in the cab while driving.
Its bizarre.

You could always have one held under a windshield wiper - found a fuel pump issue that way before.

 

[mbaird]

I have spare gauge. I will install it .
I took it hunting this weekend and again it started to cut out .
This time around 4 gals . I brought 2 5 gal gas cans just in case . After topping it off it drove fine for a few miles then acted up again . This time I was climbing a steep mountain. Maybe it’s load related? Coil ?
ECU ? Still Fuel related ?
This truck is supposed to have a CEL but I have never had it light up . I will try forcing codes .

 

[mbaird]

Single stumble or persistent?
Steady throttle or on acceleration?
Under 1/2 tank, it stops stumbling?

Runs like a champ then starts to cut out intermittently . Topping of the tank always cured it . It would always take about 8 gals regardless of drive time . Which is what led me to think it was fuel delivery related. Now it’s happening earlier.
I can’t visualize the evap system operation clearly enough and the FSM is not much help .

 

[matthon]

Is the small section of hose in the tank rated to be submerged in modern fuel?

There's a difference between efi hose and submersible hose.

If not hose will swell, lose the seal on the hard lines, fuel pressure dives, engine cuts out.

Happened to me, just a thought.

 

[mbaird]

Is the small section of hose in the tank rated to be submerged in modern fuel?

There's a difference between efi hose and submersible hose.

If not hose will swell, lose the seal on the hard lines, fuel pressure dives, engine cuts out.

Happened to me, just a thought.

Hmmm… did not know that . But the short section of hose is supplied with the pumps so I would assume so .

 

[mbaird]

This will turn out to be something simple that I overlooked due to my prejudice of assuming it’s fuel delivery related. That’s why these discussions are so helpful.

 

[matthon]

But the short section of hose is supplied with the pumps so I would assume so .

Same scenario here.
They replaced my pump twice.

I only discovered it when I swapped it the second time.

Drained a near full tank and immediately dropped it and pulled the in tank pump.

The short hose was like mush, no sealing it, no matter how tight the clamps.

Replaced with Cooper Standard PA12, used in 2 vehicles to date, didn't even need clamps, but iirc I put them on anyway.

 

[jbc426]

Runs like a champ then starts to cut out intermittently . Topping of the tank always cured it . It would always take about 8 gals regardless of drive time . Which is what led me to think it was fuel delivery related. Now it’s happening earlier.
I can’t visualize the evap system operation clearly enough and the FSM is not much help .

Does this happen even if the car is being driven smoothly on level road with no motion that would cause fuel slosh? If so and it's happening earlier, it's not likely a cavitation issue. You should pull the sending unit and pump and check the lines for leaks with air pressure and soapy water.

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On a related note, most of the so called baffled tanks they sell are not very effective at feeding air free fuel when below 1/3 to 1/4 tank. Using a surge tank system with a stock tank is vastly more effective when driving road course type roads, as any air sucked in from the tank is constantly purged from the surge tank prior to being fed to the engine.