Sputtering/loss of power at hard acceleration

[Blackascoal]

1994 IDIT, 152,000 miles. Auto. Engine all stock. Truck starts and runs perfect, idles great and drives perfect until I floor it. When it reaches a higher RPM it sputters and makes a sound like an exhaust leak snd sputters, hesitates and has reduced power. I have to let up on the accelerator coast a little bit and then can continue driving. It returns to normal after about 20 seconds. Does this on both tanks. I always add a fuel additive when I fuel up. Have changed the fuel filter and fuel pump (factory mechanical). Injection pump and injectors replaced 22K ago by previous owner at the dealer. Any ideas? Thanks for your help.

 

[Big Bart]

So we need more info to help you.

1) Is this a new problem (Did not do this before.) or are you just now noticing?
2) Did you check with the old owner to see if he had this issue?
3) What is the top RPM before the problem starts?
4) At what RPM does it seem to recover?
5) Did this start after the lift pump or fuel filter was changed. (One or both could be restricting the flow.)
6) Was this formally owned by a company or gov entity. (Sometimes these IP's are governed down to prevent speeding or over reving.)

Let us know, then we can do more to suggest next steps.

 

[Blackascoal]

So we need more info to help you.

1) Is this a new problem (Did not do this before.) or are you just now noticing? This is a new problem, didn't do it before.
2) Did you check with the old owner to see if he had this issue? I've had the truck over 4 years, don't have any contact with previous owner.
3) What is the top RPM before the problem starts?
4) At what RPM does it seem to recover?
5) Did this start after the lift pump or fuel filter was changed. (One or both could be restricting the flow.) I changed both of them to try and solve the problem. First filter, nothing changed and then pump, nothing changed.
6) Was this formally owned by a company or gov entity. (Sometimes these IP's are governed down to prevent speeding or over reving.) No it was always privately owned. I am the third owner. CA truck always.

Let us know, then we can do more to suggest next steps.

Thanks for the reply. The answers to the ones I know are bolded and highlighted. I'll have to get back to you on the RPM'S. I'll try and take a video. A couple more points:
1. The truck is warmed up to normal operating temp.
2. I do not know the fuel pressure, but thought about installing a guage. On the fence with going with one that goes in the cab or one just in the engine compartment. Not sure if knowing idle fuel pressure would help me diagnose this problem, since it runs fine at idle. But am curious what it is when it sputters.

Could this be air intrusion that only kicks in when the engine is a max vacuum (hard acceleration)?

 

[bbjordan]

Sounds like fuel starvation. It might be something partially plugging the fuel line. You might try rigging up a temporary jug of fuel to the inlet side of the lift pump to eliminate anything back of the lift pump. It won't take you very far because the return line fuel will go back to whichever tank you were on.
It'll also give you a chance to inspect/replace the little hose that goes between the lift pump and the hard line. It is a common problem, as it has quite a bend in it.

 

[Big Bart]

Next I would look for a air leak between the lift pump and the fuel selector valve. You said it happens on either tank so it would not be tank specific.

1) The tank selector valves at this vintage are very suspect. Maybe the valve is good and perhaps the hoses to and from have a small air leak somewhere.

2) Often these valves get stuck in switching between front and back tanks and cause fuel starvation. (If partially closed enough to feed idle but not wide open. Or prevent the truck from starting.) So to you it would appear as the tank valve selected because the truck will still idle. However you would not actually know if it is stuck without pulling and testing the lines.

3) Check the fuel lines and o-rings by the tank selector valve, often you see oil around a leaking line. But a bad o-ring may just allow air in during a vacuum from the lift pump.

I am not aware of air intrusion on the injector return lines causing this issue. So likely you either have an issue from tank to lift pump. Or an issue of a blockage before the IP (Filter housing, line, or front of IP), or perhaps your IP is going out. Even thought the IP was rebuilt in the 22k miles, it does not insure it will not fail. If not done at a reputable shop, it very well could fail early.

 

[Big Bart]

One last thing, make sure your IP solenoid is getting a full 12v and making a solid click when you turn the key on. Sometimes the solenoid goes bad and does not open all the way. (Starvation) Or perhaps starts to fail when it warms up.

 

[Blackascoal]

Sounds like fuel starvation. It might be something partially plugging the fuel line. You might try rigging up a temporary jug of fuel to the inlet side of the lift pump to eliminate anything back of the lift pump. It won't take you very far because the return line fuel will go back to whichever tank you were on.
It'll also give you a chance to inspect/replace the little hose that goes between the lift pump and the hard line. It is a common problem, as it has quite a bend in it.

Yeah, thats what it feels like when it happens. I did replace the hoses you are talking about when I replaced the lift pump. Thank you.

 

[Blackascoal]

Next I would look for a air leak between the lift pump and the fuel selector valve. You said it happens on either tank so it would not be tank specific.

1) The tank selector valves at this vintage are very suspect. Maybe the valve is good and perhaps the hoses to and from have a small air leak somewhere.

2) Often these valves get stuck in switching between front and back tanks and cause fuel starvation. (If partially closed enough to feed idle but not wide open. Or prevent the truck from starting.) So to you it would appear as the tank valve selected because the truck will still idle. However you would not actually know if it is stuck without pulling and testing the lines.

3) Check the fuel lines and o-rings by the tank selector valve, often you see oil around a leaking line. But a bad o-ring may just allow air in during a vacuum from the lift pump.

I am not aware of air intrusion on the injector return lines causing this issue. So likely you either have an issue from tank to lift pump. Or an issue of a blockage before the IP (Filter housing, line, or front of IP), or perhaps your IP is going out. Even thought the IP was rebuilt in the 22k miles, it does not insure it will not fail. If not done at a reputable shop, it very well could fail early.

You hit on two of my next target areas: The tank selector and the IP going bad, I know that happens. I have the receipt and it say its was "sent out for rebuild." Nothing else specific.
I going to check that selector valve over the weekend. I'm going to see if I can bypass the selector valve temporally and see how it runs. Is that doable? I've never been under that to look at it.
Thank you.

 

[Blackascoal]

One last thing, make sure your IP solenoid is getting a full 12v and making a solid click when you turn the key on. Sometimes the solenoid goes bad and does not open all the way. (Starvation) Or perhaps starts to fail when it warms up.

Got it.

 

[Blackascoal]

Speak of the devil. I found this on You Tube. Sounds like this guys truck was doing the same thing as mine. He goes through the selector valve and also does the external temp gas jug. I might try that.

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[Big Bart]

I was going to say an external gas tank to the lift pump would be easiest based on the factory connectors on the transfer switch.

 

[IDIBRONCO]

If you want to drive/run the truck for longer using a separate fuel can, then run a hose from the engine return line back into the can.