454 has major stumble/flat spot at 3/4 throttle but runs great at WOT - diagnosis?

[Hog]

I notice that you keep mentioning that you experience the flat spot, "then it clears up as you push through to WOT." In other words, you are having a part throttle flat spot that clears up once you get the throttles wide open. If you are opening the throttle(changing the TPS %), you are changing the operating parameters, thus making diagnosis more difficult.

What happens if you go to WOT immediately instead of slowly opening the throttle? Or what happens if you open the throttles to say 75% and leave them there?

Obviously the A/F ratio will need to be richer while at WOT than at part throttle. PE(Power Enrichment) mode adds extra fuel when the engine is needs extra fuel during acceleration.

Sounds like the Marine TBI systems use the high pressure regulator similar to the 1994-95 TBI BBC trucks use.

peace
PAuly

[Bonzen]

I notice that you keep mentioning that you experience the flat spot, "then it clears up as you push through to WOT." In other words, you are having a part throttle flat spot that clears up once you get the throttles wide open. If you are opening the throttle(changing the TPS %), you are changing the operating parameters, thus making diagnosis more difficult.

What happens if you go to WOT immediately instead of slowly opening the throttle? Or what happens if you open the throttles to say 75% and leave them there?

Obviously the A/F ratio will need to be richer while at WOT than at part throttle. PE(Power Enrichment) mode adds extra fuel when the engine is needs extra fuel during acceleration.

Sounds like the Marine TBI systems use the high pressure regulator similar to the 1994-95 TBI BBC trucks use.

peace
PAuly

Whether you roll into it slow or stab the gas the issue still exists. If you stab it fast it starts to run really rough for a few seconds and then can barely get back to running good for a few more seconds but eventually does. if you get to the flat spot at about 75% throttle and hold it there, it will never recover. You can drive it for a mile like that if you wanted to. You have to physically push the pedal to WOT to make it pick back up again.

Yes the regulator is the orange colored diaphragm (high pressure) 94-95 GM trucks type and it is set to about 32 psi right now, according to my Fuel pressure gauge.

I actually swapped in a lower pressure boost pump and lake tested it yesterday. Plumbed it in the fuel feed line before the high pressure pump to see if i "force fed" the stock high pressure pump it would fix the issue but unfortunately it did not. So to me that tells me I have a fuel delivery issue between the high pressure pump and the injectors.

[pmkls1]

What fuel pump is it you're running ? What type of fuel pressure regulator are you running ? Is it a stock housing modified to be adjustable or is it an aftermarket housing ? Are you positive that there isn't a kink in the fuel line anywhere ? Answer those questions and I'll see if I can't help figure it out.

[lionelhutz]

I missed seeing this earlier, but I would pinch the return line while it is happening to see if the pressure rises or not. You might as well determine if the pump is not producing the pressure or if the regulator is acting up.

[Bonzen]

What fuel pump is it you're running ? What type of fuel pressure regulator are you running ? Is it a stock housing modified to be adjustable or is it an aftermarket housing ? Are you positive that there isn't a kink in the fuel line anywhere ? Answer those questions and I'll see if I can't help figure it out.

I'm running a P60962 I believe it's a CARTER that was made for INDMAR marine. it says "962 PFI HIGH PRESSURE" on it. I just put this new pump on hoping it would fix the issue but it did not. I was running the original pump that came with the boat, which said "87 TBI 25 PSI". The flat spot issue remained exactly the same with both pumps, and both pumps show 32 PSI on the fuel pressure gauge at idle and under load (except for the flat spot area)

The FPR is the stock style that is integrated into the TBI. I have replaced diaphragms and springs and played with different pressures and it changes nothing. Yes it has been modified to be adjustable.

I had much of the fuel lines replaced recently and especially the sections where there looked to be slight possible kinks but nothing has changed by doing that.

I missed seeing this earlier, but I would pinch the return line while it is happening to see if the pressure rises or not. You might as well determine if the pump is not producing the pressure or if the regulator is acting up.

Never actually tried that...good idea. The return line comes off the TBI and goes straight into the fuel tank.

[newellshk]

Even though you have played with the diaphragms and springs i can't help but think this is a fpr problem. Have you carefully inspected the valve seat that the diaphragm closes against?

Maybe try adding a fuel accumulator. They are an OEM item on some vehicles so apparently even the OEM's believe they are needed in some situations.

[Bonzen]

Even though you have played with the diaphragms and springs i can't help but think this is a fpr problem. Have you carefully inspected the valve seat that the diaphragm closes against?

Maybe try adding a fuel accumulator. They are an OEM item on some vehicles so apparently even the OEM's believe they are needed in some situations.

I'm going to convert to an Aeromotive 13301 with the high pressure spring and install a block off/bypass plate kit in the factory TBI to defeat the stock FPR. At least that way I will know that the FPR is working and holding my pressure. And if that doesn't fix the issue, at least I'll have a nice FPR when i throw the EFI in the garbage and bolt a carburator on it lol...