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

[Bonzen]

Long time lurker but here is my first post...I have a jet boat with a completely rebuilt GEN V BBC TBI motor controlled by a MEFI 1 ECM and it runs great except at 3/4 throttle when it seriously looses power at the same point every time like clockwork...engine RPM drops, sounds very rough, runs like A$$ and completely scares the hell out of you....but if I push the gas pedal "through" it to WOT it instantly picks up again and pulls hard all the way to a top speed of about 55-60 GPS. The dead spot exists only at around 3/4 throttle, or in the 80 kPa region and about 4000-ish rpm but then it's fine after that and builds quite a bit more power. Id like to figure this out so I can have a smooth powerband all the way. I have actually gone out on the lake twice and logged data & thrown about 10 tunes at it with my tuner who has a vast understanding of the MEFI software using the MEFIBURN software from www.http://mefiburn.com/ (http://www.http//mefiburn.com/) but to no avail. My tuner and I are both stumped on this issue and he cannot find anything out of place so far on all the program settings and ECM tables. It all "looks" correct or in other words nothing sticks out as a red flag to create the issue that the engine is experiencing. To an extent, we are somewhat tuning blind as I do not have a wideband 02 sensor installed so we have no idea of the A/F ratio however I do know hat the motor runs very rich and the spark plugs confirm that. Some motor information - 1994 454 bored .030 over using 10:1 compression SRP pistons. 91 octane used only with octane booster. Whole rotating assy was professionally balanced. Stock small port iron heads with all new valve train. 3/8" cromoly pushrods. Comp Cams 226/236 @ .050 112 LSA. Factory GM aluminum dual plane intake manifold. Engine shows 160 psi in all cylinders on compression test. The TBI is a brand new unit from SPR performance - www.tbiparts.com (http://www.tbiparts.com/). No vacuum leaks. Brand new TPS and IAC were shipped with the TB and both have been calibrated correctly and verified by my tuner and I to be working as they should. The TBI is a 750cfm custom setup with adjustable FPR and 80# injectors. Fuel pressure is a steady 28psi, at idle and under load. Every engine sensor and the entire ignition system has been replaced at this point(Ignition coil, pickup coil, ignition control module, cap, rotor, wires, plugs) Spark plugs are not lean looking at all and gapped to .043. The timing has been set and checked and matches the timing value in the ECM (8 degrees BTDC locked out) Fuel pump is working strong, (high pressure pump) All fuel filters are new, water fuel separator is new. Gas tanks are venting properly. Basically the motor is brand new inside and out.

The ECM had been originally calibrated by Arizona Speed and Marine for the new motor/camshaft specs, after I verified that the newly rebuilt motor ran mechanically good with the "stock" tune(or whatever it was) last season. However, I had never set the timing correctly (locked out) while running with the "stock" tune so I always assumed that the ECM needed to be calibrated for the motor to run correctly. Once I learned about setting the ECM in service mode, I locked it out and timed it properly. The motor did gain some RPM and run faster but then almost immediately the "flat spot" started to appear at 3/4 throttle. So I do not know if this is an ECM issue that AZM screwed up or a hard part issue with my engine setup...or something ignition related maybe? A little over a month ago I sent the ECM into OBD Diagnostics and had the designer of the MEFIBURN software look at it and he didn't notice anything out of the ordinary as well. He then referred me to my current local tuner for further help. I am going to get a wideband setup soon but until then I am looking for an out of the box hypothesis as to what could be causing such a bad, but on the money every time dead zone in the powerband in the exact same spot. The one weird thing I will say, is that the tach on the dash reads 3,400 RPM while the tuning software is reading 4,200-ish on the laptop when the problem occurs. Every time at that RPM give or take 100. I will also note that this problem did not exist at all with the old motor which was your standard run of the mill marinepower 330hp 454 TBI. The issue also did not exist with the newly rebuilt motor until I had the ECM recalibrated by Arizona Speed and Marine and bolted on the new bigger bore TBI which were done at the same time. This issue has been haunting me for over a year, I am out of ideas and almost out of $$ haha. Thanks guys for any and all help, as to what could be causing this really weird issue. I've never heard of a motor running great to a certain point, then running like crap, and then running great again at WOT.

[Bonzen]

Some pics of the motor, it is 100% a car/truck motor with a marine cooling/exhaust system.

[roughneck427]

I have mefi burn and im tuning on my boat also. You definatly need to put a scanner on it and get some data logs. You also need a wideband 02 sensor to properly tune it.

[Bonzen]

I have mefi burn and im tuning on my boat also. You definatly need to put a scanner on it and get some data logs. You also need a wideband 02 sensor to properly tune it.

We have scanned it and got data logs. Tweaked a few things that maybe seemed a little off, but nothing is really standing out. No holes in timing/spark or the fuel mapping that we notice. It's just so weird that it works at WOT but not before that. Wideband is coming within the month. I was looking for some other out of the box issue that could be related to a hard part or some inherent engine design issue that someone may be aware of. Sometimes the most frustrating issues are the easiest to fix if you know where to look. I have to imagine someone has had this problem before and fixed it...I'm out of ideas and places to look!

[Six_Shooter]

<snip> however I do know hat the motor runs very rich and the spark plugs confirm that. </snip>

I'd start here. No need to run an engine fatter than it needs and that can definitely cause flat spots and stumbles.

[Bonzen]

I'd start here. No need to run an engine fatter than it needs and that can definitely cause flat spots and stumbles.

I've had that theory as well, that maybe at that point in the 80kPa range it's just flooding the cylinders with fuel and it doesn't get enough air to lean it out till the blades are wide open. My tuner doesn't want to lean it out until he has a AF gauge showing the ratio just to be safe, which I totally understand. I have such a basic understanding of EFI systems and the MEFI ECU that I really am out of ideas.

[roughneck427]

Well Ive been tweaking my tune the past couple days on the trailer before I head to the lake for some runs. I got mine to idle and run file was I had to do the whole tune from scratch with the mods I have. I did have where if I gradually rolled into the throttle it was fine. But when I was a little more aggressive I would have a flat spot and start blubbering. I didn't see it at first but it at async fueling. I noticed my bpw was more than I was commanding when it had a flat spot. I found the table and going to remove some fuel from the async multiplier. will they give you a copy of your tune or do you have it?

[Roadknee]

Two things come to mind:

Are you absolutely certain fuel pressure is steady at 28 psi when the problem occurs?

Are you running an aftermarket ignition box (holley, Mallory, MSD, etc.) and are the various wire pairs twisted together per their recommendations to minimize interference?

[Bonzen]

Well Ive been tweaking my tune the past couple days on the trailer before I head to the lake for some runs. I got mine to idle and run file was I had to do the whole tune from scratch with the mods I have. I did have where if I gradually rolled into the throttle it was fine. But when I was a little more aggressive I would have a flat spot and start blubbering. I didn't see it at first but it at async fueling. I noticed my bpw was more than I was commanding when it had a flat spot. I found the table and going to remove some fuel from the async multiplier. will they give you a copy of your tune or do you have it?

I will get a copy of the file and post it up. My tuner has no issues with me doing so or researching this issue to the fullest extent haha. I think he wants it figured out as bad as I do.


Two things come to mind:

Are you absolutely certain fuel pressure is steady at 28 psi when the problem occurs?

Are you running an aftermarket ignition box (holley, Mallory, MSD, etc.) and are the various wire pairs twisted together per their recommendations to minimize interference?

No I am not. I know it idles at 28psi and it sits at 28psi under load. but in the exact flat spot I do not know as we haven't ran the boat with the motor hatch off staring at the gauge yet...soo it's possible something is up there. FYI I upped the fuel pressure to 30psi last weekend in Lake Havasu and it almost seemed like it got worse. WOT was still great but the flat spot was still there. As far as ignition, it's all factory GM/Delphi/Delco. Explain the wire pair twisting thing?

[PJG1173]

a flat spot can also be too much fuel. hard to tell without wide band data. I ran into that problem when I got too aggressive with my pe afr table. my WB was reading 10.3:1 when I hit the flat spot with a little "blubbering" too.

[Roadknee]

No I am not. I know it idles at 28psi and it sits at 28psi under load. but in the exact flat spot I do not know as we haven't ran the boat with the motor hatch off staring at the gauge yet...soo it's possible something is up there. FYI I upped the fuel pressure to 30psi last weekend in Lake Havasu and it almost seemed like it got worse. WOT was still great but the flat spot was still there. As far as ignition, it's all factory GM/Delphi/Delco. Explain the wire pair twisting thing?

I helped a guy on here with a 1995K1500 running a pretty hot 355. From a dead stop if he mashed the throttle it would go lean and fall on its face every time at 3,000 rpm. It would quickly recover and pull strong to the 5,200 rpm shift point. If it was rolling at 5 mph or so the flat spot was not there. I messed around with probably 10 tunes adding AE and PE AFR and it just didn't matter. Finally had him install a fuel pressure gauge plumbed to the outside of the windshield. Sure enough, at 3,000 rpm the fuel pressure dropped from 28 psi down into the teens. It would do it with an empty or full tank. Stock baffling in the tank was fine. I had him install a new sending unit and hotwire kit to power the fuel pump. Problem solved. I still don't understand why an electrical problem would cause the symptoms he experienced, but it did.

About the twisted wires, the diagram in my C950 manual shows several pairs of wires twisted together to cancel out any interference IF you run an aftermarket ignition box. Since you do not this should not be an issue.

[Bonzen]

a flat spot can also be too much fuel. hard to tell without wide band data. I ran into that problem when I got too aggressive with my pe afr table. my WB was reading 10.3:1 when I hit the flat spot with a little "blubbering" too.

I do really think that's whats going on...I think it's flooding the cylinders until WOT when it gets enough air to normalize the AF mixture.




I helped a guy on here with a 1995K1500 running a pretty hot 355. From a dead stop if he mashed the throttle it would go lean and fall on its face every time at 3,000 rpm. It would quickly recover and pull strong to the 5,200 rpm shift point. If it was rolling at 5 mph or so the flat spot was not there. I messed around with probably 10 tunes adding AE and PE AFR and it just didn't matter. Finally had him install a fuel pressure gauge plumbed to the outside of the windshield. Sure enough, at 3,000 rpm the fuel pressure dropped from 28 psi down into the teens. It would do it with an empty or full tank. Stock baffling in the tank was fine. I had him install a new sending unit and hotwire kit to power the fuel pump. Problem solved. I still don't understand why an electrical problem would cause the symptoms he experienced, but it did.

About the twisted wires, the diagram in my C950 manual shows several pairs of wires twisted together to cancel out any interference IF you run an aftermarket ignition box. Since you do not this should not be an issue.

I will check the gauge at the bad spot under load...it "may" be dropping pressure although I agree, why the electrical voltage drop there? Who knows lol.

[Bonzen]

I helped a guy on here with a 1995K1500 running a pretty hot 355. From a dead stop if he mashed the throttle it would go lean and fall on its face every time at 3,000 rpm. It would quickly recover and pull strong to the 5,200 rpm shift point. If it was rolling at 5 mph or so the flat spot was not there. I messed around with probably 10 tunes adding AE and PE AFR and it just didn't matter. Finally had him install a fuel pressure gauge plumbed to the outside of the windshield. Sure enough, at 3,000 rpm the fuel pressure dropped from 28 psi down into the teens. It would do it with an empty or full tank. Stock baffling in the tank was fine. I had him install a new sending unit and hotwire kit to power the fuel pump. Problem solved. I still don't understand why an electrical problem would cause the symptoms he experienced, but it did.

Reviving this thread from the dead because I have updates... installed the wide-band AFR gauge & sensor. CNC'd custom adapter plate...and lake tested it... AFR gauge says LEAN in flat spot. Goes from about 12:1 to 14.5:1 in flat spot and then back to the 12's at WOT. Fuel pressure gauge goes from 30-ish psi to 18-20psi instantaneously during the flat spot...but then goes right back to 30psi once I push through to WOT. It doesn't matter if I mash the gas or ease into it slow, the same thing happens. I can hold the rpms and the throttle in the flat spot for as long as I want, it never recovers until I let off or push though to WOT. I'm watching this happen on the fuel pressure gauge in real time as the issue occurs....I closed the motor hatch on myself and watched the gauge with a flashlight while a friend brought the boat up to WOT across the lake lol.

So I'm looking at a fuel delivery issue that's choking the demand at that particular rpm range but then suddenly recovers at WOT...how is this possible??

Also it has been verified that the fuel pump is not experiencing any significant voltage drop during the entire RPM range, we tested it with a voltmeter under load.

How could a fuel system run great, then stumble and drop pressure significantly, and then pick back up and run great again??? Doesn't seem possible in the universe we exist in lol! I mean it is exactly predictable every time in the exact same rpm range.

[roughneck427]

Is your due pick up good and do you have vented gas caps that the vents are not plugged?

[Bonzen]

Is your due pick up good and do you have vented gas caps that the vents are not plugged?

yes pickups are welded aluminum tubing inside the tanks, I have no reason to believe they are obstructed.

Vent lines are clear and the gas caps do have vent holes in them and are also clear.

[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...