Found this site when I searched this issue in Google....I have a GEN V BBC TBI and it runs great except at 3/4 throttle when it seriously looses power and acts like the timing is retarding itself or it's experiencing a lean condition...but if I push the gas pedal "through" it to WOT it instantly picks up and again and pulls hard all the way to top speed. It's only at around 3/4 throttle but then it's fine after that and builds more power. Id like to figure this out so I can have a smooth powerband all the way. The TBI is a brand new unit from SPR performance - www.tbiparts.com and as far as I know everything is working fine. Brand new TPS and IAC were shipped with the TB and both have been calibrated correctly according to the builder. The TBI is a 750cfm "overbored" custom setup with adjustable FPR and 80# injectors. Fuel pressure is reading at 23psi. The timing has been set and checked and seems fine. Fuel pump is working strong, (high pressure pump) Plugs do not look lean, wires look fine. The ECU, which is a Delphi MEFI 1 has been calibrated and tuned by Arizona Speed and Marine www.azpseed.com to the compression of the motor and the camshaft specs, and these are the guys that all these GM marine Delphi ecu's go to apparently. I've talked with them and they say the tune is good. I've replaced the Ignition control module and pickup coil in the distributor but that didn't help. Now here's the kicker...this is in a jet boat...but this is not a mercruiser engine. It's a "car" or "truck" motor that only has marine exhaust on it but besides that it's the same stuff as automotive. Any ideas, anything would be appreciated, thanks!

 

What are you getting with your scan tool when you read the data stream? O2 voltage and crosscounts, short- and long-term fuel trim, throttle position voltage, manifold vacuum, knock sensor timing retard, etc?

 

I don't have a scan tool and even if I did I don't think I could use it. I don't think there is even a diagnostic plug on the engine. It's a automotive style motor but there is no factory automotive harness with an OBDI or II plug. The harness and computer is all aftermarket marine stuff that was adapted to the engine. If it's going to need to be scanned then I'm going to have to take it somewhere or start throwing parts at it. But at least I know its an electrical or signal issue and not a mechanical one...the next thing I was going to replace was the coil unit since they are fairly cheap.

 

No scan tool, and you weren't even supplied with a port to connect one?

I'd start by jacking-up the various suppliers you purchased from, to see what help they could offer based on your symptoms.

 

10-4..i'll have to make some calls. I knows it's something stupid simple...a cheap electrical part of some adjustment. If the engine was mechanically bad it wouldn't run as good as it does and especially at WOT. I'm the 3rd owner of the boat so if there was some diagnostic tool provided at some point it's long gone by now lol.
(20 year old boat)

If we were talking about a 1994 Camaro here...with a pretty factory setup...what would you recommend to check or replace in this case? What does this issue sound like in an automotive scenario?

 

Can you pull a part number off the ecu?

 

yes I can get a part # but it's been flashed multiple times so I'm not sure a part number will lead to anything.

If this were happening in a car or truck what would be the things you would look at as possible issues?

 

i would goto a forum that supports your ecu. marine engines are a different ball game as they usually have no o2 feedback and other efi components disabled. you'll need someway to datalog it or else you're just pissing in the wind.

 

Finally got some MEFI SCAN feedback on the engine parameters. Basically what is happening is the ECM is retarding the timing 8-10 degrees around 65%-75% throttle which is making the motor fall on it's face. RPM's drop as well at the same time. Once again this a GEN V 454 (truck/car engine not a marine "mercruiser" motor) stock heads, slightly bigger cam and TBI unit. Fuel pressure is a steady 28psi. All sensors and the entire ignition system has been replaced at this point. Spark plugs are not lean looking at all. Runs GREAT until the flat spot and then runs fine at WOT after the flat spot. Attached are the ECU (MEFI1) parameters, if anyone knows this stuff and sees anything weird please post up!

 

I'm not familiar with that style of "report"; I can't tell what is live data and what is pre-programmed limits.

If the computer is pulling timing at heavy throttle, I'm inclined to think the knock sensor is detecting detonation. Either there's too much timing in that section of the operating range to begin with, or there's a lean-spot in the fuel curve. At WFO, the fuel mix is appropriately rich and the timing isn't overly advanced, so the engine and knock sensor are both happy.

 

That is not live data. Just programmed limits with engine off/key on.

We scanned live and pulled live data but I don't have it yet. But that's how I know it was retarding the timing. However the scan detected zero knock when the timing retarded...0.0 on the laptop display. So I'm stumped on that.... Possible malfunctioning knock sensor? And if so, how would it know to retard the timing if it wasn't working correctly?

 

Apparently, the spark table is faulty. I can't help you there. Seems like you need someone to re-program the spark advance.

 

Ok, I think that's the direction we're going. My tuner created 3 different tunes to throw at it, one lowers spark advance(or some variation of it), one adds fuel in that area, and one does both. We'll see how it changes things. At least should point us in a correct direction. Just was curious if anyone saw something out of the ordinary on the parameter file.

 

Given a lack of knock-sensor activity:
The spark table you have pulls timing, and makes you complain about how the engine runs. Why would your "tuner" create spark tables that pull MORE timing?

If the knock sensor is happy, I'd want to experiment with spark tables that hold the advance steady instead of removing advance.

 

I think I explained it incorrect. It advances to 41.1 under heavy throttle right before 3/4 throttle, then it retards sharply and quickly to 30-ish... What he did was re-curve it to only advance to 36 and then to retard to like 32 or close. I'm not 100% sure as I don't speak the language, but he in his own words, "trimmed the midrange advance" and did so because "the computer isn't pulling timing due to detonation reasons, but rather because that's what the current spark table told it to do"