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454 XL TBI Stalling Issue

390Express

Regular Contributor
1996, 454 Crusader TBi (low pressure model). Boat starts and runs, and I can keep it running if I pump the hell out of the gas. It will rpm up to around 2500, and bounce between 500 and 2500, but if I let go of the throttle, it stalls. If I keep the throttle forward, it will stall, same as if I leave it in the idle position. It will only stay running if I continually pump the throttle. It will immediately re-start, but always stalls.

I changed the top "can" style metal filter on the top of the intake. New racor R11a filter, drained the racor, new fuel filter, new fuel pump. I'm at a loss. Seems like a fuel issue, but I can try plugs and wires, cap and rotor. Has good fuel pressure at the TBI. Had a small amount of sediment, barely anything, on the regulator and in the injector bowls. I cleaned that out, no change. Injector spray looks nice. New injectors.
 
I'd be inclined to scan the ECU if the fuel pressure is adequate...

Thanks Mako. Do you have some insight as to how the scanner/ECU receives it's fuel pressure info? I can say that the gauge at the rail measures a perfect 12psi, and I loosened the top of the throttle body and bumped the starter relay and a damn good amount of gas shot out everywhere, with good PSI. I just put on a new fuel pump, but I suppose it could be faulty. I could try swapping sides, but the mechanical gauge at the rail, after the last filter, reads a perfect 12psi, and the TBI seems like it's getting adequate fuel.
 
It doesn't...…

My intent was to suggest making sure fuel pressure is adequate and, if ok, then to scan the ECU.

Fuel pressure needs to be maintained across the RPM band for the system to function correctly...the pump may be new but if there is a restriction on the suction side, its not likely to maintain it....crud in the tank or the anti-syphon valve can drive you nuts...

Is your TBI the factory version?? I ask as I have never seen a fuel rail on a TBI unit.....
 
It's not a fuel rail per se. The system has a stainless line that comes up from the Racor fuel water separator, that line goes to the GM style "can" fuel filter that mounts to the top of the intake, and runs in-line with the feed to the TBI. Just past the can style filter, there's a schrader valve to test PSI. A mechanical gauge reads a perfect 12psi at the rail/feed line. I can keep the motor running for as long as I want, if I pump the hell out of the throttle, but as soon as I let up, it will die. If I stop pumping, it dies. It won't stay running high RMP, or low RPM, but will stay running as long as I pump the throttle. It reaches around 2500-3000k rpm at the tach while pumping, but won't maintain any steady RPM.
 
when you say you can 'keep it running by pumping the throttle' is this in gear or in neutral??

Got it - the Schrader valve on the fuel line....
 
ok...got to a computer where youtube would play...

do both engines behave the same way, experiencing the stalling?

where they both running ok earlier this season?
 
Starboard engine wasn't running at all at the beginning of the year. I went through damn everything, finally swapped the TBI upper from port to starboard, BOOM fired right up and ran great. Starboard motor is running perfect. Completely rebuilt the upper from the Starboard motor, and installed it on the port motor. Was running fine for a while, then this issue started. I took the TBI upper apart last weekend, small amounts of sediment in it, but nothing really significant. A little dust like substance on the fuel pressure regulator, that I wiped off with my finger, and a couple very tiny particles of crud in the bowl of the injector base. I cleaned that out, no difference.
 
Separate fuel tanks, but runs the same regardless as to which tank it’s running off of. MAP sensor is providing both low and high voltage errors with new MAP sensor, but only provides low voltage sensor error with old MAP sensor installed.

MAP sensor is a borg Warner, factory replacement. I highly doubt it’s bad out of the box, but no apparent vacuum leaks at the manifold, TBI or otherwise. I could try replacing the base gasket. Not sure where else to turn.
 
So the errors are repeatable and correlated to the MAP sensor being used?

try running the engine w/ the MAP sensor disconnected....
 
I had the old MAP sensor laying around, boat had a brand new one installed. New sensor gave high voltage and low voltage faults, but I'm wondering if the low voltage faults are due to the boat going into a stall situation, and either losing voltage and/or losing all or nearly all vacuum. The old sensor only had low voltage faults (no high voltage faults), but I didn't run it for more than a couple minutes. The old sensor could very well have high and low faults if I run it longer. I'm assuming the sensor/ecu isn't used to the motor diving below 200 rpm or so. Boat did run better with the old sensor, which I found odd. I'll try two things next time I'm down there, running it with the sensor plugged in, but without the vacuum line hooked up, and running it with the sensor unplugged. If I understand right, running the boat with the sensor unplugged should send it into limp mode and make it run rich, which should at least get it to idle.

Side note: I am getting a gurgling around the fuel pump inlet, but I'm hard pressed to fault the fuel pump at this point. The fuel pump is brand new, replaced because the old pump was noisy and gurgling. The pressure at the line is still a perfect 12psi, but I'm wondering if I'm getting some cavitation, or if it's not feeding right from the tube in the tank/anti-siphon valve? There is a small amount of kinking near the pump, but not much. I'll investigate that further next time I'm down there. I wanted to pull apart the anti-siphon valve today, but it was Labor Day and we had friends at the marina watering hole.
 
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The HIGH or LOW faults are typically set when the signal voltage ~ the sensor bias voltage, depending on the code set....in other words, when the ECU sees a value outside of what 'normal' is (which is nominally 1 volt inside the bias voltages)….unplugging the sensor causes the ECU to substitute a nominal value determined by the TPS (going on memory.) You can also use a DVM to measure what is going on at the connector....

Usually if the pressure in the line is ok and consistent (no pulsing) the pump & plumbing are serviceable....
 
The HIGH or LOW faults are typically set when the signal voltage ~ the sensor bias voltage, depending on the code set....in other words, when the ECU sees a value outside of what 'normal' is (which is nominally 1 volt inside the bias voltages)….unplugging the sensor causes the ECU to substitute a nominal value determined by the TPS (going on memory.) You can also use a DVM to measure what is going on at the connector....

Usually if the pressure in the line is ok and consistent (no pulsing) the pump & plumbing are serviceable....

I finally had a couple hours to spend at the marina to delve into this a bit deeper. Pretty confident it's not a fuel delivery issue at this point. Under the right condition, boat will run as long as I want it to at 2,000rpms, and fuel pressure remains constant 2,000 - 3,000 rpms. It just won't idle. I had three MAP sensors at the boat, the old sensor from the affected engine, a "new" sensor that i bought last year for the same motor, and a good working sensor from the perfectly running starboard motor.

I first tried running the motor again with no change, no difference. Next I tried running the motor with the vacuum line attached, but the electronic plug to the sensor unplugged. Motor ran better, but wouldn't idle. ECU put out a nominal reading for the map sensor at approximately 4.76 volts. Motor would run great at 3000rpms, 2000rpms, but when I slowly eeked down the throttle, as soon as it got below 1700 or so, the motor would go into a stall situation, whereby a running condition could not generally be salvaged, without a complete stall. After the motor stalled, it would hard start for 5-10 minutes (barely start, only sputtering, from around 200rpm to barely 1,000rpms, before stalling again). After 5-10 attempts, I let it sit for 10-20 minutes, and I could repeat the good start condition, it would again run 2000rpms, until I pulled the throttle back, and same condition.

Next I tried swapping ECUs (the poorly running port motor had a brand new ECU from ASM, I put the factory ECU from the good running motor on it), no change. Same no idle, same good performance at 2000rpms, same stall below 1700, same no start after, until it sat. Seems like it's running rich, and needs time to let unburnt fuel clear out before it will start again. It's not smoking heavily while it's running, and there's no "oil slick" out of the exhaust (so it's not running that rich...).

Next I tried swapping MAP sensors. I put the "new" sensor in the good running starboard motor, and it ran great, no MAP sensor faults. I put the MAP sensor from the good running starboard motor in the poorly running port motor, same condition, same "low voltage" error code fault. Tested the vacuum line for leaks, no leaks.

At this point, I don't know if there's some mechanical issue causing the stall, which is causing the error codes, which is causing the inability to idle, or if there's some other electrical issue that could be causing the faulty read from a good MAP sensor. I'm guessing the former is the issue, but not sure where to look. The intake has no external leaks, and still has the factory paint on it, covering the gasket mating surface. I can't see it leaking there. The TBI has been rebuilt with new gaskets, and no apparent leaks. Any other ideas?
 
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I have seen map sensor codes set if the engine vacum is eratic such as bad valves.Hook a vacum gage up to the map sensor vacum source and see what the gage reads.If it bounces rappidly then its probably a valve issue or if its low it has a vacum leak or missfire.I have 98 7.4 crusader tbi motors and the fuel pressure is 18psi on bolth motors but my injectors may be a differant size.What is the fuel presure reading on the good running engine?
 
The 1998+ TBI motors are "high psi" (18+), the 1995/1996 vintage are "low psi" and operate off of a 10-14 PSI system. Mine stay at a constant 12 PSI.

Could very well be in need of a valve job. I'll check the vacuum next trip down there. Any idea what it should be? I can use the other motor for reference, but it'd be great to have a nominal value.
 
In post #16, you say you saw 4.76VDC out of the MAP - assuming engine OFF???

I'd be inclined to check the MAP with your scan tool and a hand held vacuum pump to make sure its behaving correctly....4.75 sounds a bit on the high side, even with the engine off....could be a marginal connector pin/socket or wire that's corroded. Being as you changed sensors with little change in behavior, its suggests something else as well.

a good running engine should be able to idle with at least 15 inHg....I've seen some idle @ 18".....and the vacuum gauge will let you assess the accuracy of the MAP...
 
In post #16, you say you saw 4.76VDC out of the MAP - assuming engine OFF???

I'd be inclined to check the MAP with your scan tool and a hand held vacuum pump to make sure its behaving correctly....4.75 sounds a bit on the high side, even with the engine off....could be a marginal connector pin/socket or wire that's corroded. Being as you changed sensors with little change in behavior, its suggests something else as well.

a good running engine should be able to idle with at least 15 inHg....I've seen some idle @ 18".....and the vacuum gauge will let you assess the accuracy of the MAP...

The 4.76vdc was from the engine scanner, it was the "nominal" or "safe" output supplied by the ECU when the MAP was disconnected, while the motor was running. That setting remained constant, and the engine would run great at 2000 - 3000 rpm, but would stall below 1700ish.

Post #16 - "ECU put out a nominal reading for the map sensor at approximately 4.76 volts."
 
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I missed the sensor unplugged nugget....and its the default value used by the ECU.

Did you have time to get the vacuum gauge connected?

also, have you gone thru the MEFI manual troubleshooting for the stall issue?
 
I know that I am reviving a 4 year old thread but did you ever find a fix? I am having the exact same issue on a 1995 high pressure tbi.
 
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