454 XL TBI rpm issue

[tomschauer]

390, on another post you say you are still on the hard. If you are running the engines, trying to set timing etc, without raw water flow, you may be chasing ghosts. With the crusaders you most likely have exhaust temp sensors co sensors etc that could be shutting you down without the proper cooling water flow.

[km1125]

I would put a timing light on the coil wire and observe that when the engine is shutting down to verify that it's losing spark. That would likely mean the ECM is telling it to shutdown.

Sounds like you verified that it's not a fuel-only issue by spraying the starting fluid (but be VERY careful doing that!)

[390Express]

Source of the post 390, on another post you say you are still on the hard. If you are running the engines, trying to set timing etc, without raw water flow, you may be chasing ghosts. With the crusaders you most likely have exhaust temp sensors co sensors etc that could be shutting you down without the proper cooling water flow.

Tom, there is no exh temp sensor, and the port motor runs fine. I haven't ran the port motor for more than a couple minutes, but the starboard motor won't stay running for more than 30 seconds.

[390Express]

Source of the post I would put a timing light on the coil wire and observe that when the engine is shutting down to verify that it's losing spark. That would likely mean the ECM is telling it to shutdown.

Sounds like you verified that it's not a fuel-only issue by spraying the starting fluid (but be VERY careful doing that!)

The bitch is, it's hard to isolate the trigger wire for the coil, due to how it's wired. (it has four wires, grouped in two plugs), see below.

I'm going to try swapping ECU's tomorrow, and picking up a code scanner/reader/re-setter from Michigan Motorz. It'll be worth the $50 just to get an idea as to what's going on. I think I can keep the thing for a week for the $50 restocking fee. I'm interested to see what it tests, shows, and allows me to reset anyhow. I may eat the $1,500 and buy the MEFI burn software. I do enough with these motors and ECUs that it will likely be worth my while. If I program 3 ecus it pays for itself.

[km1125]

Source of the post

Source of the post I would put a timing light on the coil wire and observe that when the engine is shutting down to verify that it's losing spark. That would likely mean the ECM is telling it to shutdown.
...

The bitch is, it's hard to isolate the trigger wire for the coil, due to how it's wired. (it has four wires, grouped in two plugs), see below.
....

I was thinking just an old-school timing light that clips onto the "high tension" wire from the coil to the dist cap.

[390Express]

Source of the post

Source of the post

Source of the post I would put a timing light on the coil wire and observe that when the engine is shutting down to verify that it's losing spark. That would likely mean the ECM is telling it to shutdown.
...

The bitch is, it's hard to isolate the trigger wire for the coil, due to how it's wired. (it has four wires, grouped in two plugs), see below.
....

I was thinking just an old-school timing light that clips onto the "high tension" wire from the coil to the dist cap.

Did some more investigating yesterday evening. I set a timing light on the coil wire, and aimed it at the fuel spray from the TBI, to get a better idea of what was cutting out first, fuel or spark. It happens so fast real time, it's nearly impossible to tell, so I took a video. Looks like fuel is cutting out first (video uploaded to Youtube and posted below).

Motor definitely continues to make spark as it's stalling, what's frustrating is, it also has good fuel pressure. I put on a new fuel pump and new fuel line. Wondering if it's just air in the system that needs to work itself out (but shouldn't be)... It's a return type system, that bleeds down after pressurizing, and I did plenty of priming. I've started the damn thing about 10-15 times, and it's not getting better. I also put the same type of fuel line on the port motor, primed that system in a few presses of the start button, and that motor runs fine. I'm considering changing the top metal filter, if for no other reason than it's a $10 part. I could also rebuild the TBI, again... but it has good fuel pressure.

Runs great when it starts, just dies in 15-30 seconds. It will immediately start back up. Mechanic thought that the start button may be over heating and losing contact, so I bypassed all that, no change. Hoping to get him down to the boat tomorrow or Monday.

[km1125]

Not exactly sure on the TBI's, but on the earlier engines with electric fuel pumps there was an oil pressure switch so the pump wouldn't keep running if the engine quit.

However, since you have good fuel pressure, that's likely not the case.

Then I'd lean towards the ECM cutting off fuel... but if it was doing that to stop the engine, then it would cut spark to... if it was the thing driving the spark generation like on new engines. However... yours may not do that. Do you know if there is any interconnection from the ECM to the distributor/coil, or is the coil just run right from the IGN circuit and fired from the dist module?

[390Express]

Source of the post Not exactly sure on the TBI's, but on the earlier engines with electric fuel pumps there was an oil pressure switch so the pump wouldn't keep running if the engine quit.

However, since you have good fuel pressure, that's likely not the case.

Then I'd lean towards the ECM cutting off fuel... but if it was doing that to stop the engine, then it would cut spark to... if it was the thing driving the spark generation like on new engines. However... yours may not do that. Do you know if there is any interconnection from the ECM to the distributor/coil, or is the coil just run right from the IGN circuit and fired from the dist module?

ECU def runs to the distributor/coil and effects timing. I’m sure it effects spark too, but it’s not cutting spark. May be a good idea to have someone else start the motor with either a multimeter on the fuel pump, or at least an ear close enough to listen to it, but it does keep fuel pressure, and the ignition keeps firing, which is why it’s so frustrating.

There is a fuel pump relay that is fed information from the ECU. The pump won’t kick on without a signal from the ECU (learned that from the port motor). I’m going to swap ECUs this evening or tomorrow morning, it only takes 5 min.

[390Express]

Source of the post Not exactly sure on the TBI's, but on the earlier engines with electric fuel pumps there was an oil pressure switch so the pump wouldn't keep running if the engine quit.

However, since you have good fuel pressure, that's likely not the case.

Then I'd lean towards the ECM cutting off fuel... but if it was doing that to stop the engine, then it would cut spark to... if it was the thing driving the spark generation like on new engines. However... yours may not do that. Do you know if there is any interconnection from the ECM to the distributor/coil, or is the coil just run right from the IGN circuit and fired from the dist module?

Swapped ECUs, same problem. (However, the port motor runs a lot better with the factory stbd ECU. I'm going to have a conversation with ASM (Arizona Speed & Marine) about that one... No more erratic idle with the stbd ECU on the port motor. I'm guessing, as I suspected for a while now, that the idle is set too low on the ECU that ASM sent me, and the motor over compensates, bouncing the idle between a stall situation and a high idle situation.)

Talked to my mechanic today. He essentially said that a timing light reports impedance, and just because the timing light is flashing, it doesn't mean that the ignition is still receiving power, and providing enough spark to fire a cylinder. He's convinced that there's a sensor, corroded connection, or bad switch somewhere, turning off power to the ignition/ECU.

It's supposed to rain a ton tomorrow, but I expect him at the marina Tuesday. Hopefully I'll get this figured and be wet Wed.

[km1125]

Source of the post
Talked to my mechanic today. He essentially said that a timing light reports impedance, and just because the timing light is flashing, it doesn't mean that the ignition is still receiving power, and providing enough spark to fire a cylinder.

That's just absurd. An inductive timing light is triggered by the high-voltage pulse going to the spark plug. If it's there then the timing light flashes and the spark plug fires.

Source of the post
He's convinced that there's a sensor, corroded connection, or bad switch somewhere, turning off power to the ignition/ECU.

I don't doubt that either. The issue with the 30 second running sounds fishy, like there's and issue with oil pressure or some other critical signal. Some engines used two crank sensors. Would start on one then switch to the other. I think Oldsmobiles did that back around 2000. If the one sensor was bad it would have a similar symptom. Start and run, then die. I don't know if Merc ever did that... do you have any clue if there are one or two crank sensors?