Engine stalls when put in gear

[Alanjam8]

Port engine stalls when put in gear. Idles then stalls.

[mjk1040]

Engine makes and models would help. Carbed or TBI?

[Alanjam8]

Thanks for response, Mike! It’s a 1997 throttle body 40’, 454 Carver gas engines. I have a new fuel pump, water pump, checked most vitals. Motor will not idle if the MAP plug is plugged in. When plugged in, it stalls. If unplugged, it idles. But, if it’s plugged in and the temp sesnsor is unplugged, it WILL stay idling! And vice versa for those 2. But either scenario, when put in gear, it goes thump and stalls. I’m thinking it may be the ECI board went bad?

Boat sat for awhile without starting (my bad) for around a year due to personal issues. I had the Starboard engine rebuilt because it had locked up, and been trying to get the Port side running for a few months now. Had area mechanics work on it, but we are stumped ( they are just general mechanics, not certified).

So, right now it starts, idles at around 800 rpm (without eIther MAP or temp sensor plugged in) but stalls as soon as put in gear. Prop and shaft are free turning and have been checked.

[km1125]

Does it have an IAC valve?

You could measure the TPS sensor to make sure it doesn't have a bad spot. You could also swap the MAP and/or IAC with the other engine to see if the problem follows those devices or stays with the engine.

Could also be something as simple as the "minimum air" adjustment on the thottle body. Similar to the idle screw in a carb, but usually it's a allen type screw.

[mjk1040]

As km1125 said, its a good place to start I think.

[pepmyster]

Yup start with that, My IAC went bad causing this, 2004 VP 8.1,

[bud37]

My quick take is that the idle speed is too high for some reason....probably should be between 650 and 750.....also chances are the engine is not getting fuelled correctly, creating a lean stall. Change some sensors from one engine to the other like has been suggested....do you have someone with a laptop and program to access the ECU to watch the goings on in real time ?

How's the gasket/seal on the map sensor ?

[km1125]

Just an FYI...

When you remove critical signals from the ECU, it defaults to the "best knowledge it has" and fuels and sparks the engine with it's "best guess" on what the engine needs. If it doesn't have even that most basic info, the engine won't run at all. That would be the case with a bad crank sensor... in most cases, the ECU would have no idea how fast the engine is turning, so it has no idea how to fuel or spark.

If you remove the MAP sensor input, the ECU looks at throttle position and engine speed and just calculates that the manifold pressure (vacuum) would be and guesses at how much fuel you need. That's one reason why the idle speed can vary from what the ECU would prefer. Same goes with the Temp sensor.

You can measure the resistance of the temperature sensor and compare with the correct values to see if it's correct, or even close. One thing that will happen with temp sensors is they will (falsely) tell the ECU that the engine is warm when it's not. This is like trying to run an old carb without a choke. Might run, but can't run with a load until it warms up.

[Alanjam8]

I have a 1997 Carver with twin 7.4 crusader inboards that use the throttle body injection carbs. One motor runs perfect while the other stalls at idle. You can start it and set rpm at 2000 but after 10 seconds it drops off and stalls out. However, if we unplug the MAP, the engine idles fine, but stalls as soon as you put it into gear. Fuel is injecting the entire time. The IAC and TPS have been replaced. I have taken the TBI carb off the good motor and put it on the bad motor and everything runs fine so it’s not the ECU or Mass Air sensors or fuel rail pressure. The problem lies within the TBI carb unit itself. I’ve replaced the injectors and all gaskets but it still does the same thing…..the engine starts right up, idles fine, and after 10 or so seconds stalls out. Physical inspection shows no issues.

Here is what has been done:

New IAC
New TPS
New Injectors
New Gaskets
Pulls 18” vacuum when running
Have installed 3 IACs that were confirmed good but none are actuating with this TBI unit in use…all 3 work well with other engine TBI unit
Have tried 2 different TPS….no change…both worked on other engine TBI unit.
Other TBI works well on both motors

Perhaps need a new throttle body?

Your feedback is greatly appreciated.

[Viper]

Is the fuel pressure regulator built into the TB or separate from but attached to it? If separate, try swapping it with the other side and see if the symptom follows the part.