7.4XL TBI won't idle.

I have a 97 Wellcraft with a Mercruiser 7.4XL TBI. I have checked all sensors and wiring. Engine will start and run for 3-5 seconds and shut down, smokes like its flooded. It will keep doing this. The injectors in the TBI are pulsing as required but the engine will not idle. If you make it run, once it will run on its own, when you give it throttle it bogs down and black smokes, then it will slowly respond to the throttle adjustment. TBI has been rebuilt with new injectors and regulator, new TPS, IAC, cap & rotor, plugs & wires, fuel filter. Tank has been cleaned out and all lines flushed. With the spark arrestor off the TBI, once it runs and shuts off, the butterflies in the TBI are real wet with fuel but not puddled. No alarms or lights to indicate problem. What am I missing?

Re: 7.4XL TBI won't idle.

sounds like you need to connect a scan tool and check for codes - is there a MAP sensor that you know of??

a scan tool with a data items list will be your best bet if there are no codes - could be a sensor is acting up but not enough the send a code, or a sensor isn't working but not sending a code

unfortunately for you the MEFI system is now a museum piece and there is not much to note with a scan tool for codes, etc...

if you disconnect one sensor at a time and note NO change in the performance i would focus on that sensor

Re: 7.4XL TBI won't idle.

The scan tool may not give you codes, but it does tell you what the sensors are telling the engine.
For example. If your engine is running rough and rich when warm, it will not set a code, but by looking at the outputs of the sensors on the scan tool you could see that the ECT sensor shows the engine is only 40?F telling the ECM it needs extra rich fuel air mixture, even though the engine is actually 160? F.
Testing and finding the ECT sensor has failed and replacing the sensor will cure the problem.
The scan tool is just a tool you need if you want to work on EFI engines.

You also need a serial number for your engine, not just year to get the right information. Merc does not go by year on anything, just serial number.

Re: 7.4XL TBI won't idle.

dubs283 said:

sounds like you need to connect a scan tool and check for codes - is there a MAP sensor that you know of??

a scan tool with a data items list will be your best bet if there are no codes - could be a sensor is acting up but not enough the send a code, or a sensor isn't working but not sending a code

unfortunately for you the MEFI system is now a museum piece and there is not much to note with a scan tool for codes, etc...

if you disconnect one sensor at a time and note NO change in the performance i would focus on that sensor

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Yes there is a MAP sensor directly behind the TBI. My problem is all I have is automotive scan tools and no way to connect to the Mercruiser controller.

Re: 7.4XL TBI won't idle.

I have come to realize this system is definantly antique, after researching replacment parts and diagnostic information. I am a automotive tech and have been for approx 20 years, just trying to help a friend that took this boat to the local dealer and was charge over $1400.00 to change out all of these parts and clean tank, and boat is still exactly as it was before.

Re: 7.4XL TBI won't idle.

kwallace said:

just trying to help a friend that took this boat to the local dealer and was charge over $1400.00 to change out all of these parts and clean tank, and boat is still exactly as it was before.

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so this local dealer installed all these parts, charged your friend all that money and didn't realize the problem wasn't fixed??

i would help your friend either get his money back, or take the boat back to the shop to get it fixed properly

if i install a shift cable in a baot and tell my boss that it still dosen't shift properly what do you think my boss says to me??

Re: 7.4XL TBI won't idle.

dubs283 said:

so this local dealer installed all these parts, charged your friend all that money and didn't realize the problem wasn't fixed??

i would help your friend either get his money back, or take the boat back to the shop to get it fixed properly

if i install a shift cable in a baot and tell my boss that it still dosen't shift properly what do you think my boss says to me??

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Exactly!! He has tried to talk to this shop and they calim it was running fine at there shop. Problem is now is back in the slip approx 30 miles away and that shop acts as if they won't even want to talk about it. I guess if i was in there shoes, I wouldnt either. I am a service manger over a truck shop and our reputation is EVERYTHING. If i tell a man his truck is fixed and he is ready to haul a load of produce across the country, what kind of phone call do you think I get if the truck only makes it half way?? I have tried to talk to that shop but I guess i may have visit them in person. I was just trying to help out the guy because he is so frustrated.
I am not a marine technician by any means, but have worked on anything from a lawn mower to a 800Hp diesel engine. But regardless, it bothers me when I can't figure something out. I appreciate all of you guys and your help.. Thanks

Re: 7.4XL TBI won't idle.

REally sounds like the classic symptom of the MAP sensor that the TBI system had. If that or the HEI distributor pulse is erratic, this symptom will occur. Only from automotive experiance though. Not sure for your boat but sounds plausible.