strange idling problem

[nickwad]

I have just bought a 1998 Maxum 2300 with 5.7 TBI mercruiser. It always starts easily but sometimes it imediatly cuts out, if i keep it running on the throttle it will rev fine but then wont idle most of the time .
It had code 21, 22 and 34 TPI high voltage, TBI low voltage and Map sensor low voltage . when I got it the idle was very high and rough , I found the ground and signal wires to the TPI were cut . after repairing it now has this slow idle cutting out problem.
I have cleared the fault codes and they have not come back, the previous owner fitted a new TPI and IAC .
When it does idle ok after a while the IAC starts to hiss loudly and with the flame arrestor off it can be seen to be opening up more, Is this the ECM trying to save the engine stopping because of another fault ? there is a steady spray from the injectors at all times , I picks up very well with no hesitation when the throttle is snapped open.

Any ideas please ?

[mmwjr]

Welcome aboard!

Under Documents of this site you can find Mercruiser manual #24 which has a troubleshooting guide for fuel injection systems.

[nickwad]

Originally Posted by mmwjr

Welcome aboard!

Under Documents of this site you can find Mercruiser manual #24 which has a troubleshooting guide for fuel injection systems.

Thank you for the welcome ! I have downloaded the manual already and been through every dignostic check . I dont have access to a scanner as in Thailand but I have found a way to read and clear fault codes.

It is not throwing up any fault codes now .

[Phillbo]

I would replace the IAC again. They are know to be bad right out of the box sometimes.

[mmwjr]

Originally Posted by Phillbo

I would replace the IAC again. They are know to be bad right out of the box sometimes.

Use a OEM IAC folks have had issues with others.

[nickwad]

I found the problem ! it was the fact that the replacement engine came from a boat with a shift interupt system but my boat does not have one . the plug in the loom needed a jumper . This has completely cured the idle problem .

I took it for a test in the sea but have another dying problem.
Fuel pressure is 34 psi , cleaned out the tank, new filter. ( the old filter had a tiny bit of dirt in it and a few drops of water only when i rinsed it into a clear container) While the test pipe was connected i removed the pressure gauge and put the end of the pipe in a can , it still idled perfectly and filled a 5 liter can very quickly . so I guess i have not got flow problems ? ( the MIL gave a warning periodically when dumping into the can ,probably low fuel pressure ?)
It sarts and idles perfectly and revs clean on the flush hose but in the water it is ok up to half throttle but then on opening the throttle more it dies away , close the throttle and it picks up again . the symptoms are EXACTLY like fuel starvation . I have no fault codes up after the sea test.

What else could cause this ?

My test voltages on the map sensor are a bit low 2.6 v at atmospheric (ie engine off ) and 0.65v at 20 inches Hg

could this cause dying under load ?

[mmwjr]

So your boat has a Bravo drive and not an Alpha? Curious as to what wires you jumpered as I would think the interrupt switch wires should be left isolated.

[nickwad]

Originally Posted by mmwjr

So your boat has a Bravo drive and not an Alpha? Curious as to what wires you jumpered as I would think the interrupt switch wires should be left isolated.

My boat has an alpha drive , its had a replacement engine from a boat with a shift interupt switch, mine doesnt ,the plug on the loom has nowhere to plug in .
I tried putting a jumper wire in and it cured the stoping on idle with MIL sound!

I then had a dying problem above half thottle, No codes coming up fuel pressure 30 psi enen when dying problem was present , Map sensor was testing at 0.65 v at 20 ins hg and 2.67 v atmospheric ( 45m above sea level ) should be 1-1.5v and 4.5-5v roughly.
I went out yesterday and it wouldnt rev past 2500 rpm , changed the map sensor and it cured it .
I have a very slight problem very occasionally now of revs dying and fuel pessure dropping at near full throttle , I guess this is iether water in the tank or dirt as the boat has sat for 3 yrs. checking today will report.

[mmwjr]

All Alphas require a shift interrupt switch to prevent wear on the dog clutch. Sounds like your harness is either wrong, modified or missing some wiring.

[nickwad]

Originally Posted by mmwjr

All Alphas require a shift interrupt switch to prevent wear on the dog clutch. Sounds like your harness is either wrong, modified or missing some wiring.

Thank you for the info ,
Is the shift interupt switch the one on the gear change cable plate on the starbourd exhaust riser ? if so where can I find insructions on how to adjust it , I guess it is critical that it interupts just as the dogs engage.

its a bit difficult to know where all should be as its my first mercruiser and it was partly dissasembled and in a terrible state.

On the occasional drop in fuel pressure and hesitation, i checked the new water separator fuel filter after 40 mins running and it totally free of water and dirt.
Is it likely its the fuel pump ? or colapsed fuel lines ? or ....

[mmwjr]

Under Documents of this site you will find Mercruiser manuals, I believe #24 is the one you need for your engine and has the details of the shift interrupt switch setup.

[nickwad]

Originally Posted by mmwjr

Under Documents of this site you will find Mercruiser manuals, I believe #24 is the one you need for your engine and has the details of the shift interrupt switch setup.

what puzzles me is that the switch on the gear change plate is a normally open circuit ( closes breifly when putting in gear ) But my engine will only idle and run good with the shift interupt plug on the harness jumped 9closed circuit). also the plugs are a different style and the wires are black purple not white and black.
I have traced the blk and purple to the ECM it is deffinatly the shift interupt circuit.

I cant find anything on adjustment in the manual 24?

any ideas please?

[mmwjr]

attached is the wiring diagram for the 1998 TBI engine. The shift interrupt switch has a white wire coming from the ECM and a black wire to ground.