1995 Mercruiser 5.0, 4B, TB ignition problem Help!

[Braymo]

I have to crank it 15-20 seconds none stop holding the trottle wide open for it to start dead cold or warmed up. weak battery or only one on won't start. Fuel is good, floods easy, changed coil, wire from cap to coil, cap and rotor, plugs, fuel seperator, same problem. spark seems to be weak at start up and idles rough. throttles up smooth and plenty of power and top end. what is an ignition amplifier? would the ignition sensor cause this problem? Think it could be a bag ground (tach, kill switch)?

 

[winterfunguy]

Re: 1995 Mercruiser 5.0, 4B, TB ignition problem Help!

Pull the cap and rotor and check the hall effect pickup (the part the reluctor ring passes through). If you see rust or corossion on it replace it. If you know anyone with a lab scope they can easily test its operation for you. I will bet that this is your problem.

 

[Braymo]

Re: 1995 Mercruiser 5.0, 4B, TB ignition problem Help!

Its easy enough to check. thanks for the tip.

 

[dingdongs]

Re: 1995 Mercruiser 5.0, 4B, TB ignition problem Help!

may also not be getting the full 12v to the coil while cranking.test with a multimeter.if not run a 12v+ direct to coil and try again.this is only for testing!!!!!

 

[dubs283]

Re: 1995 Mercruiser 5.0, 4B, TB ignition problem Help!

I have to crank it 15-20 seconds none stop holding the trottle wide open for it to start dead cold or warmed up...floods easy...spark seems to be weak at start up and idles rough.

sounds like you have a flooding issue with your carb, have you checked to see fuel flowing down the throats of the carb at idle/shut down??

how do you know you have weak spark at start-up?? do you have weak spark when cranking, tested with a spark tester??

 

[Braymo]

Re: 1995 Mercruiser 5.0, 4B, TB ignition problem Help!

I tested 12v at the coil with a multimeter but not while cranking. I haven't ruled out the carb flooding. I have never used a spark tester is it simple and cheap? I have a multi meter.

 

[Don S]

Re: 1995 Mercruiser 5.0, 4B, TB ignition problem Help!

What were your actual volt meter readings on the + terminal of the coil while cranking? and the actual battery voltage of the battery. Yes, it makes a difference.
Do you have a Thunderbolt IV or V on your engine. Being a 95, it could be either.
IF you aren't sure, post the serial number of your engine.

 

[Braymo]

Re: 1995 Mercruiser 5.0, 4B, TB ignition problem Help!

Serial number: OF414292

 

[Braymo]

Re: 1995 Mercruiser 5.0, 4B, TB ignition problem Help!

The old coil I replaced says TB-IV

 

[Braymo]

Re: 1995 Mercruiser 5.0, 4B, TB ignition problem Help!

I tested the coil while cranking at it went from 12V to 8.5 while cranking.

 

[winterfunguy]

Re: 1995 Mercruiser 5.0, 4B, TB ignition problem Help!

Next step is to do a voltage drop test. Take the leads from the meter and put one on the coil pos side and the other lead on the bat pos side. You should read 0 with out cranking...crank the engine and moniter the voltage...should stay 0. This test tells you if you have a voltage loss between the coil and the battery. Also check your actual battery voltage while cranking, it must stay above 10.5v to have the ignition work.

 

[Braymo]

Re: 1995 Mercruiser 5.0, 4B, TB ignition problem Help!

I did put a hot wire from the battery to the + coil. meter read 10.5 when cranking.