No spark again

[burning_up]

I am trying to repair a 1997 Mercruiser 5.7 LX. I had occasional rough running all season at cruising speed but it only lasted 10 seconds or so and I attributed it to bad fuel. One day it happened and the engine stalled. It would start but I could not keep the engine running more than a few minutes. The troubleshooting guide said replace the ignition module and after replacing the distributor sensor, ignition module, distributor cap, rotor and spark plug wires the boat ran better than ever for about an hour at cruising speed and it was back where it started. It again looked like intermittent spark so I again replaced the sensor and the module and the knock module. This time the engine ran well only long enough to adjust the timing and idle jets. It again began starting and stalling and now it shows no spark with a spark gap tester while cranking.
I found a loose contact at the ignition switch and repaced it and also ran a wire directly from it to the coil to bypass the deadman switch at the shifter although I never really saw a voltage drop there. I have tried disconnecting the momentary shift kill switch. The last time it was running I checked that the alternator stayed <=14.5V as specified and when it started stalling I disconnected it with no change. It appears to me that somehow the ignition modules are being burned out. I read in another post that they may be sensitive to low voltage but when the engine is cranking I would think they have to tolerate a voltage less than 12V. The original module was discontinued so I installed the recommended replacement 861253T1 and the compatible sensor. I also tried a new coil along the way but switched back to the old one at some point.
Does anyone have an idea what to do next?

 

[Don S]

Re: No spark again

What is your engine serial number. That 861253T1 is not the right one for the LX. For a 97 5.7LX, I see part number 806574T4. Have you looked at the connector to make sure it's got good clean shiny connections? Are you SURE the ICM is bad? Look at the distributor and make sure the bushings are not worn out and allowing the rotor to contact the sensor.

 

[burning_up]

Re: No spark again

Thanks for the quick reply. The serial number is 0K023033. The new module from quicksilver came as a kit that required cutting the wires and replacing the connectors so they are shiny new. It is marked 5.7L ALPHA. I did see some indication on the rotor that it had scraped the sensor but I don't feel much horizontal play in the distributor shaft. Vertical play seems worse. I am not sure if the module is dead. I can detect pulses at the coil wire with my timing light but nothing at the plug wires.

 

[Don S]

Re: No spark again

You do have the wrong module, you have an LX and need the 806574T4.

I can detect pulses at the coil wire with my timing light but nothing at the plug wires.

If you are referring to the coil wire that goes from the coil to the center of the distributor cap, you may just have a bad coil wire. Have you replaced the spark plugs. If you have a dead one, or a single dead plug wire, it could cause the coil tower to crack and short to one of the small terminals on the coil.

Have you run these tests to determine the problem?

 

[burning_up]

Re: No spark again

I was told the original modules are no longer available and the one I was sold is the generic replacement for all the alpha 5.7L motors. There are supposedly only slight differences in the advance curves. I tried your number at a couple of sites and got "not found" responses.
I changed plugs, wires, distributor cap and rotor. I swapped coil wires back and forth several times.
I know that flowchart intimately and it always points to replace module.
Do you think contact between rotor vanes and the hall sensor is relevant? Obviously if it were grinding it up it would be. I will replace the distributor next but I don't want to destroy another module. I have access to test equipment. Do you know of a procedure to test it off the motor?

 

[burning_up]

Re: No spark again

Don,

I repeated the test this evening and I again got no spark from the original coil. Then I switched to the new coil and in the dark I saw a strong spark from the base of the coil tower to the positive terminal. Looking at it with a magnifying glass I could see a hair line crack just like you predicted. I will replace the coil again and let you know. Thanks for the info.

 

[burning_up]

Re: No spark again

Don,

I installed a new coil and the engine runs fine. I guess the original was arcing internally and the new one between the terminals. All three actually give the same resistance measurements between the terminals so the windings are intact. Anyone need a slightly used 5.7 Alpha ignition module and sensor? As I said there is evidence that the rotor vanes are hitting the hall sensor so a new distributor is next but that was not causing the intermittent spark problem. Thanks again