1996 Mercruiser 4.3 LX+ Runs rough after 1800 RPM

[trbojet]

I just replaced the head gasket on my 4.3. Thundervolt 4. Ran great before the head gasket. I put the engine back in the boat and started it up. Runs great at idle but when I start to add power it starts to break up and runs bad after about 1800 rpm's. This was done in the driveway and not under load. I've had numerous people look at it and they agreed it sounds electrical. Like the system is shorting itself out somewhere. I had the distributer out and re-aligned the rotor to point at cylinder 1 TDC in the distributer cap when I reinstalled everything. I set the timing at 8 degrees BTDC at idle. I replaced plugs, plug wires, distributer, coil and cleaned the carb. I took the gray wire off the tach and still had the same issue. I disabled the interrupter switch and there was no change. Compression on all cylinders was 180. When I replaced the coil, I took a reading at both the negative and positive side of the coil. With the key in the on position I had 12.3 volts on both sides. Is that normal? The (- ) grey side was 12.3 and the (+) purple side were both 12.3. I tested the coils (old one and new one) and they were both within limits.

If the ICM (back of distributer) fails does it fail all together and get no spark? Can there be a partial failure?
If the ignition sensor (inside distributer) fails will it work partially and get no or intermittent spark?

I've read a lot of posts and the posts that were close to my issue seemed to never get resolved or there was never a follow up with the solution. Thanks.

Glenn

 

[Scott Danforth]

plug wires mixed?

check for arcing of wires

and yes, the pickup in the dizzy can be intermittent.

 

[alldodge]

If distributor has the original pickup in it, there has been a grounding issue. The new pickups have an extra wire coming from it for a better ground

http://www.mercruiserparts.com/892150q02-sensor-assy-ign

 

[trbojet]

The plug wires are all correct. I've checked that numerous times. I have an older pickup, there is no extra ground lead.

I'm just curious why there is positive voltage on both Positive and Negative sides of the coil with the key on. I thought the one side labeled (-) was supposed to be ground. (The side with the gray wire.)

 

[alldodge]

The Neg side is not ground but does get switched to ground to cause the coil to fire. This is what the pickup module does for ignition. In older days with points the Neg side would show a ground if the points where closed, new ignition it doesn't happen with the pickup just sitting

 

[trbojet]

Is there anyway to test the pickup?

 

[Rick Stephens]

Is there anyway to test the pickup?

Unfortunately, by replacing it.

By chance did you watch the timing light to see if you are getting appropriate advance when throttling up? You should be over 20º BTDC at 1800 RPM.

 

[trbojet]

I'll check again....I know it was up there pretty high.

 

[alldodge]

Since your getting spark even though not at high rpm's it probably wouldn't do any good to run through the test flow, but this is the only way to check why it wouldn't spark

 

[trbojet]

Thanks. I ran thru the flow chart earlier. It just seems to drop out around 1800 rpms. The only thing I can think is that the sensor is faulty and not seeing the pulses at the higher rpms.

 

[trbojet]

I ordered a new ignition sensor. Should be here on Monday. I'll keep troubleshooting till I get it.

 

[trbojet]

Installed the new ignition sensor and still have the same issue. Replaced the plugs still the same. Re- checked TDC with cylinder 1 and checked that the correct wires were going to the correct cylinder.

 

[trbojet]

Also, I am over 20 degrees at BTDC at around 1800 RPMs.

 

[alldodge]

Replaced coil and pickup, timing at 8 BTDC, get 20 BTDC at 1800 rpm, plugs and plug wires are in correct order and there is no issue with the carb.

Take a timing light and place it on the coil wire to see if it starts missing above 1800. If that's good move the light to each plug one by one.

If all are firing and there is no miss then I'm out of ideas

 

[trbojet]

Will do.........I'll post an update tomorrow. Thanks for your help.

 

[kulle]

I'm experiencing a similar issue with 1993 4.3...runs/idles fine a, can rev up ( in drive way) - but seems to stall soon after i give it load in water. I recently had my oil pressure switch replaced as boat would not stay running. Tested my coil....primary was out of spec range and actually increased resistance after running for awhile ( not sure if its causing a voltage drop) -replacing it as it appears to be old. Thinking about replacing ignition sensor in dist as well. Any update on your issue?

 

[trbojet]

SOLVED..........The issue was that the distributer cap was not properly seated in the base. The notch was filled with crud and wouldn't allow it to properly be seated. Go figure. I couldn't believe it. Runs great now.