Tohatsu TLDI MD90A Sooty Spark Plugs

Last summer I was trying to fix and diagnose some problems with my engine on this thread: http://www.marineengine.com/boat-forum/showthread.php?466248-Tohatsu-MD90A-surging-around-800-1000rpm&p=695407#post695407

I concluded that the diaphragm in my fuel regulator may have been broken which was causing the problems I was experiencing. Due to Corona, I was not able to get back to the boat until now and now I have some quarantine days to work on it again.

I did the following:

• Had injectors professionally cleaned.
• Replaced fuel regulator.
• Replaced fuel regulator hose with a non Tohatsu hose but I don't think this is an issue.
• Checked the thermostat is working correctly.
• Bought a new battery to make sure this is kicking out enough juice.
• Measured air rail air pressure at between about 72 and 78psi (it fluctuates from 72, up to 78 then down to 74 psi with the engine cycle, see notes below).
• Measured air rail fuel pressure at between 80 and 90 psi (it fluctuates from 84, up to 90 then down to 80psi in time with the engine cycle).

Engine started fine but after 15 minutes or so of running at idle or close to it, I pulled one of the plugs and it was still black and sooty. I was hoping that changing the fuel regulator would have fixed this but it seems not. My understanding is that they should not be sooty, even when running at idle.

I hope someone might be able to provide some more insight on what I should check next. Some questions:

1. Should the air and fuel pressure be pulsing like this? Here is a video of the air pressure (the fuel is similar, just 10psi higher): https://1drv.ms/v/s!AmWxme0O8FrMqkt87sLhzMJsrF-6 I assume that since the air compressor is a piston and the fuel pressure is dictated by the air pressure, both will exibit this pressure fluctuation. The video is not slow motion, but on my phone you can clearly see and hear that it fluctuates in time with the engine.
2. Does it matter that my pressures seem to be a touch too low? It seems to be clear that the fuel is 10psi higher than the air, so this is good. I don't have access to shop air.
3. I did not replace the Spark plugs with new ones, just cleaned them enough to really see the soot come back. I assume that brand new ones will just soot up again. Is this ok?
4. Is it safe on a TLDI to pull the HT cords of the spark plugs one by one while the engine is running to make sure all cylinders are firing?
5. What other things are worth checking before I get out of quarantine and get the boat on the water?

Any help would be much appreciated. Thanks a lot in advance.

Here is the slow motion video of the fuel pressure reading: https://1drv.ms/v/s!AmWxme0O8FrMqlGVEDWKRt7VDA1w

The pressure may fluctuate as the regulator comes on and off.
Plugs really shouldn't be too sooty. I would try it at speed to see if that helps.
No need to pull wires; if running on only 2, you would hear it, plus, you can look at the plugs to see if they are wet.

Hi Paul,

Thanks for the reply.

I tested my air circuit by pumping (with the motor off) with a digital pump and the air regulator vented at 75psi. It seemed to hold pressure well. The pressure measurement with the motor running varies a lot (between about 72 and 78psi) and when slowed down with slo mo video, you can see it is exactly in time with the pistons firing, 72, 78, 74 and then repeats. It is not reaching 74/75psi and then dropping slightly before increasing and venting again. I cannot figure out if this is normal! Of course, the pressure might drop slightly when the air injectors open but this behaviour seems strange. Perhaps my cheap gauge is just crap.

It did occur to me that if the reed valve in the compressor was not closing fully, it may result in increased fluctuation. I also found some milky slime in the air regulator which I think is due to water in the air supply, right? I have therefore ordered the gaskets for the air compressor so I can replace these and check the reed valves at the same time, perhaps I will find an issue.

I still don't know how this would result in sooty plugs though! I will launch in the next few days and monitor the plugs more closely.

As a side note, I have found that my battery cables are 16mm2 (equivalent to about AWG5) for a total length of about 4m (6-7feet) and wondering if these are too small for this engine? I don't want any benefit of that new battery I have being wasted due to undersized cables. Problem is, I do not know how much current the engine draws while cranking. Starter motor is 83Amps but I assume draw is much more. Any advice here? It turns over and starts fine but given the issues created with these engines due to low voltage at the motor, I'd rather be sure. Was thinking of increasing to 25 or 35mm2 (AWG3 or AWG2).

Thanks a lot for your help.

Your cabling sounds robust enough. Yes, sheck the compressor valve to be sure it seats well. There is always a fair amount of water in the air, so the milk is not surprising.