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Problem RPM Diesel

10-30-2023, 05:16 AM
My question is about a motorboat with a Nanni inboard engine. Nanni has marinized the Toyota 1KZ engine, which is also found in the Toyota Hilux, Prado, ...

The story begins like this:
We experienced engine trouble at sea, where the engine lost power, as if you were driving into a net. We immediately shut off the engine, and upon opening the engine compartment, a cloud of steam greeted us, and we saw coolant in the bilge. According to the dashboard, the temperature did not exceed 90. The boat was then towed to the harbor, and there we began to search for the cause of the problem.

The following tests were performed in our search for the coolant issue:

- Tested the coolant cap, which was in perfect condition.
- Pressurized the cooling system, and there was no leak; the pressure remained stable.
- Disassembled and checked the water pump, which looked like new.
- Tested the thermostat, and it opened at the correct temperature.
- Conducted a CO2 test above the expansion tank, which came back positive, indicating exhaust gases in the coolant.
- Performed a compression test, which was fine.
- Removed and sent the cylinder head to a specialist, who pressure-tested and resurfaced it, confirming its integrity.
- While the turbo was off, we had it overhauled, and it was in good condition; new seals and bearings were installed.
- Cleaned the coolant cooler.
- Installed a new head gasket and reassembled all the components.
During the test run, we encountered the same issue again. Air bubbles entered the cooling system at 2000 rpm, and coolant was pushed out. The CO2 test was positive once more. As a result, we removed the cylinder head again and pressure-tested it ourselves at 7.5 bar with hot water, first on the coolant side and then on the exhaust gas side. Nothing was detected. Ultimately, we decided to install a new cylinder head because we were sure about all the components and could find no other cause. The cylinder head we purchased was not from Nanni but based on the motor code, which is a 1KZ engine. We observed that both the 125hp and 140hp versions of the Toyota engine use the same cylinder head. Our engine has a power of 200hp, but all the inputs, outputs, and mounting holes are the same. The cooling water problem was resolved during the test run, so it must have been a hairline crack after all.

However, we now have a new problem. We cannot get the engine to rev above 2600 rpm. In our quest to find the cause, we performed the following checks:

- Pressurized the exhaust manifold, both on the exhaust and coolant sides, but found nothing.
- Had the injectors retested, and they open at the correct pressure.
- Disconnected the 4 injector lines at the injector and collected diesel during startup; all provided an equal amount of diesel.
- Verified the clearance, camshaft length, valve diameter, intake and exhaust port size, and valve length on the new cylinder head with pre-mounted valves and camshaft. These measurements matched those of the old cylinder head and the specifications in the workshop manual.
- Checked the timing of the timing belt.
- Cleaned and pressure-tested the intercooler.
- Set the wastegate turbo to 1.16 bar, and it opens at the correct pressure.
- Adding extra diesel while sailing made no difference, only more smoke.
- Pressurized the bypass of the small filter located between the intake and the injection pump with an external pump set at 1.3 bar while sailing. We observed that the injection pump is fully compressed by turbo pressure, and the turbo pressure is at the correct level.
- Adjusted the injection pump timing, resulting in more smoke but no increase in RPM.
While searching for a solution, we also performed maintenance and found very fine, shiny particles in both the pre-filter and the fuel filter. This led us to suspect the injection pump. It is a mechanical rotary injection pump from DENSO. We disassembled it and had it checked by a specialist who was puzzled why we brought it in, as he had never received a pump in such good condition. All tests were fine.
To be sure there was no blockage in the fuel system, after installing the tested injection pump, we connected both the supply and return lines directly to a jerrycan placed on the injection pump. This resulted in an additional 50 RPM due to the elimination of filter backpressure.
Finally, we inspected the hull and, in particular, the propeller with a camera. There was minimal fouling, and nothing was found in the propeller.
The propeller shaft is easily turned by hand.
During the test run, we observed that the engine revs up to 2600 rpm. Beyond that point, there is still 1/4 of the throttle lever left, but pushing it further does not increase the RPM, and the engine sound remains the same, with a little extra smoke. The engine starts very well, and up to 2600 rpm, it sounds and responds as if nothing is wrong.

Can anyone help us in our search? We have exhausted all our ideas.

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