Join Date: Jul 2000
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Re: Yanmar 3gm30
1979 - if original engine, would be a 3QM30
The 3QM30 (and successor model Yanmars) has a notoriously BAD starter primary circuit - panel wiring is non-tinned and results in significant wire and connector corrosion which leads to low voltage/high amperage problems in the (primary/solenoid) circuit of the starter. That you did successfully start after you used ALL batteries to start sort of validates that the problem is in the primary circuit. A better way to start such an electrically corroded-up starter primary circuit system .... turn on the key at the control panel, open the engine compartment, take a screwdriver and use as an electrical 'jumper' between the main battery to starter connection and the 'other' small electrical connector which isn't the input primary circuit connector on the starter solenoid --- just momentarily apply the 'jumper' between these starter/solenoid 'taps'. If the engine starts immediately and with no 'struggling' by the starter; then, the wireing from the control panel to the starter's solenoid is 'kaput' and needs to be remedied. The usual 'worst' corrosion between the control panel and the solenoid in the primary circuit on a 3QM30 is on/in an engine mounted fuse holder (located portside aft of the engine). Replace the fuse holder for one with TINNED components and connectors.
CONTAMINATED FUEL PROBLEM OR BLOCKED INJECTION ELBOW ????.
Slowing of the engine to 1/4 speed with full throttle applied =
1. fuel delivery system blockage OR 2. Failing high pressure injector pump.
Contaminated fuel delivery system probabilities are
A. (most common) blocked filter(s) due to tank fouling by a fungus - filter surface will be 'black'.
B. fuel line blockage by fouling by bacterial 'strings' ... normally when the fuel lines become blocked by bacteria and the engine stops; the 'strings' will decompress after a while after shutdown .... and after a 'rest' time and 'decompression of the strings' will allow some fuel supply to the engine upon attempted restart, and one can 'restart' and run the engine for a short interval before the engine become fuel starved again, .... and again, ..... and again.
In either cause, the entire fuel system usually needs some severe remediation including a thorough clean-out of the tank AND fuel lines, followed by sanitization.
The reason that either of these problems happened: Failure to continually use/add a biocide with the fuel .... starting with a CLEAN fuel tank / fuel system.
EXHAUST SYSTEM BLOCKED AT INJECTION NOZZLE TO WATERLIFT.
For quick identification of such a problem - remove exhaust manifold bolts from engine and, ... 'pry' the manifold away from the engine by ~1/4"-3/8" which allows exhaust gases to escape directly to atmosphere; Shut off raw water to engine. Run engine (for a brief time ONLY) with exhaust manifold 'loose' but still connected to exhaust system .... if noticeable increase in engine rpm, then the injection elbow is probably blocked and the injection elbow most probably needs to be disassembled and cleaned; ........
.... AND, the raw water pump checked for proper water 'outlet' flow of ~3 gallons per minute flow at 2000 rpm engine rpm, at the exhaust outlet at the stern with bucket and stopwatch; ..... AND the heat exchanger checked for 'fouling' and degraded heat transfer characteristics.
An an injection elbow mixing chamber that is running 'too hot' will/can quickly become blocked ... with carbonates 'precipitating' out of the raw cooling water. The usual cause: insufficient raw water flow or otherwise fouled engine raw water cooling system.