The stock Jabsco raw water pump is not the problem. The stock Lehman-designed raw water pump drive coupler is. It has a fatal flaw in its design and manufacture but it either works or it's failed. There is no middle ground. And when it fails the raw water pump stops turning right now. So from your description of the problem the Lehman drive coupler (if your engine is still equipped with the stock raw water system) is not the cause of the problem.
There are a number of things that can cause overheating in an FL120 (or other simiilar engine). The ones we've encountered so far are:
1. Leaking coolant pump. As on a vehicle, these pumps have a finite life and when the seal(s) go the pump begins to leak or blow coolant.
2. Collapsed raw water hose. If the raw water hose leading to the raw water pump is old, soft, or of the wrong type, the suction from the pump can cause it to partially or completely collapse. But when the pump is not running the hose will appear normal.
3. Blocked or partially blocked raw water intake. This can occur for a number of reasons--- barnacles or other growth, grass/weed that's been sucked up into and jammed inside the through-hull, too much bottom paint on the external strainer (f your boat is so equipped) that's closed down the holes or slots. We've even experienced a weird, crystalline growth that gradually closed off the port raw water through-hull to the point where the engine began to run hot. The diesel shop had to chip it out with a screwdriver. They said they'd seen it only a few times before, and the timing was such that it had obviously been "growing" in the through-hull before we bought the boat. Whatever it was, it has never come back.
Other causes could be:
1. A partially blocked heat exchanger--- main, lube oil, or transmission fluid.
2. A partially blocked or delaminated and collapsing raw water hose between the various heat exchangers.
3. A failing or failed thermostat that is no longer opening up enough when the coolant gets hot.
4. A worn raw water pump (the Jabsco unit on the end of the drive coupler). Problems could be a failing impeller or a too-loose fit between the cover plate and the front surface of the impeller and the wear plate and the rear surface of the impeller. If the impeller is not a tight fit between the cover and wear plates, it in effect loses "compression" and the water flow will be reduced. It will still pump water but perhaps not enough to keep the coolant temp where it should be. If this turns out to be the problem the pump can be overhauled with a new impeller, cover and wear plates, and bearing/seal. In fact I believe this pump is still available new off the shelf from Jabsco.
However.... are you SURE the engine is actually running too hot? A coolant temperature gauge reading can vary with the resistance in the line connecting it to the sensor or ground. Also, the temp sensor itself could be a bit flaky. This is NOT to say that you should assume it's the gauge or the sensor and continue running the engine as-is. The fastest way to kill a Ford Lehman 120 is to overheat it. Even for just a few minutes. The head gasket will go right now followed by the head warping and there is also a risk of severe cylinder wall damage. I have heard this directly from people in the UK with decades of experience with the base engine (Ford of England Dover engine) in trucks and industrial and agricultural applications.
We have had three precautionary engine shutdowns in the thirteen years we've owned the boat, each of them due to cooling issues. One was a leaking coolant pump that gradually dumped coolant into the drip pan during our first run with the boat from Tacoma to Bellingham after we'd had the boat trucked north from California. The other two were due to blockages of the raw water intake through-hull. In each case as soon as we noticed the coolant temperature starting to climb we shut the engine down, tied off the shaft, and finished the trip on the other engine.
Learning the FL120s vulnerability to overheating is what prompted us to adapt the use of a small oven timer at the helm set for a five minute countdown. The timer beeps, and whoever is at the helm scans the engine instruments and then puhes the restart button on the timer. It's too easy to get distracted when running a boat and looking out the windows or at the nav equipment or talking to friends and a long period of time can go by between engine gauge checks. Sure, the FL120 has oil pressure and coolant temperature alarms but I learned a long time ago that the purpose of an alarm is to let you know that the piece of equipment the alarm is monitoring has just failed. I got the idea of the timer from two places--- Carey of this forum does the same thing and the trans-Australian railroad. We also put thin pieces of black tape on each instrument (except the tachs) matching the normal indication of that particular gauge. That way the briefest of glances tells the helmsperson if a reading is off.
You can get as sophisticated about this as you want. A number of people on the Grand Banks owners forum have installed raw water flow alarms on their boats. Some have temperature sensors on their oil, transmission, and main engine heat exchangers. These are all great ideas. I've just not been motivated enough to make the effort, hence the oven timer
One thing I don't know that RickB or someone might be able to tell me is if the EGTs (pyrometers) on our boat would be of any value as overheat indicators. I'm inclined to think not as the change in the EGT readings might be too subtle to mean anything until it's way too late.
PS--- I want to add that, regardless of what suggestions I or anyone else on an internet forum gives you, an overheat situation is not something to be taken lightly.* It is my opinon that unless one is well aquainted with their engine's systems and is real good at troubleshooting, the problem is best turned over to an experienced and reputable pro.
-- Edited by Marin on Saturday 30th of July 2011 08:43:46 PM
