Oct 122014
 
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If you know me or my blog, you know I watch Lake Powell. Most of my posts are about the water level increase expected each spring and early summer. But many boaters have reported weather problems at the lake that weren’t related to dust storms, thunderstorms or wind storms. Triple digit temperatures are a common occurrence on Lake Powell. The high temperatures can cause engine problems.

Waterskiing at Lake Powell

Waterskiing at Lake Powell

On of the intended benefits of fuel injection was to prevent vapor lock. The gut reaction of most “gear-heads” and marine mechanics is that a fuel injected engine can not suffer vapor lock. I’m now a believer that there is a mechanism for vapor lock in a marine engine.

Marine engines run in a tough environment. The stern drive motors are typically inside a small engine compartment with limited ventilation and exceptional noise insulation. It’s a great environment for containing heat. The lower unit of the stern drive takes up fresh water and pumps it to the engine and support systems. It’s a great way to cool a motor while it’s running. But when you shut down a boat’s engine, there isn’t a natural heat release path. You can run the engine compartment bilge blowers to provide air flow. But there is no radiator like in a car. In the Lake Powell’s summer temperatures, blowing 110 degree air over a 155 degree engine has little effect.

To protect the fuel, a fuel cooler is a part of the cooling system. The fuel cooler’s intent is prevention of fuel vaporization due to high engine compartment temperatures. The water comes in from the outdrive, flows through the power steering cooler, then the fuel cooler, before going to the engine’s thermostat housing. This doesn’t work after the engine is shutdown, either.

Over the last few years our boat driving habits at the lake have changed. We used to spend lots of time on the beach with brief runs for water-skiing or wake boarding, or sightseeing. Now, we tend to do longer runs with plenty of canyon exploring and trips to Rainbow Bridge with friends. This means we run our engine longer and the compartment and engine gets thoroughly hot. Here is a temperature profile of the engine during a summer weekend trip. I measured the temperature on the top of the thermostat housing lifting eye. So, it wasn’t in direct contact with the housing or engine.

Engine compartment temperature profile, July 18-19, 2014.

Engine compartment temperature profile, July 18-19, 2014.

The heat build up is fairly quick. Idling the engine after a run provides a fair amount of cooling. But after shutting down the engine, the cool-down is much slower. Opening the engine hatch provides a better cool down of the top of the engine. Shorter runs result in a much faster cool down. We had to open the hatch because the engine wouldn’t start; we hypothesized vapor lock. But the engine compartment temperature had cooled much at this point. Shouldn’t it have started?

There is a theory about some marine engines. The theory is that some of the engine cooling water flows backwards after engine shutdown. This takes very hot water out of the engine and back the way it came. The first stop on the way out is the fuel cooler.

My son and I tried to replicate the outcome. We took our boat on a long, roughly 40 mile round trip. We went straight back to our slip at Wahweap Marina and shut down the engine as quick as possible. We watched the temperature on top of the thermostat housing lifting eye, and the air temperature inside the fuel cooler box, just an inch or so away from the real heat exchanger.

Thermostat housing and fuel cooler temperature

We watched the fuel cooler temperature quickly move from 101.5 degrees F to 121F at 15 minutes, and hit a top temperature of 124F by 30 minutes. At 45 minutes, we thought the temperature had stabilized and attempted to start the engine. It turned over, but didn’t start. We took a video the fuel being injected at the throttle body when cool, and again when it failed to start.


The fuel spray in the hot video is quite different with spurts, drips and gaps in the flow. The cool fuel spray is well atomized. What a difference!

On other trips, I tried to measure the temperature of the cooling hose that allows back-flow to the fuel cooler. Since the data logger I used had a self-protection turn off at 140F degrees, it only produced a few minutes of data as the temperatures climbed above 140F. Back-flow was the likely issue.

Mercruiser is the manufacturer of my engine. They have provided a tech bulletin about this vapor locking issue on their V-8 engines.

Mercruiser Vapor Lock Tech Bulletin

One of the solutions Mercruiser gives is to insert a check valve in the cooling system to prevent back-flow. I opted for this option. I installed the check valve in the cooling line between the fuel cooler and the thermostat housing. When I removed the existing hose, I found that it was mostly empty. To me, this validated the back flow problem as an issue. The top part of the engine coolant drained out via the fuel cooler. With the included instructions, the valve was remarkably easy to install. An experienced mechanic could do it in probably five minutes. I invested 45 minutes. Here’s a picture of the installed valve.

Cooling line check valve installed. Blue with yellow in the lower right.

Cooling line check valve installed. Blue with yellow in the lower right.

To test the repair, we did a high-speed run to Rainbow Bridge. When we shut down the engine, I watched the fuel cooler temperature. At 15 minutes, the temperature had dropped by 0.3 degrees. In other trips since then, it has occasionally increased, but never above 103F.

Every pump has a low pressure intake, and high pressure discharge. The fuel pump takes it suction on the outlet of the fuel cooler. The pump’s suction can vaporize the heated fuel, even if it is below boiling point. I wanted to record this some place because many boaters at Lake Powell to get hit by this phenomena.

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