Volvo penta 7.4gi loss of top speed perhaps tied to slight thermostat overheating?

[Lou C]

but did you test it?
a 160 stat should start to open at 155-160 and be fully open by approx 165 you should use a radiator thermometer to verify and measure how far it opens, if it opens too late or not far enough you can have running too hot issues
as Ronald Reagan once said trust but verify
also:
what do the passages in the stat housing look like? Make sure there are no bits of old impeller vanes in there. Make sure the bypass port is open....
besides testing for air getting into the cooling water flow, you may need to test for exhaust gas getting into the cooling water. This is a tough to track down cause of overheating the bad news is that it is caused by leaky head gaskets.

 

[wmubronco]

Hoping to get down there tomorrow to check a few more areas out with the guidance from you guys. Thanks for the info.

 

[wmubronco]

correct, also look at them to see they are not damaged or corroded away

Very tough to see in there. Long screw driver is not really hitting anything in the cavity.

 

[alldodge]

Your right, forgot the old 7.4's has the vanes further back and down
Get a piece of stiff wire and use it to see if you get any kind of feel. Only other way is to remove the back cover of the pump

 

[wmubronco]

Your right, forgot the old 7.4's has the vanes further back and down
Get a piece of stiff wire and use it to see if you get any kind of feel. Only other way is to remove the back cover of the pump

There were two tiny ports I could unscrew but neither could really show any visible signs of the vanes. On either end of the larger diameter hose that runs between pump and thermostat, I should be able to check pressure generated by pump by covering with my hand and someone revving the rpms for a quick second or two, no? Only other option would be to remove the pump assembly from front of engine to inspect.

 

[alldodge]

I should be able to check pressure generated by pump by covering with my hand and someone revving the rpms for a quick second or two, no?

No, its called a pump but there is no pressure or suction developed, or not enough to notice. Its a circulating pump which means only that it moves water thru the motor when restriction is low. Like your hand going thru water, it does something, just not a lot

 

[wmubronco]

Talked to mechanic over the phone who knows the boat fairly well and helped me out troubleshoot last year. He thinks not all 8 cylinders are firing and recommended I perform a compression test on each.

Any recommendations for where to pick up a cheap tool? Harbor freight? Autozone?

 

[alldodge]

Many local auto parts stores types have tool rental programs.
Yes HF would be a cheap place to get one

 

[wmubronco]

Was able to get the compression test done, took photos of each cylinder and also the 8plugs (which are fairly new this season);


SP3:

SP7:




Compression ranged between 130-150psi on each cylinder, my ratio is 9.1:1, so should be all good there. Called my mechanic after this and he questioned my prop, which I currently have a 14 1/4 x 21 , 3-blade stainless. I said that I should try an 18inch pitch, does this make sense? Did not have time to pull the prop to check the hub yet, but want to do this next time I am down there, perhaps it's slipping at the higher RPMs. Mechanic also mentioned maybe my exhaust is blocked leaving the boat, which would not allow the hotter water to get out of the engine quick enough. What would be easiest way to check this?

Also, he said to replace the two sensors going into the throttle body, does this also make sense? Can be seen here;



I also met a fellow boater this weekend who has the 99' model year of same boat, 245 Sundowner, which mine is the 98. He said he had loss of top speed, and ending up replacing almost everything I had listed above. It ended up being an $8 part for the siphon-valve for the fuel lines.... which is starting to make me wonder about the fuel system not supplying enough fuel. I will have to pull filter off next time I am out there to check to see if there's any separation in the gas.


I guess I can rest a little easier knowing compression is good, and not having to go down the line with pistons/rings/gaskets.

Getting closer..... appreciate any further insight.

 

[alldodge]

First and foremost your mechanic is a parts changer, he should not be allowed around a motor he (him self) is not spending his own money on. When needed, listen, smile and then after he leaves forget what he said

Your plugs are showing your running on the lean side. Running lean increases heat and can also destroy the motor if it gets to lean. Your plugs should have a bit of a tan color to them, those are white

Your compression is good

Yes your antisiphon valve can restrict fuel and if it does it can cause a lean condition

For now lets forget about the overheat issue and work more on the fuel issue. Measure fuel pressure at the rail and should be 39 +/- 3 psi. This is measured from idle thru WOT

 

[wmubronco]

Thanks for quick response, I'll hopefully get down there tomorrow or Friday. I'm guessing if is low pressure, it's either blocked or fuel pump is on it's last leg?

Will add there is a slight hesitation when I do open the throttle to get out of the hole. I like where this path is going and to meet the fellow boater the other day who was basically in the same shoes, just with a 1 year newer boat, it had to of been a sign...

 

[wmubronco]

After looking at some other threads, I'm leaning towards two inexpensive routes, 1) fuel pressure regulator screen (apparently a bulletin out to replace this?), and 2) anti-siphon part should be cleaned/replaced.

Just want to make sure I am looking at the right two parts and their location on the fuel system.

 

[kenny nunez]

In addition to the anti siphon valve try to pull the suction tube out of the tank and check there is no fine screen filter inside of the tube. Then the best way to check a tank problem is with a 6 gallon outboard tank connected to the fuel pump .

 

[wmubronco]

Regulator and anti siphon location in this diagram?

 

[MI CC]

Anti-siphon valve is the fitting coming out of the gas tank that the gas line is connected to. Just replace it as it could look clean and still not be working correctly.

 

[wmubronco]

Anti-siphon valve is the fitting coming out of the gas tank that the gas line is connected to. Just replace it as it could look clean and still not be working correctly.

Good deal, as this was the $8 part that saved another guys boat who I met over the weekend who described some of the loss of power he was having that seems like mine.


As far as a fuel pressure tester, this should work for the Schrader valves, no?

https://www.autozone.com/test-scan-...ection-pressure-tester/273356_0_0?spps.s=4352

 

[wmubronco]

Went to boat yesterday in an attempt to clean out some screens on the fuel system but did not have any luck. Had the fam on the trip and my window to troubleshoot was about 20 minutes. Took a few pics to try to identify where the potential problem could be (the two pictures with green circles are the areas I was able to check but they were free of any blockages), as I worked my way from low pressure pump to high pressure pump and then inlet to engine;






Since there were no blockages at these two points, I figured fuel could get to the low pressure pump and also into the engine. My guess is either somewhere in between or right at the fuel tank.

My next trip out there needs to be solo so I actually have the time to work. After looking around I cannot seem to locate the fuel pressure regulator, maybe I do not have one on my engine?

Also figured I will rent the fuel pressure tester from auto store and bring multimeter to test power to both pumps.

Any other ideas at this point?

 

[alldodge]

What was the fuel pressure reading?

 

[wmubronco]

What was the fuel pressure reading?

I did not get a chance to do this. I had minimal time to work on it and thought I could try to find the "screens" that may be preventing gas glow. Did not seem to have luck since only checked two ports on the lines.

If my preasure drops or goes higher than spec, what would be my next checkpoint?

 

[wmubronco]

First and foremost your mechanic is a parts changer, he should not be allowed around a motor he (him self) is not spending his own money on. When needed, listen, smile and then after he leaves forget what he said

Your plugs are showing your running on the lean side. Running lean increases heat and can also destroy the motor if it gets to lean. Your plugs should have a bit of a tan color to them, those are white

Your compression is good

Yes your antisiphon valve can restrict fuel and if it does it can cause a lean condition

For now lets forget about the overheat issue and work more on the fuel issue. Measure fuel pressure at the rail and should be 39 +/- 3 psi. This is measured from idle thru WOT

Should be okay to run pressure test in neutral or do you recommend to perform while in gear?