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

[wmubronco]

74FJPBYC

Hello all, been battling an overheat issue since last season, which I was able to suppress, but now has shown it's ugly side again. I feel I am very close to resolution, so let me give you a quick background how last season ended.

2019
Chased the overheating for several outings, ended up putting a new seawater pump and thermostat in. Still a ramping up on temp. gauge. Ended up having a mechanic out there with me, we removed the thermostat and were IR gunning various points on engine and thermostat housing.... mechanic says, "your engine sounds like it's struggling, when was the last time you changed the plugs?" Next outing, checked plugs and the gaps were toast. All new plugs went in, and voila, no more overheating. It was my understanding the full firing helps reduce heat generated in the cylinder heads and full push out for exhaust. Great news, solved the head scratching annoyance once and for all.

Well, fast forward to this season...

2020
Thermostat was still out, and the first 3 or 4 outings, my temp gauge barely went past the first line (VDO dash), wow amazing, still in business from all the maintenance from previous season. As this season went on, started seeing temp. gauge slowly creep up, and currently it will rise up to the red line giving the temp. chirp alarm when I run maybe 2 minutes on plain. Not good. I thought maybe plugs went bad again, so put new set on, nope. Here's the new maintenance performed over the past 2 weeks;
- New plugs
- New spark plug wires
- New fuel filter
- New oil filter and oil change
- New distributor cap and rotor (see pictures of old ones)
- Checked impeller (see picture of current new one from end of last season)
- Reinstalled thermostat today (See pictures of VDO temp. gauge, first one rising temp, 2nd is almost red line, 3rd is slowing RPMs to get temp. back down)

Checked water flow as much as possible, getting great flow to the thermostat from the seawater pump which goes through heat exchanger, then PS cooler, then oil cooler and back up to thermostat. There was a blocked line (small hose on top of thermostat that goes back to reservoir which has the autosiphon valve filled with debris). Cleaned this out in hopes this would help, but no luck, temp. gauge still crept up to red line. After using IR gun on various spots, I think I have narrowed it down to hot water being restricted in the thermostat. Both cylinder heads were around 130F, but the thermostat was reading upwards of 180F.

With all that said, I thought I would also add I have lost top speed when the overheating started up this season. Usually could run at 3000-3200rpms, and get around 28mph. Now I am only able to maybe top 20-25mph at about 3400rpms. I mention this as I believe the weaker speed may be related to why the hotter water is not exiting the thermostat/engine.

I hope to reach a solution soon as I suspect it being something minor. Perhaps manifolds/risers are my next investigation. I look forward to the great insight this forum always provides. Cheers!

Patrick

 

[kenny nunez]

THis is just a shot in the dark but have you ever thought of by-passing the the water intake from the outdrive and converted to a through the hull intake? There could be a restriction between the drive and the raw water pump.

 

[alldodge]

Not finding your motor directly, think some letters are off

Should be a 7.4 G (as GM) saying a Gi , ok got that, but the F is where things don't add up

Is this a closed cool or open cooled motor?

Have you tired using a piece of clear hose between pump and thermostat housing?

 

[wmubronco]

THis is just a shot in the dark but have you ever thought of by-passing the the water intake from the outdrive and converted to a through the hull intake? There could be a restriction between the drive and the raw water pump.

I have not thought about doing this. As I mentioned at the end of last season and the first 3 or 4 times out this season, temperature was not an issue.

 

[wmubronco]

Not finding your motor directly, think some letters are off

Should be a 7.4 G (as GM) saying a Gi , ok got that, but the F is where things do add up

Is this a closed cool or open cooled motor?

Have you tired using a piece of clear hose between pump and thermostat housing?

Open cooled, as the water exits out of the exhaust near outdrive. Did not try the clear hose between pump and thermostat, as this is part of the engine overheating diagnosis from Volvo Penta guide.

 

[alldodge]

I'm wondering if your sucking air, attach a short section at the thermostat housing

 

[GA_Boater]

Open cooled, as the water exits out of the exhaust near outdrive. Did not try the clear hose between pump and thermostat, as this is part of the engine overheating diagnosis from Volvo Penta guide.

Open or closed, the water still exits in the same places.

 

[Scott Danforth]

a few things

that motor should be able to spin up well over 4600 RPM. if you could only muster 3400 RPM, you have additional issues (like fuel system, timing, etc.)

check the incoming raw water line at the outdrive. the stock fitting is a check every year, replace when you do bellows thing
check your manifolds if they are plugged

overheating is a sign of lack of water flow. either water flowing in, or water flowing out.

 

[wmubronco]

Open cooled, as the water exits out of the exhaust near outdrive. Did not try the clear hose between pump and thermostat, as this is part of the engine overheating diagnosis from Volvo Penta guide.

Misspoke, it is an open cooled system.

 

[wmubronco]

a few things

that motor should be able to spin up well over 4600 RPM. if you could only muster 3400 RPM, you have additional issues (like fuel system, timing, etc.)

check the incoming raw water line at the outdrive. the stock fitting is a check every year, replace when you do bellows thing
check your manifolds if they are plugged

overheating is a sign of lack of water flow. either water flowing in, or water flowing out.

I can get it up to 4000rpms today, but still only pushes just over 25mph, this was not the case earlier in the season. I don't know if I ever pushed it past 4200rpm last season.

 

[wmubronco]

What I am scratching my head at, with respect to losing the top end speed recently and the overheating is that neither condition existed during the beginning of this season.

Fuel system - I replaced fuel filter this season and had a full take of ethanol-free gas (just over 60 gallons). What other components should I check?

Timing - new distributor cap and rotor put on (put on in same orientation as old cap and rotor), and all new spark plug wires. I am a newbie on the timing subject, is there anything I can check without the need of a mechanic?

Again, trying to wrap my head around how my two conditions started to occur recently...

 

[Scott Danforth]

4200 RPM is still low if you never mashed the throttle against the stops to bring the engine RPM up to where its WOT range is, how do you know that this just started?

what is the contents of your fuel filter look like
what is your fuel pressure?

you need a timing light
did you try a clear hose like AD mentioned?

 

[wmubronco]

4200 RPM is still low if you never mashed the throttle against the stops to bring the engine RPM up to where its WOT range is, how do you know that this just started?

what is the contents of your fuel filter look like
what is your fuel pressure?

you need a timing light
did you try a clear hose like AD mentioned?

Haven't been back out there yet, marina is about a 45 minute commute from home.

Throttle all the way down, and hitting 4000-4200rpms. I suspect the problems starting recently as at the beginning of this season I was hitting 32mph at about 3600-3800rpms, and I did not have any overheating.

Since I replaced fuel filter, didn't check the old one's contents. I will pull the new one off and dump it to see if there's any water in fuel. I do not known/have a have to check to fuel pressure.

I will use the clear hose method on various spots to see if I'm sucking air.

On another thought, if my manifolds are risers or elbows were obstructed with debris, wouldn't they also have a rise is temp? When the alarm starts, the IR gun shows 128F'ish on the manifolds, and 136F'ish on the risers. I would suspect these to be creeping above 170F if there was water flow restriction.

 

[alldodge]

The exhaust might not increase in temp if restriction is in the block. Disconnect the large circulating water pump hose and check the vanes on it. Place something like a screw driver inside to hold the vanes and see if water pump pully can be turned. Looking for broken vanes or vanes have come loose from pump shaft

 

[wmubronco]

The exhaust might not increase in temp if restriction is in the block. Disconnect the large circulating water pump hose and check the vanes on it. Place something like a screw driver inside to hold the vanes and see if water pump pully can be turned. Looking for broken vanes or vanes have come loose from pump shaft

Am I turning the pully using the screwdriver? More I think about the engine water pump, I am starting to think this is where the water may not be being pushed at higher speeds.

 

[alldodge]

Am I turning the pully using the screwdriver? More I think about the engine water pump, I am starting to think this is where the water may not be being pushed at higher speeds.

The circulating water pump on the front of the motor just moves water in/out of the block. Your just checking if the vanes are intact and the vanes have not broken loose from the shaft. Look up inside after removing the big hose. Use the screw drive if things look good to see if the naves move with the belt still installed, do try hard, just want to know if its broken free or not

 

[wmubronco]

The circulating water pump on the front of the motor just moves water in/out of the block. Your just checking if the vanes are intact and the vanes have not broken loose from the shaft. Look up inside after removing the big hose. Use the screw drive if things look good to see if the naves move with the belt still installed, do try hard, just want to know if its broken free or not

Right, so trying to spin the vanes on the inside of the water pump to see if it drives the pully. If it slips and does not, that's an indication the inside shaft is busted up, no?

 

[alldodge]

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

 

[Lou C]

The thermostat you replaced, is it for sure a marine unit? Most Volvos use a 160 stat. Take it out, get a radiator tester thermometer and put it in a pot of water suspended by a coat hanger. Put the pot on the stove and test it, it should be mostly open at 160 or 165 or so. It sounds like from the reading you have taken the stat is opening at too high a temp. Your elbows temp at 128 is not really excessive if you were running on plane. I have found with the OMC/Volvo style thermostat you get the most accurate temp reading (closest to the dash gauge) when you take an IR temp reading on the intake manifold right below the stat housing. Mine will usually read 10 degrees lower than what the dash gauge reads when checked there.

 

[wmubronco]

The thermostat you replaced, is it for sure a marine unit? Most Volvos use a 160 stat. Take it out, get a radiator tester thermometer and put it in a pot of water suspended by a coat hanger. Put the pot on the stove and test it, it should be mostly open at 160 or 165 or so. It sounds like from the reading you have taken the stat is opening at too high a temp. Your elbows temp at 128 is not really excessive if you were running on plane. I have found with the OMC/Volvo style thermostat you get the most accurate temp reading (closest to the dash gauge) when you take an IR temp reading on the intake manifold right below the stat housing. Mine will usually read 10 degrees lower than what the dash gauge reads when checked there.

Thermostat is rated at 140. Brand new one as well. Even if removed, the housing is running around 170-180F right around the temp sensor.