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Stumped, 79 351w running hot at idle

Johnnye23

Johnnye23

New member
My 79 351w is running hotter than I like 185-195 at idle,comes down to 160/170 when pulling a skier at approx 3000 rpms. Here is a list of what I have done thus far .
1. New impeller in raw water pump
2. New wire reinforced hoses installed, as hoses were removed visually checked connections, trans cooler for any debris
3. raw water pump flow test 15 sec/3000 rpm was good.
4. New thermostat 143 degrees
5. Removed circulating pump inspected bearings/impeller all looked and felt good( note it looks to be a bi directional pump).
6. Removed sender and gauge and set up on kitchen counter with a battery ( desperation mode here) placed sender in a pot of water with known good thermometer and verified gauge was reading accurately.
7. Using a fake/lake idle temp is only 170 in the driveway

I would think it was a flow problem because the temp decreases with RPM/boat movement . Could it possibly be a head gasket
or cracked head ( no water in the oil) ? I plan on doing a differential leakdown today and will post results. As well as a vacuum test at idle.

Anyone have any ideas or suggestions they would be well received. Thanks in advance . :confused::confused::confused:
 
you need t measure the raw water pump's flow at idle as well as cruise...you say it was good - what was it?

when was the last time the exhaust elbows were changed (or inspected)?
 
makomark said:
you need t measure the raw water pump's flow at idle as well as cruise...you say it was good - what was it?

when was the last time the exhaust elbows were changed (or inspected)?
Click to expand...

I could not find any specs for a flow test at idle , at 3000 rpm it moved 5 gallons in 15 secs. I have not inspected the risers as of yet , I was heading to the hardware store this morning to buy some clear 1" hose and replace the hoses going to the manifolds to watch flow and check for bubbles . If you think I should pull and inspect the risers I will gladly do it and cross that off the list too. I'm really stumped here thanks for you input
 
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most pumps are sized to deliver at least 7.5 qts in 15 seconds, with no pressure head...

Since you didn't indicate their age, I'd be inclined to start with the exhaust elbows as they are high on the list of cooling issue problems.
 
makomark said:
most pumps are sized to deliver at least 7.5 qts in 15 seconds, with no pressure head...

Since you didn't indicate their age, I'd be inclined to start with the exhaust elbows as they are high on the list of cooling issue problems.
Click to expand...


Thanks for the direction I will yank risers and update thx !
 
Removed risers no signs of any restriction no water intrusion into exhaust manifolds The gaskets for risers
were installed in correct orientation as per PCM instructions . image.jpgimage.jpgimage.jpg
 
Had same problem on a 302. Check the Raw Water inlet under the Boat and the Exhaust Manifolds; you have already checked on the Raisers but did not mention the Exhaust Manifold. The Thermostat is quite sensitive and the PCM issue should be used.
 
I'd say being the elbows are clear, you need to check the pump's output at idle...if less than spec'ed, I'd investigate the raw water pump. you said you changed the impeller...what did the rest of the internals look like?
 
Checked the hull pickup and no obstructions,orientation was correct .Removed the exhaust manifolds ,removed inlet and outlet plugs. Found no sediment or debris.Took a hacksaw blade and fished it thru as best I could but didnt find any blockage. I removed a freeze plug from the block no sediment found in the motor. Did a leak down ( cold motor ) had 6% on all cylinders but #7 . Was getting air thru the valve cover though I must not be on TDC with the piston.Checked two more times ( rotating engine back to #7 TDC) and it showed 40% leakage. I went ahead and removed the l/h head and it looks like brand new. I found no evidence the headgasket was leaking anywhere.I layed the head on its side and filled #7 intake with fluid and got no leakage past the valve.Poured some oil in #7 cylinder and let sit no sign of any drainage. Bore looks good no visual grooves or scratches or of course cracks. I am really scratching my head ,as I expected to see some serious damage in the #7 cylinder . I am inclined to think I just didnt have #7 at TDC when doing the leakdown. The engine has been gone thru at one point pistons are .020 over size and heads are using the smaller diameter sparkplugs . Any ideas guys ? Dayum I sure make extra work for myself.
 
Did you check the raw water pump's product at idle? results?

did you inspect the raw water pump when the new impeller went in or did you just swap impellers?
 
makomark said:
Did you check the raw water pump's product at idle? results?

did you inspect the raw water pump when the new impeller went in or did you just swap impellers?
Click to expand...

I did do a good visual inspection makomark . I took the brass plate and as a precaution and did a quick surface clean up using a flat surface and some 500 grit wet/dry paper. Installed new Sherwood impeller and new O-ring and as a precaution . Also a thin coating of rtv to edge seal in addition. Made double sure that the 3 case half bolts were snug. Any possibility a shaft seal could be in backwards and air is getting sucked in the pump ? There are no external leaks from the RWP.


When I get the heads back on I will update the idle RWP flow asap. Thanks again for all the input .
 
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Anything is possible regarding the seal but I'd expect it is ok unless somebody serviced the pump at the onset of the cooling shortfall.

How were the "fingers" on the cam?
 
Good pics but I can't see enough detail in them...for the first photo, are the tips of the cam's fingers pointed or blunt? also, is the top set of fingers symetric to the bottom set?

photo 2 - looks like a radial imperfection across the lower three fingers - real or a photo artifact?

pump body - looking into the impeller cavity, can you feel any step where the impeller shaft recess meets the annulus where the vanes ride (is the dark circle "higher" than the rest of the back wall in the body?)
 
Replaced raw water pump,sending unit ,head gasket and cleaned all grounds ! Took the boat out and it ran flawless at 160 degrees all day long

Many thanks to MakoMark for his suggestions and input. I'm one happy guy !
 
Glad you got the pump together and can go boating now...

One other suggestion: Now that the engine is "right", take and IR temp gun and get temp readings so that you will have a good baseline for troubleshooting in the future....
 
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