2000 GT-40 Overheats at planing speeds

After 19 years of good and faithful service, my 2000 SN (Ford PCM GT-40) has engine overheating problems. Idles nicely at 165F at the dock and underway at 4mph, but when up on plane at say 25mph gets up to 200F after just a few min. The port (passenger) side exhaust system runs much hotter than the starboard.

Cooling water system output I think is fine. Replaced water pump impeller (old one was shredded - more on that later) & thermostat; strainer is good. Tested the output with the boat in the water; flow from the thermostat to the port side exhaust manifold filled a 3 gal bucket in 1 min; on the starboard side it took 50 sec.

Exhaust system on port side runs very hot. I replaced the exhaust pipe & 16" hose leading to the muffler. There was some disfiguration on them, I assume caused by high temps. Took off the riser/elbow and it seemed fine, put in a new gasket and re-assembled. Flushed out the exhaust manifolds & got some crud out. Engine still idles fine but overheats at speed; no improvement.

Have searched for pieces of impeller vanes in all the usual places, but have found very little, which worries me. Could they have flushed out? Or are they trapped somewhere in the internal engine block cooling water passages?

Checked the exhaust ports, leading into the ex man with my new temp gun. Port side runs about 20F hotter than starboard side. But the Ex riser on the port side runs about 55F hotter than the starboard.

So -- does all of this indicate that my problem is a bad ex manifold on the port side? I can get a new one (man + repair kit) from DIM for $327.   Or -- are those pesky old impeller vanes hidden somewhere in the block and causing my problem, necessitating a very difficult engine disassembly?    Thanks in advance for any advice!

You can verify / confirm that your temp sending unit and gauge are both working correctly with your I.R. gun. You may want to run at full speed and get the temp to the 200 degree mark that you mentioned. Quickly back off to idle and kill the motor. Then shoot the temps of:

- Thermostat housing
- Intake manifold at cooling water inlet
- Water pump (not raw water pump)
- Exhaust manifold risers
- Block near heads at each cylinder
- Etc.

That should give you a better picture in real-time. I'm wondering if debris or a piece of the old shredded impeller is causing some kind of obstruction.

JQ

JQ:
Thanks, good suggestion, I'm away from the lake this week but will try next weekend. But I would say that the dash temp gauge seems to be working properly.

Hmm. I took off, drained, & re-installed w fresh teflon tape:
- exhaust man & block drain (port)
- exhaust man & block drain (stbd)
- U tube drain by water pump (stbd side)
- transmission cooler plug

All seemed to go smoothly. I don't have a torque wrench, but tried to tighten all back firmly. No stripped threads or anything. Where would you suspect an air leak?

TRBenj,
Thanks. I already pulled the port (hot) side riser off; inspection seemed OK, and installed a new gasket. But haven't pulled the port exhaust manifold yet. I'm worried about breaking the bolts... But if that's where the problem is I'll have to go for it. Any suggestion on a penetrating oil that might help get the old bolts out?

When you say to possibly swap the gaskets, do you mean reverse the front/back? I thought the correct way was to have the blocked water passage go towards the exhaust outlet. I put the new one on that way - same as the old one.

Let the bolts soak with atf+acetone for a bit... but the best defense against breaking those bolts is heating them up a few times. Hot hot.

Not swapping gaskets, swapping risers. If you find minimal/no blockages then swapping side to side will allow you to see if the problem follows the riser or not- and help isolate where the blockage is.

Like HW said, you’ll have riser cooling issues if the gasket is installed backwards.

Keno, thanks. I did take off the inlets (90 degree brass fittings) on both sides. Looked w flashlight, probed around with my little finger. Got some crud out (sort of like rusty sand). I had hoped to find large impeller chunks, but did not. I also removed the ex man drains, and flushed w garden hose. Also back-flushed. Got some sandy crud out, but not any satisfying size impeller blade pieces. Afterwards, no improvement in the overheating...

Argh! I did not check the INLET to the water pump! Will do asap. I assumed that the flow would take any broken impeller vanes downstream - to the thermostat, exhaust headers, etc. Thanks for the suggestion. If it works it will save me a ton of time + $!

If your impeller is found to be failed and missing chunks you need to search inside the cooling hoses until you find all the large pieces. A small piece should blow through but the larger ones can plug things up. I have found chunks blocking the thermostat inlet, not sure it that was inspected yet on your boat..

I seem to remember reading not too long ago here on CCF about an overheat issue at higher RPMs that was a hose collapsing on the inlet side of the RWP. As I remember is was fine at idle and lower RPMs and would collapse at higher suction and starve the RWP.. Also have you checked the Transmission cooler inlet screen for possible partial restriction????

Bad baffle in my muffler caused temperature differential between 2 risers. It also caused a loss of top end power.

The muffler would also pillow up when engine revved in neutral. Sound was different too.

Success!
After receiving good advice from bfootr & others, I checked the water pump INLET. Tons of broken impeller chunks found blocking the entrance to the pump housing. Cleaned everything out and now boat runs great! I had been looking downstream of the impeller only. Lesson learned!
THANK YOU to all here who offered suggestions and saved our July 4th family ski vacation!

Gotta love the knowledge available on this site !!!