Operating Temp Fluctuations - Maxum Boat Owners Club

Dustin07 · 06-05-2020, 03:26 PM

I don't think I have any issues but I am curious about something. Hoping mmwjr Mike will reply since I think he and I have the same motor.

Cruising at about 3k-3200 rpm I'm sitting around 21knots by GPS and normal cool operating temp, around 185-190 I believe (gauge shows 25 degree intervals so it's tough to know exact.

Bump throttle up to around 3600+ RPM and cruising speed raises to around 27knots + still very smooth everything sounds good. Operating temp raises to right about 200F perfectly but stays there. I was at first concerned it was over heating

so bumped it back down to 3200 for a half hour or longer, nice cruising speed again back around 21 knots, temp drops back down to that 185-190 region.

Raise it up to get back up to about 27-28knots and temp raises to about 200. but it stays there, so it's not out of control.

I seem to remember my old CTS-V with the LS2 Motor doing the same thing, I feel like during summer time that car would find 205F as it's high point when pushed hard but wouldn't pass it.

are these normal changes in your boats?

mmwjr · 06-05-2020, 04:37 PM

If you have closed cooling 200* may be ok but if it’s an open raw water cooled engine that’s to hot. Open cooling should be 160*. Use a IR temp sensor and take readings of the engine thermostat housing and exhaust components when you out there running and see these conditions. The helm gauge may be off. The sender is in the thermostat housing and gets ground by two brass eyelets in the gasket, these are know to get bad with age. Have you changed the impeller lately? How old is the exhaust?

Dustin07 · 06-09-2020, 03:27 PM

So I (and coincidentally two other people) had posted this exact thing in the maxum group on facebook and it seems like everyone had exactly the results you just posted. People either ran 185-200, or they hung steady at 160-175. No discussion of cooling system was there, so I bet the difference was just that.

My exhaust was redone in 2016 so it's sorta in that grey area. a replacement now seems a little early but I was planning to do risers this fall.

The impeller was planned to be swapped out this fall and both the surveyor and previous owner thought that sounded correct (I think it's 2 years old now).

Looking up the impeller for our motors it seems cheap so I may just go ahead and order it anyways. I've never done an impeller before so I'll need to research that a bit and see if it's something I can do myself.

mmwjr · 06-09-2020, 04:58 PM

The Bravo drives use an inboard engine mounted belt driven raw water pump. You can find the Bravo manual under Documents of this site. Basically remove belt and two hoses. Remove two bolts holding pump bracket to engine block. Disassemble pump swap impellers and reverse to install.

Phillbo · 06-09-2020, 10:47 PM

I use a little dish soap on the impeller to work it back into the pump housing.

mmwjr · 06-10-2020, 02:27 AM

Quote:

Originally Posted by Phillbo

I use a little dish soap on the impeller to work it back into the pump housing.

So do I.

shrew · 06-10-2020, 01:26 PM

At 200 the overheat alarm should be coming on. Just because someone else reports the same thing, doesn't make it right. Bravo III's had an issue with corrosion that starts to constrict the water intake as it comes through the transom assembly.

I had this issue on my 2800, and it manifested itself by increasing temps with increased rpm's as well. replacing the transom assembly resolved the issue. Once the transom assembly was removed I could see the intake hose being crushed.

Dustin07 · 06-15-2020, 05:01 PM

awesome that's important feedback, thank you! working on impeller now and I may look at risers as well.

Dustin07 · 07-23-2020, 04:07 PM

So we had a good cruise last weekend but it wanted to creep up in temperature again whenever I broke over 3k rpm. Stopped by my local marine mechanic and we discovered the raw water pump is shot. We should have that fixed and be back on the water by weds/thursday of next week to see if that's the only issue. fingers crossed.

mmwjr · 07-23-2020, 04:29 PM

Quote:

Originally Posted by Dustin07

So we had a good cruise last weekend but it wanted to creep up in temperature again whenever I broke over 3k rpm. Stopped by my local marine mechanic and we discovered the raw water pump is shot. We should have that fixed and be back on the water by weds/thursday of next week to see if that's the only issue. fingers crossed.

So you never replaced the impeller?

Dustin07 · 07-23-2020, 04:49 PM

actually that's what led to doing the whole water pump. I was going to do the impeller but noticed that the pump was not performing/wobbling a bit on the pulley. the shaft is pitted and the bearings are shot. so the timing to do both is perfect now. Plus we noticed the belt is getting quite worn so doing all three.

Calguy · 07-24-2020, 12:36 AM

My re-powered 1995 SCR 2400 had a leaking sea water pump a few months ago. Replaced by a Merc 2008 Horizon 350 for Bravo 2. I replaced pump with the Hardin Marine Stainless Steel sea water pump. It was within 20.00 of the Merc OEM pump. The swap was very easy. Running temperatures have always been between 159 and 165 with FWC. Best of luck to you.....

Dustin07 · 07-31-2020, 05:01 PM

The water pump, impeller, and belt have now been replaced. We have two heaters on the boat and they may be poorly placed above the motor, which could be creating both a small coolant leak as well as flow issues so we are bypassing those since I have no need for them anyways for now. Temps are getting better, stay tuned...

Dustin07 · 07-31-2020, 05:02 PM

scratch that, we have three heaters on the boat, one wallas in the cabin, a red dot, and then another that vents at the helm. since they are only working when the motor is on I have no need for cabin heat while cruising, so we are bypassing them. This winter I may reconfigure them better.

mmwjr · 07-31-2020, 06:46 PM

How are they plumbed, in parallel with each other? Supply hose should come just before the thermostat and return on the suction side of the circulating pump.

Dustin07 · 07-31-2020, 07:30 PM

I'll be back on the boat here in a couple hours and will try to get you a picture. we also took apart the heat exchange and found some sea weed in there. we are getting much much better water pressure now. Boat maintaining temp at 3500rpm with what was 3000 rpm. however it still wants to approach 200f after running a bit.

we are replacing the thermostat too which appears to be the wrong one for this motor.

next week will probably be the exhaust elbows.

jparsons121 · 08-02-2020, 01:50 PM

While you’re at it, check the power steering cooler for blockage. In most 5.7applications this is the first choke point after the pump.
I’d also suggest doing the volume test outlined in the manual to be sure the flow is in spec.

Dustin07 · 08-03-2020, 02:54 AM

So we are getting over 10 gallons per minute of flow. New water pump new impeller obviously, new serpentine, new thermostats. Bypassed the heater, checked the power steering. Only thing still showing hit is elbows the the mechanic and I are planning on doing those on Wednesday

Took it out this week and man this motor wants to run. It starts so nice and sounds good. But anything over 2k rpms and the heat breaks 180+.

If I hit 3k it still goes to about 190+.

If I go any higher it tries to touch 200f but I won't let it I slow down and it drops back to that 180-185 range.

At that point if I gun it again it immediately cools off very fast back down to a nice even 175. However it starts creeping back up.

Water flow is very good. Risers are nice and cool. Last item we can think of is elbows as they are warm.

We also took apart the orca heat exchanger and found a ton of seaweed in it but that didn't fix the issue.

Dustin07 · 08-03-2020, 02:54 AM

Sorry for the typos...replying from the phone ...

jparsons121 · 08-03-2020, 01:29 PM

Ok if you verified 10 gal/min at the pump output, that’s in spec (Merc calls for 7.5 gal/min at exactly 1000 rpm in the water not on muffs). Only two remaining things come to mind. 1) Restriction on the transom tube. Had this issue on my BII’s where mussels partially blocked the transom hose. Flow test at 1000 rpm wouldn’t show an issue as it was only partially blocked. However, at higher rpm there wasn’t enough flow to keep up with the demand.
2) Restriction in the output side of the system. Since you have gone thru most of it, I’d say that the next suspect is the elbow/risers.

Good luck!