Riser (elbow) temperature

[saf]

What is riser (exhaust elbow) normal temperature at WOT for fresh water cooled 5.7 V8? I've installed temperature sensors in both of my risers after having problems with overheating (the problem was air leak in the outboard water intake line, now fixed). I verified temperature readings with a remote thermometer. With outboard water temperature around 60F I am seeing riser temperatures around 70F below 1200rpm, the temperature goes up to 175F at 2500rpm and starts pushing 212 at WOT, at which point I drop rpms to avoid overheating. The engine temperature is rock solid around 150-160F. Both risers are new and heating up to roughly the same temperature. I cannot verify that riser temperature is actually reaching 212 since I have to stop to make the reading and temperature drops quickly after dropping rpms. Does any of you have real time temperature readout from risers? What temps are you seeing?

 

[Lou C]

Ok your engine is fresh water cooled but as far as I know most Volvo do not have a provision for closed cooling the manifolds so they must be raw water cooled. In that case your raw water flow from the heat exchanger may not be enough to keep the exhaust cool at higher rpms & load on the engine. Sounds like you need to test the impeller out put and the raw water side of the h/x may need to be cleaned. I have Volvo style exhaust on my raw water cooled OMC and the hottest they get is about 125-135 after coming off plane.

 

[saf]

Update:
Did the raw water pump test. I pulled the hose going from heat exchanger to port manifold and did the "bucket" test. The water flow is solid (no bubbles ) about 8 G/min @1000rpm. From what I read this is about normal.
I verified with a remote thermometer that both risers heat up above 160 under load, meaning that both of them have to be blocked (seems very unlikely for new risers/manifolds). What else I might be missing ?

 

[Scott Danforth]

You should be closer to double that flow

Bask flush the raw water line

Pull the end caps off the HX and clean it

 

[saf]

You should be closer to double that flow

Are you sure about that? I was reading merc instructions (all I could find) for similar engines (https://www.google.com/url?sa=t&rct...60t00_6a.pdf&usg=AOvVaw3CS2RLWk5-KwX8QhAzJUfU) and they say that for 5.7 fresh water cooled engine the raw water flow should be 28G/min @4000rpm, and for the "bucket" test the pump should produce 9.5qt in 15sec @1000rpm. My test was a bit under (8qt in 15sec ) but my volume measurement was not that precise, it could have been 9qt, l was trying to make sure that it was in the ballpark (technically, the flow has to be measured right after the pump, but it is harder to do). Are mrecs that much different from VP to require almost double water flow?

 

[Lou C]

maybe you have enough entering the HX to keep the engine cool but not enough exiting it fast enough to cool the exhaust. Remember the raw water comes in the HX, picks up heat from the antifreeze side and then exits to the exhaust. If the flow of it exiting is slowed down by crud in there it could cause what you're seeing. Also, is this a factory install? If aftermarket are you sure that the HX is big enough?

PS there is a document floating round the net Volvo Penta Overheat Diagnosis see if you can find it, it has very good diagnostics in it...

 

[Scott Danforth]

Yes im sure of the VP raw water pump flow. I also know the merc Alpha and Bravo flow rates

 

[saf]

Yes im sure of the VP raw water pump flow. I also know the merc Alpha and Bravo flow rates

Sorry Scott, did not mean to sound like I was questioning your knowledge, merely trying to make sure that I provided accurate information. I could not find any specs for VP raw water pump flow (that troubleshooting guide that Lou mentioned only gives pressure readings, which requires building a special rig to measure), so, could you please confirm that I understand this correctly: for VP 5.7 fresh water cooled engine I should be seeing ~16 gallons per minute flow rate @1000rpm after the raw water pump? If so,, am I correct to assume that VP wants ~50G/min flow @4000rpm?

 

[Scott Danforth]

the jabsco pump on a VP flows 24 GPM at about 3000 RPM in-situ.

at about 1000 RPM, you should be about 15 GPM flowing into a bucket (~ 1psi back-pressure) if I remember the pump curve.

the curve is not linear and is limited by pressure.

typical causes of low flow on a VP installation are:

• bad fitting at the drive and sucking air
• debris in the fitting and impeding flow
• debris in the PS cooler and impeding flow
• damaged impeller bore (should be smooth)

other things that can lead to low flow in-situ:

• plugged elbows
• plugged manifold
• plugged heat exchanger

 

[BRICH1260]

Sounds like you are not getting enough water flow through your manifolds and risers. In addition to checking water pump output you might check the hoses for obstructions or clogged water passages in the manifold and risers themselves. How old are the risers, maybe time to replace?

 

[ripjmk]

Scott, I have to dissagree with you on the pump curve. These flexible vane impeller pumps are positive displacment pumps and as such flow is directly proportional to rpm and the curve is linear. The pressure is limited by the strength of the impeller but as proven by cases of hoses bursting (when the by-pass port gets plugged with pieces of old impeller vanes) that pressure is quite high. Low flow rate can be caused by a worn impeller or an air leak on the suction side of the pump.
The only curve i could find is for a general engine cooling pump with a 1-1/4 hose connections, but it will be close to that being talked about. In any case the shape of the curve will be similar. https://pdf.directindustry.com/pdf/jabsco/18940-0010/14649-727601.html

 

[saf]

Sounds like you are not getting enough water flow through your manifolds and risers. In addition to checking water pump output you might check the hoses for obstructions or clogged water passages in the manifold and risers themselves. How old are the risers, maybe time to replace?

This is just it, risers and manifolds are new (well, almost new, replaced them last year). I went through the entire raw water system last year while looking for the air leak (found it in the outdrive lower unit out of all places). Replaced the raw water pump in the process (brand new Volvo pump), replaced the impeller this year again, just to be safe. And I'm still getting risers overheating - driving me crazy. I wonder if my cooling system was put together wrong somehow. This is a replacement engine done by the previous owner, I wonder if they installed something out of spec. I am getting the flow meter, will put it in the loop to see what the in-situ water flow is at 3000 rpm...

P.S. And yes, I did make sure that the gaskets under risers are correct (all water passages are open).

 

[ripjmk]

Have you checked the trim pump oil cooler for water flow? A restriction in the sea water pump suction line could cause the pump to cavitate at high rpm. In the water not on muffs, in neutral get someone to rev the engine while you feel the pump suction hose and see if it want to collapse in on itself.

 

[BRICH1260]

I`d pull the water supply hoses going into the manifolds and ultimately the risers and briefly start the engine to see the water flow into them. I don't have a fresh water system so I am generally guessing, but do the manifold supply hoses feed from the thermostat housing like on mine? If so, could you have a blockage in your thermostat housing?

 

[Horigan]

Before even doing that, have you confirmed the raw water passages of the heat exchange are clear? This involves taking the end caps off and running the appropriately sized rod through each of the passages. Better yet, then run diluted hydrochloric acid or barnacle buster through just the heat exchange after rodding the passages.

 

[Lou C]

Before even doing that, have you confirmed the raw water passages of the heat exchange are clear? This involves taking the end caps off and running the appropriately sized rod through each of the passages. Better yet, then run diluted hydrochloric acid or barnacle buster through just the heat exchange after rodding the passages.

that's what I said...the exhaust on this set up is fed not from the thermostat housing like with a raw water cooling system but from the raw water exiting the heat exchanger. Remember first the raw water has to pick up the heat from the antifreeze side, and then is obviously hotter than when it entered the exchanger and now it has to cool the exhaust. It may be that even when everything is functioning as well as it can be the exhaust will run a bit hotter with the heat exchanger than a raw water system. However even with raw water cooling, the water cools the engine first before cooling the exhaust, once the thermostat has opened. Questions, how old is this set up, how many years has it been used, are you sure the heat exchanger is sized right for the engine (probably is because engine does not overheat) etc. Has it ever been cleaned?

 

[saf]

Thanks everyone for your input. Did a lot of testing over the past couple weeks, here are the results:

1. Did a clear hose test while on the plain - no air is being sucked in
2. Took both caps off of the heat exchanger and looked through tubes. I see no signs of corrosion of buildup, looks pristine.
3. Did multiple tests of flow rate, both in-situ and "bucket fill". For in-situ measurement I installed a simple (glass tube type) flow meter right before the heat exchanger. Double checked the flow meter readings with "bucket" measurements (used a 5 gallon bucket to improve the accuracy) - readings are consistent. So... drum roll ... at 3000rpm I see 18G/min in-situ flow. At 1000rpm I see 6.5 in-situ flow.
4. Did the vacuum and pressure tests per VP overheating diagnostics manual. First, I connected a 1and 1/4" tee to the incoming raw water line (right before the pump) and measured the vacuum. I see 2inHg @ 1000rpm and 8inHg @3000rpm. Did the same with a 1" tee in the outgoing line (right after the pump). I see 5-15psi @ 1000-3000rpm. According to VP manual, for my outdrive (DP-SM) I should see 2-19inHg vacuum @idle-3000rpm and 2-10psi respectively. So, looks like a blocked water passage downstream, right?
5. So, i did a radical (dangerous) test by diverting the flow straight into a bucket (bypassing the heat exchanger and exhaust risers for a short period of time) - I am getting 7.5G/m at 1000rpm (no head pressure, obviously). Still too low!!!
6. Bypassed the intake (pulled the intake hose off of the steering oil cooler and extended it with another 1" ID to suck raw water directly from overboard) - no change in flow rate!!!

I am pulling my hair at this point! WTF can it be? Again, this is a brand new (VP) pump.

the jabsco pump on a VP flows 24 GPM at about 3000 RPM in-situ.

at about 1000 RPM, you should be about 15 GPM flowing into a bucket (~ 1psi back-pressure) if I remember the pump curve.
...

Scott, this is a jabsco pump you are referencing. Is it one of their "high flow" pumps? Is it possible that the VP pump is rated lower? (sorry, reaching for straws here). Could you please point me to that jabsco pump (exact model, etc)?

Is it possible that the problem sits with my engine tune, i.e. exhaust being too hot or something? (again, reaching for straws here)

 

[ripjmk]

Have you taken the new pump apart to inspect the impeller? You may have burnt the tiips up when you first started it! At this stage it would be worth checking.

 

[Senior B]

When the new manifolds and risers were installed, do you know if any gasket sealer was used? The spray stuff doesn't cause problems but I've seen folks use to much RTV type sealant that resulted in blocked water passages. Just another thing to check. Good luck.

 

[Scott Danforth]

been looking for the pump curves and cant find them.

with a good HX, at idle, the 7.5 GPM you measured should be fine. at 2500 RPM, the flow rate would be significantly more

when you had the clear tube hooked up, did you get bubbles? did you get bubbles coming up on plane?

have you back-flushed the raw water line from the RWP to the drive? this should dislodge any debris.

adding a hose over the transom to suck water directly would only add restriction.