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5.0 MPI manifold/riser temp discrepency

S

stillcrazy36

Contributing Member
Recently acquired 2004 Chaparral with 5.0MPI with dry joint exhaust and single point drain system. I am old school 350 guy and need info on water flow with this system. The problem is that BOTH riser elbows are cool to touch but the STB. exhaust manifold is hot (about 175+) the port exhaust manifold is warm(about150) . Now the kicker is that the engine temp is steady 160. Engine has 100 hours. I will drop the lower unit and do an impellor replace and remove the strb. riser and check the plate/gasket, but is there something that would block the water flow to (or out of )the manifold ? I see check balls, restrictor plates and crossconnected supply and drain connections.
 
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Pulled the exhaust manifold. UGH ! used to be easy but Merc has seen to it that everything is now in the way and harder to get at.. I just removed a set from a '88, 350 in minutes. Anyhow..Manifold the looks good, no build up and flushed good but the riser gasket is a restrictor type with a 1/8" hole on the riser front supply side and a 7/8" hole aft.
After looking at the supply piping it looks like the risers get their water first (normal) and then the usual config for Tstat and circ pump EXCEPT, at the riser where full pump pressure is combining with the water trying to get out of the manifold. This may explain why risers are cool. Might also point to impellor for not enough volume.
I also tested the "check ball" in the manifold piping. It doesn't really do much. It slows down flow in one direction, so I am still pondering it's affect. The rubber sealed fitting it is in, is another Merc joke !
 
I had a 2008 5.0 MPI in a chap 215, its just the nature ofthe beast in the 305's, that is how they seem to run. changing the impeller is good preventive maint., dont need to pull whole drive to change impeller. get a manual and when doing the job be especially careful of the little O ring that is the oil seal and remember to put the motor in forward gear.

personally I liked the one point drain system, beat removing a bunch of plugs. look on the bottom of the manifold and see if you have hose clamps to the cooling lines, if not install some before one blows off.
 
Do you live where it can freeze ? I live in South Fl and no need to drain, in fact I believe that allowing air,oxygen rich, to come in contact with wet cast iron is detrimental. Boats that get little regular use around here have short manifold lives. Those that are used commercially last longer. All the extra lines add maint. and opertunity for failure.
Funny that you mention the clamps on the manifold lines. Does Chaparral overlook them ? I just lost a starter on the boat I just purchased because the supply hose clamp (cheap China? SS) rusted off and sprayed the strb. side of the engine with hot salt water.
Thanks for the tips on changing an impeller for the newbees. I've been doing it for 20 years and good advise.
 
There is a lot of things I thought chap either overlooked or the parts went to the lowest bidder? Quality Control seems to be a big issue that there lacking in in my opinion. My boat had scuppers for a cockpit drainage system, the thru hull fittings were higher than the scuppers in the cockpit, so instead of the water draining out , it drained in, can't defy gravity.

I have to winterize my boat each year, water gets drained out and -100 rv anti freeze goes in.
 
I agree, after looking around I see a lot of "Rusty" stainless below deck. I also saw the bilge blower suction hose secured so that the end was flat against the bottom severly limiting air flow. Plain stupid but I'll stop there ! I'm glad the anti freeze replaces the vacant water areas.
I Will repost after changing the impeller and getting starter bolt out.
 
good luck, beware of those rusty hose clamps, there sharp and like to bite, also watch out for the fiberglass instead of gel coat inside the engine hatch, I get all kind of unexplained brush burns.
 
Finally got around to dropping the lower unit and replacing the water pump impeller. The Old one looked new ! I hate to pop for new manifolds but that's on the list unless someone has another idea. Only the starboard manifold gets hot. Both risers are cool and the Port manifold is 25 degrees cooler than the starboard. I pulled the riser and manifold flushed and looked OK (doesn't mean much) It has a "dry Joint" and a "Restrictor" plate. Is this the normal set up?
Engine temp holds 160. Hot manifold is at Idle and low RPM. so I don't think it's sucking air. It also did it on a hose.
 
Everything back together and test ran it yesterday. Shot temps around the engine. 160/162 on exhaust manifolds, 90/92 on riser elbows, 160 on Tstat housing, 160 on engine temp. Looks good ? for replacing an impeller that had just taken a little set. Picture taken a couple of days later and it relaxed some. Chap water pump impeller_0228.jpg
 
That is what they usually look like after even a short amount of time, like a year.
You did the right thing, and now you will be an exper for next time. Cheap insurance.
You re-used your old manifolds and risers, right?

My 6.2 MPI runs a 170 T'stat. The manifolds run at 180-190; and the risers at 85-90 usually.
So that is pretty consistent with what you are seeing.
The restrictor hole gaskets with the small hole forward is the normal setup for RWC engines.

Rod
 
There saying 3-5 years supposedly with the new impellers, I pulled mine after 3 and it still looked brand new. don't be surprised if an IAC bites the dust with the 5.0's.
good luck with the Chap.
 
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