mercruiser 5.0 mpi overheats at idle

[nauticool]

Re: mercruiser 5.0 mpi overheats at idle

That's really helpful, thanks

 

[nauticool]

Re: mercruiser 5.0 mpi overheats at idle

Hope you're all still here!

Boat was lifted back in today. Overheats straight up to 200f. rev to 1500rpm and drops to 160f. motored back to my dock ok, about 30mins, but as soon as low speed docking, manouvering, it began to overheat.

both exhaust elbows are same temp

used lazer temp gauge just above sensor on thermostat housing and shows 180f, generally running 10f behind gauge. i guess this is right as its not the water temp.

so whats next?

changed every servicable part on the outdrive and the thermostat.

shall i pull the hose off the transom, now im back in the water, to see how much water im pumping for 30 seconds. how much should an alpha 1 gen 1 pump per hour?

could it be the noisey cool fuel pump?

trying to keep my patience here but it looks like were back to the drawing board

 

[JustJason]

Re: mercruiser 5.0 mpi overheats at idle

shall i pull the hose off the transom, now im back in the water, to see how much water im pumping for 30 seconds

Hmmm.. Now that sounds familiar. Do you have a rubbermaid bucket handy?

Depending on your gear ratio- you should get at least 1.5 quarts running the engine at 1000rpms for 15 seconds. (do this on a stone cold engine). Then shut her down and reconnect the hose. Service manual 37 page 6a-15

 

[nauticool]

Re: mercruiser 5.0 mpi overheats at idle

good results from outdrive, way above minimum and no air in the water. removed hose from power steering cooler and did as instructed.

1000rpm for 15 secs = 4 quarts

550rpm for 15 secs = 2.1 quarts

Can you tell me what the part in the pic is and what it does?

http://www.youtube.com/watch?v=0CPwGTwLMz4

its after the power steering cooler and before the fuel pump

i think we must be looking at a blockage/restriction somewhere, i guess i will have to take everything apart one by one to confirm it is clear or just take the pipe off after the fuel pump and run to see if i still have good flow there?

Thanks everyone, at least i dont have to have the boat lifted again

 

[nauticool]

Re: mercruiser 5.0 mpi overheats at idle

can see on a parts diagram its a check valve

could there be a problem with the circ pump in the engine? do these ever fail? im assuming they have a brass impelller....

 

[achris]

Re: mercruiser 5.0 mpi overheats at idle

can see on a parts diagram its a check valve

could there be a problem with the circ pump in the engine? do these ever fail? im assuming they have a brass impelller....

If it's a proper MARINE pump the impeller is brass.... If someone's done a bodgie job, then it could be an automotive, in which case it will be pressed steel, and salt water and pressed steel only has one outcome.

 

[nauticool]

Re: mercruiser 5.0 mpi overheats at idle

Still stuck but not willing to give in!!!

I have confirmed the plumbing, post #40, and good flow of all of the hoses on the flow side of the raw water.
Every hose has been removed and checked.

is there no oil cooler on this engine?

If you look at post #40 you will see two hoses going to the block with quick release couplers on the ends #10. I pulled the stbd one and water came out of the hose and began to drain the block. Then i pulled the port one and water came out of the hose but not the block. I put a plastic zip tie into the block and a load of black junk came out and it drained. tried engine again but no difference. pulled the hose again and block drained straight away. so this wasnt a problem but the first blockage i have found.

i think the next, and last, place to go is the circ pump. is there anything i need to know before removing it? will read the manual first.

Could there be a blockage in the head, or block? is it possible and is there any other solution other than to remove the head???

could there be something wrong with the engine that is making it run hotter at idle?

Where should i connect the buzzer at the helm? there could be a problem i dont know about as the only way i have any warning is through the buzzer.... which is missing...

the more i think about it the more i am convinced this is a problem that has got worse. looked at a video taken last year and the gauges show 180f at 3500rpm approx 30 knots. now i can barely dock without it overheating.

This is a massive learning curve for me and i am very grateful for all of your help. Really cant afford to take this to a dealer plus i dont know one i can trust enough. Hopefully with enough time and patience i will get there!

 

[achris]

Re: mercruiser 5.0 mpi overheats at idle

There should be an oil cooler for the steering pump, but no cooler for engine oil.

Buzzer positive connects to any purple wire in the instrument loom, buzzer negative connects to the brown/dk blue wire that has come up from the engine (in the harness).

Do you have a buzzer yet?

Chris.......

 

[nauticool]

Re: mercruiser 5.0 mpi overheats at idle

oh, ok, power steering cooler is clear.

Dont have a buzzer yet, will be taking the circ pump off on sunday hopefully.

Thanks Chris

Glenn

 

[Don S]

Re: mercruiser 5.0 mpi overheats at idle

The more I look at your systems design, I would be that your Alpha Pump is the problem. It doesn't have near the volume output as the Gen II drives, and at idle, with the way that drain system and the extra openings to the risers, manifolds and engines are setup, I would just bet there is not enough volume from the pump to cover it all.
I'm trying to find some specs on water output volume between the Gen II and the older Alpha, just to see what the difference is. But I know it's substantial.

 

[nauticool]

Re: mercruiser 5.0 mpi overheats at idle

Don, if you take a look at service manual 37 6a-15 it shows the bravo pumping up to 6 times the water in a 15 second period.
Jason put me onto this. However my alpha one is now in top working order and pumping twice the minimum specified which is half the minimum of a bravo.

Would it be possible to make this system work as it did with the 5.7 previously fitted?

Given the fact ive run out of places to look i am also thinking there is no blockage or component fault

 

[nauticool]

Re: mercruiser 5.0 mpi overheats at idle

Apologies Don, just read your post again and realised you are talking about the gen II alpha and not the bravo

 

[Don S]

Re: mercruiser 5.0 mpi overheats at idle

Ok, for the Alpha and the Alpha I Gen II pump output, it's identical at 1000 rpm.
Below is a portion of the #15 service manual that shows the Alpha I and Gen II drives as having the same output.
This is consistant with manuals older and newer. (Except #37) that shows only half that flow. A mistake no doubt.
It might be a good idea for you to actaully do the test and see what you come up with. I really doubt you have a problem with the circulation pump.

 

[nauticool]

Re: mercruiser 5.0 mpi overheats at idle

Don,

post #85 shows the test results under these conditions. i also tried at idle

 

[Don S]

Re: mercruiser 5.0 mpi overheats at idle

Yeah, that's why I usually avoid long posts over many weeks. Too much to remember, and too long to read each time.

 

[nauticool]

Re: mercruiser 5.0 mpi overheats at idle

would it be best to start a fresh thread stating my problem and everything ive done already?

 

[achris]

Re: mercruiser 5.0 mpi overheats at idle

Yes please....

Start new thread and just bullet point what you've done, and what you've tested.

Chris........

 

[nauticool]

Re: mercruiser 5.0 mpi overheats at idle

this thread has now been simpilfied and moved to

http://forums.iboats.com/showthread.php?t=378344

 

[DS_G]

Overheating that resolves with speed usually indicates a partial obstruction in the manifold water input on these raw water engines. While the impeller and its water input should be checked (and replaced every 1-2 years), if the engine cools normally at high speed, then a bad impeller, thermostat, or circ pump are less likely to be the problem for obvious reasons.
However, at low rpm/low impeller output, a partial obstruction in the lower manifold may allow cooling water to bypass the manifolds. So no cooling occurs until the rpm and pressure increases enough to force flow past the obstruction. BTW, the manifolds remain cool in this situation, unlike a riser obstruction that causes hot risers.

this obstruction can only be seen by opening the manifold input water ports, which is very difficult without removing the manifold. Inspection of the upper manifold at the riser-manifold joint, does not show this obstruction. The solution is to replace the manifolds +- risers.