Cracked riser on my 7.4 merc?

[mjk1040]

Viper, going to replace my impellers when on the hard this winter. And in case things are missing, where do you suggest one should back flush from and how far up the system. I'm fresh water cooled. Mike

[Viper]

Don't replace the impellers until Spring. Just my preference. I mean why put new ones in now and have them take a set from sitting for 6 months right from the get-go?

Back-flushing depends on how you're wired in a plumbing sense but usually on inboards, the raw water will pass through oil coolers first (engine oil and transmission fluid) depending on application, check valves, cool fuel modules, etc. then on to the heat exchanger before exiting through the exhaust. It's usually enough to back-flush from the output side of the last cooler/heat exchanger in the circuit as those are typically where things such as impeller and marine debris will get trapped with the first oil cooler after the pump being the first trap. If you isolate one at a time and flush, you reduce the risk of clearing out one and passing it on to the next in line down the circuit. Follow the raw water intake hose along its path and take note what it's going through before it gets to the exhaust manifolds.

[Harvester_1]

Hey MIKE, VIPER....
i have a late update about my cracked riser/ overheating issue
I did go ahead and backflush the system after i pulled the coolers (trans, oil) to inspect them visually and didnt find anything. Then i changed both impellers.
By the way, to answer your question Mike... i have an in- line t-valve right after my main through hull water intake shut off valve. The previous owner put it on both engines for winterizing so i clamped a garden hose on it and fed it through a window and pulled the hose from one of my exhaust manifolds and back flushed from there as well as each joint in the system like the coolers and such. As far as i could see, that would make it go through the entire system backward from that point to discharging overboard through the garden hose. i watched it go through the pump too b4 i closed it up.
Anyway,
I got 2 new risers and gaskets then got busy. It turns out that there was a little tear in the gasket causing the coolant to slowly go out with my exhaust mixing with the water. So that answers why it didn't overheat during the sea trial and the previous owners 8 mile trip to my Marina to begin with. I believe it took those few trips to finally get the coolant level low enough to the point where it went airbound. The cracked riser wasn't causing any issues at all but I'm sure it eventually would have so i changed both sides just to be even. It's funny the stuff i find sometimes when pulling these systems apart. It usually does turn out to be something simple that I've over complicated in my head b4 I've even looked at anything. So, long story short ...i changed a gasket and riser, pulled the cap off the heat exchanger, started the motor, and topped off the coolant and..... presto, it's fixed. I don't regret taking the long way around a simple fix cause now i understand the closed system on my engine very well and the new parts i put in I'm calling preventive maintenance so it's all good. The whole issue was like an episode of CSI for boating... and I've solved the case.....
Hope this helps someone else out with similar problems.... good luck with your impeller Change Mike..... I'm working on replacing my shifter cables now. They are too stiff for my liking and it seems like a fairly easy job, i guess I'll find out.

[Viper]

Good to hear all is well. Thanks for the follow-up

[mjk1040]

Thanks Viper for all the info, and I'll wait I'll spring then and change the impellers. Have you done them in the boat or do you prefer to pull the pumps and do them on the bench? Glad to hear all is well Harvester. CSI for boats, I love it! Mike

[Viper]

Source of the post .....Have you done them in the boat or do you prefer to pull the pumps and do them on the bench?...

I've done them both ways Mike, but it's certainly easier to pull the whole pump off and do them on the bench if you're talking about Mercs and replacing the housing also which is what I do automatically. There is a right and wrong way to hook them up as they were originally designed for standard and counter rotating engines so make sure you mark at least one of the hoses and the port it hooks up to. The ports are marked LH and RH on the body of the pump to reflect engine rotation. You want to plumb it the same way.

When you remove the pump, you'll have to remove the belt, now is the time to check it and replace if necessary.

Mike, your vintage might have the seawater pump with the fuel pump attached. If it does, let me know.

[mjk1040]

No Viper, Crusader 7.4L XLi's. Sea pumps are separate and belt driven. Plain old Sherwood's. Housing isn't replaceable, but the cams are.

[Viper]

Easier to get at than the Mercs!

[Harvester_1]

Yes viper. I agree mine are 7 4 Merc's. It was no small feat to get to that pump housing but my advantage is that I'm small and skinny so i was actually able to grab a seat in front of the engine to get everything off. It was a tight fit for even me but i sqeezed in there for most of it. I did just as you described. I marked the hoses with a white soap stick pencil then removed the whole pump to change the housing and o rings. I got a great service manual strait from "Mercruiser" in the mail. It helped alot .My belt seemed ok. No shine on it or worn edges. I measured it to see how much stretch it had beyond its original size and it was dead on. So i just used the same belt. The engine i was working on was my starboard side so the pump was on the outside left front of the engine at the very bottom. The mechanical fuel pump is right on to off it just about. The port engine has the pump stareing me right in the face so I'm gonna tackle that one in the spring. But yes it was quite a job but like i mentioned b4.... it's all another learning experience and makes me more familiar with my Merc's

[Harvester_1]

Also, my fuel pump is seperate from the water pump. It did see the type you mentioned in that merc book i have. Glad i didn't have that one. Looks like us needed a good hydraulic press for that type.