Calling out experts, SBC engine problems.

[Shrom CVX-18]

Long story short. The boat (CVX-18) SBC 358 Mild performance mods. Ran last year at between 140- 170 degrees temp was. I figured I would need to replace Impeller. Cause it varied between rpms.  So this year I took it out it ran around 170 and then it over heated at higher Rpms around 200 degress.(short period of time) I then pull it from the lake and replaced the Impeller. The Impeller was obviously bad cause when I pulled it out, it kept it "bent" shape. Keep in mind first time out it ran like a champ besides heat issue...

So All should be good right? Now it runs again between 140-170 degrees. Runs half decent. But after I run it. Stop let it sit while we swim. Then go to start it again, It won't run correct. In fact I can hardly get it to idle. And it seems to miss fire as I rev it to get it to run. This happened twice. I plan to replace the coil. And I bought a new water/fuel separator just for good measure. Any thoughts?

Seriously considering selling the boat...

[Rich_V174SS]

In regards to the impeller, after a while they do curl over and hold that shape when they get older. I hope when you replaced it you used an OEM Mercury impeller and not an aftermarket one like from Sierra because they are garbage and will give you problems.

The misfiring issue could be a bad coil, or if you still have points ignition you could need a new set with a condenser. Water in gas could also cause similar symptoms, as can a bad key switch or a sticking shift interrupter switch.

[Plugcheck]

Sounds like heat soak, likely an issue with the ignition system.  I would take a close look at the whole system from cap/rotor, plugs, wires, points, etc.  A bad coil can cause this, but also moisture under the cap, corroded connections, and bad points/condenser.  If not already, consider electronic ignition.

[Shrom CVX-18]

Thanks guys sounds like I was thinking in the right direction at least. I'll check all that stuff out closely.. Probably time for a good tune up.



The impeller was new one my dad happen to have. But he opened it up before I ever saw what brand it was. Would not surprise me if it was Sierra. I'll keep a close eye on temp. ( as I usually do ) 

[carlsoncvx18]

What kind of shape is your exhaust manifolds and risers in?

I had a over heating issue that i replaced the impeller 4 times and did not go away.

Ended up the my exhaust riser were plugged up restricting flow.

Removed the risers as the the exhaust manifolds were new and soaked the risers in muratic acid until the s$%t quit flowing out of them.

Over heating issue went away.

All the over heating as mentioned above was at higher rpm settings.

Good luck.

[Shrom CVX-18]

What kind of shape is your exhaust manifolds and risers in?

I had a over heating issue that i replaced the impeller 4 times and did not go away.

Ended up the my exhaust riser were plugged up restricting flow.

Removed the risers as the the exhaust manifolds were new and soaked the risers in muratic acid until the s$%t quit flowing out of them.

Over heating issue went away.

All the over heating as mentioned above was at higher rpm settings.

Good luck.

Probably have to do that also.   Thanks!

[CVX Fever]

That's what I was thinking Jeff when I read his first post thinking about your overheating issue a few seasons back.

Not quite apples to apples, but I remove the raw water pump on the Bravo drive for Blind Date every fall and inspect the the impeller. It mounts on the front of the motor as an accessory like an alternator so it's super easy to do.  After one season the veins will be bent over, and they will stay like that until I put it back together the next spring. And it will pump like that for many seasons after. So I'm not so sure that your impeller was the culprit. It could be, but honestly I would have expected the overheating to be a problem at displacement speeds more than when your up on plane.   

I guess if you continue to have overheating problems you know where to look next.

[Shrom CVX-18]

I'm not going to roll out the risers. Its not to difficult to pull them, So i guess I should just a head and do it. Sense not running at correct temp can effect tuning a little. Probably best to just take care of it all now.

[BiPolar75ss]

Hey there bud. Maybe I can help a little bit. I've built more small block Chevy than I can count and lots of marine and auto knowledge. Anyway...I can't remember if mercruisers have thermostats in them, I thought they did... If so replace it. Buy "copper one" not the metal ones cuz they rust then fail cuz of all the water in, then water out and with heat added as well with no fluid makes rust. They make a MARINE thermostat. As for temp I'd run a 190. Small blocks like and need 170 to 200  temp roughly to make a good combustion and run good I run a 190 in mine and at wide open. 6500 rpms I'm at 195 to 180... Then when I settle it down it runs 175ish and idle 170. We all run the same water and we have plenty big enuff radiator aka the lake lol and we stay cool. Don't bother testing old one cuz its not accurate, just replace it then test it, all thermostats have a small gap I'm them they don't seal 100% so it will stay cool at idle even if its bad. It probably isn't opening fully causing your boat to get all hot headed:) then BEFORE YOU PUT NEW ONE IN spray a can or 2 of WD-40 in the water neck and let it sit for a hour or 2 or over night to break up sand, rust whatever then flush the crap out of it thru the thermostat housing. Now put new one in with a piece of mind your motor is clean.  Your impeller was fine, curve is normal thing and they are naturally stiff, and when they go its usually in pieces or a dummy move , poping it off out of the water to make sure it starts before you head to the lake lol  guilty of it. And for the letting it sit then don't wanna start or feels as if its cold blooded or flooded... If it has a carb on it you will be looking at a new needle n seat cuz more than. Likely its leaking the extra pressure out of the line and filling the carb past the float level and leaking into the carb by the squirters. And if its a quadrajet carb there nactorious for it and the acelorator plunger as well, i do it every spring cuz todays fuel sux! awd you want it rebuilt get ahold of me im a wizard with them things I drag raced using one for 10 years and will out perform most hollys and stomp edelbrocks performer. Hope this helps sorry I made a book but like to explain things clearly. Probably would be a good time to change cap rotor and wires and plugs as well... If you put ac plugs in they are R45 or R44 STOCK. if you have R43 in it now go to a R44 that could be another reason for heat. To hot of a plug. Ok lol need anything just holler

[Shrom CVX-18]

Hey there bud. Maybe I can help a little bit. I've built more small block Chevy than I can count and lots of marine and auto knowledge. Anyway...I can't remember if mercruisers have thermostats in them, I thought they did... If so replace it. Buy "copper one" not the metal ones cuz they rust then fail cuz of all the water in, then water out and with heat added as well with no fluid makes rust. They make a MARINE thermostat. As for temp I'd run a 190. Small blocks like and need 170 to 200  temp roughly to make a good combustion and run good I run a 190 in mine and at wide open. 6500 rpms I'm at 195 to 180... Then when I settle it down it runs 175ish and idle 170. We all run the same water and we have plenty big enuff radiator aka the lake lol and we stay cool. Don't bother testing old one cuz its not accurate, just replace it then test it, all thermostats have a small gap I'm them they don't seal 100% so it will stay cool at idle even if its bad. It probably isn't opening fully causing your boat to get all hot headed:) then BEFORE YOU PUT NEW ONE IN spray a can or 2 of WD-40 in the water neck and let it sit for a hour or 2 or over night to break up sand, rust whatever then flush the crap out of it thru the thermostat housing. Now put new one in with a piece of mind your motor is clean.  Your impeller was fine, curve is normal thing and they are naturally stiff, and when they go its usually in pieces or a dummy move , poping it off out of the water to make sure it starts before you head to the lake lol  guilty of it. And for the letting it sit then don't wanna start or feels as if its cold blooded or flooded... If it has a carb on it you will be looking at a new needle n seat cuz more than. Likely its leaking the extra pressure out of the line and filling the carb past the float level and leaking into the carb by the squirters. And if its a quadrajet carb there nactorious for it and the acelorator plunger as well, i do it every spring cuz todays fuel sux! awd you want it rebuilt get ahold of me im a wizard with them things I drag raced using one for 10 years and will out perform most hollys and stomp edelbrocks performer. Hope this helps sorry I made a book but like to explain things clearly. Probably would be a good time to change cap rotor and wires and plugs as well... If you put ac plugs in they are R45 or R44 STOCK. if you have R43 in it now go to a R44 that could be another reason for heat. To hot of a plug. Ok lol need anything just holler

   



Id like to pick your brain a little. Do you have an email i can reach you at?