Pump should have nothing to do with it beings you say it is only heating to 150 degrees.That's perfect with that thermostat.Good thing your changing that impeller though sense it's showing some cracks.They take on that folded back look from running in the pump body.It still probably fits in there tight though.New ones are perfectly straight so when you install it make sure you fold them back the right way for the rotation of the pump.Also that wouldn't explain the steam only coming out one side.

How cold was the water you were putting into it? I was winterizing my boat last year and the garden hose water temp was probably around 40 degrees. Of course it's gonna steam when it hits the exhaust then.............. But mine was steaming out of both pipes, not just one. Are you talking out of one motor, or one pipe of one motor?

Thanks for all the info guys. I put the new impeller in and a new sending unit on the thermo and that got that fixed. So i took it to the lake and did a few trial runs and now i have to do some tuning and timing. Anyone have any words of advise on msd ign. Wasnt very happy with speed and power kinda doggy. Hope i can get it out. Thanks all agian for all the info.

MSD,,definitely use a reg timing light !!! NO DIGITAL OR ADVANCED LIGHT !!!

MSD's work great.They really helps keep your plugs from fouling at idle speeds around the dock when running bigger cams and carbs.What cam,intake and carb are you running now?Do you know what your compression ratio is?I don't know how you are setting your timing but you should set it when it is fully advanced.Hook up your timing light and direct it on the timing mark.Have some one throttle it up while you watch the timing mark advance.When it stops advancing and holds steady,usually about 2500-3000,set it at 38 degrees.If your balancer isn't numbered that high you can get a timing tape to put on it at your local speed shop.Need to know your balancer diameter though.This would be a beginning for your tuning.Your doggieness surely has nothing to do with the MSD.

38 ??????? mine is at 36 and i run racefuel,,,are u kidding ?!

That's what it says in all my books on building high performance big block chevys.That's what the guys at all the local speed shops have told me also.I've been running my engines at 38 degrees total advance for 25 years and have never had a problem ,and that's on pump gas.

Merc Carbed 500's are listed at 36 if I remember correctly.

If you bought used exhaust manifold then that might be a problem. If they are cracked, or the riser/manifold gasket is leaking, you may be getting water dripping back into the head. When it hits the exhaust valve it creates steam. You may also be getting reversion. Same steam condition.

Daredevil, my whippled 540 is timed at 36!!

Could be ,,,but a stock 454 .......36 ???? more like 32 !!