Flushing the cooling system with..... baking soda?

[78 GMC Jimmy]

My SBC 400 has a dirty cooling system due to the previous owner using WAY too much "stop leak" in the cooling system.

I already flushed it thoroughly once before. I got about 3 inches of stop leak grit in the bottom of a 5 gallon bucket. Inside the radiator hose was a thick layer of brown sludge. I put radiator flush in there and drove it for a day, and then flushed the whole system with a garden hoes (thermostat out), then flushed with distilled water and then added antifreeze.

That, combined with a 160 thermostat solved the per-ignition problem that would not allow the engine to start / cranks very slowly before starting when the engine was hot. Cylinders overheating due to being insulated with crud?

So everything was fine until the warmer weather showed up. Now the temp gauge is climbing higher than it used to, and the engine is very difficult to start until it cools off. Once it cools off, it starts extremely well.

I have already hose flushed it again with the thermostat out, but there is still plenty of crud in the cooling system. I wish I could send plastic pellets through the system to scrub the whole system, but thats not an option of course. So..... Baking soda? Anyone ever try this before? Online it says the ratio would work out to two cups of baking soda, run it till its hot, and then flush again? Maybe try this a few times in a row?

Thoughts on the baking soda idea?

 

[HankScorpio]

I had a Jeep Cherokee that was filled with stop leak. I hooked up one of the flush kits to the garden hose. T stat out, radiator cap off, heat on and flushed that thing for a couple hours running in the driveway. Water spilling everywhere. No overheating issues after that. Running right at 195 in the desert. We actually had vapor lock issues from high under hood temps but the engine never overheated again.
I didn't need any additives. I wouldn't think baking soda would hurt anything. Not sure if it will really clean out a system though. That stop leak stuff is like clay. Until I ran a crazy volume of water through the system it was still present. I could do a quick flush and it would still be all gross in a week.

 

[78 GMC Jimmy]

I had a Jeep Cherokee that was filled with stop leak. I hooked up one of the flush kits to the garden hose. T stat out, radiator cap off, heat on and flushed that thing for a couple hours running in the driveway. Water spilling everywhere. No overheating issues after that. Running right at 195 in the desert. We actually had vapor lock issues from high under hood temps but the engine never overheated again.
I didn't need any additives. I wouldn't think baking soda would hurt anything. Not sure if it will really clean out a system though. That stop leak stuff is like clay. Until I ran a crazy volume of water through the system it was still present. I could do a quick flush and it would still be all gross in a week.

Thanks for the info. Maybe I will try the method you recommend, flushing for hours.

 

[tarussell]

Just a thought - having a SBC 400 that’s prone to head cracking and gasket blowing issues when the cooling system is in fine shape it might be wise to consider why the P/O chose to put so much stop leak or something much more aggressive like “Blue Devil” in it in the first place
Something you might want to keep an eye on so nothing gets damaged beyond repair.

 

[78 GMC Jimmy]

Just a thought - having a SBC 400 that’s prone to head cracking and gasket blowing issues when the cooling system is in fine shape it might be wise to consider why the P/O chose to put so much stop leak or something much more aggressive like “Blue Devil” in it in the first place
Something you might want to keep an eye on so nothing gets damaged beyond repair.

I appreciate the advice. I thought that there might be a head gasket issue, but so far there have been no sign of it. The only leak that I ever found was at the inlet on the radiator. It leaves a drip down the side of the radiator after driving for a long time. I replaced the hose, and thoroughly inspected the radiator in that area and cant find where the leak is. Its SUPER small, and the fluid evaporates quickly, plus the fluid level never really goes down in the radiator. I think it loses maybe a drop of fluid per heat cycle. So maybe they previous owner was trying to address that issue with the stop leak. But ya, I will keep an eye out for signs of head gasket issues. When the engine is cool it starts and runs fantastic - if I didn't know better I would just throw the thermostat away and run 12v electric heaters in the cab.

 

[eodcoduto]

I’ve put CLR in before and run that until it was hot, it got a lot of junk out of the system.

 

[78 GMC Jimmy]

I’ve put CLR in before and run that until it was hot, it got a lot of junk out of the system.

Might have to give that a try. After flushed the system thoroughly and adding a box of baking soda, I idled / drove it for maybe 1.5 to 2 hours. Opening the radiator, the fluid is not transparent. It looks like hot chocolate. Guess my work is not done yet......

 

[tarussell]

Don’t forget to run it with the heater on while flushing or you’ll end up dumping a bunch of that crud in your system once it gets cold.

 

[78 GMC Jimmy]

Good advice. I run the garden hose through the heater core in both directions with each flush cycle. Crud comes out and then it clear up. Then I flush the other direction and more crud comes out then clears up.

 

[78 GMC Jimmy]

Is there any part of the distributor advance system that is effected by engine temperature?

 

[Wes Harden]

if you are still running a thermal vacuum switch, the vac advance could be plumed through that.

 

[Blue85]

Don’t forget to run it with the heater on while flushing or you’ll end up dumping a bunch of that crud in your system once it gets cold.

Does a 78 have a valve to the heater core? I know in the 80's the coolant is always flowing and the heat controls just affect the air flow.

 

[tarussell]

Does a 78 have a valve to the heater core? I know in the 80's the coolant is always flowing and the heat controls just affect the air flow.

I didn’t think about the constant flow of some years ? That’s a good point.

Regardless it sounds like the core has that crap embedded in it fairly well if it continues to get nasty after flushing it both ways several times.

 

[78 GMC Jimmy]

When I disconnect the heater hoses from the water pump / manifold, I cash run water though either direction - so I assume that there are no valves to stop the coolant flow.

 

[Wes Harden]

Yeah no valve, heater core has constant flow. Is use as bypass when t Stat is closed

 

[78 GMC Jimmy]

Yeah no valve, heater core has constant flow. Is use as bypass when t Stat is closed

I was wondering why I didn't have a bypass hose. That makes sense. Thanks for the info.