Leaky 400 RB... Any suggestions???

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73SwingerBuild

73SwingerBuild

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Disclaimer- This is not an A-body, but it's a big block Mopar (72 Roadrunner with a 400/727.... close enough? Hope so. It's really bumming me out.

I guess I'll start this one from the beginning. I was driving my Roadrunner one night around 10:00pm and came to a light on the highway next to my house. I took off (perhaps slightly motivated to accelerate a bit hard per the turbo Grand National next to me) and upon driving about a half mile and glancing down, I saw my temperature gauge rising... I eased off the throttle and took the first exit ramp. The gauge wasn't quite pegged (close) so I decided to limp it home (~1 mile) barely pushing on the gas. I turned onto my street and could smell the coolant so I decided to shut her off and open the hood. Mind you, the gauge was indeed pegged by now.

Besides the coolant pissing out the overflow, I could see what remained of my water pump belt tangled in the fan... I was not happy. Anyways, we pushed her home, changed the belt, and she ran great.

I kept driving her (it's been about two years) but continued to notice a small leak coming from the engine. It's been getting a little worse over time and tonight I tried to diagnose it. It looks like (based on the trail of grime) that the passenger side cylinder head is leaking coolant towards the rear. There is also a small leak looking like it is coming from the drivers side.


I want to make it clear that there is no coolant in the oil or oil in the coolant. I have changed both and they are both clean. It is my thinking that the overheating caused one of the following:
1- The head lifted slightly and is allowing coolant to seep out under the head gasket, but the cylinder is still sealed.
2- The sealant on the exhaust manifold studs came loose when it overheated and every time it gets warm, the coolant pisses out the stud.

Anyways, any thought/comments would be appreciated. I thinking I might have to pull the exhaust manifolds and re-seal the studs. Am I missing anything?

The first photos are of the drivers side, and the last two are of the passengers side (I'm sure most of you could figure that out... just wanted to clarify).
Thanks all. I really appreciate it.

-Mike
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Worst case you warped a head/ the block

I would CERTAINLY try your approach first. I assume at this point you have not tried to just retorque the head bolts? If the bolt threads are leaking, they would probably be seeping into the oil.

So at the very least I'd do a careful retorque, and I 'spose resealing them is not a bad idea.
 
It does look like it is coming from higher than the head gasket. My bet is on exhaust studs. If it still has steel shim head gaskets you could have hurt one of them too...
 
I doubt I warped the block, but I do worry about the heads...

I have not tried to retorque the cylinder head bolts, but I probably should pull the valve cover along with the manifold. I did try tightening the exhaust manifold bolts and they did tighten up a little bit, indicating that they were somehow loose... Sealant vaporized when it overheated? Still not quite sure. I'm very hesitant to pull anything apart as the engine was just rebuilt about 6 years (3,000 miles) ago.

When you say "if the bolt threads are leaking, they would probably be seeping oil", which bolts are you referring to and where would the oil be coming from?

Thanks a lot for your response. Much appreciated for sure.

-Mike
 
The bolt threads or stud threads for exhaust manifold are usually the culprit. I use hightemp red RTV on them.
 
I was told that the Copper RTV worked best on those. I'm going to order some valve cover gaskets along with new exhaust manifold gaskets. I plan to retorque the heads, reseal the valve covers, and reseal the studs going into the water jackets. If that doesnt fix it, time to pull the motor. I hate leaks!!

Thanks for your comments again. Good to have support before you go about a project like this.
 
Usually when they get really hot they change shape. With 5 bolts around each hole, it's rare for a big block to really "pop" a gasket. But that doesnt mean they can't seap or leak after... I agree - that one pic shows clearly the coolant's coming from above the head flange. I dont think, unless the engine was rebuilt and it did it from day one, that the exh studs are a problem. I'd be much more concerned with heads cracking around the "shared" exhaust flange in the center of the head. Also, overheating can wound pistons, and almost always hurts the rings. So if it pegged the gage, and it's leaking, it may be worth your while to pull it and inspect things anyway.
 
moper said:
Usually when they get really hot they change shape. With 5 bolts around each hole, it's rare for a big block to really "pop" a gasket. But that doesnt mean they can't seap or leak after... I agree - that one pic shows clearly the coolant's coming from above the head flange. I dont think, unless the engine was rebuilt and it did it from day one, that the exh studs are a problem. I'd be much more concerned with heads cracking around the "shared" exhaust flange in the center of the head. Also, overheating can wound pistons, and almost always hurts the rings. So if it pegged the gage, and it's leaking, it may be worth your while to pull it and inspect things anyway.
Click to expand...

Thank you for your response moper.

I am going to pull the manifold tonight. I think the overheating might have caused the seal in the studs to burn up, or perhaps dissolve, as the area right beneath the rear stud is the main cultprit, but I don't know what is causing the leak near the "shared" exhaust flange. It looks very dry, so it looks like it may have sealed itself up, but I am going to inspect the head there anyways while the manfiold is off. The rebuild was fairly fresh, so I'm hoping the rings are okay. I changed the oil immediately after and I run a magnetic drain plug that came back clean.

I'm hoping resealing the studs will work. Either way, I'm going to try that before I pull the entire motor. I know for a fact that pulling the motor will require me doing a cam swap and a carb upgrade... Isn't that how it always goes? lol
Thanks again.

-Mike
 
moper said:
Usually when they get really hot they change shape. With 5 bolts around each hole, it's rare for a big block to really "pop" a gasket. But that doesnt mean they can't seap or leak after... I agree - that one pic shows clearly the coolant's coming from above the head flange. I dont think, unless the engine was rebuilt and it did it from day one, that the exh studs are a problem. I'd be much more concerned with heads cracking around the "shared" exhaust flange in the center of the head. Also, overheating can wound pistons, and almost always hurts the rings. So if it pegged the gage, and it's leaking, it may be worth your while to pull it and inspect things anyway.
Click to expand...


X2 I'm with Moper
 
I had a 400 that my radiator cap stuck on - While towing up the approach to the Tappan Zee brodge... the radiator exploded, blew the open element out of my air cleaner, shut the engine off, bent the hood hinges, and bent the radiator around the fan. the combination of presure drop and excessive heat (gage did not work as it turns out...) instantly boiled out all the coolant from the engine - only fluid left was in the heater core. I had to swap the entire cooling system (radiator, water pump and housing, coolant and water included) from my race car into the truck on the side of the road. That engine went on to run for 4 more years and is now a 500" stroker. The heads and block were fine, it never seaped a drop. But, the ring seal suffered and it started to use oil after that. They are really tough engines...
Normally exh seepage will seal itself. Which is why I say unless it was rebuilt or done it since day one, it's probably not the studs. But, a little time spent can't really hurt at this point.
 
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