Uneven temps bank to bank, SBC

[Pablo]

388 Stroker, new build.

It's running way hot on one side. I figured this out because my temp gauge sender is on the drivers side head but I have a CTS mounted for datalogging on the manifold on the passenger side of the manifold by the head.
My temp gauge was reading way cool, like the engine wasn't getting up to temp but noticed my datalog temp going way high (like 220 while the gauge was indicating around 125).
I confirmed this temp differential with a second gauge first added to the driver's side of the intake manifold and then added to the passenger side head which sits at the rear. I also swapped sending units and coolant temp sensors. I have a cheapy IR thermometer that corroborates what's going on too. Finally my hands can tell a big difference from one side to the other.
I pulled the pump and thermostat and put my borescope in the block and as far as I could go down the manifold into the head. I could not see any obstructions. Pulled the water pump apart and it looks great. I even poured water down the t stat hole to observe flow out of the block inlets and the rear sensor port on the head. There is flow and it appears to be even from side to side. I also blew through the water jacket with compressed air and could feel flow at the opposite ends.
I pulled a few plugs just as a quick check, 2,8, and 1 look the same. The carb is new, holley 750 track warrior. The performer rpm plenums feed both sides of the engine anyway, and to top it off I have a 1" open spacer.
The engine idles really nicely, it also runs great and has tons of power.

There seems to be no other symptoms, I'm not getting air bubbles out of the radiator or any water consumption. The only thing I can think of is some obstruction that I couldnt readily observe, or some freak problem I've never seen before.
I suppose I could check lash but I am very meticulous when it comes to setting lash and am confident that it's right. Not ruling that out as a problem but I'd like to hear ideas.

Can anyone think of any reason why it's doing this?

[pdq67]

Years ago, I found a decently sized piece of casting flash in the water jacket of my 406. Had a good time pulling it out, it was hard to grab on to..

Or was it in one of my heads?? It's been too long ago...

pdq67

[MadBill]

Has this block and/or heads operated satisfactorily in previous builds or are they new to you?

[Pablo]

The heads yes. The block only ran a short time awhile ago on a build that wiped the cam so I don't know for sure if it was doing it then.

One thing that I did differently this time was that I went with cometic MLS gaskets. I'm wondering if I have a leak into the cooling system. Would a leak down reveal this as well as one of those combustion gas testers for the radiator?

[MadBill]

Any significant cylinder pressure leakage into the cooling system would result in the cap venting. Also, it wouldn't likely affect just one bank, no matter where the leak was.

[Pablo]

Well I just did a leak down for the heck of it and my numbers seemed pretty good for a cold engine. Every cylinder was at 2-3%.
No bubbles in the coolant system which goes along with what you said.

I'm about out of ideas short of pulling the manifold so I can get a better look at the coolant ports.

[Mean Racing]

If you are using a thermostat- it sounds like the thermostat bypass is plugged, some head gaskets block the passage and some water pumps have no provision for it. Drilling a couple holes in the stat will cure the problem but take longer to heat up of course. If you are not using a stat I would suspect the water pump has an issue.

[BOOT]

Check the exhaust

[cjperformance]

Check the exhaust

^^ DEFINETLY a good start before you rip it apart.

[peejay]

I've seen a head gasket installed backwards cause one side to run signficantly hotter than the other. One side running cool and the other side boiling the coolant out of the head.

The passages are generally closed or very small up front and open in the back, so the coolant is forced to run to the back of the block and then up into the head and forward. Get it backwards and the coolant just goes up from the water pump to the outlet on the head and doesn't go near the rear three cylinders.

Usually caused by well-meaning but inattentive people installing both head gaskets the same way up instead of looking for "front" markings. One will be upside down relative to the other, if they are not marked left and right.

[Pablo]

The thermostat has bypass holes drilled. The water pump also has the bypass restricted. I used all of these parts on my previous build without this problem.

The head gaskets are Cometics. I don't recall them having a front or back, in fact I don't recall any SBC HG having a front or back. Isn't that a Ford thing? Maybe it's an improperly made gasket and I didn't notice?
I'll check the exhaust though I'd be surprised if something was in there. This thing rips at WOT.
I guess that leaves lash (?), some obstruction deep in the block, or something got into the head and is blocking flow. I have no other theories.

[Pablo]

Well guys I figured it out.

I tried a million things but long story short I disconnected my cooling fans to let it get hot because I wasn't getting any flow out of the thermostat. When the T stat finally opened, all of the sudden the temps evened out bank to bank.

So I took out the thermostat and this solved the problem. Temps are pretty even now.

The only thing I can figure is that the holes in the t stat were enough to allow flow through the drivers side head in sufficient quantity to keep the t stat shut. The passenger side on the other hand seems to not have been developing the same pressure and flow, probably due to the bypass.

Why it's doing it on this build is one thing I haven't figured out.

Now I'm debating installing a new T stat without holes drilled or if I want to just keep running without the T stat and let the electric fans control the temp. The engine seemed to come up to temp in a decent amount of time and this isn't a street car - there's no heater or anything like that.

Pretty weird situation eh?

[cv67]

moroso makes some restrictors that fit in place of the Tstat wouldnt know what size to recommend though.

[peejay]

The thermostat has bypass holes drilled. The water pump also has the bypass restricted. I used all of these parts on my previous build without this problem.

The head gaskets are Cometics. I don't recall them having a front or back, in fact I don't recall any SBC HG having a front or back. Isn't that a Ford thing? Maybe it's an improperly made gasket and I didn't notice?

I have not seen any head gaskets that did not have a front and back. Water circulation through the engine is defined by the size of the passages in the head gasket.

This, incidentally, is one place that screws people up on Audi engines. The later DOHC engines had MLS gaskets, which can be used to replace the head gasket on SOHC engines. The problem is that the SOHC engines had a single water outlet between cylinders 1 and 2. The DOHC engines had an external water manifold that had coolant exiting the head between each cylinder. So, the water passage theory in the gaskets was massively different. You need to modify the SOHC heads to have coolant takeoffs like the DOHC head if you want to use the DOHC head gasket...

[cjperformance]

PhotoPictureResizer_180226_125310125-1024x768.jpg

Run the thermostat with no extra holes, plug the factory bypass in the pump and add in a bylass line from near the thermostat and into the pump inlet as in this pic. Works a treat!