383 Vortec/TBI Build Thread

[77 K20]

Working through some issues still, besides the tune. Some solved, some still lingering.


4. Temp gauge issue. There are two temp sensors: one in one of the cylinder heads with one wire and one in the intake manifold by the thermostat with two wires. The two wire one controls the ECM, and fortunately that one is working fine. My data log sessions show a very consistent 175-185 deg f after it's warm. But the one in the cylinder head controls the temp gauge on the dash and it is wonky. It'll be pegged hot at one minute and show 190 the next. And I don't see any consistency. It's a new temp sensor so I'm thinking I might have bought an incorrect one. I believe I have an OEM sensor that I can put in there so I'm going to try that and see what happens, but if anyone has any ideas, let me know.

More bugs to work on besides these, but that's enough for now!

On my truck I was getting temperature fluctuations while on the highway with the coolant temp sensor in the head (which goes to my VDO coolant gauge). I have another coolant temp sensor in the manifold for my fuel injection and that one was pretty steady. In my case it was called out in my GM crate engine manual:

"Any small block engine, regardless of year, that uses Vortec heads, will require an external coolant bypass line from the intake
manifold to the 5/8" hose nipple on the water pump (passenger’s side). Suggested routing is from the 3/8 NPSF boss on intake
manifold to the water pump."

Something about coolant/water will boil in the head then force its way out somewhere else then cooler coolant then comes rushing back in. In my case if I remember right this would happen every 10-20 seconds while going down the highway. I added a coolant bypass and the issue went away. Others say instead of a coolant bypass just drill 1-3 holes in your thermostat to allow coolant to bypass.

 

[theblindchicken]

Yeah, the starter is what I'm afraid of. It's a high torque unit from an early 70s truck, cast iron nose. I rebuilt it with new drive gear and brushes, but there definitely could be an intermittent problem. I have a more modern unit with aluminum nose and enough parts to rebuild it as well - I'll swap them out if I keep seeing problems.



Pretty sure it's not the battery - it's brand new (even though that can sometimes be the cause!) and starter is acting like it's not taking a load at all - headlights don't dim one bit. But it definitely could be a ground issue. I'll be looking real close at that next time it happens. Not timing related - it won't crank at all.

Thanks, all!

Blow through about 30 optima yellow top batteries a month (give or take) at my work (electric go karts). Some last well over a year, some last less than a few weeks.

So always worth giving it a test before you spend hours digging into electrical issues.

I also had an issue where if the starter gear sticks to the flywheel, you lose all power in the cab, no crank, no radio, nada. Pop it loose, and power is regained.

 

[4WDKC]

On my truck I was getting temperature fluctuations while on the highway with the coolant temp sensor in the head (which goes to my VDO coolant gauge). I have another coolant temp sensor in the manifold for my fuel injection and that one was pretty steady. In my case it was called out in my GM crate engine manual:

"Any small block engine, regardless of year, that uses Vortec heads, will require an external coolant bypass line from the intake
manifold to the 5/8" hose nipple on the water pump (passenger’s side). Suggested routing is from the 3/8 NPSF boss on intake
manifold to the water pump."

Something about coolant/water will boil in the head then force its way out somewhere else then cooler coolant then comes rushing back in. In my case if I remember right this would happen every 10-20 seconds while going down the highway. I added a coolant bypass and the issue went away. Others say instead of a coolant bypass just drill 1-3 holes in your thermostat to allow coolant to bypass.

Interesting since the older blocks have ports that recirculate to the water pump..

 

[Daveo91Burb]

On my truck I was getting temperature fluctuations while on the highway with the coolant temp sensor in the head (which goes to my VDO coolant gauge). I have another coolant temp sensor in the manifold for my fuel injection and that one was pretty steady. In my case it was called out in my GM crate engine manual:

"Any small block engine, regardless of year, that uses Vortec heads, will require an external coolant bypass line from the intake
manifold to the 5/8" hose nipple on the water pump (passenger’s side). Suggested routing is from the 3/8 NPSF boss on intake
manifold to the water pump."

Something about coolant/water will boil in the head then force its way out somewhere else then cooler coolant then comes rushing back in. In my case if I remember right this would happen every 10-20 seconds while going down the highway. I added a coolant bypass and the issue went away. Others say instead of a coolant bypass just drill 1-3 holes in your thermostat to allow coolant to bypass.

Cool, thanks for refreshing my memory - when I put vortecs on my 76 vette years ago I think I looked into this but it’s never been a problem (that I know of). But the block is original so I’m thinking it has the internal bypass.

I guess maybe I’ll consider drilling the t-stat but some other posts I’ve read make it sound like as long as you have coolant always going thru the heater core - which I do - you shouldn’t need to as that’s your bypass. And wouldn’t this be a problem only when the t-stat is closed?

 

[77 K20]

The older blocks had a internal bypass built in, but the heads do not. Since the heads don't the newer blocks didn't have the bypass either. I read where some drilled out the vortec heads to have this bypass. Many deemed this unnecessary.

I spent probably 2 weeks researching the external bypass. I got tired of reading thru a full spectrum of completely different responses. Some never did a bypass and never had issues- some are not even running heater cores. Others say the rapid temp changes caused their vortec heads to crack (which they can be prone to). I thought running thru the heater core would be good enough but then I saw strange temperature fluctuations. I replaced my thermostat thinking it was faulty. New one did exactly the same thing. I then did the bypass and now the temperature is stable.

Then a lot of people say if you run a reverse flow coolant pump things will be ok (for a serpentine belt). The V belt water pumps favor the right side of the engine. Who knows... maybe? I'm running a V belt.

Lastly some said it makes a difference where the heater core returns to. Water pump or radiator. Mine goes to the radiator.

Just a few things to consider I guess.

 

[Daveo91Burb]

The older blocks had a internal bypass built in, but the heads do not. Since the heads don't the newer blocks didn't have the bypass either. I read where some drilled out the vortec heads to have this bypass. Many deemed this unnecessary.

I spent probably 2 weeks researching the external bypass. I got tired of reading thru a full spectrum of completely different responses. Some never did a bypass and never had issues- some are not even running heater cores. Others say the rapid temp changes caused their vortec heads to crack (which they can be prone to). I thought running thru the heater core would be good enough but then I saw strange temperature fluctuations. I replaced my thermostat thinking it was faulty. New one did exactly the same thing. I then did the bypass and now the temperature is stable.

Then a lot of people say if you run a reverse flow coolant pump things will be ok (for a serpentine belt). The V belt water pumps favor the right side of the engine. Who knows... maybe? I'm running a V belt.

Lastly some said it makes a difference where the heater core returns to. Water pump or radiator. Mine goes to the radiator.

Just a few things to consider I guess.

Thanks much for this info - really helps. I was having the same frustration as you - so many different accounts on the internet of what to do and what worked and what didn't. All I know is I have a heater bypass and my dash temp gauge is still showing hot and that's with two different head sensors. I started it up cold today and just let it idle for five minutes and it was already creeping well past 210. Yikes. And you don't have to tell me about Vortec heads cracking. Only one of the two from my vortec core motor were good, ended up paying an extra $100 core charge to the rebuilder there. And same thing when I did my Vette motor years ago, although that crack wasn't as bad and was repairable.

Your post and my experience made me decide: not going to mess around with relying on the heater bypass or drilling holes in the t-stat, just too risky. I took a look at my setup tonight and bypass from manifold to water pump will be easy, just have to find the right fittings. I'm even able to get to the plug in the water pump without taking anything apart. Any chance you could post up a pic of your bypass setup? what size hose did you run between the fittings?

 

[77 K20]

I have a weird setup with an Edelbrock manifold that has a coolant heat plenum below the TBI.

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So between the coolant sensor and the thermostat I pulled the plug from there and send it back to a port near the TBI. Coolant then runs under the TBI and out a port on the other side. From there it returns to the fitting on the water pump. For the life of me I can't remember if it was 1/2" or 9/16".

And drilling holes in the t-stat never made sense to me either. I didn't even try that.

And I have this picture of when my engine was new and I was running a carb at the time. This is the same manifold as you have (I think).

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Since I didn't have a coolant temp sensor I put a 45* fitting in there and then a straight fitting on the water pump. The fittings were very close together- so I bought some of that really flexible high temp silicone hose and was able to squeeze that in there to connect it. I didn't have any coolant temp issues with this setup. When I put in the fuel injection I got rid of the bypass and then I did. (its been a LONG learning experience).

That is why I like this forums so much. There is always something to learn or think about new ideas.

 

[Daveo91Burb]

I have a weird setup with an Edelbrock manifold that has a coolant heat plenum below the TBI.

You must be registered for see images attach

So between the coolant sensor and the thermostat I pulled the plug from there and send it back to a port near the TBI. Coolant then runs under the TBI and out a port on the other side. From there it returns to the fitting on the water pump. For the life of me I can't remember if it was 1/2" or 9/16".

And drilling holes in the t-stat never made sense to me either. I didn't even try that.

And I have this picture of when my engine was new and I was running a carb at the time. This is the same manifold as you have (I think).

You must be registered for see images attach

Since I didn't have a coolant temp sensor I put a 45* fitting in there and then a straight fitting on the water pump. The fittings were very close together- so I bought some of that really flexible high temp silicone hose and was able to squeeze that in there to connect it. I didn't have any coolant temp issues with this setup. When I put in the fuel injection I got rid of the bypass and then I did. (its been a LONG learning experience).

That is why I like this forums so much. There is always something to learn or think about new ideas.

Thanks a lot for the pics and great info. Yeah, your last pic is pretty much where I'm going. Only difference is I'll probably use the port on the top of the manifold (you show a black plug in yours) since my ECM temp sensor is in the front position. Or I may switch them around, whichever has the cleanest hose routing. I'll post pics when I'm done plumbing it in.

 

[Daveo91Burb]

Got my bypass put together tonight and it seems to solve the problem. Let it idle for a while and then took it on a test drive - cylinder head coolant temp never went above 190. Before I just had to let it idle for a few minutes and gauge would be well over 210 and still climbing before I shut it off. Still not sure why it’s needed when the circulation thru heater core should be doing the same thing, but oh well it works.

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My big worry now is my Corvette. I’ve been running Vortecs for six years now without a bypass and that car has a vacuum modulated valve on the heater circuit - I don’t think coolant circulates thru the core unless the heat’s on. And the temp sensor is in the intake so it wouldn’t see high temp in the cylinder heads. Looks like adding a bypass to that is going to be a near-term project.

 

[4WDKC]

Got my bypass put together tonight and it seems to solve the problem. Let it idle for a while and then took it on a test drive - cylinder head coolant temp never went above 190. Before I just had to let it idle for a few minutes and gauge would be well over 210 and still climbing before I shut it off. Still not sure why it’s needed when the circulation thru heater core should be doing the same thing, but oh well it works.

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My big worry now is my Corvette. I’ve been running Vortecs for six years now without a bypass and that car has a vacuum modulated valve on the heater circuit - I don’t think coolant circulates thru the core unless the heat’s on. And the temp sensor is in the intake so it wouldn’t see high temp in the cylinder heads. Looks like adding a bypass to that is going to be a near-term project.

this is what your heater control valve looks like

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for it to return hot coolant to the system while bypassing your heater core it would need an additional hose connected. This is why it was causing your engine to be hot and needing the re-circulation hose connected.

 

[Daveo91Burb]

Thanks. I understand what you're saying but some on the net say just the act of the coolant flowing through the heater core is enough circulation to get that hot water out of the heads. Definitely not true in my case. And for my Corvette, I don't even think I'm getting that circulation, unless the heat's on since it has one of these type control valves in line in the heater hose coming from the manifold:

https://goo.gl/images/vRGKcZ

 

[4WDKC]

Thanks. I understand what you're saying but some on the net say just the act of the coolant flowing through the heater core is enough circulation to get that hot water out of the heads. Definitely not true in my case. And for my Corvette, I don't even think I'm getting that circulation, unless the heat's on since it has one of these type control valves in line in the heater hose coming from the manifold:

https://goo.gl/images/vRGKcZ

either way when you have the climate control set to cool even with ac off it stops the flow through the heater core.

 

[yevgenievich]

On my ls motor and tbi I have coolant valve that bypasses heater core and let's the coolant circulate

 

[Daveo91Burb]

either way when you have the climate control set to cool even with ac off it stops the flow through the heater core.

OK, now I see. That valve must be behind the firewall on our trucks? I just assumed the coolant always circulated, even if HVAC wasn't on heat. Guess that explains it......

 

[yevgenievich]

on trucks there is no heater core bypass. The control panel does not have the vacuum valve on hot/cold selector that cars tended to have.

here is the valve on my TA

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