Jake Knight

Hey everyone! I'm kinda at my wits end so i want to share my problem and see if anyone has any diagnosis.
I have a 1985 C4 with a new crate motor from Jeggs. There are approximately 800 miles on the new engine.
I'm unable to get the temperature to stay below 230 or higher.
When I press on the gas, the temperature goes up. When I let it coast, or idle, it goes down.
Here are the steps I've already taken.
1. Replaced the thermostat.
2. Removed the thermostat.
3. Verified the temperature on the gauge was similar to the engine using a laser temp sensor.
4. Replaced the water pump with a new one.
5. Replaced the coolant temp sensor and plug just above the water pump.
6. Verified the fan was turning on around 228.

None of those things have helped the temperature stay where it was before I replaced the engine. Previously it was about 190 at all times.

I haven't:
1. replaced my radiator
2. cleaned out my radiator
3. bought a new vette

Any help you might be able to offer would be incredible!
Thanks,
Jake

3D-Aircrew

Hey Jake ... welcome to the forum.
Does it overheat when on the highway with a lot of airflow or does it only overheat when going slow?

Search the forums on this topic and you will find many posts that will tell you 230 is normal temperature for emissions. If it doesn't go higher then you may not have a problem. Do you have an '85 FSM?

I have a MemCal chip on my late '86 that programs the ECM to command the aux fan to come on at 160 and rarely does the temp reach even 190. I'm not sure of the '85 if it even has an aux fan. But checking the radiator is definitely a priority. It's good you replaced the water pump because it makes no sense to put a 33 year old water pump on new motor.

good luck

s carter

Long shot but worth asking, could the intake gaskets been installed backwards. I think if YOU remove the thermostat and look inside you should be able to see onto the head coolant port, if you see a blockoff plate I believe that is backward.

PatternDayTrader

That's not a long shot. It happens all the time. The good news is it's pretty easy to fix.
Someone else needs to chime in on weather or not you can see the block off from where the tstat sits. I dont know.

s carter

I always installed them plates rear and just confirmed it in the only book that I have that covers the 85 that is a Hays book and that is showing open by Thermostat and blockoff plates rear.

PatternDayTrader

Agreed. I just dont know if you can see or feel in there through the intake, but you probably can.

ex-x-fire

So you have the stat out, take the radiator cap off and look in there. Rev the engine, is it like a water fall in the radiator?

Jake Knight

It overheats at all times. The temp just keeps going up as long as I keep pressing on the gas. When I let off the gas then the temp goes down, but once it's over the 230 or so mark it rarely gets below 220. Like I can't even get it to 70 mph because the temp keeps going up. Highest I've seen in is 258. That's when I get really nervous.

Jake Knight

Thank you I will take a look and see.

Jake Knight

I will try that today and see what I find. There is some movement when it's idling, but I haven't looked at it when there is pressure on the engine.

VikingTrad3r

Jake Knight

Interesting! Yes my oil and coolant temp have been going high! The temp on the exhaust manifold was like 480 degrees or something like that coming right off the block! Is there an easy way to check it? Do I just need to replace it?

VikingTrad3r

topfuel67

My friend's Tahoe just gave him this same problem. We replaced thermostat, waterpump, flushed radiator and more. Nothing helped. Finally pressure tested the system and found a rotted overflow hose wasn't allowing the system to pressurize. Without pressure the system doesn't cool enough. Autozone rents the radiator pressure test kit for free. You'll hear where it is leaking with the engine off using the kit.

Red 91

Are you sure you have the correct waterpump? Some turn rt. and some turn Lt. Good Luck!

Mr6spd

Jake,

Have you verified that your ignition timing is correct?

radar502

I would up grad to a 2-3 row Alum. radiator . If that's an Org. one it's shot . Most people have not realized that the outside temp has gotten hotter over the years and stays hotter longer so need to go with the flow and make sure their is NO air in the system . I just put a Champion in my 93 I know cheapeee but it works fine . The old radiators have small flus in them to start and over the years fill up with water deposits . I would say new radiator and if doing that up-grad to a alum. one .. Then if still over heating start looking at other things .. This is not rocket science hot water goes in the radiator cools goes back in motor ..

Jake Knight

Yes I was able to verify that the water pump is working and the coolant is moving through the system. Thank you!

Jake Knight

Yes, I did check the timing and it is correct and the engine runs and sounds great!

Jake Knight

This is definitely on my list. The problem I see that seems to indicate the radiator might not be the cause is that as I slow down, the temp goes down. When it idles, the temp does not rapidly increase. It ONLY happens when I accelerate. As long as my foot is on the gas, the temperature continues to rise. Then, after being driven for about 30-40 mins, the oil temp starts to go up.

I think I have a plugged exhaust system or cat. It was over a year in the process of rebuilding the car and the exhaust sat open on both ends for quite a while. We had many problems with mice so I'm thinking a nest could have plugged it. I'm going to replace the catalytic converter and see if that solves the issue.

If not, the radiator is the next step

Thanks for taking the time to comment!