View Full Version : I diagnosed the problem but need the right solution

09-18-2008, 02:04 PM
I was hoping someone could help me figure out the right solution for this. I bought a 1978 FJ40 in Nov and have been rebuilding it since. The PO had thrown a rod in the transplanted 350 so that has been part of the rebuilding process. About a month ago I finally got him started but have been battling a cooling issue since. Just yesterday I finally figured out the issue but I am not sure of the correct solution.

It appears that the heater system flow is messing with the thermostat. Without going through every scenario I have tried over the last month, those of you that have had heating issues know what I am talking about, I will simply tell you the scenario that gave me that answer.
I currently have my heater looped so that on hose goes straight from the water pump through to the front and rear heaters before it loops back through and into the top of the intake manifold next to the thermostat. When the heater is on (coolant can flow through that heater system) my engine runs around 185. However, when the heater is closed (ie the valve is closed that allows coolant to flow through the system) my engine will heat up past 225. I usually open the valve before I let it get any hotter and in less than 30 seconds it is back down to 185. To test my theory that the change in flow was messing up the circulation of the system I went ahead and just looped one of the hoses straight from the water pump into the top of the manifold. The system stayed at 185.

So, I am not sure if I am missing a bypass or a valve somewhere or if my heater hoses are hooked up incorrectly but something needs to be adjusted to get the pressures or flows or whatever is messing with the system to flow correctly.

Anyone have any thoughts? I have read a few suggestions including replacing my high flow water pump with a lower flow one, installing a high flow thermostat, and drilling a 3/32 hole in my 180 degree thermostat (which I tried and has not worked). Anybody have any experience with this type of issue or can offer any insight on how my heater should be hooked up? I do not have AC to hook up. Also, anybody have a better explanation on the exact physics of what is going on in the engine?

Thanks in advance and I look forward to meeting more of you at the next meeting.


09-18-2008, 04:05 PM
Some of the old mini trucks did this too. The heater loop would return such cold water to the t-stat that it would close!

The fix? a toyota factory t-stat with 2 openings. a low temp small one and a big hot one. Toyota part# 90916-03070. Whatcha gonna do with a Chevy? You got me there. Will the Toyota t-stat fit in the Chevy t-stat housing?

Talk to Marco?

Can you plumb the heater loop to return farther away from the t-stat so it can mix with the hotter engine loop water better?

09-18-2008, 04:37 PM
Yeah, but when the heater is open is when everything is fine. When it is closed is when I have the overheating problem (so when no fluid is being returned from the heater system).

I am also assuming that fluid is being pumped up through the water pump side, through the loop and out the manifold side. I guess it is possible that it could happen the reverse way depending on how the pressures are at the different locations in the engine. The pressure could be so high at the manifold side that it is shooting fluid out my manifold and into the water pump thus reducing the flow rate by too much.

I did consider hooking it up to a different location but thought I would throw it out here to see where people suggest. I saw that Cool cruisers has a T connect that they have inserted in the middle of the hose the connects the thermostat hosign to the radiator.

09-18-2008, 04:44 PM
sorry, at the end of my second paragraph I meant to say that maybe the flow is going the reverse way and into the water pump reducing the flow rate and allowing to water to cool in the radiator. Maybe the water pump is moving the fluid to fast to cool off.

I have heard of reducers too limit the flow of high flow water pumps. Of course I am not sure of the purpose of a high flow water pump if you are going to do that.

09-19-2008, 06:39 PM
Someone else suggested that I might have an air bubble. I had tried to burp it but could have done it wrong.:confused: Is anyone else runnign a high performance (high flow) water pump with a 180 thermostat on a SBC 350? If you are with no problem and your heater is hooked up the same as mine, maybe I do have an air pocket.

leiniesred, you think I should try looping it into the top radiator hose instead of from the water pump?