Overheating Mystery

OK Guys...I'm baffled. I have read through the forums, but haven't found anything exactly like my problem. Most of the fixes I've found, haven't worked.

Here's what's happening: I have a 95 Cobra SVT 5.0. I put in a new short block 2 weeks ago, and have spent the last week rebuilding the T5 and getting it installed. I drove it for the first time yesterday so it has less than 45 miles on it. But today it started overheating BAD. There is no steam, no leaks, no blown gaskets. I checked the head compression and all cylinders are running 90-110psi and holding the pressure. I've checked for vacuum leaks and found nothing (I even sprayed carb cleaner all over the engine looking for leaks). The water pump has no visible leaks, no sign of bearing wear, and is pumping water through the heater core. During the engine build, the pump was inspected and all looked good. I replaced the thermostat (TWICE - once with a 180 and now with a 195), still no change. I put in a mechanical temp gauge to rule out a faulty dash gauge, and it's showing the same results. I've replaced the temp switch and the temp sensor. I checked the oil for water and it's clean and golden clear. There is never any dieseling or knock. All in all, everything looks good. Except that it's getting extremely hot, extremely fast.

When it heats up, and it really heats up fast, it never seems to open the thermostat. At 210-220* the upper rad hose is still cool and flimsy. I'm not seeing any flow in the radiator, and it's not kicking on the fans. I ran the diagnostic and the self test kicked on the fans with no problem and showed no CELs. I directly connected the fan to power and ran it on high to see if it would help reduce the heat, but it didn't help.

Here's the strange part - when it heats up, it will go to about 212*, hang there for about 30sec, then JUMP to 230-240*. I turn off the engine asap when this happens.

This is the strangest overheat problem I have ever encountered and I am totally out of ideas.

Sorry this is so long, I just wanted to show what all has been done.

Any ideas??

Got to thinking about this last night and have a theory to add to the issue. Kinda makes sense, but still doesn't totally solve my problem... I said that I wasn't seeing any flow in the radiator, the fans weren't kicking on, and that the thermostat didn't seem to be opening. Well, if I'm correct, the fans run off the radiator temperature, not the engine temperature, so if no hot water was ever flowing from the engine into the radiator, then the radiator isn't going to get hot (which it isn't), therefore never kicking on the fans. And there is a very good chance that the thermostat IS opening, but there just is no water flow to push through the opening, out to the radiator, making the hose continue to stay cool and flimsy. Also, if there were no water flow at all, that would definitely make the engine heat up extremely fast.

Make sense? I think it might be a dead flow issue.

Is there a way to undeniably check the water pump, or the radiator?

Thanks, I'll check the fan under A/C load today.

I've narrowed it down some more. It's not getting water into the engine from the radiator. There definitely is no flow at all.

I did some testing yesterday on the water pump. First I removed the output hoses on the pump and cranked the engine, there was no activity from the output, not even a drop. So, I put all the hoses back, did a back flush of the system (engine and radiator), then filled the radiator. I fired up the engine and watched the water level in the radiator...nothing. If the pump was pulling the water as it should, the water level in the radiator should have dropped a bit, but it just sat there, not even moving. I pulled the pump, but can't find anything wrong with it. I will replace it today anyway because it is pretty worn, but I somehow doubt that will fix my problem.

If it's not the pump, what else would prevent water from being pulled into the engine from the radiator? There was literally NO movement in the radiator water at all. No circulation, no level change, not even a slight ripple. When I did the engine flush, I got good flow through both the engine and radiator.

Here's the latest - COBRA90GT, I checked the fan on A/C Max, all is working. I installed the new water pump, and I am seeing water circulation in the radiator, so it is pulling all the way through the engine and back into the radiator - BUT, the heating issue continues... I fired it up and watched the temp gauge. It hovered at 180-185 for a while, then started climbing to 210 and up. Once it hit 230, it SPIKED (in less than a second) to 245!! The needle just skipped everything in between and hit 245 and kept climbing. I killed the engine as fast as I could and started checking everything. Again, no steam, no leaks, no hissing, no smoke, nothing! Just a hot engine.

The upper radiator hose is hot and full of pressure, so we know the thermostat is opening. The driver-side of the radiator is cool, the passenger side is pretty warm (which is typical).

So, in short, we have:
-new short block with less than 50mi on it (driven only one day)
-new coolant temp sensor and temp sender switch
-two temp gauges (stock dash and mechanical) both giving the same results
-new 195* thermostat (did the same thing with 180*)
-new water pump
-new rad cap
-checked hoses for blockage
-fans are working
-no steam, no leaks, no hissing, no smoke
-compression check on heads showed good compression and pressure hold
-clean golden oil (5qts)
-did cooling system flush and had no issue with water flow through engine passages.
-heats slowly to 200, then speeds up to about 230, then spikes to 240-245.

WTF????

Thanks guys. Took another look at it last night. Looks like there is a very small amount of steam coming from the tail pipes. It's been pretty windy here the last few days, so that would make it almost impossible to notice that small of an amount of steam. Looks like I have a head gasket issue. Odd that it passed a compression check, but stranger things have happened. Steam in the exhaust is usually a typical sign of a bad head gasket. And that would definitely explain why I can't find the problem elsewhere in the system. I probably didn't get a good seal on it when I re-installed the heads to the new block. Luckily, I never let the temp get to (or over) 250*, which is usually the kill temp for a head, so they may not be warped or cracked. But I'll have them checked, just in case. Guess I'm tearing the engine back down again. LOL!

Thanks for all your help!

Well, I had the heads reconditioned and put them back on, along with a new water pump and 3 core radiator. It's STILL overheating! I figured I'd try to repost in "94-95 5.0 tech", so this post is continued there:

http://www.stangnet.com/mustang-forums/819385-5-0-overheating-problem.html#post8248070

Hey guys, the problem is fixed. Rather than me go through all the steps I did in troubleshooting, you can follow the full thread here: http://www.stangnet.com/mustang-forums/819385-5-0-overheating-problem.html

Thanks to everyone who chimed in on this. Your input was extremely helpful!

Mike