427 S.O. Overheating Questions

[elmariachi]

I have been helping a buddy try and diagnose his overheating side oiler and so as a last resort, I thought I'd start another seasonal overheating discussion. I've tried to provide as many details as possibleso PLEASE READ THIS BEFORE YOU POST and ask me if the __________ has been checked or changed:

This engine is a 12k mile medium riser, 500HP, Edelbrock heads, Cometic head gaskets, MR intake, 850 Holley DP, 9.5:1 compression, solid lifters, typical setup with external Mocal oil cooler. Car would show idle temps of 180F, NEVER ever exceeding 190F on the hottest days and down-the-road-temps at 165-180 depending on ambient temps. Never in 12k miles has it showed any signs of heat buildup or overheating. Then the bypass hose blew. The car overheated to 240-250F before he could get it shut down and off the road. Bypass hose was replaced, refilled and burped and ever since, idle and low speed temps rise slowly past 180 and on up above 200 and the car either has to be shut down or run above 80 down the road to cool it off. With a blown head gasket looming, we have worked to eliminate the easier/cheaper stuff first:

Step 1: Check temp gauge. Using IR gun confirmed temp gauge is working.

Step 2: Check for signs of blown head gasket. Oil is clean, plugs all clean and showed no issues but replaced anyway. No moisture or smoke in exhaust pipes. Ran chemical test on coolant TWICE, no signs of combustion gas in cooling system. Also checked cylinder head temps with IR gun and all of them are close to one another 250-300 while engine is at 185-200.

Step 3: Compression test (not a leakdown). All cylinders were 140-150 psi. A little lower than you'd like but all relatvely close.

Step 4: Ensure no air pockets trapped in cooling system. Had the system pressure tested by reputable and knowledgeable Superformance dealer. Test revealed leaks in radiator (car is 15 years old) so a NEW Fluidyne Cobra radiator was installed so system could be pressurized and tested (so much for cheaper stuff first.) Problem persisted after test/evac so a BIG single puller fan and two pusher fans were installed. No resolution. Replaced the water pump, radiator cap, all hoses and 180F t-stat. Radiator surface temps show 140 at the middle and bottom of the radiator while the IR gun aimed at the temp sender and dash both shows 200-210F. Fans and radiator are apparently doing a great job yielding a 40+ degree differential. On a related note, water level is constant in the expansion tank (no bubbles, leaks or overflow), but both water pumps show an unimpressive amount of flow at idle through the tank. System is running distilled water and water wetter, as it always has.

Step 5. Check base timing. Possibly the distributor was somehow bumped and idle timing retarded during adjacent plumbing repairs. Checked base timing, 18* with 36 total, no issues.

The radiator seems to be doing a great job of dissipating the heat from the coolant, but circulation is still suspect with both water pumps (once of which is a fresh rebuild.) Once the car heats to 200-210, you can shut it off, let the fans run for 2 minutes and then start it, and the engine instantly draws in the cool water from the radiator and the temp gauge plunges to 170-180. But over the course of the next 5 minutes it builds back up to 200+. All of this even with a 3' diameter shop fan running on high jammed against the nose.

Before you post, keep in mind that all of this started when the bypass hose blew. So its not pulley drive ratios or the like because that has not changed. If its a blown or leaking head gasket, could we be getting enough combustion gasses to heat the adjacent water this much but not enough to show up in either the chemical test, compression test, on the plugs or out the exhaust?

Thanks in advance.

Jim