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Old 12-12-2015, 07:54 PM   #1
dphc
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3.5L overheating - myths vs what works

I've read a lot of conflicting information about 3.5L overheating problems and just want to share what worked for me. Short answer - heater core, and open the bleeder screw (only) when filling an empty system.

There are people who say you need a special funnel to add coolant, repeatedly purge air from the system while the engine is running, fill your cooling system (not your refrigeration system) while it's under a vacuum, only use an OEM water pump, reverse the hoses to the heater core, drill holes in your thermostat, etc. I'm guessing that last one actually might hold some truth because from what I can tell, the extra spring loaded disc on the thermostat only opens under pressure, so maybe it won't open if air gets trapped behind it? But anyway...

After my original water pump failed on the highway, I had my car towed to the nearest shop where they replaced it with a Bosh. From then on, I experienced unusually high engine temperature at moderate speed (30-50 mph) and low engine RPM, but not at idle or on the highway. I replaced the thermostat a few times, tried those odd engine running purge procedures, but then the problem would creep back up, and seemed to get worse with the heat on.

Someone somewhere suggested downshifting when necessary to keep the RPMs around 1800-2000, and it worked - the engine temperature would immediately drop back to a normal range. If high RPMs don't lower your temperature, then you might have a different problem like a restriction.

After more than a year, I finally started smelling coolant when using the heat, so I knew the heater core had a leak, but I believe the leak was causing problems all along. Early on, the leak was probably small enough to where it would not leak under pressure, however some leaks only occur under vacuum - when the system cools, you have an effective vacuum. Seems like air was getting drawn in with each cool down, which would end up back near the thermostat and cause problems next time I ran the engine.

So as a test and temporary solution, I bypassed the heater core with a 5/8-3/4 heater hose adapter - you can get them for like $3 at O'Reilly sort of places. High temperatures immediately stopped. A warning on these adapters - unless you can find a metal one, try not to use the plastic one for more than 6 months or a year. I had one fail after about a year and a half. The one I replaced it with was staring to sort of bend inward after only a few months. Maybe I just didn't have it far enough away from the exhaust system.

(If you bypass the heater core and the problem persists, then there's not much to be gained by replacing the heater core unless you already smell coolant when the heat is on. You might have a leak somewhere else in the cooling system, such as the crossover at the rear of the engine)

When I took out the old heater core, the leak was barely visible - just a bit of coolant residue about 1/2 the size of a dime. Not even enough to drip into the heater box. I installed the new one completely empty. Just hooked up the hoses and got right on the highway. I experienced no unusual high temperatures and did not need to purge any air.

Whenever I replace and refill the coolant, I just open the bleeder screw on the passenger side top of the radiator (hear the headlight), add coolant to the reservoir until it starts coming out of the bleeder screw, then close it. I don't mess with it any further - the o-ring seems to get stuck if it's opened and closed too much. The coolant level will still settle in for a week or so (so I check it regularly), but after 5-10 warm-up cycles, it is stable. I do not experience any unusual high engine temperatures and do not need to purge any air.

I was told that the system is supposed to self-purge - I guess that's what the 1/4" hose is for at the top of the coolant tank. If you've got a leak there, even if it's small, you're better off replacing the tank. It's easy enough, and it's nice when you can actually see the coolant level inside. Also, the low coolant switch on the old one could be stuck.

If you're using dex cool, I also recommend getting the pellets from the dealer. I take 2-4 of the large ones and let them dissolve in new coolant overnight, then shake bottle before adding to the tank.
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2002 Oldsmobile Intrigue / 3.5L / 230,000 mi

Last edited by dphc; 12-12-2015 at 09:30 PM.
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