This overheating seems to be a problem with a subjective solution. For all of those people that assisted or advised me on this topic in my last thread, thank you. For all those reading this for the first time – your opinion is needed and will be respected.
Background
2002 Oldsmobile Silhouette
3.4 litre
Current Km. – 179K
Purchased used in 2009 with 129K

Major service at 79K at GM dealership for right (back) head gasket kit under warranty

First Incident of Overheating

March 9, 2009 – overheated on highway returning to Toronto from Hamilton on the day that I purchased the vehicle from chief mechanic at GM dealership. Pulled over to side of road and allowed to cool down. Re-started vehicle and continued on my trip with all systems (cooling) normal. No further incidents until spring of 2010.

Second Incident occurred in Spring 2010 – overheated twice prior to continuing to operate normally

Did not check coolant level and adjust if needed in either incident 1 or 2 as technician always checked coolant levels at every oil change and found no problem with levels.

Third Incident – fall of 2010 – vehicle overheated to the degree that the coolant temperature warning light came on and within a few minute engine performance dropped (as designed) to protect the engine. Vehicle was serviced at that time, radiator flushed, thermostat changed, re-filled, pressure tested. Result was that coolant stayed within normal operating temperatures after that – 99% of the time with a random overheating incident every 2-3 months.

Observations

The engine never overheated during the extreme cold of two winters (with heater running) or extreme heat of 2 summers (with a/c running). Vehicle only ever seemed to overheat with the weather change in the spring and the weather change in the fall. This led me to believe that a vapour lock was the culprit.

Fourth Incident – spring of 2011 – overheated twice prior to continuing to operate normally

Fifth Incident – fall of 2011 – overheated on highway, high enough to trigger temperature warning light. Cooled vehicle down, got vehicle to dealer. Dealer pressure-tested, determined leaks at lower intake pipe and at water pump, low coolant levels, also computer tested and found a misfire code. Tested low speed and high speed fans – both were operating normally and triggering within acceptable normal temperature range. Dealers’ recommendations: replace lower intake pipe, replace water pump, replace head and intake gaskets. Based head gasket recommendation on misfire code and stated that coolant must be leaking through gasket, lowering the coolant level which caused overheating.

Sixth Incident – vehicle overheated approximately 2 weeks after being at dealer, checked coolant level and found to be still low – had assumed dealer had topped it up, but they had not. Topped-up coolant level. No more incidences for next couple of weeks.

Seventh Incident – ongoing: vehicle operates normally for approximately 3-4 days after topping up fluid levels, overheats, then I top up coolant and good for another 3-4 days. Took vehicle to a radiator (coolant) specialist. His observations: slight leak at water pump, but not at seal, slight leak at intake pipe (residue evidence only). Slight vapour at exhaust – minor head gasket leak. Pressure tested – no drop or loss of pressure. Their recommendations: 1) head gasket leak is minor – add a can of stop leak after removing and cleaning radiator 2) replace intake pipe and water pump.

My Observations and Conclusions

1) With all incidences of overheating up till the 7th, it appears that the coolant level was low, so I added coolant. Randomly, that put a temporary end to the problem, however when the coolant level dropped the problem returned. However, I have never observed any fluid accumulation in my driveway or any location where I have parked. For example, I topped up the radiator last night before I retired. At that time when I restarted the engine and idled it for circulation, the heater was pumping out warm air and the coolant temp was normal. Then when I drove the car this morning, it overheated within perhaps 10 blocks and constantly blew cold air from the heater. The car sat all night after topping up the coolant and there was no evidence whatsoever of fluid accumulation in the driveway, beneath the car, if from a leak. When I checked the coolant level after the engine cooled, the rad level was low. However, the coolant level in the overflow reservoir was higher than normal.
2) I have always noticed that the coolant level in the overflow reservoir gets higher from the last time that it overheated. Therefore, I can only conclude that the coolant that I think that I am loosing through a leak is actually going to the overflow reservoir, as I have noticed that it will continue to fill the overflow reservoir tank until the tank eventually blows coolant through the top vent of the tank. I thought that perhaps there was a blockage in the backflow line to the tank or a bad rad cap, but both check out to be ok.
3) Most of the time at the beginning, if the heater started to blow warm air, if it was previously blowing cold air and the car overheated, then the coolant temperature would drop. However, now, as I suppose that the “root” cause of the problem has grown worse, I have noticed that randomly the heater continues to blow cold air and the coolant temp still drops, OR the heater starts to blow warm air and the coolant temp remains high. I have also noticed recently that the heater will blow warm air when I accelerate the vehicle until it returns to almost normal operating temp and then it will start to blow cold air even while driving at a higher speed. The coolant temp rises rapidly when the vehicle idles,(e.g. at a stop light) however slowly drops when I continue driving.
4) My thoughts on this are as follows: yes, there may be an underlying head or intake gasket leak that is part of the problem, however, I do not think that it is the “root” cause of the overheating problem. One thought is that there is a blockage somewhere, whether it be a physical blockage or an air blockage. A second thought is that it could also be an electrical problem in regards to a faulty sensor or switch. Anyone who knows which electrical parts on the 3.4L that may affect the coolant temperature, please advise! Actually, both of these things could be happening.



My Question

1) I was thinking of the following action plan – flush the cooling system, replace any damaged or leaking hoses, replace the water pump and the thermostat. Test and/or replace any electrical parts that may affect or control the cooling system. Re-fill the cooling system and add a bottle of head gasket fix. Start vehicle and pray.

Anyone have an opinion on that plan, it would be appreciated

One comment stood out for me: "it overheated within perhaps 10 blocks and constantly blew cold air from the heater".

It sounds to me like you are losing coolant and overheating when the coolant gets low. Then, you top up the coolant but don't bleed the system thereby introducing air into the cooling system. Air in the system can cause the engine to overheat even though the HVAC is blowing cool air.

If you already replaced the head gaskets, you may have a warped head.

I would get the system pressure tested again to see where you are losing coolant. When you top the system up, make sure you open the bleeder screw and get all the air out.

Thanks 2001 Bonneville, however, I am removing both bleed screws while topping up and topping correctly into the rad and not the overflow tank. System has been pressure tested twice so far, first time last fall, no visible fluid leaks. Second time, last week at rad shop. System held pressure without visible reduction. Existence of 2 leaks, one at the intake pipe (substantiated only by the residue - no liquid visible) The second leak was at the water pump, again residue only, no visible liquid and residue was not at the pump seal, where you would expect it. The only time that I have ever seen fluid from the system is when it expelled antifreeze through the blow of vent on the top of the overflow tank. I topped it up last night and did not move the vehicle and yet when it overheated again today, I let it cool down and it took about 3/4 of a litre to top it up again. Has not overheated since second top up. No evidence of antifreeze leak on driveway.

I am losing coolant, but the evidence of a leak at intake pipe and water pump + a potential leaky head gasket or intake gasket does not account for the volume.

pressure doesnt simulate the heat of the engine, the leak may only happen when hot and get burned off by whatever hot engine component it drips on. my old 3400 was leaking from a bad joint on that metal tube that goes from the pump area to the tb. didnt leak till hot and no puddles on the engine. that was a good one. once i figured it out though i just put some jb weld on it and painted it black and it was fine forever. did they ever clean the fins of the rad. not inside outside and the evap core is in front of that as well, alot of times you pull those and backflush it to regain cooling capacity. you got to figure that van with the tiny engine compartment is very taxing on the rad so you need all the efficiency you can get. those sealant bags head gasket fix are no good either unless you are planing on replacing the engine after it seals up the coolant passage ways

Thanks for that advice, seems logical, so I investigated again this morning. Found evidence of leak at two connections (the metal tube that you talked about was one of them). However, it seemed that the leak volume was not enough to justify overheating the next day after topping up the coolant. Went out this morning about 6 a.m., drove 3 block trip at 6 and return 3 blocks at 6:45. Engine still cold. Waited until 8 a.m. and opened rad cap to check coolant level. Low and behold, as I released the rad cap through the first of it'* two release points, it blew back fluid all over me. 1) there should not have been any pressure in the system after only driving 6 blocks and cooling for over an hour 2) the fluid was ice cold - not even warm.
That led me to believe that it the coolant system was NOT blowing back but as I released the rad cap, the overflow tank was now emptying back into the rad. The overflow tank was still 3/4 full when the engine was cold this morning and that should not be the case. That tank should fill as the coolant heats and empty as the coolant cools. So to me at this point, I do not think that the rad cap is operating properly. So, tomorrow, I will have the coolant system flushed, hoses repaired and replaced, rad cap changed and cooling system recharged and see what happens at that point.

Cross your fingers for me and yes, I will return the can of stop leak and get my money back.

Thanks again for your advice

X2 on JW'* comments relative to the sealant bags. You may get a short term fix that causes long term problems.

Fix all those small leaks that you don't think are large enough to be a problem. Your assessment on how insignificant they are my not be correct.

Any signs of coolant in your oil? Check for foaming on the oil dipstick.