I saw this question at first and was wondering myself. A water pump may be time consuming but certainly not the price of a replacement engine. Then I read more.
I can completely understand the mechanic preferring to replace the entire engine with this many unknowns. It has been overheated many times and a stop leak type product has been added. I personally would rather have one with an obvious head gasket problem than something masked by stop leak. He is probably afraid that you will spend a bunch of money on the timing belt and water pump only to find out that there are other more serious problems requiring an engine rebuild or replacement. I deal with the general public doing service work myself. Some people always want something done cheap, even if you tell them the job you just did will have to be redone, possibly in a short time and for the same amount of money if the other part I suggest replacing fails. They are all fine with this until the repair fails in a short time and then they demand I redo the work for free. I now have them sign a form and that is that. Most either walk away or agree to having a proper repair performed.
Here is what I suggest. DO THIS WITH THE ENGINE COLD. Remove the radiator cap and start the engine. If you see what looks like air bubbles in the tank, exhaust gasses are getting into the cooling system. That can happen from a blown head gasket, a cracked head, or cracked block. Movement of the coolant is normal but bubbles are not. Also, cars with a blown HG usually have a nasty exhaust smell in the coolant. It is a certain smell that you cannot really describe but kinda smells like well-used motor oil but a lot worse.
If this is inconclusive, look for a block test kit to test for combustion gasses in the coolant.
You probably paid too much for this car but at least it isnāt a life-changing amount of money. Next time I would pay around scrap if possible when getting a car with this many unknowns. Seeing stop leak in the system would have been a deal breaker for me.
I thought the last year for the Trailblazer was 2009 .
It it too late now but maybe the next time the OP hears the word " overheated " when looking at a vehicle they run for the hills to find something else.
Now that we have more information, I regret making such a quick condemnation of that mechanic. I now see he had a better view than we did of the whole situation.
Please keep us here informed of what you find and decide. Good luck.
If youāre interested in opinions, based on the details you provided, I would not put one more dime into the existing engine. Way too much cost and risk. I actually know someone that rolled the dice, spent almost as much as a good used engine installed and the ārepairedā engine failed in a few months due to latent damage from prior overheating that wasnāt addressed. Iād listen to db4690ās advice regarding how to prep the replacementā¦good luck!
I once drove several hours one way to look at a car that I planned to repair and drive for myself. The owner indicated it had ājust stopped runningā and that he had it towed home and was going to jsut sell it as is. Before leaving, I put all my spare parts for the car in the trunk and headed off. I was planning to try and drive it home and he agreed that I could diagnose and repair the car at his location.
Anyway, I got there and found it was much rustier than described. Then I turned the key and tried to start it. There was a clunk and something was locked up. I opened the oil cap and could see bits of metal inside the valve cover. I pulled the dipstick. There was no oil on it but plenty of metal bits as well. Then I opened the radiator cap and it was full of stop leak.
He knew that I had come from quite a distance and made a special point to be home when I got there. I honestly donāt think he realized what was going on but could tell I was losing interest quickly. He had the title in his hand and was ready to sign it over. I explained to him that the problems with this car were pretty major and that I was sorry but didnāt have any interest in it. He then asked if I would take it for a lower price and I was like āno but thanks.ā I didnāt even make him an offer. He did admit that although synthetic oil was used during the last change, he hadnāt changed the oil or checked the level in about 10,0000 miles.
I later saw the car with the price cut in half. He said āSomeone who looked at this car said the engine is locked up and will need to be rebuilt or replacedā so donāt think he was trying to screw me. He just didnāt know. I have been a little more reluctant to go long distances for non-running cars since this time.
Basically, you need to know when to cut your losses or invest the money to make things right.