Possible head gasket or just air lock?

[91 LSC SE]

Hi Everyone!
Looking at buying another M8 with 107k miles. These are beautiful rides! Only issue with this new one is random overheating. I have been reading overheating/headgasket threads for over an hour, searching way back. Found a post from xtriggerman who made his own lower t-stat housing - nice!

Do you think its a small head gasket leak or trapped air? And how easily/fast do these aluminum heads/blocks warp?

1st test drive was short, didn't have plates on her:
The coolant bottle was empty. Owner topped her up with water before test drive. We let her run while I looked her over. Took about 25 minutes checking the usual - heat, telescopic steering, neon light etc.

She, had the rad, hoses and water pump just replaced:
- didn't overheat at idle, or idling for 25 min.
- hauls ass, no lack of power, did two WOT's and then let her run for 35 minutes after that and didn't overheat.
- no white smoke at tailpipes
- no sweet smell at tailpipes
- no milky oil, but oil looked new and may have just been changed

It did go below freezing a couple degrees for a couple nights after the 1st test drive. I asked the owner to add coolant, he said he did. Though when I arrived the 2nd time the coolant bottle liquid was clear, not green and not to the top. This means, that the newly added water moved into the cooling system (coolant bottle was empty, maybe coolant system wasn't full) or it has a leak or it is burning it. Didn't look like it was leaking, but hard to tell, as car was parked on grass.

2nd test drive a week later:
Coolant bottle was half empty. Figure some coolant may have entered cooling system or burnt off. Again water was added without burp last visit (cap seized, couldn't remove) I added concentrated coolant to fill bottle. Then let her run for 5 minutes and did a hydrocarbon test with the Lisle 75500 combustion leak detector. Tested clean, fluid stayed blue.

Temp gauge was half way (normal), started drive with two WOT's and then driving 50mph for less than 2 minutes the gauge went to 3/4. Put her in Neutral and let her coast for a minute and temp back down to half way again. Continued driving and temp went back up to 3/4 and continued rising this time to the top white line before red lines. Tried to accelerate 35% of throttle, but motor bogged and RPM's would not increase. Eased off and she kept current speed, but didn't have the power. Still in Normal range, but at the top of the top white line just under red, almost touching red at this point. I was testing right by owner's home, and right by house when this happened, so I stopped the test drive and pulled in the driveway. She made a pre-ignition sound and stalled. We open the hood and saw a small amount of smoke from the driver side head strut area (Left bank) for about 2 seconds, then gone. Let her idle for a minute and she ran good, then rough almost stalling, and then good again - temp still at top white line. Shut her off, waited for 10 min. then started her up and she idled with temp gauge going back down to the middle, staying there for 25 minutes no issues. Didn't go back out on the road. Though I revved the motor, 2 or 3 times repeatedly, she was building RPM's easily and revving normally, sounding like a beast, gauge staying in the middle entire time.

Could my leak test been false due to adding new coolant? I added coolant and let her run for about 5 min. to warm up a bit, but didn't look like any coolant was cycled into motor or vs versa. Think maybe it didn't get a chance to mix with coolant in cooling system?

If it didn't have lose of power, I would be certain its an air lock. Owned my current M8 for 12 years and never had a over heating issue, so do not know if its normal to loose power when at the top almost over heating?

 

[xtriggerman]

Could be some air got to the pump. I found the high pipe into the heater at fire wall held air that no one talks about when burping the system. My first test drive after burping looked good sitting but after a 2 mile run, it got up to the red and I feathered it 2 miles home. I didnt try and kind of throttle, just enough to crawl home. Burped it again and let air out of that heater core top pipe, burped again or tried to. everything was good after that. These cars will fool you with the burp thing very easily. Its possible the PCM cut fuel and retarded the ignition off the temp sensor as well since they are a computed ignition. Just a guess tho. If your in Canada, why in Gods name is the guy putting water in the aluminum block? Hard water will grow barnacles in there..... Hopefully it was at least distilled water. Anyway, The disappearing "water" is a big problem. If its a external leak maybe its still a buy. If the coolant is all water and overheated to 210 or so, it could just evaporate out of the oil. Water is a real bad idea to put in these all aluminum blocks. I wish you the best of luck figuring this all out. Personally from what you said I'd be hard pressed to buy it unless a head job is not a big deal for you to do.

 

[91 LSC SE]

Could be some air got to the pump. I found the high pipe into the heater at fire wall held air that no one talks about when burping the system. My first test drive after burping looked good sitting but after a 2 mile run, it got up to the red and I feathered it 2 miles home. I didnt try and kind of throttle, just enough to crawl home. Burped it again and let air out of that heater core top pipe, burped again or tried to. everything was good after that. These cars will fool you with the burp thing very easily. Its possible the PCM cut fuel and retarded the ignition off the temp sensor as well since they are a computed ignition. Just a guess tho. If your in Canada, why in Gods name is the guy putting water in the aluminum block? Hard water will grow barnacles in there..... Hopefully it was at least distilled water. Anyway, The disappearing "water" is a big problem. If its a external leak maybe its still a buy. If the coolant is all water and overheated to 210 or so, it could just evaporate out of the oil. Water is a real bad idea to put in these all aluminum blocks. I wish you the best of luck figuring this all out. Personally from what you said I'd be hard pressed to buy it unless a head job is not a big deal for you to do.

Thanks Xtriggerman! Greatly appreciated. I too was concerned about the water. Thought it might freeze in the block. He didn't want to test drive with low coolant and didn't have any. Don't know if it was distilled water. I figured If I buy it, I would just change coolant using the coolant vacuum refill tool I bought to change coolant in mine 5 years ago. It worked perfectly without any burping.
I haven't changed a head gasket and know its a labor intensive job. We did try to remove the cap to burp, but it was seized/rusted - not being my ride and without a replacement cap, didn't want to push too hard.