Experienced opinion on when to reuse MHG?

[quickstudy]

Actually more about when it is necessary to refinish deck and head - sorry about the mistaken title for the thread. Barely have driven the car since I did a full rebuild on 7m Turbo, redecked, head done professionally, but reused my titan motorsports MHG with the copper sealant spray, torqued ARP studs to 80. I'm getting slow bubbling coming into the cooling system and slowly losing coolant to overflow, eventually leading to overheating. Otherwise running well. My best guess is that the old MHG, already compressed and conforming to the old surface contours, didn't seal to the newly refinished surfaces. Would it be reasonable to try just a new MHG without sealant, without pulling the whole motor and refinishing again? How much needs to come off the head to accomplish this? Can it be done quickly by leaving the manifolds on? I've done two full rebuilds, but never tried just doing a quickie head gasket swap with the motor in the car. I have only driven a few hundred miles total on the rebuilt motor.

Thanks for your thoughts!

 

[quickstudy]

Actually not so slowly overheating. Got the right temperature sensor in. I was surprised that it overheats within 1-2 minutes of starting the engine, just sitting at idle. I didn't even know that was physically possible. I wonder if that's the water temp or just the temp of trapped gasses in the coolant passages. Especially weird because compression tests were good after I put the engine together.

I guess I'll be stripping the head again and resurfacing the head and the deck. I'm expecting the deck will just need some polishing, so I'll leave the engine in. Found a good procedure with pics. Darn, I was soooo close!

 

[757_supra]

I was surprised that it overheats within 1-2 minutes of starting the engine, just sitting at idle. I didn't even know that was physically possible. I wonder if that's the water temp or just the temp of trapped gasses in the coolant passages. Especially weird because compression tests were good after I put the engine together.

Have you checked your thermostat? If it's old and opening sluggishly, that can cause your quick heating up. Also, did you fully purge the cooling system? It tends to trap air and cause problems.

On the head gasket reuse, my personal opinion is that after doing all the work, using a brand new head gasket is cheap insurance, as I only want to do it once. Do a compression check, if you haven tore into it already, and see if it's low one one or two cylinders. Good luck.

 

[quickstudy]

Thanks for your response. This isn't sitting well with me and i'm giving it a week or two before tearing into the job.

1) I did compression test it and all were within specs, though some a bit lower than others. Sadly, I can't find my data sheet to say what cylinders were relatively lower or higher. the compression was raised in the rebuild, so I factored that in.
2) The thermostat is new. I checked it in hot water before installing.
3) I did burp the engine when I filled it. Did this many times, but kept getting air bubbles back in. I can drive it and only lose coolant slowly, but once I got the temperature guage working, all the tests for the sender and guage per TSRM check out, tried 4 different senders, I found the temp was going hot almost instantly. Now I'm afraid to drive it at all.

I will of course recheck both compression and thermostat as i tear down to change the head gasket, just to be sure. At least there's a little hope.

I'm not worried about the work of changing the head gasket, I'm just confused about what went wrong and afraid that it will still be the same when I put a new head gasket in.
Agree with you on new head gaskets. Lesson learned.

 

[quickstudy]

Block test was negative.

actual Radiator coolant temp at the pressure cap after idling 20 min was 170 degrees with a separate thermometer. The dash guage was off the scale.

I'm checking the thermostat tomorrow. Will do compression and leakdown anyway to be sure. Since this thread is mistitled, I'm going to jump on another similar thread at this point, but if anyone does reply here I will be checking back.

 

[hvyman]

http://www.cygnusx1.net/supra/Library/TSRM/MK3/manual.aspx?S=BE&P=41

You can check the sensor.

 

[emericaskater285]

On the head gasket reuse, my personal opinion is that after doing all the work, using a brand new head gasket is cheap insurance, as I only want to do it once.

I have always thought reusing head gaskets was a dumb idea. I feel the people that do reuse them get lucky if they seal and for a long time. I rather spend $100+ on a new mhg than have to tear the motor back down after just reassembling it.

 

[quickstudy]

http://www.cygnusx1.net/supra/Library/TSRM/MK3/manual.aspx?S=BE&P=41

You can check the sensor.

I checked that sensor over and over with resistance at different temps, got a new one, pulled an old one known good from a donor car, but I will do one step more. This time I'm going to remove it, leave it connected, and test the whole sensor/guage system with a cup of boiling water. That will leave no doubt whether the readings are correct on the dash gauge. More this evening.

 

[quickstudy]

I have always thought reusing head gaskets was a dumb idea. I feel the people that do reuse them get lucky if they seal and for a long time. I rather spend $100+ on a new mhg than have to tear the motor back down after just reassembling it.

I'm a convert. Even if it turns out to be okay, it was totally not worth the aggravation of wondering if it's okay. I have a new one on order, and I'm even thinking of swapping for the old one anyway just to eliminate one source of uncertainty in the future.

 

[quickstudy]

I forgot to mention that my coolant reservoir was relocated slightly more towards the right and is mounted maybe 2cm lower than stock position when i put in a large intercooler. I wonder if that is giving some spillage from the overflow tubes. If so I just have overheating or gauge issues and not overpressurization of coolant.

 

[hvyman]

The coolant reservour placement doesnt have an effect on the cooling system.

 

[quickstudy]

Good news. The gauge is not working, will replace. Strange that resistance test checked out fine, but pulled the sensor and it maxed out the gauge at room temperature. Got your post hvyman, don't doubt you, but I'll try extending the neck of the coolant reservoir, test the radiator cap, and see how the coolant level and temp hold up. With negative block test, and overheating in doubt, I assume that BHG can be considered less likely and I'm looking for another source of slow coolant leakage. The only time I thought I heard coolant boiling, after 1 hour idling, i didn't put a thermometer in it and the coolant level was 1/2 gallon low. My friend suggested pressurizing the coolant system and looking for coolant in the cylinders, as well as the leak down test, if the problem continues. No one likes unfinished diagnostic threads, so I'll post f/u if it comes to testing pressure/compression.