Failed MHG install? Now what??

[rbutton]

Apologies for the long post.

I recently put an HKS 1.2mm MHG on an 86.5 NA with a refurbed head (mirror finished) and a block that was only surface cleaned. Installed ARP bolts to 85ft lbs. Was thrilled to get 180-190 psi reading on all cylinders, and the car ran stronger than it ever has in the 20 years I've owned it. :icon_bigg

Then the temps hit 90+F this week and I started having cooling issues. Car spat out most of it's coolant in a parking lot after a 20 minute highway drive on Friday. Maybe the caps bad, so I flushed system, refilled, and installed new radiator cap. The next day the coolant temps were going from 1/3 to 2/3, back down to 1/3 constantly over a 20 minute drive in the city. Drove home with windows open and heater on full blast - that was fun! New fan clutch arrived that evening so I installed it and that seemed to help on a night drive later that day.

Today I drive 20 miles in the morning to play golf, no cooling problems at all. On the drive home (now 93F out) the gauge is right around 40% all the way with A/C running. I get off the highway to grab some chow and while sitting in the drive-thru I notice the gauge is pegged high!. I quickly pull over and shut off the engine, the temp starts coming down slowly.

Pop the hood and I can hear a strong hissing sound, seems to be coming from the back of the engine - like around #6 cylinder. Don't see any coolant or steam or anything, just hissing for a few minutes. I then loosen the radiator cap and get some boil-over into the overflow tank for about 2 minutes. After I get the cap all the way off I had to put about 6-7 pints of water in the radiator just so I could get home.

Do I have a bad MHG install? Why did I get such great compression readings at first?

What is the hissing sound?

Recommendations on what to do next?

P.S. - The two OEM head gaskets that I've had to replace over the past 20 years have always had "crushed" or deformed exhaust-side coolant gaskets on cylinder #5 and #6.

 

[super.secret.supra.club]

iis there leaking of any coolant? did you check the hose in the back if the head to make sure the hose is clamped properly

 

[IJ.]

So you didn't machine the block and timing case?

 

[rbutton]

iis there leaking of any coolant? did you check the hose in the back if the head to make sure the hose is clamped properly

As far as I can tell, coolant only getting out through radiator overflow

---------- Post added at 11:24 PM ---------- Previous post was at 11:23 PM ----------

So you didn't machine the block and timing case?

Correct. I don't have equipment to remove the block. Last HG I replaced lasted 15 years, but it was OEM.

 

[IJ.]

MHG on a twice BHG'd block was always going to end in tears, replace it with an OEM and you might get lucky for another 10>15.

 

[rbutton]

MHG on a twice BHG'd block was always going to end in tears, replace it with an OEM and you might get lucky for another 10>15.

Why the great compression readings when cold, then BHG when hot? I thought the ARP bolts would hold things together better when the going got tough?

 

[IJ.]

Block and Head expand at 2 different rates, might have been leaky when cold and sealed up when hot

Luck of the draw kinda thing.

 

[rbutton]

Block and Head expand at 2 different rates.

Yes, I understand that. According to this site:

http://bobmay.astronomy.net/misc/tempcoef.htm

aluminum expands at twice the rate of iron. At hot temperatures the force on the head gasket/bolts should be greater than when cold.

So in the past when I had problems with the OEM gasket it was due to the loosening of the stock bolts as the head expanded and cooled continually. I would sometime go to tighten the head bolts and find the rear exhaust bolts very loose.

I thought the ARP bolts were supposed to help eliminate this problem. If not, then what's the point of the higher strength bolts and higher install torque?

 

[IJ.]

Bi metal engines "move" the idea is to have a perfectly flat surface on each side of the uncompressible MHG so it will clamp down and seal, no flat/flat no seal, a composite actually works better with a rough finish.

The Upgraded hardware prevents lifting the head under load, the initial Tq settings aren't that much higher than stock, more clamp pressure doesn't automatically equal a better seal as you end up pulling the deck up around the stud holes and deforming the head. I posted a cutaway of a 7M head they're NOT thick/sturdy at all.

 

[chris_2jz]

Hey if it helps at I had all I was similar symptoms your having and for a while I couldn't find the problem and then one day I saw a puddle of coolant falling from the fire wall comes to find out the hose that goes from the back of the head to the firewall had a leak. I replaced it and it fixed my problem. Hopefully its something as simple as that hose

Sent from my Galaxy S phone

 

[hvyman]

You either machine both for a mhg or dont even bother. Cant just slap a mhg on and call it a day.

 

[suprarx7nut]

Always machine the block for a mhg. Always.

Every. Single. Time.

If you can't pull the block, you can't use a mhg. Plain and simple. This is exactly why.

Pull it apart, machine the head again or at least have it checked by a shop and use an oem Toyota gasket.

Sent from my ADR6400L using Tapatalk

 

[rbutton]

Hey if it helps at I had all I was similar symptoms your having and for a while I couldn't find the problem and then one day I saw a puddle of coolant falling from the fire wall comes to find out the hose that goes from the back of the head to the firewall had a leak. I replaced it and it fixed my problem. Hopefully its something as simple as that hose

I couldn't see any coolant leaking at the back of the engine, although I did have to move the big water union from my old head to the new, so I guess it's possible that I didn't get a good seal when re-installing.

However, I think other posters are probably right that I need to abandon the MHG and go back to an OEM.

The good news is that IJ says the MHG is "uncompressible", so I should be able to pull it off and re-sell it.

 

[rbutton]

In reviewing pictures I took of the block I found some cracks between exhaust-side water jacket ports on cylinders 2 and 3. Also looks like a small crack from water jacket to head bolt hole.

I reviewed close-ups of all other areas and found no other cracks.

Should I be concerned? I don't normally have problems with cylinders 2 and 3.

 

[bioskyline]

doesnt look like a crack, looks more like a ridge? have you run a straight edge over it to see if its flat or not?

 

[CyFi6]

If you look, you can visually see an indentation from the compression sealing ring of the stock head gasket around the cylinder, this is going to reduce how much the MLS is going to seal in the most important area. One of the biggest reasons the block needs to be machined even if it is super clean. Like IJ said, since the MLS cannot compress, low spots are left with little pressure against the gasket and are likely to leak.

 

[hvyman]

Take some pictures of inside the coolant port and the threaded hole. If it goes inside id junk the block as your never going to fully fix that and alwasy have problems.

 

[AJ'S 88NA]

Yep^^ Doesn't look good from those pics.

 

[4U2QUIK]

I see what looks like rust in the hole when i zoom in real close. pretty much means there's a crack between the port and the threaded hole. That's all bad.... Time to find a new block.

 

[hvyman]

Ya looks like a straight up crack even from my non zoomed in picture. Thats not casting mark either as it would have been machined out.