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Dreaded Vq25Det Headgasket...

DMM

By DMM,
in Four Door Family & Wagoneers

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DMM Apprentice

DMM

Posted
Posted

So i recently purchased an m35, knowing it had an overheating issue. I knew it would 99% be headgasket blown! Only 70km on the clock.

Thankfully the motor looks on great nic on the inside! I was also happy to note that the coolant didnt blow through into the oil galleries - which has made the repair job much easier!!!

These things are built tough, ive played with vq35's a hell of a lot previously and RB's. I am falling in love with the design of these motors. Thick sleeves, large coolant volumes around the sleeves which the 3.5 block doesn't have. Good pistons.

That centre blanking cap in the middle of the valley I think is used for a coolant control valve in the US market? Inside mine looks to have boiled under there and acted as a small air lock. I'm going to link from here and the factory bleeder valve to a bubble tank - hopefully alleviating air lock issues in the future!

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DMM Apprentice

DMM

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Scotty_m35 makes a bolt up coolant mod that works similar to the us version and also has a nice bleed valve added in. We'll wprth the investment

Any pics or links to the system he uses? I am just about to order an fittings and weld ons.....

Brick Mentor

Brick

Posted
Posted

Any pics or links to the system he uses? I am just about to order an fittings and weld ons.....

Search the forum for scotty's m35 coolant mod.

Pretty reasonably priced for the product and the quality of his workmanship.

Also Google scotty's customs. He has a website and produces a lot of bolt on m35 parts that are proven to produce the goods.

Also look at his dump pipe, it's the best there is

PN-Mad Grand Master

PN-Mad

Posted
Posted

Oh, to answer the question on the extra coolant gallery, it was a feature on some of the other motors in some of the other motors inthe VQ range, that part of the "towing package" would feature an additional coolant passage with separate thermostat.

Here are the parts :)

http://www.z1motorsports.com/350_g35/product_info.php?products_id=3011

Good job opening it up, and sorting out the issue btw.

scotty nm35 Grand Master

scotty nm35

Posted
Posted

Except those parts won't fit the M35. ;)

Overheating doesn't mean the head gasket is gone in these, so anyone reading this later, check you have properly bled the cooling system, or fit my coolant mod kit before ripping the engine out. Please. It will save you a lot of headaches.

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scotty nm35 Grand Master

scotty nm35

Posted
Posted

Definitely not, plus the outlet pipe to the radiator is completely different.

No idea why you would need another two thermostats either, just another part to fail.

My kit uses the heater line to bypass coolant from the block, rather than relying on the one small pipe at the back of the engine. It also replaces the plastic bleed assembly with alloy which is another common failure point.

DMM Apprentice

DMM

Posted
  • Author
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Except those parts won't fit the M35. ;)

Overheating doesn't mean the head gasket is gone in these, so anyone reading this later, check you have properly bled the cooling system, or fit my coolant mod kit before ripping the engine out. Please. It will save you a lot of headaches.

Correct! However - these being open deck - often will warp the heads. The whole VQ range from my experience does not suffer to well under heat - ive seen them where they even warp through the cam tunnels!!!

Of course i didnt just take a punt - i knew it was head gasket before ripping it out.....

scotty nm35 Grand Master

scotty nm35

Posted
Posted

Often? I've had mine at 150C at the track on many occasions, it never lifted or warped the heads, even at 40psi up the straight... Being alloy block changes things, as the expansion rates are similar, unlike iron block/alloy heads.

If the heads are warping that bad there is an airlock causing no coolant to flow, which is the common VQ fault I tried to alleviate with my coolant mod design. As the bore and heads overheat they cause flash boiling which exacerbates the problem.

DMM Apprentice

DMM

Posted
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Often? I've had mine at 150C at the track on many occasions, it never lifted or warped the heads, even at 40psi up the straight... Being alloy block changes things, as the expansion rates are similar, unlike iron block/alloy heads.

If the heads are warping that bad there is an airlock causing no coolant to flow, which is the common VQ fault I tried to alleviate with my coolant mod design. As the bore and heads overheat they cause flash boiling which exacerbates the problem.

Well when you are running ARP head studs i would hope you don't have issues!!! FYI i've gone back to OEM studs - ditched the angular torque and run my own torque values (105NM final value). Angular torque on a used block is a waste of time and generally ends in uneven torque values.

The issues with OEM bolts and open deck comes down to contact surface area. Nothing to do with iron alloy mixes....that wasn't even on the pallet for discussion haha. Like you say - flash boils - but the residual heat in the open deck decreases the dynamic torque of the studs and allows warping and reduced pressure between bolt spreads. When the pressure between the cylinders lifts obviously they blow through. I have never actually seen a VQ blow through on the outside of the open deck and lift to an adjoining oil passage - always happens middle of block between coolant chambers.

I've blanked off my radiator cap. Fitted my own bubble tank with the pressure cap, teed off from turbo water feed (base of turbo), centre of valley, teed off top of thermostat and bridged and removed OEM plastic bleeder.

DMM Apprentice

DMM

Posted
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Posted (edited)

I must say however I do prefer the VQ25 block over the same era VQ35 blocks. These blocks are much more boost friendly with a thicker sleeve and larger volume coolant chambers surrounding the sleeves. Also the OEm metal gaskets are very good - however I don't like they fact they pre coat them with the teflon/graphite mix. I threw mine in the light costic wash for a few hours to get rid of the coating :)

Edited by DMM
Ben C34 Grand Master

Ben C34

Posted
Posted

Could you give the reasoning behind not wanting to use angle torque on a used block?

It is my understanding that angle is used to remove the error caused by varying amounts of friction acting on the thread, interfering with the actual clamp load and stretch of the bolt.

How does the block being used negatively affect that?

scotty nm35 Grand Master

scotty nm35

Posted
Posted

I only fitted the ARP's after running unlimited boost, lifting a head. I also ran the car hard for a year before the head gasket finally let go in a big way on the dyno. Like you say the extra meat of the bore is what helps us VQ25 guys, comparing it to the VQ35 is silly, they are very different as you have found.

OEM gaskets here, with extra Hylomar spray. 105nm sounds like a lot on stock bolts...they will have twisted nicely. Did you lube the threads at least, and clean the threads in the block?

DMM Apprentice

DMM

Posted
  • Author
Posted

Could you give the reasoning behind not wanting to use angle torque on a used block?

It is my understanding that angle is used to remove the error caused by varying amounts of friction acting on the thread, interfering with the actual clamp load and stretch of the bolt.

How does the block being used negatively affect that?

Ive found in the past angular torque stretches the block threads heavily. There have been some situations where a customer has requested a certain bolt with angular torque and it pulls the thread. The only way i've found against this is using helicoils to increase thread depth contact area. Mathematically when you use stretched alloy the thread bolt only touches around 60% of possible mating thread in the block.

If it was all brand new then nothing wrong with angular as yes you are right - helps eliminate error.

I only fitted the ARP's after running unlimited boost, lifting a head. I also ran the car hard for a year before the head gasket finally let go in a big way on the dyno. Like you say the extra meat of the bore is what helps us VQ25 guys, comparing it to the VQ35 is silly, they are very different as you have found.

OEM gaskets here, with extra Hylomar spray. 105nm sounds like a lot on stock bolts...they will have twisted nicely. Did you lube the threads at least, and clean the threads in the block?

No twist - prepared in layers. Cleaned and oiled of course. Process as follows:

1)Spun bolts in and out 3 times to distribute oil evenly and wipe off excess after each spin.

2)Check block temp - anything over 25degree at 0.15% NM for every 1 degree over 25.

3) Torque all bolts to 65NM. Let gasket squish and settle.

4) Torque to 90NM - let set over night

5) Release bolts, remove and check oil coverage. You will see ghost marks on bolts if oil coverage is poor, otherwise will have nice even moist look.

6) Nip to 90NM again - just touch 90, no need for high tension load.

7)Torque to 105NM and finished.

No bolt groan, no binding, no twisting process! Used this many time over the years on RB, SR and VQ engines. A lot have seen the loads of bathurst etc without engine failure.

DMM Apprentice

DMM

Posted
  • Author
Posted

It won't matter how tight the bolts are done if you don't fix the coolant issue. Surely you would have known about that with your VQ racing expertise?

I chose to do my bolt tensions that way to reduce twist or shift. Not to stop a head gasket from lifting. Gaskets are designed by nature to be the weak point.

Coolant issue is as you have said - failure to flow out the back of the block/heads. The two passages out of the rear of the heads are tiny and restricted from flow heavily. This is why is I'm adopting the centre relief in the valley to help with flow.

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