Blown Headgasket

[Randall]

Well just put about 400 miles on the motor and after pushing it pretty hard one evening next start up cranked then immediately stopped as if there was too much timing but eventually turned over and started up. I then noticed that my idle vacuum was a little low around 17.8 to 18.3 inches where it has been at about 20 or so. It drove fine so i did not think much about it. Today I went to start it up and same thing but eventually starts i drive it around and as im pulling into my driveway i realize that my temp gauge is reading 215 in megasquirt. The radiator was about a gallon short of fluid so the temp could have been hotter.

 

So i do a compression test: 

 

1. 145

2. 134

3. 134

4. 139

5. 138

6. 141

 

I then take it for a spin and it starts misfiring at low RPMs so i start pulling plug wires to find number 5 the culprit.

I then pull number five spark plug to find it drenched in water.

 

So i pull the head and to find this:

 

 

 

Cylinder 5 was blown at the water jacket and all other cylinders except number 3 looked like they where close. 

 

Could this be detonation? (no signs of detonation on plugs or piston tops). Should i be tightening the ARP head studs tighter than 60 ft lbs? This is also a standard fel-pro gasket.

 

Here are my AFR targets:

 

 

Here is my timing map:

 

 

This is at 10 lbs non-intercooled .040 over pistons and "A" cam on stock P90.

 

 

[skirkland1980]

Your timing had to be too advanced at the time of the failure though your map looks ok. I would try to find a problem with the distributor or something.

[skirkland1980]

Did the jb weld come out of the distributor?

[Randall]

I don't believe so. I was having issues with noise on the tach line but I think I fixed those because I had the shielding wired wrong. If i set the timing to fixed 20 degrees and hit it with a timing light it showed 20 but as the revs went up it seemed to fall below 20 degrees and become a little jittery with a random spike above 20 every now and then. I didnt get to check if the shielded wiring fix helped because the headgasket let go. 

 

Im using the old 75-76 VR distributor but the later zx distributors seem to have a better VR setup. I have been reading up on the VR signal settings and noise and everyone says to use rising edge but looking at the waveform output from the FSM and the msextra manual it should be set to falling edge which is what i have been using but my symptoms seem to follow that of having the wrong edge trigger.

 

thinking about getting one of these: http://brickems.com/brickrpm/

[skirkland1980]

_{You shouldn't use 20 degree trigger angle. Try 60.}

[skirkland1980]

Oh, we've already covered this.http://forums.hybridz.org/topic/111407-first-start-280z-turbo-on-ms2/

[Xnke]

That is characteristic detonation damage. Cylinder 5 runs the hottest in a stock unmodified head, and will detonate first most of the time.

 

Running an engine short on coolant will readily increase the chances of detonation even with stock timing. Get that head checked for warp before you put it back on, it only takes once to warp an aluminum L6 head, although you may get a pass on it.

[dexter72]

I would also close the 0.35 spark plug gap to 0.28 

[Randall]

_{You shouldn't use 20 degree trigger angle. Try 60.}

I'm not using a 20 degree trigger angle. I used a 20 degree fixed timing since i don't have a dial back light.

I am using a 0 degree trigger angle though. Couldn't get it to run with a 60 degree trigger angle and correct timing.

 

That is characteristic detonation damage. Cylinder 5 runs the hottest in a stock unmodified head, and will detonate first most of the time.

 

Running an engine short on coolant will readily increase the chances of detonation even with stock timing. Get that head checked for warp before you put it back on, it only takes once to warp an aluminum L6 head, although you may get a pass on it.

Yeah I plan on taking it to my machine shop again just to have it checked out. Although when removing the nuts off the ARP studs most felt like they had pretty close to the same resistance so it probably didn't get that hot but would rather not take any chances.

 

I would also close the 0.35 spark plug gap to 0.28 

I am using BPR7ES with a gap of .30

 

 

Also what are the odds that I bent a rod from the coolant in cylinder 5? It seemed to be running fine and revved fine other than the misfire at idle.

Edited March 28, 2013 by Randall

[Xnke]

It's unlikely you bent a rod. Trigger angle of 0 would require next-cylinder timing, trigger angle of 60+ allows this-cylinder timing.

 

Trigger angle of 78 is what we use here, on dizzy type cars, and have no problem getting the timing set correctly. You need to set the trigger angle less than 20 or greater than 50 to get things running correctly, and you need to make sure your trigger angle and your this-cylinder/next-cylinder timing is set correctly. Other than that, it's cake.

[Randall]

It's unlikely you bent a rod. Trigger angle of 0 would require next-cylinder timing, trigger angle of 60+ allows this-cylinder timing.

 

Trigger angle of 78 is what we use here, on dizzy type cars, and have no problem getting the timing set correctly. You need to set the trigger angle less than 20 or greater than 50 to get things running correctly, and you need to make sure your trigger angle and your this-cylinder/next-cylinder timing is set correctly. Other than that, it's cake.

Just curious are there any benefits of running this-cylinder/next-cylinder? Also do you use rising edge or falling edge triggered? Looking at the VR output in the FSM for the dizzy and reading the msextra manual it should be set falling edge if red distributor wire is used as tach input and green is grounded.

 

I was using a trigger angle of 0 and the timing was spot on but if i revved the motor the timing would jump around and fall bellow the commanded timing with an occasional jump above the commanded timing.

[Xnke]

I use the ZXT optical distributor, trigger on the falling edge, and trigger angle of 78. I would say you need to setup the VR dizzy following the instructions in the Megamanual, and set the trigger angle about 78, and make sure your advance mechanism is completely locked out.

[dexter72]

Cool Randall, was thinking that since you had stated earlier that you were running .035 plug gap, the spark was blowing out under boost. Which can lead to possible hesitations, pre-ignition, and possible detonation depending on your tune.

 

Just thinking: Early on with my setup, I could hear the engine pre-ignite under medium acceleration, it would hesitate also. My spark plug gaps were .038 to .035. Did some searching on this site and I lowered the gaps to .025 and never had the problem again.

 

Are you positive you never heard any detonation or pre-ignition while under boost?.

 

Have you thought of richening up the afr's a tad after 3500 rpm's?.  

Edited March 30, 2013 by dexter72

[Randall]

Well I have a knock light set up and it would go off after 4000 but I attributed that to noise since that is everyone else's experience with the knock sensor.

 

There was what felt like slight hesitation under boost but only in 3rd and 4th.

I think that it was probably do to the random advance spikes I would see when hitting the motor with a timing light.

 

Think I may reduce the gap even further to .25 and get a scope on my VR sensor to see what's happening.

 

[Randall]

Got info back from the machine shop.

 

The head only needed 5 thousandths off so it wasn't bad at all. I also had them cc the head and it came out to 55 cc this seems off because stock is 53.5 cc

 

Can anyone explain why my chambers would be larger?

Would spark plug used effect this?

Edited April 2, 2013 by Randall