Hello Forum. '94 3.9L V8i. Closer inspection revealed several MLS exhaust gaskets have blown out at the heads after about 5 years, mostly weekend driving. Not sure when it happened. Still driving fine. I wonder what I did wrong. Is this a common failure?

Also wondering which is better for replacement, MLS or composite? And do I need to drop the pipes?

It has always had lots of clicks and pings as it cools down after a drive.

 

Pretty crazy. I have MLS, going on 2 years. All seems okay.
Do you have cats or another downstream exhaust issue that might have pressurized the system?
Also - I would think those blown areas would be pretty noisey and/or throwing off the O2 sensors. maybe not?...

 

Maybe throwing off the O2 sensors. So, I am experimenting by disconnecting the O2s (only two on the 94). Going to check gas mileage vs about 12 mpg prior. Working on the first tank now. CEL turned on. So far there has been one driveability issue. The engine will hunt once or twice then stall at idle, only once in a while, only at zero speed. Often idles just fine. Drives great at speed. Will report mpg soon.

I can prevent stall with the gas pedal, so trying a bit tighter adjustment on the throttle cable. Yesterday I adjusted the CO trim on the MAF back to where I found it originally (1.87 volts) vs RPI suggested value (1.80 volts) for the last couple years. When I parked, the engine hunted a bit, but did not stall.

As for noise, it is not very noticable, except a knocking sound when low rpm acceleration or starting up a grade. Telltale black streak at cylinder 2.

I bought MLS and Composite gaskets, but haven't decided which I will use yet. I like the look of the Felpro made in USA double composites I got from Rock Auto. The single composites from A-B look kind of cheap and are two different styles. The Victor Reinz German mls from AutohausAZ look great too. I am leaning towards the Felpro composites since they might conform to surface divots a bit better. I think the mls tends to beat down the aluminum surfaces over time.

 

I switched over to the D2 style exhaust manifold bolts (12 point with spacer) on all my D1's, they don't seem to loosen up or give any issues like the old RRC/D1/D90 style used to.

 

Thanks for the advice. I bought D2 bolts from Lucky8. The individual bolts were about half price of the 16 set. Good thing I bought a couple extra from AB that had the spacers for a great price. One of the Lucky bolts was for the big end caps or such.

I decided to use the composite Felpro gaskets. Just need to put it all back together.

Do you have any advice about torque. The bolts came with some thread lock material pre-installed, and they go in stiff. Do I need to give 'em more torque than specified in the D1 shop manual? Or maybe in and back a little, then torque?

Bank 2 cyl 2 & 8
Shiny D2 spacers in the background, not torqued yet. The gaskets failed by blowing out the head side layer, cylinders 2 and 8 were really bad, as shown in the photo. The bolts were easy to remove, like not very tight. Cylinder 7 was bad too. Was not really load, rather sounded like knocking when lugged down (like during slow uphill climb).

 

Once I get all the bolts started, I just torque em down starting in the middle and basically doing a HG style pattern. I honestly go by feel on those. Once they're all the same I let the engine go thru a few heat cycles then I check them again. I've seen some with loctite and with the heat those puppies will have it should be fine.

 

I just replaced one that was cracked and have everything snugged up and was looking around to find better torque specs when this thread popped up to the top......

The factory service manual I have says 20 Nm, which is way too loose. Googling around shows service data saying 55 Nm which seems like too much. Somebody on LRO claiming 38 Nm but with no source. That seems more reasonable but at this point I don't know. Pretty sure I went by feel 10 years ago when I put this one on and it's been fine so I'll probably just do that again too.

I recently upgraded to the fancy digital snap on torque wrench so I'll see what my "feel" ends up being.

 

Went for a test drive yesterday. Guess what: no more clicking and clacking and ticking after shutting down the engine. Seems there is lots of expanding and contracting going on at the manifolds.

Still performing the gas mileage experiment with O2s disconnected. Stalled again at idle, so tweaked the throttle tension a bit more. I'll reconnect the O2s after a few more miles.

 

Following up on this......my "feel" was right around 50 Nm. So I went to 55. Seems all good but now I've got another leak that I can hear lol.

 

Results of gas mileage experiment are in. After 1/2 tank without O2s = 10.8 mpg. With O2s was 12, so 10% reduction in gas mileage without O2s for my 94 3.9L V8i.