Hi all, I’m back at things after recovering from two surgeries, hopefully I can keep the content coming on at least a semi-regular basis now 🙂
Ok, today we are looking at how to replace the exhaust manifold gasket on a non-turbo 6 cylinder FG Falcon. So why would you need to do this repair? Typically a leaking manifold gasket will produce a ticking sound, usually more pronounced at idle. Have a look and listen to the video below to see what I’m talking about.
A handy way to figure out where exactly the exhaust leak is is to grab a length of hose (1 metre of garden hose will do), place one end up to your ear and with the engine running move the other end over the manifold where it meets the cylinder head and where the exhaust pipe to manifold flange is. You will hear a flutter noise where there’s a leak and where there’s no leak you won’t hear anything other than normal engine noise. Give it a go, you’ll be surprised how effective it is at finding the problem areas.
Ok, so if you have confirmed that you have a leak around the manifold area the first thing to do is to let the engine cool down for a while. The cast iron manifolds retain a lot of heat and doing this job even while the engine is still warm is a pain in the backside.
Once it’s cooled down we need to remove the heat shield. There is three 13mm bolts across the top and two 10mm bolts towards the bottom.
With the shield removed the oxygen sensor wiring connector can be unclipped.
At this point I like to spray the flange bolts with wd-40, both the flange in the centre of the manifold and the manifold to exhaust pipe flange bolts, this can help a lot when undoing them. Also now I like to loosen the three 13mm nuts holding the centre flange together. No need to remove them but loosening them now can make life easier once the manifold is removed from the car.
Next up we can remove the two 14mm manifold to exhaust pipe flange nuts. These can be undone from up top, or with a deep socket and long extension from below if you jack the car up and support it on stands. Keep in mind that the exhaust will drop down once both nuts are removed!
With these removed the remaining 9 manifold to cylinder head bolts can be removed and the manifold removed from the car. BE CAREFUL as the manifold has a bit of weight to it and the oxygen sensor needs to be treated as fragile. On the bench we can then remove the three 13mm flange nuts and seperate the two halves of the manifold.
Once the manifold is out we can take a look at what we’ve got. Our FG had a couple of loose bolts towards the rear where the leak was so it was fairly obvious why it was leaking, this may not always be the case though and you may have to look a bit further to find a reason for a leak. Black or dark soot around any of the ports on the cylinder head or on the manifold itself is a good indicator of a leak.
To be sure you’re not putting a manifold back on that is warped first clean the mating surface with a single-sided razor blade or give it a rub with some sandpaper and place a straight edge along both sides. A small variation can be taken up by the gasket but if it’s more than a couple of millimetres I suggest getting a replacement manifold or getting that one machined. Reusing it will result in it leaking again in no time.
If the manifold checks out ok I would now clean the cylinder head mating surface in the same way as you did the manifold.
Now we need to replace the flange gasket between the two manifold halves. The old gasket can be levered off evenly using a flat blade screwdriver and the new one should just slide on nicely.
NOTE – If buying your parts from Repco this flange gasket is listed as a “muffler flange gasket”, part number JF040 (The manifold gasket is part number MG3207). Not sure why the gasket is listed like this but it caused a bit of confusion when we ordered our parts!
At this point I bolt the two halves of the manifold back together but I don’t tighten the nuts right up as the halves will more than likely need to rotate a bit to fit correctly to the cylinder head.
We had a slight warp in the rear manifold section and decided to use a high temp sealant on both sides of the gasket. This is not absolutely necessary and fitting them without any sealant is fine, I just wanted a bit of sealant on the gasket to help take up the warp in the manifold.
To refit the manifold to the car I suggest fitting the top centre bolts to the manifold as a way of holding the gasket in place and then move it into position, being careful not to knock the oxygen sensor or loose the gasket.
Although three of the manifold bolts are used to secure the heat shield, I suggest that you fit all 12 bolts, tighten them all up and then remove the three top ones when ready to fit the heat shield. The reason for this is to get an even pressure across the manifold to help the gasket seal and to limit any warpage of the manifold.
With the bolts it is important to start all 12 before tightening any also. If you are having trouble getting some started a large philips head screwdriver inserted into the bolt hole above or below will help to move the manifold around enough the get the difficult bolt in. Be sure not to cross-thread any bolts!
I always tighten the centre bolts of each manifold half first and then go to the outside bolts. Once again this helps to get even pressure across the manifold and gasket surfaces. Oh, and don’t forget to refit the engine lifting hook at the rear.
Next up we can tighten the three 13mm nuts that secure the two manifold halves together and lift the exhaust pipe up and fit the two 14mm lower flange nuts (you may need an assistant or a jack to help with this – be sure not to cross-thread the lower flange nuts). All of the these nuts need to be done up evenly and gradually, doing up one side further than the other will result in an exhaust leak.
With everything bolted in place and tightened up the oxygen sensor wiring can be reconnected and the engine can be started to check your handy work.
If you’re happy there are no more leaks the three top manifold bolts can be removed (first, third and last) to fit the heat shield.
Fit the heat shield in place, refit the top bolts and refit the two lower 10mm bolts and enjoy your exhaust leak free Falcon!