Plenum butterflys and compression test. DIY with pics

[Micko66]

G'day Guys and Gals,

I recently bought a 3.5 DOHC 24 valve NJ Paj. It has a bad miss and a few other small issues. One of which is an air leak at the front of the inlet plenum chamber where the diaphram switches the butterlfys for low/high rev optimisation.

So I figured I'd pull the plenum off and do a compression test.

It's not too hard really. Mostly just 10mm or 12mm bolts/nuts.


Remove engine cover. (See pic of plenum chamber, pic 1). Remove battery and air cleaner including MAF sensor. Remove throttle body wiring, coolant tubes and vacuum lines. Someone in the past had glued the TPS harness clip to the TPS (see pic 2). So I just removed the throttle body and put it to the side and left the harness attached.

Undo the temp gauge, power steering sensor and ECU temp clips. Remove harness clips from the module on top of the plenum chamber. I'm not sure what that module is called but I suspect it has something to do with the tacho not working. Note also that someone in the past had glued a harness clip on (see pic 3). Remove module. Wedge harness out of the way. These harness clips all seem to be unique fit, so they can only be clipped into one thing, good for reassembly. :-)

Undo throttle cable and all vacuum tubes from plenum chamber. There are a few but they will be easy enough to relocate.

At the back of the plenum chamber on top there are a couple of bolts/nuts that secure the fuel lines and some sort of air hose, tricky to undo.

If it's a hot day have a cool drink in your favourite cup. (see pic 4) Go Tigers! :-)

The plenum chamber is secured down by 4 braces (from memory) around the outside, these will be obvious when you get into the job. Undo only the bolts you need to. There are 7 bolts that go through the top/middle of the plenum chamber. The one at the back is tricky so you'll need to remove the cruise control/throttle cable bellcrank cover and loosen the cables (see pic 5).

It now should all be loose and come off (see pic 6 of upside down plenum and butterflys).

You'll see the butterflys which redirect incoming air either through the long or short route. That's to assist in low or high speed engine power. The theory is (I think) that the longer path speeds up slow RPM air to assist in low speed power. The shorter tracts are for high RPM power. However, you'll see that 2 of the butterflys on my plenum are missing!!!! Gone!

I know that there should be 6 butterflys there. Someone has removed 2 of them. I know that someone has been here before because the plenum was siliconed in! What's worse is that whoever removed the butterlfys put the screws back, or at least 3 of the 4. You can see that in the pic. One of the butterfly screws in inlet tract number 5 is missing. (see last pic).

So I did a compression test and guess what? 180 PSI spot on across the board except for number 5 cylinder which read 30 PSI, I even did a wet test...same thing (although I doubt the validity of wet tests on V configuration engines). I guess that screw fell into the combustion chamber and did some damage.

Why 'o why would someone remove 2 butterlfys and put the butterfly screws back in loosely?

SIGH.......

Next job is to pull the drivers side head off and see what it looks like in there.

Here's hoping it's just a valve. I doubt it's a holed piston because the engine didn't blow any smoke.

....to be continued.

Micko

[Martynhpajero]

G'day Guys and Gals,

I recently bought a 3.5 DOHC 24 valve NJ Paj. It has a bad miss and a few other small issues. One of which is an air leak at the front of the inlet plenum chamber where the diaphram switches the butterlfys for low/high rev optimisation.

So I figured I'd pull the plenum off and do a compression test.

It's not too hard really. Mostly just 10mm or 12mm bolts/nuts.


Remove engine cover. (See pic of plenum chamber, pic 1). Remove battery and air cleaner including MAF sensor. Remove throttle body wiring, coolant tubes and vacuum lines. Someone in the past had glued the TPS harness clip to the TPS (see pic 2). So I just removed the throttle body and put it to the side and left the harness attached.

Undo the temp gauge, power steering sensor and ECU temp clips. Remove harness clips from the module on top of the plenum chamber. I'm not sure what that module is called but I suspect it has something to do with the tacho not working. Note also that someone in the past had glued a harness clip on (see pic 3). Remove module. Wedge harness out of the way. These harness clips all seem to be unique fit, so they can only be clipped into one thing, good for reassembly. :-)

Undo throttle cable and all vacuum tubes from plenum chamber. There are a few but they will be easy enough to relocate.

At the back of the plenum chamber on top there are a couple of bolts/nuts that secure the fuel lines and some sort of air hose, tricky to undo.

If it's a hot day have a cool drink in your favourite cup. (see pic 4) Go Tigers! :-)

The plenum chamber is secured down by 4 braces (from memory) around the outside, these will be obvious when you get into the job. Undo only the bolts you need to. There are 7 bolts that go through the top/middle of the plenum chamber. The one at the back is tricky so you'll need to remove the cruise control/throttle cable bellcrank cover and loosen the cables (see pic 5).

It now should all be loose and come off (see pic 6 of upside down plenum and butterflys).

You'll see the butterflys which redirect incoming air either through the long or short route. That's to assist in low or high speed engine power. The theory is (I think) that the longer path speeds up slow RPM air to assist in low speed power. The shorter tracts are for high RPM power. However, you'll see that 2 of the butterflys on my plenum are missing!!!! Gone!

I know that there should be 6 butterflys there. Someone has removed 2 of them. I know that someone has been here before because the plenum was siliconed in! What's worse is that whoever removed the butterlfys put the screws back, or at least 3 of the 4. You can see that in the pic. One of the butterfly screws in inlet tract number 5 is missing. (see last pic).

So I did a compression test and guess what? 180 PSI spot on across the board except for number 5 cylinder which read 30 PSI, I even did a wet test...same thing (although I doubt the validity of wet tests on V configuration engines). I guess that screw fell into the combustion chamber and did some damage.

Why 'o why would someone remove 2 butterlfys and put the butterfly screws back in loosely?

SIGH.......

Next job is to pull the drivers side head off and see what it looks like in there.

Here's hoping it's just a valve. I doubt it's a holed piston because the engine didn't blow any smoke.

....to be continued.

Micko

I think your motor swallowed the butterflies... They break in half.

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[Micko66]

If the motor swallowed the butterflys how come 3 screws are still in the shaft?

Micko

[Martynhpajero]

If the motor swallowed the butterflys how come 3 screws are still in the shaft?

Micko

They break in half... The plastic shaft bushes wear, the whole thing rattles and the brass butterfly's fracture down the middle. My own DOHC had a burnt exhaust valve on the driver side front cylinder when I bought it. The butterfly shaft Bush at the front was really worn and I reckon it caused a lean mixture on that cylinder, enough to burn out the valve.

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[Micko66]

You were right Marty. I found 2 halves of the number 6 butterfly in the head. But I can't find the number 5 butterfly bits.

Micko

[Micko66]

As Marty suggested the butterflys split into 2 pieces (see pic) and fall into the inlet side of the head. They are made of iron/steel so I manged to fish them out with a magnet. The number 6 cylinder pieces were in there, the number 5 pieces aren't in there.

The plenum has been off before, there is silicone. Because the number 5 butterfly pieces can't be found and they are too big to fit through the inlet valve if someone previously had removed the broken bits? I also wonder what happened to the second screw that holds the number 5 butterfly went. Perhaps that did end up in the cylinder and has caused the compression loss?

Micko

[Allan2]

Watching this with interest.
Allan

[Micko66]

Plenum butterflys and compression test. DIY with pics (update with more pics)

Update.

I got the head off. It appears that a valve is bent. I can get a feeler gauge in behind one of the exhaust valves (see pic 1) in number 5 cylinder (the one with zero compression).



Someone (Mr Magoo?) has been in this engine before and perhaps tried to fix the butterfly issue before. Number 5 and 6 butterflys in my plenum chamber are gone (see my first post). I initially found 2 butterlfy halves in number 6 inlet port but not number 5. But you will see that a screw that holds the butterfly on in number 5 cylinder is gone! To make this riddle more complicated I found one half of a butterfly in number 4 inlet port. Like WOW!!! Plus I found some bolts and nuts in the valley and around the inlet manifold. I can only begin to imagine what previous "mechanics" have done.

Anyway..... The butterfly halves will NOT fit through the inlet port, past the valve and into the cylinder. They are too big (see pic 2). They will just sit in the inlet port rattling around. But the screws that hold the butterflys on will fit. I suspect this is what happened in number 5, the screw fell off and fell into the number 5 cylinder, bending an exhaust valve on it's way to spending the rest of its life in the catalytic converter.



Number 5 piston is ever so slightly pock marked (see pic 3). Whew.... No real damage there, so the piston and cylinder wall are good to go around again.



Next step is to remove the exhaust cam and replace the valve, re-seat it and check its integrity.

Does anyone have a serviceable exhaust valve for a DOHC 3.5? I'm not looking forward to what Mitsubishi will charge me.

Micko

[Martynhpajero]

I put aftermarket exhaust valves in mine. Wasn't much money. Maybe $20 each.

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[Micko66]

I put aftermarket exhaust valves in mine. Wasn't much money. Maybe $20 each.

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Do you remember who you sourced them from?

Micko

[Martynhpajero]

Do you remember who you sourced them from?

Micko

It was a local autopro store.

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[erad]

Mick:
Talking with you today, I empathise with you about missing butterflies. I had a Range Rover automatic, and my daughter had it when it stalled. How do you stall an automatic? When she started it up again,it revved madly and would not go back to idle. She phoned me and I told her to take it to the local NRMA for them to fix it.

It was idling at 3000 R/min. The NRMA man looked at it and said "Is it on gas? Switch it to petrol" so she did and it idled at 3600 R/min. He then fiddled with the gas mixture and got the idle down to about 2000 R/min. At that point my daughter rang me and told me what was happening. I told her to stop the man before he did some real damage...

I went down in my wife's car and daughter drove that home. I drove the Rangie home myself, up Brown Mountain. It didn't overheat according to the temperature gauge, but obviously it got very hot internally because after that, as I subsequently found out, two cast iron cylinder liners came loose in the aluminium engine block. The NRMA man had leaned the gas mixture so far out I was amazed that it even fired, let alone ran.

Naturally, I started looking for the problem. I very quickly found it. There were 2 butterfly valves controlling the throttles. They each had a 12 mm dia poppet valve held closed by a lightweight spring. These valves were designed to limit the engine vacuum on the overrun (and reduce the oil being sucked up past the rings and valve stems). The poppet on one of the butterflies had disappeared - guess where.... There was a hole about 10 mm diameter in the butterfly - no wonder the engine was idling at 3000 R/min.

So I decided that who needs anti-vacuum butterflies on the engine anyway - I would solder some shim onto the butterfly and close it off forever. I then looked at the other butterfly, and low and behold, there was a one cent piece soldered onto it! Later on , whilst I was trying to diagnose the cause of massive clacking noises in the engine (loose cylinder liners) , I saw some dent marks in No 5 piston crown, where the poppet valve had gone through and ultimately been hammered enough until it got spat out the exhaust. Every time I go past the NRMA place down the coast, I remember how the 'Mechanic' had so skillfully stuffed up my engine. Oh the joys of having a daughter who wanted to borrow my car... She still isn't that mechanically minded even to this day.

[Micko66]

Hi guys and gals,

The job is done.

I got a new valve for about $25. I used a SOHC gasket kindly given to me by member erad (thank you again :-) )

A few things to note, the inlet manifold and plenum chamber gaskets between the SOHC and DOHC are different. The SOHC and DOHC head gaskets are the same.

I unscrewed the existing butterflys in the (dual tract) plenum chamber and removed the shaft. (see pic 1). The front end where the shaft went in, I tapped and plugged with a hex head plug I just happened to have. (see pic 2). The engine will never again swallow a butterfly screw.

Engine started right up but one of the hydraulic lifters is noisy. I did this job on an extreme budget, so I haven't changed the oil yet. Now that the engine works and seems that it will last I'll change the oil, fuel filter, oil filter, plugs, coolant, alternator and AC belt.

These 6G74 engines are pretty strong. When the head was off it became clear that it had swallowed something before. There was previous damage in the number 1 combustion chamber.

Next big job is the clutch.

Micko