Misfire

[NJV6]

HI,

I have some sort of misfire (and so does the vehicle!). The motor is a Quad cam 3.5 V6. It is more than running on 5 cylinders, but it just doesn't sounds quite right, not as even as it was or should be.

It is most noticeable at idle - as mentioned it still runs on 6 but not as healthily as it should.

Has 104,000km and had new plugs at 97 000 when the cam belt was done. They are 100,000km platnium plugs and quite a mission to get at.

They have 3 coils which supply 2 plugs each.

I am not a modern electronic dude. and have no idea where to start.

It is worst when hot hot, such as a long working climb off road. It is not very noticeable when cold. But it is regular & not intermittant. Unfortunately I can't recall wether it did it before the plugs were changed.

's?

[hudson44]

It sounds like the injectors might be a little dirty. To test this just clamp the return fuel line at idle and if the idle improves then you will need to remove the injectors and get them ultrasonically cleaned.

[NJV6]

Does clamping the return basically 'up' the pressure to make them work properly...??

I did run it dry once...... On the way to the airport of alll places. But that was a couple of years ago.

Good plan tho, I've managed to get all plugs out (didn't have to remove as much of the inlet manifold as I expected) and the plugs look kinda normal, not all the same but no huge standouts.

I'll take the coils to be tested somewhere tomorrow. Or any ideas on what teh resistances should be??

[hudson44]

Yeah not sure of the resistance readings but if it's only a half missfire then i would be leaning towards injectors. A faulty plug or lead/coil pack would normally only give a deffinate, full missfire.

Clamping the return line just ups the fuel pressure allowing the injector to spray more fuel. It's a good indication of faulty injectors. Most of the mitsy's of that era are prone to dirty injectors.

Actually just a thought. While you've got the plenum/manifold half apart it wouldn't be hard to remove the injectors and have them flow tested. Not sure about over there but we used to get a mobile guy out to flow test and clean them.

[NJV6]

I didn't have to remove as much of the inlet as I had expected to get at the plugs. I wonder if I should get them checked as a matter of course......

[hudson44]

Yeah i would. From what you describe it does sound like an injector problem. I would give the throttle body a clean as well while your at it. Let us know what the outcome is.

[NJV6]

Hudson....

I seem to recall somewhere you being a mitsi mechanic...?

How much work is it to get the injectors out?

[hudson44]

Yeah man i'm not a specific mitsy mechanic but i have had a lot to do with mitsy's. In saying that i haven't had that much to do with the quad cam 3.5's. I'm pretty sure they can be a prick to remove the plenum chamber (top part of the manifold) as there is a lot to pull off to gain access to the injectors.

If it's not that hard to get back together from where it is now i would do that and clamp the return line off when the vehicle is hot and at idle and see what difference it makes. This will tell you whether or not the injectors are dirty.

[NJV6]

Ok. cheers. I get the coils back tomorrow as I dropped them in to be tested.

Its not too hard to put back together as it stands, just all the throttle body assembly which has water pipes into it for cold idle i presume.

I wonder if any of this could be related to the occasional pinking I get when under moderate load at 2000 rpm ish most noticeable when it has been hot as in sitting idling.

[-Scott-]

I wonder if any of this could be related to the occasional pinking I get when under moderate load at 2000 rpm ish most noticeable when it has been hot as in sitting idling.

Often due to a lean condition, which could (again) point to injectors not flowing enough fuel. Low injector flow could also be due to low fuel pressure - any idea how it's regulated?

Clamping the return line may only bring pressure back to where it should be - can you get somebody to measure the fuel pressure before you clamp it?

[NJV6]

Hmmm. good thought Scott. I always use 95 petrol and 98 in the places where it si available. The DOHC is designed for 96.

I'll ask about for fuel pressure.

Cheers
Glen

[NJV6]

Coils came back tested ok.

Clamped return line and it increased RPM very slightly and made the misfire much more noticeable, the most I have ever noticed it, almost to the point of fully dropping a cylinder I'd say.

I'm guessing fuel pressure is pretty good as the rubber that I clamped swelled noticeably.

Next guess?

Buy another set of plugs?
Compression test?
Or still dicky injectors?

With the set up of two plugs per coil I can't just pull the leads off can I to potentially see which one it is?

[hudson44]

If its pinging under load then it's running lean and i would tend to still lean towards injectors. Without looking at the vehicle it's hard to tell. Maybe take it somewhere that has a scan tool to see if any codes have been logged but i'm still leaning toward injectors.

[NJV6]

No code's have been logged. I put it on the Mutt or whatever the thing Mitsi have is. Well actually that was a little while ago, i wonder if anythign ahs changed.

I am putting off the injectors as its such a major job! lol

I can't even get a screw driver to them to hear if they are clicking properly.

Ah I've got all day tomorrow, I guess I could pull the bugger to pieces!

Do you want me to run a video clip!!!!???

Anyway I do appreciete your help.

I just had another thought.

It only did it since it went in to get the cam cover gaskets changed, (one had a major hemorage...)
At the same time, the cam belt was done, new tensioners, water pump and spark plugs.
Initially I thought they must have got the timing wrong on maybe one cam on one bank that would make it miss a little at ldle but no the timing is definately correct. (its had 3 cam belts in 5000km - long story, but thats how I know the timing is right.)

[-Scott-]

If the pressure increasing made it worse, I'd lean towards a dodgy spray pattern - fuel isn't atomised properly, so it can't burn properly. Increased pressure made the dodgy spray pattern worse.

So yes, I'd say injector(s) is number one likelihood.

Can you disconnect one spark plug at a time, and see if you can narrow it to one particular cylinder? I don't know if that simplifies anything, going after only one injector?

[NJV6]

Its hard to tell. To check one side is easy, just unplug them but the other side where the coild are bolted to the plugs is a bit iffy.

Tried that - didn't come to any conclusions... except it runs not to badly on 4, pop's and farts a bit but sits there idling away.

I have pulled the fuel rail this evening so can take all that in on Monday to get checked.

Thankfully you can just sneak it out wihtout having to drain coolant.

[NJV6]

While the injectors are away I tested the leads for resistance.

Came out at one at 14.7 on 20k ohms and 2 at 18.3 on 20k ohms

So I figure that is 14700 and 18300 ohm's. Is that correct? - i'm not up with this fangled whodickory multimeter thingiemajig

[hudson44]

Yeah that sounds within specs. I think the specs are 8k ohm (8000ohms) per 30 cm of lead so i would imagine one is longer than the others. Will be interesting to see the flow rates of the injectors.

[NJV6]

Injectors came back - all good.

Tested them at 5000 & 10000 rpm and all 6 the same.

[hudson44]

Damn. Next step would be to grab a compression guage and do a comp test.

[-Scott-]

Injectors came back - all good.

Tested them at 5000 & 10000 rpm and all 6 the same.

There's your solution - only drive at 5000 & 10000 rpm.

[NJV6]

Bit the bullet and ordered a new set of leads.

Will try and borrow a compression tester.

Cheers guys.

[NJV6]

Got Compression guage. Found it.

Front to back

----------160
180
----------185
170
----------185
100

So what the .... has caused that to happen. 1 cylinder a bit to low i'd say.

What do I do now? I am no home mechanic but I guess I'm up for a top end rebuild.

[ferret]

You'll need to figure out what's causing the low compresion, could be a number of things, headgasket, valves, rings cracked head. I'd be getting a leak-down test done to check the rings and if that's all good, a co2 test on the coolant to check the headgasket, a co2 test isn't always conclusive, but it's a pretty good indicator as it'll show if there's combustion gasses entering the coolant path. If both of those tests come up ok, my money would be on a bent or burnt valve.

[NJV6]

It is now all back together.

Runs once again, pretty dam well for something that low on one cylinder.

If I put my mind to it, I can hear choof kinda in the exhaust.

Going for a leakdown test next.

Thanks for all your help peeps.

[hudson44]

If I put my mind to it, I can hear choof kinda in the exhaust.

A choof normally means a burnt exhaust valve.

[NJV6]

Yea but lets be honest here, isn't that the best thing for it to be....?

[NJV6]

Finally got something done here.

Burnt valve number 6
So thats ok - I guess

BUT

the other one that was a bit low (150) is a ring.

So I need to decide, replacement motor or rebuilt current one.

What do i do........

O and more to the point - WTF has this happened at 104000km. I wonder if the K & N that was in it when I got it has anything o do with it.....

[NJV6]

Update time

Engine was out before christmas.

Who has experienced a Push Pull Clutch before.........?

ARGH, what an episode, never mind - it'd sure be alot easier if I had tiny hands that could fit into the bellhousing hole.

So then Monday to the engine specialists people. Went for a look yesterday and said the motor is in perfect condition in the bottom end and not even going to bother pulling it down.

The exhaust valves had a bit of crap, sludge build up on the valve stem which was preventing one valve from seating properly.

I need to drive it harder.

So thats pretty good news I guess and hopefully will be back on track to get it back in the vehicle in another week or two.

It is getting a new rear main as that leaks away and of course the normal Mitsi thing, Valve stem seals.

[NJV6]

Operation complete and finally back in.

Push pull clutches....... Bollocks to them.

Anyway, started 1st pop and settled down to sound like a V6 Diesel. Rattled like no tomorrow but I had been warned it might - new lifters.

After 5 minutes they settled down.

Took for a spin, admired the smoke coming from under the bonnet for new gaskets, came to a corner in the road and Bang - off came the steering shaft that goes into the steering box.. Thankfully only going 50km so just busted into the undergrowth - then remembered what the finall remaining bolt was for....

Seems to go well, idles nicely and $2650 later I'm quite pleased. 300km in 6 months is not ideal.... So it shall be used.

Glen