Misfiring, but only irregularly

[mikeloadsasaabs]

Just come back from another trip to the UK in my 2004MY Aero auto estate. I noticed on the motorway that, when flooring the throttle fairly quickly, the car misfired and wouldn't pick up for a few seconds unless I released it slightly. Seems to be when it's under load, and only at speed, around 80+mph (well, it was on a German autobahn  ). I always use the high octane fuel. It's never done it before under the same circumstances, although to be fair it was June when I last travelled across Germany hence reached the relevant speeds.

I have two theories. Firstly, when I put the new plugs in a couple of months back I set them to 1.1mm, whereas I usually use 1.0mm. Secondly, I have never cleaned the throttle body in the 4 and a half years I've owned the car (in fact, never had to clean it on the last 9-5 either). I can easily adjust the plugs, but won't find out if I've fixed it for months, so I'd rather tackle the most likely cause first. Has anyone else had a similar problem? Starting etc. is no problem.

And never having cleaned a throttle body before, any tips?

[Steve440]

Could be a partially blocked fuel filter.

Had a similar thing on a Ford a few years back, fine all the time and started easily but at higher speeds it would feel breathless and misfire.

The throttle body is best cleaned once removed, I used carb cleaner and brake cleaner with an old toothbrush.

Steve

[mikeloadsasaabs]

I guess I might give it a whirl. Any idea how long it's likely to take to remove & replace on the 9-5?

[Steve440]

Not long, probably take as long to jack the car up as to replace. A jubilee clip and 2 banjo bolts.

If your quick you wont loose much fuel.

Steve

[RussJ]

I'd suspect the first signs of a failing ignition cassette.
Symptoms are identical....

Russ

[TomPaine]

I'd suspect the first signs of a failing ignition cassette.
Symptoms are identical....

Yes, though strange it didn't throw a CEL. It's normally very quick to pick up misfires on the DI.

[phoenix]

A jubilee clip and 2 banjo bolts.

MY04 might be bayonet release fittings on the fuel filter. Not sure.

You have yo remove the plastic cover first. Allow time for penetrating oil to soak in to the screw otherwise it's likely to snap.

[Saabman]

Sounds like the failng DI fault I had on my old 9-5

[Audax]

I'd suspect the first signs of a failing ignition cassette.
Symptoms are identical....

Yes, though strange it didn't throw a CEL. It's normally very quick to pick up misfires on the DI.

Not really, the early stages of DI failure will not give a CEL. You'll just count a few more misfires with Tech-II and possibly the extended DI test may catch it but otherwise it's not uncommon for them to give problems months before you'll get the CEL.

[TomPaine]

Not really, the early stages of DI failure will not give a CEL. You'll just count a few more misfires with Tech-II and possibly the extended DI test may catch it but otherwise it's not uncommon for them to give problems months before you'll get the CEL.

Well, in both my cases, previous 9-5 and current, the CEL was tripped at the first hint of a misfire. I'd never have known there was a problem had not CEL said there was.

[mikeloadsasaabs]

Not long, probably take as long to jack the car up as to replace. A jubilee clip and 2 banjo bolts.

If your quick you wont loose much fuel.

Steve

I actually meant the throttle body when I asked about how long to remove & replace  ! I've done fuel filters, but never fiddled with throttle bodies.

[mikeloadsasaabs]

Well, I've just removed the plugs to regap them, and to my horror found they looked like this:

Cylinder 1


Cylinder 2


Cylinder 3 view 1


Cylinder 3 view 2 rotated 90°


When I last replaced them a couple of months ago they were fine  . This probably explains the burnt oil smell that I noticed a few days ago. I'm now concerned as to what's causing it. I can only think of turbo oil seals, or valve guides. All cylinders seem to be affected, and if you look at the two views of cylinder 3 taken 90° apart, you'll see that there seems to be a definite difference between the two sides, perhaps where the petrol/air (and oil it seems now) mixture enters the cylinder, although the same effect doesn't happen on the outside cylinders. Oil consumption isn't particularly noticeable, although I did put half a litre in today I can't recall where it was on the dipstick after I topped it up before (it was still well above the minimum today). Any thoughts?

[sgould]

Is it definitely oil and not soot? 

If its turbo seals you should see smoke on starting.  And a lot of oil in the inlet pipes (there is always a thin film from normal breathing)

If its soot from running rich, you should see higher fuel consumption.  Maybe a lambda sensor is failing.

[mikeloadsasaabs]

Mmmm, lambda sensor. Yes, come to think of it they were sooty rather than oily, and a few times recently the fuel consumption hasn't been waht it was. Hadn't appreciated that a failing lambda sensor could cause that sort of thing. Bit difficult to tell with the smoke on startup, it's been pretty cold here and there is always a visible exhaust trail.

Is there any way of testing a lambda sensor? There's no CEL being thrown. I guess at 8 years old it's not excatly in it's prime!

[sgould]

I'm not 100% sure, but I think that the lambdas only look for a difference between themselves to check if the  cat is working. 

The first sensor out from the turbo is the one that checks the mixture.  But they don't do anything on starting up.  That's done on preset maps.  Lambdas only come into play once the engine is up to temperature.  At least ats what I was told.

I don't think that you can check them.  We had one cleaned on the 9-3 but that threw a CEL.  Maybe only because the second sensor after the cat was clean.

This might sort out the plugs, but I don't think it will fix the misfire. I'm sure the lambdas don't react that quickly.

I'm not even sure you are running rich.  If its cold and you are doing short journeys, the engine will be running rich.  It will be just like having the choke out.  You really need to check the plugs after a run.