Lately, my Series 3 has started intermittently misfiring, always under load. Most noticeably accelerating from a stop, the throttle will drop, the car will stumble and then backfire. Then it goes back to running beautifully. Sometimes it misfires a few times in a day but then days can go by when it doesn't misfire at all.

My mechanic drove the car for 2 days and had nothing but solid running but he did check the wiring harnesses (original but still supple, not arcing), the spark plugs (all look normal), the distributor cap and rotor (again, normal).

In the last few months I've had timing chains adjusted, the belts changed, the fan blade/clutch replaced, the O2 sensor replaced and a new alternator put in.

Any thoughts about what we might check next?

Thanks,
Rhett

 

Hello Rhett and greetings from down under. Have your checked fuel delivery up front, fuel filters etc? If partially blocked, starvation symptoms could appear, such as you describe, especially under load with higher fuel delivery volumes . . . followed by recovery when fuel system replenishes.

Best wishes,

Ken

 

Fuel filter in the boot, yes. Up front, no.

Will check. Thanks.

 

or maybe a bad fuel pump, if not both.

 

Fuel pump is less than a year old but that's certainly possible.

 

so when you step on the pedal hard, it loses its breath, coughs, and backfires?

 

A backfire in the exhaust suggests excessive fuel.

A backfire in the intake manifold suggests lack of fuel.

The later is much more common than the former so I agree that the fuel system merits some investigation

Cheers
DD

 

Yes. It's pronounced enough now that I'm certain it's either a backfire or afterfire, not a misfire.

 

All three???


A misfire can be either!!! Or neither. No fire at all.


Still no clear evidence as to whether it is an ignition issue
or a fuel issue.


I suspect the former.


Carl

 

heh heh

"misfire" can mean different things to different people.

I generally think of a misfire as being what you'd feel if, let's say, you were to unplug a spark plug wire and run the engine. But, more broadly, it can mean any incorrect 'firing' and be caused by either fuel or ignition.

A lack of fuel can cause a 'lean misfire'....which can be slight.

If the lack of fuel is severe you can get a backfire in the inlet tract and this will almost always be accompanied by a large hesitation in throttle response: press the throttle, engine doesn't respond or responds weakly, accompanied by the 'pop' or 'woof' of a backfire in the inlet tract

Too much fuel can be impossible to ignite and you'd likewise have a misfire. Gross over-fueling can eave raw fuel in the exhaust system which can sometimes ignite....giving a huge bang and often blowing mufflers wide open! This is very rare nowadays as fuel injection system typically don't overfuel to that extent, even if malfunctioning

Cheers
DD

 

Thanks Doug. What you describe in the paragraph above is what I'm experiencing. From within the cabin, it's hard to tell where the pop/woof/bang is coming from, even with the windows open but even the biggest bang isn't gun-shot or muffler-busting loud. For whatever reason, the car only starts to mis/back/after-fire when the engine is hot and then only under load. If I feather the throttle, it feels like a misfire but if I floor it, the car behaves as above.

"Thankfully", the situation has worsened to the point where I could take it by the shop on my way to work and demonstrate it for my mechanic. After demonstrating it for a couple of miles, his thought went not to fuel delivery but to ignition.

 

It wasn't the ignition coil...

 

Got the car back this afternoon. Seems the coil wire had a tremendous amount of resistance which was degrading the ignition under load. New, ignition wires and iridium plugs seems to have sorted it.

We'll see. Car doesn't seem any more responsive but it's back to it's smooth and peppy (relative to a series 3) self.

 

Correction: You'll probably think I'm high but my Series 3 does actually seem noticeably quicker since I got it back yesterday. Possible the iridium plugs and new wires make that much difference?

 

Well, that was my guess from afar. Ignition. But, I couda woulda, shouda guessed HT wires.


They degrade over time. Resistance increases gradually. And, then in last stages, when warm resistance becomes too much and a weak spark is delivered, and at times, none.


Had you warmed it up and looked under the bonnet in the dark, you may or may not have seen an Impressive light show!!!


I doubt any difference between the performance of any quality plugs over each other.


But, two forces at play.


1. It is running better than it was on marginal ignition, even pre mis fires.


2. Seat of the pants, a smooth running car does feel quicker.


Glad the mystery was resolved with a relatively simple fix.


Carl

 

Yes, me too. Thanks.