How is this diesel engine stalling pattern caused by dirty air filter?

Question

I happened to be negligent enough to leave the air filter unchanged in my Hilux KUN26, engine 1KDFTV — for 5.5 years and 47k kms (30k miles).

The vehicle was seemingly all fine until one day I started it cold, drove it out of my gate, went out to close it and heard the engine stall while idling. "Wow" I thought. "What the hell was that?"

So I started tests: engine would consistently stall (if idling) after about 0.5-2 minutes after start. If accelerated, it would go fine. But when put back to idle, it would stall again pretty soon.

A quick search identified potential causes, and I went to check the air filter. Jeez! So, I got a new one. Here they compare:

So I replaced the air filter and started testing again.

• 1st attempt — same as before. I thought, was the cause something else, not air filter?
• 2nd attempt — same as above.
• 3rd attempt — runs for about 10 minutes and stalls.
• 4th attempt — same as above.
• 5th attempt — runs for 40 minutes no problem before I turn it off.

So, apparently (and hopefully) the cause was indeed the air filter.

But I am struggling to understand the mechanics of the above behavior. If the stalling was caused by insufficient air pressure on low revs due to the air filter being clogged, then replacing it should have fixed the issue immediately. But instead, the issue appears to have gradually faded away.

Perhaps, the immediate cause was dust contamination of some engine parts due to the old filter letting some dust through? And now, the flow of clean air has cleared that out? But how exactly would that cause the engine stall on low revs?

Update

As the comments suggest, the air filter may have only masked the real issue. Indeed, there was another thing that well could have been the cause:

Before I first encountered the issue, the fuel tank had only about 15 litres of fuel and I was going to refill it. I added about 50ml of diesel injector cleaner additive in the fuel tank (which I have virtually always been doing before refilling the tank). And right after it the problem first surfaced.

I am, however, hesitant to blame the injector cleaner because I have been using it for ages and never had any issues with it.

Answer 1

I can’t see this being the issue. If the previous day the vehicle was driving fine, then it must have been getting enough air into the engine to support the fuel that was being injected as required for normal driving. So the small amount of fuel required to keep the engine idling would have had more than enough air.

If the air filter had broken up and blocked the air intake, or had become soaked in water, then maybe a sudden failure would be possible, but it wouldn’t explain the issue after changing the filter.

Diesel engines don’t need to throttle the air supply like gasoline engines need to, so they run at maximum air intake even at idle (some do have a throttle for EGR purposes). You would have expected issues driving the vehicle at speed long before issues at idle.

This sounds more like a fuel supply issue to me, or maybe there is a faulty sensor that is confusing the ECU.

Answer 2

there is another possibility (and i am a layman, so sorry for the less technical wording)

i had a similar behaviour on idle sometimes

in my car (2007 Isuzu D-max) the engine block gets ventilated (and this gets returned into the inlet, to burn off). This outlet has a rubber + spring, if it does not function properly the pressure can rise too much and cause a "misfire" (not a misfire but the valves behaviour is changed). It feels like a stall, momentarily, just once and then the engine continues normally.