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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:

enter image description here

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.

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  • Can you describe more about how this happened? You opened the fuel tank, added some injector cleaner, then closed the fuel tank and tried to start the engine. The engine started, then stalled? Did you add fuel? Did you do anything else? How many times did you start and stall it? Did you install the filter the same day?
    – HandyHowie
    Commented Jan 27, 2023 at 11:50
  • @HandyHowie It first stalled after I added injector cleaner but before adding fuel (and that was after 2 weeks of not using the vehicle). Second time it stalled when I was approaching the fuel station (already in neutral gear, just about to stop). Fuelled up, didn't add anything else. Drove back home, did some tests — saw it stall after 10 mins idling. Two weeks later received and installed the new filter and proceeded testing.
    – Greendrake
    Commented Jan 27, 2023 at 12:00
  • 5th attempt — runs for 40 minutes no problem before I turn it off - suggests it's nothing to do with the filter - which, after all, has remained constant though all five scenarios - but rather, some blockage in the feed system Commented Jan 27, 2023 at 23:43
  • @RobbieGoodwin Yeah, apparently. I can now confirm that the issue was only occurring when the engine was cold. Today I, after a couple of initial stalls I was able to heat it up and drive 150 km without any issues.
    – Greendrake
    Commented Jan 28, 2023 at 4:38
  • Maybe a faulty engine temperature sensor.
    – HandyHowie
    Commented Jan 28, 2023 at 9:18

2 Answers 2

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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.

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  • If the issue has nothing to do with the air filter, how can we explain that changing it has seemingly made the problem fade away? Coincidence? Random fluctuations? Like if I did more testing without changing the filter, I would have observed the 40 minutes of non-stop idling too?
    – Greendrake
    Commented Jan 27, 2023 at 10:04
  • @Greendrake - My suggestion is, there's another issue that the air filter has masked. If you've kept poor maintenance on the air filter, have you done the same for fuel or oil filter? How about the oil itself? Have you used different fuel station lately? There are a ton of things with which the air filter was only part of the reason. I'd continue to look and see if there is anything else which may be at fault. If it had just been the air filter being the problem, it would have fired up directly after replacement. You wouldn't have had attempt 1 thru 4, it would have gone straight to attempt 5. Commented Jan 27, 2023 at 10:33
  • @Greendrake it’s hard to say whether it is a coincidence or not. It just doesn’t make sense that the filter caused the issue when it was working fine one day then not the next. The dirty filter looks ok other than being dirty. The dirt will have only restricted air flow. Was the new filter installed on a warmer day than the day it was failing (for example)?
    – HandyHowie
    Commented Jan 27, 2023 at 10:52
  • @Pᴀᴜʟsᴛᴇʀ2 There was another thing indeed. Please see the update.
    – Greendrake
    Commented Jan 27, 2023 at 11:13
  • @Greendrake - As you surmise, the injector cleaner probably isn't to blame either. My suggestion is you may have had (or still have) water in the fuel which was picked up and sent through the injectors in a large mass, which caused the issue. The not wanting to run for several tries, and then running what seems just fine after all the attempts kind of leads me to believe this. Again, the air filter as suggested by your original question, does not seem to be the fault for it stalling. Commented Jan 27, 2023 at 11:32
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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.

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