8
  • 1998 I6 - 4.0L
  • All stock
  • ~175,300 mi
  • 13.8 mpg - mostly city

A few months ago, I began to hear a noise that sounds similar to a diesel engine, which begins a few seconds after starting the cold engine. Once warmed up, or as soon as driving or revving, the noise goes away completely. An M4A sound file is linked under What

I primarily make short trips ~9 mi, alternating back-roads and highway.

Where: Loudest at the bottom of the block. Stethoscope doesn't reveal any distinct source.

When: A few seconds after starting, until warm

What: Knocking, sounds similar to hammer striking to solid metal plate without resonation

Why: I'd love to know

Through forums, I've seen many different hypothesis as to what it might be, none of which seem to fit:

  • Piston slap - I imagine this would occur all the time, in the first few seconds after starting, and get worse with revving. I haven't removed the oil pan to check for skirt fragments, but have taken a magnet to it to listen for pieces that might "snap" to it.
  • Loose flex plate bolts / cracks - all bolts were torqued, no visible cracks on inspection
  • Stuck/clogged lifter - I pulled the valve cover and looked for rods that weren't delivering oil, loose rockers. All appeared OK, bolts were still torqued. I would expect this to also make noise as soon as I start the engine, or while revving, before warm.
  • Spun bearing - I haven't removed the oil pan to check - but imagine if this was the issue, she would have broken down since I first posted this.
  • Dirty oil - Ran sea foam in the oil for ~100 mi, changed previous full synth out @ 3k mi (came out like midnight), replaced with full synthetic, Valvoline Maxlife 10w-30 subbing 1 qt with Marvel Mystery Oil (though I always used Castrol GTX high-mileage before this and the last change).

Other observations:

  • Intermittently, a few seconds after starting, the engine will "moan" for a second or two. I've caught this about 3 times in the past few weeks. Serpentine belt is def a bit loose, will squelch if revved immediately after starting
  • Tach quivers during idle, engine appears to miss intermittently. (this has been ongoing)
  • Hard starts when engine is cold. (This was addressed in another question, but returned)
  • Oil pressure varies greatly. Needle moves a bit during idle at around 20, stays steady around 50 while driving. This used to jump, or not register at all until I wiggled the engine sensor.
  • Radio head unit illumination will go out intermittently. Punching it temporarily fixes it and relieves stress.

UPDATE: Oil was a quart low at the last change (3k mi)

UPDATE: It appears that with the cold weather (it's 40F out atm), the noise has disappeared. Possibly unrelated: CEL light went on, and back off - transmission malfunction codes were thrown, noting shift solenoid B (gears 2/3). Higher RPMs were required to get the vehicle moving, intermittently, but has since gone away.

Community
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ethesx
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3 Answers3

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It could be that your engine has hydraulic lash adjusters for the valves. These remove the need for valve clearance adjustments as they work on oil pressure. The trouble is that they have very small inlet/outlet holes to admit oil, and often get clogged up, restricting the flow of thick, cold oil at engine start and causing the tapping noise. When the oil is hot and flows better, the adjusters operate properly. I know you said that the noise appears to be louder from the bottom of the block, so I can't say it definitely is this, but I can suggest it's just a side effect of the stethoscope transmitting the sound directly through the block, since there's a big air gap between the cylinder head + valves and the top of the engine where you'd touch a stethoscope to.

My 1985 Supra has an L6 with hydraulic adjusters and has quite a reputation for the tapping noise on startup.

Edit: forgot to say, you can buy oil additives that are supposed to clean out the lash adjusters and remove the tap. I can't comment on their effectiveness as my engine does not suffer (yet). As I understand, adjuster tap is merely annoying and otherwise harmless to the engine.

Gargravarr
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Knocking sounds reflect worn out parts, which had got so much tolerance than oil can't fill the gap any longer. It could also be the oil itself, being to thin or too old (and got thinner). If the noise is only in the bottom then it could be the bearings in the crankshaft, or the displacement bush also in the crankshaft at the extremes of it. However a "diesel engine" kind of rhythmical continual tapping noise is typical heard when the valves or the camshaft are worn, or there is too much valve clearance. If the engine has oil plunges to move the valves, one of them may be getting "lazy"

Aram Alvarez
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Could you have a tiny fuel system leak that is slowly bleeding air into the fuel line somewhere? This would add air bubbles to the initial start which might take a moment to clear. Im my experience this will throw the timing considerably when started, which will cause a knocking in some engines.

xtfer
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