- 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).
- 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.