I posted a test drive video here:
(please go to youtube video description for timecodes of when symptoms occur during test drive video)
1999 Acura TL with two intermittent symptoms:
1) get an intermittent noise (about 1 second) when car is decelerating (foot off gas pedal or on brake pedal). Sounds like noise is coming from location of catalytic converter. Noise sounds like a metal grinding noise. No noticeable effect on driveability when it occurs. Noise only occurs after car has warmed up.
2) there is a loss of power when accelerating from a stop, particularly going up a hill. Sounds like car is struggling. Noise sounds kind of like anti-lock brakes being activated. Noise appears to be from engine. Only occurs when car has warmed up.
Both symptoms are intermittent. But I can reproduce Symptom #2 100% of the time on my test drive at the same location (going up hill from a stop) each time. However, Symptom #2 does occur when accelerating from a stop on a flat road as well.
Car seems to drive fine on freeway (but did experienced Symptom #1 once when decelerating on freeway off ramp). Car runs fine when idling in park, even with engine hot and across RPM range. Symptoms seem to occur during city driving after engine is hot. The idle RPM of car when hot is bet 675 to 725, and steady.
Background
single owner car, 100k miles. well maintained. No OBD error codes. transmission fluid service every 30k miles. last transmission service 10k miles ago. fluid level is good and fluid appears good. throttle body and EGR passage in intake manifold were cleaned about 15k miles ago. Spark plugs were changed maybe 20k miles ago. Car had misfire (P0300) from single bad coil about 10k miles ago. That coil was replaced and car drove fine afterwards. Car was driven approximately 100 miles (short trips) with the misfire.
Car was driving fine until approximately 3 months ago. Got a P1491 (EGR valve lift insufficient) and P1399 (random misfire) with symptoms of stalling and rough engine at idle. Tested EGR valve with voltmeter and saw valve appeared to be stuck open. Replaced EGR valve. Saw that the replaced valve did indeed have piece of carbon keeping valve open. After putting in new EGR valve, car seems to run fine. But after car is warmed up, symptoms #1 and 2 occurred. Symptom #1 may have occurred very intermittently prior to replacing EGR valve, but not sure.
I went ahead and replaced the new EGR valve with the old EGR valve after I cleaned the old EGR valve and removed carbon from passage. Car had no EGR symptoms (no stalling at idle) after putting old EGR valve back in. Symptoms #1 and 2 still occurred. Drove the car with EGR valve disconnected (electrically) but symptoms still occurred. So I ruled out EGR valve as cause of problem.
I have also tried to drive car with transmission fixed at 2nd gear, so there is no transmission shifting, but symptoms still occur.
So far, work done on car after symptoms occurred:
1) changed air filter (was due for replacement anyway)
2) changed engine oil (was due for replacement anyway)
3) replaced upstream O2 sensor (was due for replacement anyway)
4) replaced brake switch (old switch measured 10.5 V instead of battery voltage)
5) measured for exhaust backpressure at upstream O2 sensor location: 0 psi at both idle and 2500 rpm. test was done on cold engine.
6) bled cooling system using spill free funnel. Car has had slight coolant leak (approx. 3 oz in 6 months) at lower radiator house connection to engine block. It's possible that some of the leaked coolant may have deteriorated wiring, but wiring seems OK when I checked.
CURRENT HYPOSTHESIS
1) idle air control valve is stuck close after engine vacuum from decelerating. This may be cause of noise in Symptom #1? Then the stuck idle air control valve restricts intake air and causes Symptom #2 when coming off a stop.
2) just did scan using Launch x431 scanner and found 4 error codes related to TCS and will check those. (e.g., TCS relay, 5V reference voltage error codes). Plan to drive with ABS fuses pulled out to disable ABS/TCS to see if ABS/TCS is causing the symptoms.
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