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Low idle at altitude

I have a 99 Honda Accord, 94 K miles, received from Philadelphia last fall. When the car is cold, it idles when stopped (e.g. at a light) fine, but when it warms up, it idles so low it feels like it will stall. Also, once parked, or sometimes idling, a smell of rotten eggs comes form the exhaust. The check engine light remains on, though I replaced the gas cap. Is altitude the culprit? How to fix? It was serviced before I received it and I’ve changed the oil at 3 K miles once. I mostly drive low miles, locally. Thanks!

When the check engine light comes on it means that an error code has been stored in the computer. There are hundreds and hundreds of error codes. One or two of those might involve a gas cap problem.

Many auto parts chain stores will read your error codes for free. So find one of those, have them read the codes. Post the exact codes - they look like P1234. Don’t post only some “text” or what someone said the code means unless it comes with the exact code.

Here is the directions for setting the idle. You can have a friend monitor the tach while you turn the screw. we had 2 Fords that we had smelled of rotten eggs from the exhaust. It is do to sulfur in the fuel.

Disconnect the EVAP purge control solenoid valve 2P connector.
Connect a tachometer.
Start the engine. Hold the engine at 3,000 rpm with no load (in Park or neutral) until the radiator fan comes on, then let it idle.
Check the idle speed with no-load conditions: headlights, blower fan, rear defogger, radiator fan, and air conditioner are not operating. Idle speed should be M/T 700 ?50 rpm A/T 700 ?50 rpm (in Park or neutral)

Adjust the idle speed, if necessary, by turning the idle adjusting screw (A) 1/4-turn clockwise or counterclockwise. NOTE: Do not turn the idle adjusting screw more than 1/4-turn without checking the idle speed.
After turning the idle adjusting screw 1/4-turn, check the idle speed again. If it is out of spec, turn the idle adjusting screw 1/4-turn again.
Idle the engine for one minute with heater fan switch on HI and air conditioner on, then check the idle speed. Idle speed should be M/T 770 ?50 rpm A/T 770 ?50 rpm (in Park or neutral) NOTE:

This can’t possibly be the procedure for a 1999 Accord. A Wiki search says idle speed is controlled by the computer. Idle speed has been non-adjustable on OBDII cars for many years.My 2004 Ford clearly states on the underhood sticker idle speed in non-adjustable. Something is out of wack here.

I am not getting any response on my “you have it wrong” claim. I think what has happened is perhaps the diagram and procedure is from 1989 not 1999. Idle speed adjustment was one of the first adjustments that we lost with the introduction of OBDII controls. The idea was that mechanics were using idle speed adjustments, mixture adjustments,timing adjustments to cover up or mitigate the real problem. The EPA wanted an emphasis put on fixing the real problem rather than using these adjustments to cover up an issue.

As I suspected the procedure is for a 1989 not a 1999,take a look at a very similar picture for a 1989 Accord. Picture quality is prety bad,I probably could take some time and work on this,at high zoom you can just barely make out the same screwdriver being used in the same place.For those with access to EBSCO you can find the picture if you look up tuneup procedures for a FI 1989 Accord (strange they offered a carb in 1989).