Starting about 2 weeks ago, I have been having a rough idle with my '99 2.4l. It will idle smooth for a few seconds at a time, and then it will hiccup, dropping the rpms slightly, before returning to a smooth idle. The engine runs smooth at all other rpm ranges and acceleration is not affected. The problem has been getting progressively worse over the past couple of weeks. It has not thrown any codes. Any advice?

Also, it has platinum plugs in it which look clean (slightly lean), a clean air filter, and a fuel filter which is about 15k miles old.

I would say IAC valve in the throttle body. If you havent done this yet i would take off the throttle body, take out the sensors and clean the TB and IAC...could be a lot of carbon build up

With the engine runing ...spray some carburator cleaner around the intake manifold gasket...around injectors and rubber hoses around throttle body.'if the engine dies or bogs you have a vacumm leak, check for dry rotten hoses

A buddy of mine had a 2001 z24 that had the same prob. He sold me the car for $800 because of this prob. It was a 4" hose that was cracked in the front of the throttle body causing a major vacuum leak. $2.00 later the car runs great!! Hopefully this is your problem.

Check the map sensor, and the hose to it for a vacuum leak. Also check the IAC valve. Report back after that.




PRND321 Till I DIE
Old Motor: 160whp & 152ft/lbs, 1/4 Mile 15.4 @88.2
M45 + LD9 + 4T40-E, GO GO GO

I haven't posted here in forever, but I remember my 98 ls had the same problem. It would idle fine at a stop light for the most part, but about every 30-45 seconds the rpms would drop for a split second. I checked the iat sensor, tps sensor, replaced the vacuum line from the throttle body to the intake manifold, spark plugs....

It ended up being cause by a small almost invisible crack in the ignition coil housing. I actually had to replace that part a couple times while I still had the car. Fixed the problem every time.



MaTtMan
98 Ls AuTo

OP, did you ever figure out what was causing your issue. Mine is doing similar things.

Replaced

IAC
Front O2
Cam Sensor
Map Sensor Hose
Ignition Housing
New AC Delco Plugs
No vacuum Leaks

Still not improvements.

I have not yet solved the problem. I checked out the throttle body, which did not have excessive carbon buildup. I removed and cleaned the IAC, which had substantial carbon buildup. I checked the vac hoses for obvious cracks. Looking back, I realized that this issue began after using seafoam on the engine. I used it in the gas tank and the brake booster vacuum line. This leads me to believe that perhaps the O2 sensor is fouled from the engine burning off carbon buildup from using the seafoam. The idle has changed slightly, from the occasional hick-up, to the more constant, slightly rough idle. It seemed to help very slightly to clean the IAC. Let me know if you have any more suggestions, and thanks.

Brought it to autozone and had codes pulled. It flashed a SES light about a week ago, and it went away about 10 minutes later. Today, the scanner showed a cylinder 2 misfire. Plugs looked good. Advice?