Slightly Rough Idle

89 Thunderbird Standard V6, EFI

I'm hoping ladies and gentlemen that this is just a tune-up related issue. My bird has suddenly developed the slightest of rough idles. I recently put platinum plugs, and cleaned and recharged the K&N air filter. But is has been over a year since I done any other tune up related work to the car (Other than fluid changes). Also, in another post I started, I was told that higher octane fuel burns cleaner. I've been using 89 octane fuel, and with that being the case, could that be the reason this rough idle has developed? I'm just hoping its nothing to serious. Any and all opinions would be greatly appreciated. Thanks.....

I dont know if this information is relevant or not, but the car seems to run fine while driving. It starts fine, and there is no visible smoke from the engine area or the exhaust. My first thought was it maybe an oxygen sensor, but the check engine light doesnt come on, and that light does work.

Take the plugs out to check and clean them. You may have put too much oil on the air filter and fouled the plugs. While the plugs are out, do a compression test.

thanks 97Bird for your reply and advice. Will putting to much oil on the K&N filter really harm the spark plugs? I ask because I believe I did put to much oil on the filter while cleaning and recharging it. I was just gonna do a complete tune up sometime next week. But I'll check the plugs before I do that. Also, how do I do a compression test?

The excess oil gets sucked into the compression chamber and fouls the plug. This is something that happens a lot to people when they clean and put too much oil on the air filter of their lawnmower. The engine should be at operating temp to do a compression test. If you don't have a remote starter then remove the circuit breaker that is for the ignition. My '95 isn't in the yard to look at but on my '97 it is the third one down on the left side (side opposite from where the Mega Fuse is attached to the outside of the Power Distribution Box under the hood). The first two fuses are smaller in size and the fourth spot is empty. It is a 20A breaker. A '95 might be different so check your manual for location. Install the compression tester and have someone spin the engine over while you watch the gauge to see when it reaches the highest pressure. Count how many times it goes through a compression stroke to reach the highest mark. You will be able to tell by the sound of the engine and the guage needle movement when it is going through a compression stroke. Write down the pressure and go on to the next cylinder. Spin the engine over the same amout of compression strokes as you did to get the highest reading of the first cylinder and write down the pressures of each cylinder. All cylinder pressures should be within 25% of the cylinder with the highest pressure. A good trick is to use a piece of rubber hose (gas line) that fits snugly over the end of the spark plugs to help remove them from the hot engine. I also have one of those oven gloves that you see advertised at Christmas time to handle hot parts.

the 3.8 definitly requires a comp test anytime unusual things happen and remember that not all trouble codes turn on cel or ses lights on so check your codes also the upper plenum and lower intake manifolds tend to leak vacuum with age (causing rough or hi idle)or if bolts were not torqued to correct spec or the correct sealant on bolt threads was not used----more common than you think----