I have (2002 STS) noticed a bit of a miss when i was at idle. It was not continueous but got more noticable. I had no warning lights, no service engine lights, nothing. I got a set of plugs and replaced them thinking this was the problem. I just rolled 100k miles. after changing the plugs all seemed well. I went to drive to work the next day and about 10 miles into the drive i noticed the car was surging. not drasticly but I could feel it just the same. Still no warming lights.

I made it the rest of the way to work and on the drive home i had the service engine soon light come on. i found a place with a code reader and had it checked. the reader said: mass air censor, O2 censor, mis-fire and I think a lean faul of some sort. The tech went through it too fast to read.

on further inspection we found that the air cleaner was not conected correctly allowing air into the system after the mass air censor. we corrected this yet there was still a miss in the idle. He cleared the codes and said the car had to re-learn how to run and it would self adjust as i drove. At that point I thought he was full of stuffing and left.

So now I have: a sevice engine soon light on a bit then off a bit then on again....no ryme or reason.

I have a rough idle

I have HIGH gas consuption rate

And a great fear of taking it to a dealer *******************************.

Help! any one have any ideas?

 

an engine miss is about the most difficult issue to trace. even a good tech with proper equipment can end up scratching his head in frustration.

the one thing we need here to identify probable problems is CURRENT and history codes.

go to our FAQ and pull the codes yourself, report back.

also, when you replaced the plugs, did it get worse? if it did, you may well have damaged primary wires, coils, or any of several components.

the most likely items to be at fault is the coils, primary wires, various sensors, CO2 sensors, TPS, charcol buildup in the intake, new plugs improperly installed/gapped/damaged.
.

 

ok I will check those items, I know the gap was correct because I checked them. It may be any one of the other items you said.

I will have it read today to see what the current codes are. I may be the coil wire due to the fact that I had a hard time getting them disconected. I had no idea that changing a set of plugs would be so involved.

As for the way it ran after the plug change, it seemed to run fine for about the first ten miles and then started to act up.

Is it true that a small vac*** leak can cause this engine to have a miss?

 

A vac leak can cause a slight miss as well as turn on the Engine Light.
Some say that vac leaks are a pain to find. But a good mechanic can find them very easily. They do whats called a smoke test. They hook up to whatever end and put colored smoke through the system.
Maybe its colored maybe not, but you get the jist of it.

Have had that test done on both my wifes Bonneville and my Cadillac. They were able to find the leaks lickity split.

On my 01 Cadillac just about a month ago I received the dreaded Engine light. I checked the codes and found lean on bank 1. Before I could get it to the dealer the light went out. I talked to them anyways and they suggested that it "may" be an O2 sensor. But dont know for sure and didnt want to look at it unless the light was on.

The light came back on later so I went back and they scanned it. They also got the running lean code as well as showing a slight miss on 2 cylinders.
My problem was the intake manifold gasket. They found that by doing a smoke test.
My car was in and out the same day.

 

Was the intake gasket replacement exspensive Mrmojorisin? I have been told it is very costly.....
I was able to fix the problem for the most part. I had a mis fire showing up in the #1 and #3. upon further inspection i found that the air cleaner had come apart after the sensor and that 3 of the spark plug boots were cracked.

I still have a bit of a miss at idle but no light or code.....?

 

It cost me 600 Canadian. On my invoice they called it the Front Plenum.
The gasket itself is only 50 bucks. Its the labour that costs. They basically have to remove the top of the engine.

But Dont be goin and changin out parts unless you know thats the problem. I was ready to change my O2 sensors but waited to see what the dealer had to say. Good thing I didnt. Would have been a waste of my time and money.

Take it to someone who knows what they're doin and have them check it out.
I guess Im fortunate that I know a dealership that I can trust.
Actually ...Im not sure how far I would trust the dealer if it wasnt for the service rep that I talk too. I deal with Dave and Dave only.

So you have no engine light now. Hows your mileage. Round 18 mpg ?
What kinda miss are you feeling ? Is it at idle in park or drive ?
If in park and just very slight then I wouldnt be concerned.

If the miss is really givvin the car a shake either or then it should be addressed.
How are the RPMs at idle ?

 

Well, my milage is about 18 mpg. the miss is mostly in park at an idle. the RPMs are about 750 to 800. I noticeit some when in drive but not as much.

I am starting to wonder if whatever it is also is effecting the speed censor. the reason I say this is that when I am at a stop in drive and i let my foot off the brake the rpms jump to about 1200 almost imediately. It also will stay at the higher rpms when coming to a stop. once i am stopped it will fall back to about 750 to 800.

 

im keeping an eue on this post, have similar problem check out "99sts cold rough idle" post..Have a miss when its cold, but still present when warm as well just a little lighter..also my fuel millage just recenlty last few took a dive!

 

I feel like I need to take it to the dealer but I can not bring myself to pay the rates they want. I love my car but at some piont you have to concider that the purchase of another car is going to be cheaper than the thousand dollares here and there for the parts and labor......

 

If your average mpg is at 18 I think thats standard. 750 is the right RPM at idle.
RPMs jumping to 1200 when releasing the brake from a stop is interesting.
I dont think that is a vacuum issue. I would lean toward idle control or something of that nature. Possibly something electronic.