Hi guys, i am new to the thread but been viewing as guest on this site for a while and its been very helpfull. So just registered but i have a 99 STS w/ N* course and had car 3 years know pretty decent amount about it and cant seem to figure out why it has a rough idle when cold? especially weather changing now and its even rougher. it been like this a year now but living florida its usually always warm. I have no codes on the dash, seems to smooth out once it reaches full warm up, when ideling if i rev up to about 1500 RPM goes away. tends to feel like its running out of gas sort of when really cold. know its not plugs and wires since changed all them recently normal maintance. I think i have a guess but want to see what everyone thinks first.
Thanks guys-mike

New plugs and wires.....how about coils?

Well i think with coils it would be doing itthroughout all different stages of engine temperature/engine RPM. it would be present at any given time instead of when its just cold..?

There are no codes in the system,im going to check again right now actually but other wise people are telling me to have someone with the techII scan tool check it out and see it it sense something thats off

I say coils because that's what was wrong with mine, same sympytoms you describe no codes either. Cropped up at this time of year too back in 06. It's worth a check. Just look at the coil posts to see they are not rusted or otherwise corroded and that wires are set properly.

Ok thanks for input, i will check them at daylight tommorow. I have been thinking if not the coils could it possibly be of the following....Engine coolant temp sensor? Idle air control valve? least thinking MAF? Just curious never had bought coils but do they run for this type car? last time i looked i saw they were in sections of 4 i think 2 cylinders each pack correct? I will check them all, im guessing there each indiviualy replaceable right?

I never jump to the worst case scenarios first, often little things need attention and presto it's fixed. Coils are pricy though, I paid $89 CDN. You are right two cylinders per pack, they are individual in pairs and come off pretty easy, they are all attached to one large bracket bolted on the rear valve cover.

I don't think a bad idle can be attributed to the coolant temp sensor, the IAC would cause surging not rough idle, MAF probably more of a high speed issue.

When the car is cold the system is feeding it more gas, I mean here in Canada we rely onfast idle to keep our cars running in the morning this time of year. When the mixture is rich it seems to me that your ignition system can't deal with all the fuel so you get the roughness. Once it warms and leans out it gets better. I still think you are looking one or more of at plugs, wires, coils. Another possibility is a vac*** leak at the intake, bad hose, plenum gasket etc..

I just logged on to check the price and caught your post. The coils are sold seperatly correct? like 2-2-2-2 Guessing auto zone should carry? looking for rusty or corrision signs theywould be under the coil right, take it off and check and were plugs plug into them too right.(might not be a bad guess as i live off a salt water canal) and if was a va*** leak that could cause run rough cold and better warm

just found this info and pricing on ac delco pack. It looks to me that even thou there numbered there all same corect? this one pictured doesnt have any cylinder numbers on it
http://www.autopartswarehouse.com/details/QQCadillacQQSevilleQQAC_DelcoQQIgnition_CoilQQ1993 1999QQACD555.html

Yes only the factory OEM parts are numbered, they are all the same. Good idea to get AC Delco. If you do change a coil(s) good idea to get some dielectric grease. Might as well treat all wires and posts too.


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im thinking plug wire/plug also.... its funny though no codes???? 99stsguy, do you know how to pull codes? dont take affence
rickyd

Hey rick, umm yea I think did right for what I can do. Codes I looked into and checked were the ones in dic. Held on/off button and up arrow wit key on. Found couple old history codes nothing to do with whats now. And I checked them on site I had to decifier them. Oh by way guys don't kno how much difference this makes but car is 99 wit 143k all stock pretty well taken care of and found records on plugs wires and changed them last thanksgiving day. All ac delco/ gm parts. And this didnt start until about march April 07 area. Really appreceiate all the input
mike

Just came in from pulling the coils off, everything look A ok, no corrision rust etc. All wire plugs were on snug removed N* cover all wires are still in great shape no look cracked/ arched. Coil post were clean clear. Now just think if a plug or two fouled out or something? next im going to pull first 4 easy plugs out, check them.

if any body is looking at this post for reference or info- i am in process of ordering new coils(pack). I will keep posted the status as it changes and once install new pack.

lol looks like im posting to my self now...but if it helps....I changed coil packs about 5-6 days back. So far same conditions still present. Rough when cold way better when warm but still vaguelypresent when warm. Help me out guys! O no codes either
Thanks-Mike

Fuel pressure? If you can check it, what isit anddoes it hold after pump stops running (key or prime)? Small vacuum leak at intake or boot between t/body and intake manifold. What type of fuel do you use? What kind of driving do you do?

Im not sureof the fuel pressure, but i do know if it helps that the fuel pump is brand new-fuel filter is new. Still shows same signs before/after change. At the intake i know the boot your talking about, usually found on eldorado but not on mine sts. Was at cadillac and we checked over that. I use 93 oct and fuel injector cleaner 3k mi (if it evenhelps)usually chevron/sunoco almost always. I have tried regular few months back when gas was really high..no difference. the type of driving i do is usally same highway 24 mi round trip morning city 22 mi round trip night. i like to WOT merging highway 50-80 no hesition. i would classify my self "spirited driver"

the only thing i can say, but peole keep denying it is..a year back i changed my radiator. installed new one filled up with new 50/50 dex cool. The minute i started the car i instantly felt the "rough idle" as if it just began that moment! i am very picky/obsservant and tend to notice "everything" so i keep thinking till now that somehow the new coolant..fouled or upset the ECT sensor. Everyone denies it but im very close to getting a new one just changing it. i have read online that that also controls fuel/air input mixture. and wont throw a code if bad.

Have you checked fuel pressure regulator for leaking fuel at vacuum hose connection. New pump and filter are good but regulator controls pressure. Correct pressure is more important on start up than when engine is warm. Carbon build up on can be a factor also during cold operation. Scanner is going to be required to monitor temp and O2 sensors. EGR valve not completely seating also possible. Grounds alway can figure in. I think you need a professional type scanner to figure it out. Also type coolant should not kill ect sensor

ok guys i know i am a year late, but i just came back from the shop and had it hooked up to the scanner. We checked every sensor, fuel, air, va*** leaks, settings possible. We went throu each setting, at different rpms..etc.. Everything checked out perfect. Engine temp,o2 sensors,iacv,maf,etc..Mechanic actually said it couldnt run better even with the miss for a vechicle this age and millage. What he did mention after the scanner, that it could be a valve not seating properly. He wants me to come back and do some check with a va*** gauge when engine is completely cold, And posibly a compression test.
Any ideas please reply

couple days ago i got the P0300 code, hooked up scanner it was 305 to be exact, and also pulled p0174, and 171 po420. always had 420. But the other three all pointed to either bad MAf,injector, o2 sensor or va*** leak. Well based on the comp they hooked up last month..my o2 sensor(s) are good, MAf as well, and said no va*** leaks. So hat leaves me injector. I put a screwdriver to it (number 5 cyl) and rest them, they all sound same. Next i unplugged it, the engine dipped in rpm then came right back to the same. I put it back and went up a tad then back normal..(immediately) so i tried another injector did the same thing..also yesterday did the seafoam cleaning thru intake procedure, kinda the same cant tell to much maybe better throtle response. i also bought another injector gonna pop it in there see if anything changes..post any thiughts~
MN