Envoy with dreaded rough idle, no codes, UPDATE FIXED!

[fatbutweak]

Another Envoy with the dreaded rough idle. 04 with 200k, been a great truck, maintained well,
no major issues. No Codes
Having an issue with a rough idle condition, goes away at about 700 rpm. Accelerates fine and top end is fine. Just a pronounced misfire at about 500 rpm.
Actions taken:
Cleaned TB, run BG44k through tank.
Cleaned Camshaft actuator solenoid. Was clean even after 200k, first time I've pulled it.
Pulled plugs, look fine. Have about 50 k on denso plugs.
According to torque app, engine runs about 202 degrees F
Compression test: all cylinders except number 3 at 170 psi, which is at 160 Put a little oil on top of piston, wet tested at 170
Changed motor mounts with oem.
Air Filter has about 10k on it, fuel filter at about 25k.
Not sure where to go from here, I don't want to just throw parts at it. I would appreciate any help.
I have a bad feeling it's the valves, but am still in denial about that, lol

 

[Mooseman]

When you cleaned the TB, did you disconnect the battery for at least 30 minutes?

 

[mrrsm]

Your compression is too high to be overly concerned about bad valves just yet. Perhaps tightening the Intake Manifold Fasteners to seal up any vacuum leaks would help. With such high mileage on your Motor, those I-M Plastic "O"rings have long since flattened out. One other thing to investigate would be cleaning out the Engine Oil that has collected inside the MAP Sensor... as it can cause sketchy idling when clogged..

 

[TollKeeper]

I thought these engines really only liked acdelco plugs?

 

[fatbutweak]

Sorry, I made a mistake in the op. They are AC Delco 41-103 plugs

I thought these engines really only liked acdelco plugs?

When you cleaned the TB, did you disconnect the battery for at least 30 minutes?

Yes.

 

[Mooseman]

Check the intake manifold bolts. They can get loose. Since you have Torque, did you check your fuel trims and O2 sensor voltage output?

 

[fatbutweak]

Check the intake manifold bolts. They can get loose. Since you have Torque, did you check your fuel trims and O2 sensor voltage output?

No, I will check it out this afternoon, thanks

 

[gmcman]

What you say it's rough at 500 RPM, are you reading the tach needle or a scan tool? These motors idle at 600 RPM give or take and a worn motor mount will produce a felt vibration at idle that goes away just above idle.

If you have never changed the mounts, I would consider that a possibility.

Only use AC mounts, the aftermarket is very hit or miss, biased to miss, on damping the 10 Hz frequency at idle.

 

[Mooseman]

He did say he replaced the mounts with OEM.

 

[fatbutweak]

It seems to be fixed now, not exactly sure what it was that solved it I've been battling this for over a month now, and it's gone away.
Smooth and quiet as when it was new.
Anyway, thanks all for the input.

 

[gmcman]

He did say he replaced the mounts with OEM.

Yes he did.

My bad.

 

[fatbutweak]

Yes he did.

My bad.

Well after a week or so, the rough idle is back. Not as pronounced as before, but it's still there. Not sure what to check next

 

[gmcman]

There's a few things you can try.

First you need a vacuum gauge and check the vacuum at either the front port on the intake, facing the pass side (may or may not pull vacuum) if it does hiss then check there, otherwise you can use the vacuum port on the throttle body.

See what it's pulling and see if the needge fluctuates rapidly or not.

25k isn't terrible, but it wouldn't hurt to change it....I personally don't like to go over 15K or 1 year but that's just me.

Did you check ALL of the intake manifold bolts and verify that they were not loose?

You could also change the fuel pressure regulator. I changed mine and it smoothed out the driveability a little. The thing about the FPR is you can't really check the fuel pressure at the rail, so it's not money wasted IMO at 200K miles to replace it.

 

[fatbutweak]

I changed the FPR about a month ago because the rough idle started up coincidentally with a strong gasoline smell in the car.
Traced the gasoline smell to a leaking FPR, gasoline smell fixed but rough idle continues.
I admit I didn't check the intake bolts behind the little hose, but none of the other bolts were loose and I sprayed some starter fluid around that area with no change in rpm. Is there a trick to getting that hose off? It was late and I was tired and I didnt want to break something getting that hose off.
I will try to track down a vacuum gauge this weekend.
Thanks

 

[gmcman]

Also, while your trying to find the cause of your rough idle...not sure where you purchase or what type of gas you use, but I would recommend only using 87 octane from a busy station.

Don't use other octanes as they usually mix at the pump with 93 which likely sits in the tanks for long periods.

 

[jhenry999]

My 2003 Trailblazer 4.2 with 200K miles had the same problem. Sometimes mildly ragged idle, sometimes not. Drove me crazy. Check the coils, cleaned the throttle body...all the usual things. Then I learned that the Mass Airflow Sensor in the air intake tube could get dirtied up and cause a rich mixture...which is what it seemed like. In 200K miles I had never cleaned it or even thought about it. Went to the auto parts store and bought a spray can of CRC MAF Sensor Cleaner. A 5 second spray was all it took. It purred like a kitten on a 300 mile round trip to Orlando today when it got Average 24.5 MPG on the car's computer! A new record for this car.

 

[fatbutweak]

After chasing this for a couple of months and not wanting to throw parts at it, I broke down and bout six new coil packs and 6 new plugs. Well, that did the trick.
Been about two weeks and even with 200k on the clock, runs pretty damn good.
I can't believe that did not cause a misfire code, it was pretty bad.
Thanks to all who contributed

 

[mntegra01]

My 2003 Trailblazer 4.2 with 200K miles had the same problem. Sometimes mildly ragged idle, sometimes not. Drove me crazy. Check the coils, cleaned the throttle body...all the usual things. Then I learned that the Mass Airflow Sensor in the air intake tube could get dirtied up and cause a rich mixture...which is what it seemed like.

Just gonna throw this out there... A 2003 4.2 does not have a Mass Air Flow sensor. They have an Intake Air Temp sensor in the tube right next to the duct box that says 4200.

The reason I am here is that I am having the same random jerk idle at 600 RPMs. Its like a stutter... I also can rev it to 2000 or 3000 RPM and still feel it.

I have new mounts and spark plugs. I guess probably what I need to check is for vacuum leaks around intake. If I go that far, I may as well replace the valve cover gasket and spark plug tube seals. I can probably do a fuel induction service here at work, but I might just grab a TECH 2 and see what data I can get from it first.

It purred like a kitten on a 300 mile round trip to Orlando today when it got Average 24.5 MPG on the car's computer! A new record for this car.

Also this is hard to believe when I barely get 14 MPG, I really wish I could get just 18-20 on the TB Shorty 2WD. My 4 cylinder Honda gets 25 MPG.

 

[budwich]

get some obd data... You can readily check misfire counts on each cylinder which may tell which / any cylinders that are in trouble to help focus your attention in those areas. Further, obd data for fuel trims at idle and at constant medium throttle will also provide an insight into other areas. Go from the results. First, guess would be misfires at individual cylinders.

 

[Mooseman]

Which spark plugs did you install? This engine ONLY likes genuine ACDelco 41-103 (there are fakes floating around).

 

[mntegra01]

I got them from work. I work in parts dept at a GM dealer. They are OEs. I just looked at misfire data on TECH 2 and no misfires at IDLE or while ramping up the RPM. I looked at fuel trims but I have no idea what I am looking at. I will have to do some research on that.

EDIT:
my long term trim was about -7 while messing with it earlier. I did not really pay attention too much earlier as I didn't know what to look for.

 

[TequilaWarrior]

Mine has idled like warm dog crap from the first day I owned it. I've been able to make some improvements by doing everything you mentioned. I also replaced all the intake gaskets and did a thorough cleaning of the throttle body, intake manifold, injectors, and head while I had everything apart. I also replaced the CPAS, O2 sensor, and exhaust manifold.
It now idles "ok" but not what I would call "good". I have the same MPG you mentioned. I was only able to improve it to about 16 MPG after all that work. Some of these engines are just "thirsty".

 

[budwich]

ok... thanks for the more info. Next place, when was the throttle body last cleaned?
In terms of the trims, "nominal 0" is the "target" but obvious deviations are normal... but as you "move" away from 0 in either direction, it maybe a slight indication of fueling / air issues. Since you don't you appear to indicate that your truck (year ?) doesn't have a MAF, running "parameters" are largely gleamed from from the MAP and O2 sensors (of course, others too). MAP's are relatively cheap to replace but do some more checks before heading that way.

 

[mntegra01]

Thanks budwich and TequilaWarrior.
2003 LS 4.2 , it's in my signature.

The idle bothers me because I think it should run perfectly smooth and no twerk/jitter at idle. It isn't a huge deal and everything else is great with the truck.

 

[gmcman]

@mntegra01 I would recommend picking up a can of BG44K from the parts dept and run it through on a full tank. Anytime I hear of these issues or experience them myself, that's my goto treatment then I reassess the situation after a tank or 2 of fuel. You may have a small build up of carbon and barely holding a valve off it's seat.

Take an analog vacuum gauge and see if the needle is steady and not fluctuating, it's ok for it to wander slightly as the TB adjusts idle speed, but not rapidly bouncing, if rapidly bouncing then an intake valve could not be fully seating.

Take a thick piece of paper like a piece of manilla folder and hold it agajnst the exiting exhaust flow, if it snaps back to the tailpipe then you have an exhaust valve not seating fully.

If you have a compression issue on a cylinder you could get a random stumble or misfire, I've also had coil packs get weak, maybe pick up a new AC coil and move it around from Cyl 1-6 and see if the miss goes away.

Running the BG44K through will not only clean any carbon deposits, but will help clean the injectors if dirty.