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

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


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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..


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I thought these engines really only liked acdelco plugs?


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


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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.


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


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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.


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.


Well-Known Member
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.


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.


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

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