Engine doesn't idle as smooth as it used to

[Merkava_4]

2000 Buick LeSabre

Check engine light is off and no codes stored. About twice a year I'll get the P0451 code which is the fuel tank pressure sensor circuit. I read somewhere that the culprit is an overly sensitive PCM and the only fix is to re-flash it. So I'm not too worried about it. I just erase the code whenever it comes up, which is very seldom.

The engine doesn't idle real real smooth like it used to. It used to be imperceivable as to whether it was running or not. It used to be the only way to you could tell if it was running is to look at the tachometer.

So. . . . . .you guys have any ideas as to why it wouldn't idle as smooth as it used to?

 

[ARB1977]

Have you tried a new gas cap? It's possible since you got a fuel tank pressure issue.

 

[Merkava_4]

Gas cap seems to be fine since they check it at the smog shop every time it gets smogged.
It's been smogged three times since 2005 and it's passed with flying colors every time.

 

[63Marauder]

ever thought of a tune up?

 

[Merkava_4]

The car's had new spark plugs and ignition wires installed at 85K. Car has 112K on it now. I keep the oil changed regularly and the coolant changed once a year. Air filter is a Fram AirHog that never seems to get dirty. EGR valve has been replaced awhile back because the solenoid burned out. The one thing I haven't done yet is clean out the throttle body. And I've never cleaned the MAF sensor yet either.

 

[95busa]

Pcv

 

[demarpaint]

Check the PCV, check the wires, especially on the rear bank of cylinders. Run a bottle of good FI cleaner through her. As a last resort you can try hot water to remove carbon via a vacuum line. Just don't hydro-lock the engine.

 

[pbm]

When I had my Buick Century I would clean out the TB occasionally and it really seemed to smooth out the idle.
Mine had the 3.1 motor and a lot of black carbony junk accumulated in the TB.
I would remove the hose and spray TB cleaner in without letting it stall...black [censored] would pour out (into my paper towels) and she'd run rough while I did it but smooth as silk afterwards....

I also ran a bottle of Regane, Techron etc...just before each oil change...she was running like new when I sold her at 135K ish...

 

[Trav]

If this is a 3800 with the MAF sensor in the throttle body do not clean the throttle body with the MAF in place.
Remove it and clean it separately, TB cleaners can damage the MAF sensor. Have the LIM gaskets and upper plenum with improved EGR chimney been replaced?

 

[Vikas]

Are these cars known for worn motor mounts? On some makess not so smooth idle can be attributed to them.

 

[simple_gifts]

Maxlife, j/k.

 

[zoinks]

Clean the throttle body. At your mileage it will help the idle and you'll avoid letting it get to the point where it won't idle at all.

 

[Trav]

Originally Posted By: Vikas
Are these cars known for worn motor mounts? On some makess not so smooth idle can be attributed to them.

Not really as much as a longitudinally mounted engine or ones that use side mounts like some Japanese V6's, the dog bones on these take much of the torque strain.

 

[bigt61]

Add a bottle of Techron to a full fuel run. I usually add a bottle a year to my 94 Bonneville w the 3800. It's cheap at Costco and even cheaper when it goes on sale about every 3 or 4 months.

 

[Merkava_4]

Originally Posted By: Trav
Have the LIM gaskets and upper plenum with improved EGR chimney been replaced?


Not since I've owned it. And I don't know if the original owner had them changed either. I keep an eye on the coolant level and check the oil for water every week. Got the car in 2005 with 83K on the clock. Car has 112K on it now. Car is completely stock original except for the high flow air filter.

 

[Mark_Walk]

+1 on dirty fuel injectors. Also, what about an O2 sensor going bad?

 

[nitehawk55]

Originally Posted By: zoinks
Clean the throttle body. At your mileage it will help the idle and you'll avoid letting it get to the point where it won't idle at all.


TB gets overlooked . My Ford 500 has to have it cleaned about every 60-80K km or it has a start-stall issue and idles rough . Easy job and once done it purrs like a kitten

 

[Cujet]

My 130K mile Jaguar X-Type used to idle perfectly, and amazingly smoothly. Recently, I noticed it has become a bit rough on startup and at idle.

I cleaned the TB, replaced the plugs, cleaned the PCV, cleaned the mass airflow sensor, air filter and so on. No change. I started to think that maybe valve sealing and ring sealing were part of the problem.

Then I thought about using 2 tanks of double dose Techron. That helped quite a bit. Coupled with a fresh M1 5W-40 TDT oil change and it idles like new.

 

[loyd]

This is just my opinion, but an air filter that never gets dirty is, to me, defective! Especially living in the Central Valley of California where there is lots of dust from agricuture.

 

[SuperDave456]

I'd start by doing more research on that air filter.
I've heard very mixed reviews on all brands that are washable and reusable.

I'd personally start by doing 2 full tanks with a full dose of a quality fuel injection/fuel system cleaner.
Redline
Amsoil

Something known for its cleaning ability.

I'd then do a short OCI, think 3,000 or 4,000 miles.

Typically I'd think that this is an electrical issue, or something to do with the MAF or another sensor.
However, if all that is good, then start with the Fuel Injection cleaner and run 1 dose on a full tank 2x back to back.