Hey all, first post here. Im on my second monte now but I've never had a problem like this. Im having an issue with the engine stumbling and roaming at idle and accelerating. When slowly accelerating the engine will stumble and its hard to get going. I really have to feather the gas to get it to go. When in neutral the RPM's will go up and down from about 500 to 1500 and wont stop until I slowly give it a little gas. If I quickly rev it up in neutral or park it picks up rpm fine but it wont catch itself on the way back down and kills.
Down the road it drives fine but I notice a very slight stumble...almost feels like a miss every once and a while and my rpm's are a bit higher than usual. It seems to make decent power in the upper rpm range also. Ive done a few hard pulls and you cant tell the stutter is there at all and rpms go just about to redline, pulls hard, and shifts fine.

Only time i really notice it is when its at idle, or when Im accelerating off idle. Almost feels like a fuel issue but the exhaust smells a bit like rotten eggs so i am leaning towards a clogged or clogging cat.

What do you all think? I just dont want to shotgun this thing when it could just be the cat. Car has ran fine since ive had it, only noticed this this last weekend after starting it back up after it was sitting in a parking lot for a few hours.

I havent yet checked the fuel filter, or pcv valve. They will be the first to get checked out before i throw a new cat on there. Also wondering if theres some sort of vacuum leak somewhere, although I didnt find anything.

Sorry for the long post, just trying to give as much info as possible so i can let you monte experts do what your good at. Thanks for any help!

 

..Welcome to the Monte Carlo Family.
Sorry to read that you are having problems with your Monte.
Please post what year/model/engine/miles that are on your Monte Carlo & it will help our expert tech member's assist in advising you what do check out. It does sound like it could be a cat.

How long have you owned it ? What has been done to your Monte ? Do you have any modification to your engine ?
Look forward to additional information...Good Luck

 

Could be anything. Age of the Car, mileage? When was the last time you did any tune-up on the car? Your MAF might be getting dirty. Do you have any modifications to the engine?

etc. etc.

 

The rotten egg smell is the only thing in your problem description that sounds like a bad Cat. The rough running issues you describe could be something entirely different. My only experience with a bad Cat was a lack of power, especially at higher RPM; it didn't really cause 'missing' or stumbling. Sounds like your issues are mostly at low RPM.

Some of the symptoms you describe could be from lack of fuel pressure or something restricting fuel flow. That's what happened recently on my old car. Fuel pump was dying causing intermittent rough running at various RPM.

Sorry I'm not much help ID'ing the issue, but sounds like it could be more than just an old Cat.

 

Sorry guys, later on I realized I should have probably put down my year and engine before i posted but couldnt get to a computer until now.

Monte is an all stock 98' LS. 3.1 engine. about 144k miles. Ive had the car just over a year now that I bought from a dealer. Last monte I had I clocked well over 200k before I finally sold it and it was still running strong. Im hoping to do the same with this one.

I drove it to work today and the thing Im really noticing is that the 2k RPM mark seems to be the sweet spot. Once it hits that the stutter/miss goes away and it pulls smooth getting up to highway speed. I put fresh gas in it today(it was at 1/2 tank) and it seems to be idling a bit better but the hesitation is still there. Going down the highway I can still feel it struggle every once and a while for a sec and then the rpm's seem to jump up a few 100 rpm like its trying to compensate for lack of power/fuel?

I just find it odd it did this out of the blue, mid trip. I headed up to watch a snocross race(about 3 hr drive) this last weekend and it ran great. Just before I got to there I filled up with gas and drove another 10 min to the race. Parked it for a few hours, and then leaving immediately after starting it up it wouldnt idle and stumbled terribly, and died multiple times trying to get out of the parking lot...which was lined bumper to bumper. My original immediate thought was that I got a bad batch of gas but I dont know anymore.

As for the rotten egg smell, that may have been a little pre-mature. Im not so sure its a rotten egg smell rather than just a really strong exhaust smell, much more noticeable when stepping out of the car. Its kinda hard for me to explain.

Not sure what to do, I havent really "tuned it up" since Ive had it but it was "serviced and inspected" at the dealer before I got it, and Ive put about 25k on it since I bout it.

Starting to wonder if maybe the fuel filter might be causing some of this?

 

Are you getting a check engine light at all? It sounds like a combination of things, including a bad cat, possibly bad spark plugs, maybe an O2 sensor even. But check some of those things first and see how it works out.

 

engine isnt throwing any codes...would it be possible to get it scanned at a dealer and see what its doing even though its not throwing any codes at me?

 

Don't go to a dealer, they'll charge you to scan the Monte. Most auto parts stores will check your car for codes for free.

 

Both the air filter and fuel filter are probably the stock units so I think tonight I will go ahead and change them out. Also might throw in a new pcv valve. its cheap enough to see if it makes any difference.
It seems to be idling better but every once and a while it still will surge up and down. Slow acceleration has improved a bit too but right around 1500 it stutters pretty bad. Actually killed on me this morning turning out of a parking lot. I am going to let it sit and idle for a bit when i get home and shoot my temp gun before and after the cat and see if theres a big temp difference. Would that be an indicator pressure is getting built up in front of it, or will it always be hotter infront of the cat? I know even a clean converter will creat a bit of back pressure but if it is clogged surely it would be an obvious difference between front to back?

Would also like to check my fuel rail pressure at idle and see if the pump could be causing an issue. Is there a schrader valve on the rail? And how much of a pain is it to get to?

Thanks for all the help guys, Just want to get this think running like it should again.

 

UPDATE: Was installing the air filter, PCV valve, and fuel filter tonight after work. With the engine running, I was just messing around looking for any cracks in the vacuum lines, ect. I finally pulled the fuel pressure regulator vacuum line and within a few secs gas came pouring out. For a few seconds with the line disconnected the engine actually ran smooth. Once the fuel comes out and the line went back on the engine instantly kills.

That's got to be the problem, Fuel was getting sucked back into the intake manifold and was probably running much richer than it should, causing the engine to load up and stutter, as well as causing my exhaust smell. My cat was probably trying to keep up with the extra fuel. I will put the new regulator on and let you all know how it does. Keep your fingers crossed.