Hey I have a 2001 S40 and it currently has problems when starting I get normal rpms at first then a second later it drops down and starts sputtering and fluctuating from 800 rpm to about 200 rpm while shaking violently and to the point where it sounds like its about to die, and it does this for a few seconds then it goes to normal rpms and runs fine. But today it actually did die and right before it did the check engine light flashed. Some other symptoms are that all the car lights dim when I turn on the AC but they tested it out and told me that the battery and alternator/electrical system are fine. Its always the first time that I start the car that it does this and after that, it starts up fine at about 1500+ rpm. and I take it to work every day at 11 p.m. since I work nights, it gets pretty cold whenever I'm starting the car. The only issue that I know of is that the front catalytic converter is pretty much dead and I was told that could possibly be the issue. If you guys could help that'd be very helpful!

 

Whether the cat convertor is functioning properly has no bearing on the car's performance. If the internals of the cat convertor fail and clog it, the symptoms are normal start and idle but no power. I've not heard of a modern cat convertor to fail internally. (1980's it was common, today not so) The test is to physically remove the upper O2 sensor and see if the symptoms change.

Is the check engine light on now? If yes then have the codes pulled. Autozone and the like will do it for free.

I would test the fuel pressure and bleed off rate. Place a fuel pressure tester on the schrader valve on the fuel rail under the plastic cover. Then have a helper start the car just like you do when it gives you trouble. Watch the gauge. Look for about 55psi when you turn the key on and start the car. It will drop a bit as the car starts. Watch the pressure to see if it fluctuates or stays steady while it acts up.

Then turn the car off and watch the pressure gauge. The pressure should stay close to 55psi and drop off very slowly overnight. It should not go immediately to zero.

Fuel pressure testers are available to buy at Harbor Freight for quite low prices. Last I checked you could also do the "loaner tool" at Autozone, Advance, etc.
Fuel Pressure Tester & Fuel Injection Pump Tester

 

The car also gets terrible gas mileage too, it's like getting 3-4 miles to the gallon and the transmission doesn't transition that smoothly from 2nd to 3rd gear. However I noticed that whenever I start the car up in warm weather it has no problem at all. However whenever I start it up for the first time in the middle of the night for work, it acts up very bad. So I'm guessing the cold weather has something to do with it? I'm going to check the fuel pressure later as I do not have the tool right now

 

Yikes, 3-4 miles to the gallon? Does your exhaust smell like fuel? Are your spark plugs covered in black soot? (Fuel Pressure Regulator) Mileage that poor should show itself other than poor running when cold. Is your coolant temp gauge in the normal range? (Coolant Temp Sensor) Do you get hot air from the heater? (Thermostat)

You can always test for a clogged cat by removing the top O2 sensor. The little hole opened by removing the sensor will make a huge difference *IF* that is the problem. Don't run it that way for more than a few minutes. Up to 1300 degree exhaust gases are pouring into the engine compartment.

 

Low MPG is very strange are you sure you are parking at a place people would not steal your gas? Other than that my bet is on a sticky Idle Air solenoid.I had this simptome you describe tho my car never did stall but after a couple of seconds the idle dropped then vent back up.One day it didn't start without keeping my foot on the pedal and had to drive home with foot on the throttle when I had to brake.Replaced the solenoid and all is good again.

 

The car has died now and it cranks but won't start. It stalled while in the middle of driving

 

okay so when we checked it out, it appears that the timing belt slipped off and so thats the reason why it died, however could there be an explanation as to why it fell off besides human error? Does that explain the rough rpms and idlings and why the check engine light came on? Hopefully putting the belt back on will fix it all

 

if the t-belt jumped or fell off then u have a timming issue plus might have bent valves. i do agree with the idler air valve. it seems like it was sticking in cold temp. due to carbon deposites. you could have removed it and tryed cleaning it with parts cleaner. but for now u got bigger issues. good luck

 

Oh that's a totally different animal you are trying to hunt down here.What is you mileage on the car and if the belt was ever replaced?Do you have a mechanic to blame for missing the 8 year or 100K miles T-belt replacement deadline?Jumping a tooth or two would explain the low MPG entirely.With the belt jumping off you'v got to be the luckiest ever not to bend the valves.I wish you the best of luck on this journey.

 

Yeah the mileage is about 130,000 and idk if the timing belt was ever replaced and so that's why we changed it just to be on the safe side. Just out of curiosity how long do these cars last? It also has new spark plugs as of 100 miles ago

 

I have a 2001 with about 180k. At about 150k miles my timing belt slipped due to being oil soaked from a leaking CVVT gear and bent all 8 exhaust valves. It took about 2 weeks from noticing oil on the driveway to damaged. My fault for not finding the issue earlier. I fixed it myself for about $500 in parts.

The older the car the more tlc it needs. The good thing is that most issues have already happened to someone else so you can find help if you like to DIY. Parts are still plentiful although getting more expensive. The junkyards are seeing more and more so used parts are more available.

The question for me is time or money. Do you like working on your cars (time) or rather have a new car and just gas and go (money)?