What would cause a car engine to idle rough at stoplights and stall only while stopped but run smoothly while driving? It is also rough to drive in slow traffic for a long time.

I did the following
- clean throttle body
- changed the spark plugs
- replaced the Intake Manifold Vacuum Solenoid Valve

But, it is still the same. May I know what could be the issue? 

coil pack failing

 

Try cleaning the MAF sensor with electrical cleaner

2 hours ago, M Lam said:

What would cause a car engine to idle rough at stoplights and stall only while stopped but run smoothly while driving? It is also rough to drive in slow traffic for a long time.

1. It would be useful to know what engine (I've already guessed it's petrol, but not which one)?
2. How many miles has it done?
3. Does it idle smoothly if the gearbox is in Neutral ?

1 hour ago, unofix said:

1. It would be useful to know what engine (I've already guessed it's petrol, but not which one)?
2. How many miles has it done?
3. Does it idle smoothly if the gearbox is in Neutral ?

It is 2.0L petrol. It is a 18-year-old car with 176,000 miles. I change its oil regularly because it has a timing chain belt and is serviced regularly. 

I just change the air filter and cabin filter too. As well as stuff below

- clean throttle body
- changed the spark plugs
- replaced the Intake Manifold Vacuum Solenoid Valve

When I drive with a Neutral around 60 miles per hour, the rpm acts funny. It keeps going up and down between 2,000 rpm and 3,000 rpm. I can hear a rough engine sound like someone pressing the accelerated pedal, even if it is neutral, and never push any pedal. But, when I had the accelerated pedal on. The rpm is stable. And when I press the brake to stop. The rpm goes up to 3,000 rpm, and after a few seconds, it goes back to 1,000 rpm. 

has the cluster been looked at, that can throw allsorts of silly faults.

26 minutes ago, Jimpster said:

has the cluster been looked at, that can throw allsorts of silly faults.

I remember once it showed the yellow engine error light. I tried to scan it. It shows P2110 or 2110 (it indicates that the engine control module (ECM) is detecting significant failures in the throttle actuator control (or TAC) system). I cleaned the throttle body and reset the error code. However, the engine error light will be turned on and off later.

Think I'd try replacing the throttle body with a good used one.  If the TPS inside is worn out then cleaning will not fix it.

6 hours ago, TomsFocus said:

Think I'd try replacing the throttle body with a good used one.  If the TPS inside is worn out then cleaning will not fix it.

Should I disconnect the battery when I try to replace the throttle body? 

I didn't disconnect the battery when I cleaned the throttle body.

Its good practice to disconnect the earth lead on the battery when doing any work on the electrical system and associated parts. Give the BCM a chance to clean itself down.

13 hours ago, M Lam said:

Should I disconnect the battery when I try to replace the throttle body? 

I didn't disconnect the battery when I cleaned the throttle body.

It is not necessary for replacing the throttle body, but there's no harm in doing it.  However, the throttle body should be relearned after having been unplugged or swapped.  For that the battery should be disconnected briefly, then after reconnecting, start the engine without touching the pedal, let it idle for a few minutes without touching the pedal, and then take it out for a drive over various throttle openings for it to learn.  I can't remember the exact sequence for the throttle body relearn but you could probably find it by searching online if the above doesn't work.

You will need the radio code after disconnecting the battery, unless you have an aftermarket radio fitted.

6 hours ago, TomsFocus said:

It is not necessary for replacing the throttle body, but there's no harm in doing it.  However, the throttle body should be relearned after having been unplugged or swapped.  For that the battery should be disconnected briefly, then after reconnecting, start the engine without touching the pedal, let it idle for a few minutes without touching the pedal, and then take it out for a drive over various throttle openings for it to learn.  I can't remember the exact sequence for the throttle body relearn but you could probably find it by searching online if the above doesn't work.

You will need the radio code after disconnecting the battery, unless you have an aftermarket radio fitted.

Thanks, everyone. 

I purchased a used throttle body from eBay and will try it when it arrives. I remember seeing that step on YouTube, too. I will update you all later. Thanks again.

give it a damn good clean before fitting

46 minutes ago, Jimpster said:

give it a damn good clean before fitting

Thanks for the tips 😀

46 minutes ago, Jimpster said:

give it a damn good clean before fitting

Thanks for the tips 😀

On 6/10/2024 at 10:10 AM, TomsFocus said:

It is not necessary for replacing the throttle body, but there's no harm in doing it.  However, the throttle body should be relearned after having been unplugged or swapped.  For that the battery should be disconnected briefly, then after reconnecting, start the engine without touching the pedal, let it idle for a few minutes without touching the pedal, and then take it out for a drive over various throttle openings for it to learn.  I can't remember the exact sequence for the throttle body relearn but you could probably find it by searching online if the above doesn't work.

You will need the radio code after disconnecting the battery, unless you have an aftermarket radio fitted.

Dear All

I just changed the throttle body and properly cleaned it before installation. I even followed the step suggested by this guy: Let the ECU recognise the throttle body by leaving the key at position 2 for a while and turning off the engine. After that, run for 15 minutes.

It's better, but the problem remains. Do you think I should run for another 30 minutes?

Should I put out clean this below as well (Red circled)

 

 

The pipe in the red circle is for the purge valve (aka evap valve).  It doesn't need cleaning, but should be checked for any splits which could cause a vacuum leak.

2 hours ago, TomsFocus said:

The pipe in the red circle is for the purge valve (aka evap valve).  It doesn't need cleaning, but should be checked for any splits which could cause a vacuum leak.

It looks fine. LOL, I run out of idea now.

By the way, I noticed the engine is extremely hot. 

13 hours ago, M Lam said:

By the way, I noticed the engine is extremely hot. 

what does the temp guage say ? possible stat or sensor ?

9 minutes ago, Jimpster said:

what does the temp guage say ? possible stat or sensor ?

It was around 80'C yesterday after an hour's drive. 
Today, the speed limit mode is on even if the engine is not hot after a short drive. If I stop the car and start again, it works for 500 yards, and the problem repeats. Unfortunately, changing the throttle body does help. It is still the same.🥲

find someone with FORscan, clear all codes take it for a run and re-read. Back up the BCM with Forscan and SAVE a copy in a safe place. Then completely reset it all to factory defaults and run again. Am erring towards the cluster. Unless anything obvious pops up.

80c isn't overheating.  The stat doesn't open until 90c.  Then the fan cuts in around 98c.  If it's over 100c, that's when I'd consider it to be overheating.

Thanks, everyone. This Friday my mechanic will use the computer to investigate for me. Something interesting is that my new spark plugs have oil on the threads.

 

4 hours ago, M Lam said:

Thanks, everyone. This Friday my mechanic will use the computer to investigate for me. Something interesting is that my new spark plugs have oil on the threads.

 

Just speculation at this point but could be a a leaking valve seal or piston rings. That said don't panic because it's not unheard-of to see a little bit of oil on a plug and everything be more or less fine.