Rpm Going Low After Heat The Engine

sampathapg · July 27, 2014

My AE110 car has a problem, After drives about 15km in city area, RPM is going low. The engine also starts shake when in idle. I switched off AC, but no change.

What is this issue? Help me.

Solved: I replaced genuine engine mounts. Now the car runs smoothly

Edited December 4, 2014 by sampathapg

maheshw · July 28, 2014

when was you last EFI tune up? Does RPM goes up when switch on A\C ?

sampathapg · July 28, 2014

On 7/28/2014 at 6:03 AM, maheshw said:
when was you last EFI tune up? Does RPM goes up when switch on A\C ?

I did EFI tuneup 3 months ago. No change in RPM when turn on or off the AC.

Rumesh88 · July 28, 2014

What do you mean by low PRM? What is the exact value? How about your cold start? Does the engine rev up on cold start and then the RPM slowly come down? Leave the engine on idle after cold start until the radiator fan kicks in. Note the RPM at this stage. Is it stable or still shaky? Or does this happen only while driving and when coming to a stop?

sampathapg · July 29, 2014

On 7/28/2014 at 10:24 AM, Rumesh88 said:
What do you mean by low PRM? What is the exact value? How about your cold start? Does the engine rev up on cold start and then the RPM slowly come down? Leave the engine on idle after cold start until the radiator fan kicks in. Note the RPM at this stage. Is it stable or still shaky? Or does this happen only while driving and when coming to a stop?

This car doesn't have a RPM meter. So I cannot read exact value. But I assume it by engine sound. When in cold start engine sound is high and rough. But after heated up the engine the sound is so smooth. I drive from Kadawtaha to Bambalapitiya daily, When I reach to Borella, the engine sound is very very low, then it's start shake in very low speeds (mostly in idle). I have to reverse the car at Bambalapitiya office car park, when I reverse the car (without accelerating) the engine hardly shakes. I noted mostly this happen when I drive in traffic jam.

Recently I have changed engine oil, ATF and power steering fluid. Spark plugs, PCV valve and all belts also new.

Please help me to solve this issue. Thanks.

Rumesh88 · July 29, 2014

On 7/29/2014 at 5:34 AM, sampathapg said:
This car doesn't have a RPM meter. So I cannot read exact value. But I assume it by engine sound. When in cold start engine sound is high and rough. But after heated up the engine the sound is so smooth. I drive from Kadawtaha to Bambalapitiya daily, When I reach to Borella, the engine sound is very very low, then it's start shake in very low speeds (mostly in idle). I have to reverse the car at Bambalapitiya office car park, when I reverse the car (without accelerating) the engine hardly shakes. I noted mostly this happen when I drive in traffic jam.
Recently I have changed engine oil, ATF and power steering fluid. Spark plugs, PCV valve and all belts also new.

Please help me to solve this issue. Thanks.

Assuming the things like TPS, IACV are OK, I just wanted to narrow the issue down to a possible vacuum leak. Sorry I thought you have a RPM meter. BTW your cold start idle sound can be high (means high in pitch because of high RPM) but not "rough". If so get a tune up done for you may have a dirty IACV etc which may be the cause for rough idle at low RPM.

When the engine is running running smooth on idle after fully warmed up and on "P" or "N" position, apply brakes and see if the idle beat changes. Also after a normal run, park the car, stop the engine and open the fuel cap while listening to the noise of air being sucked in to the tank. These are DIY things to identify if there is a vacuum leak from brake booster or fuel tank side of the EVAP control.

sampathapg · July 29, 2014

On 7/29/2014 at 8:07 AM, Rumesh88 said:
Assuming the things like TPS, IACV are OK, I just wanted to narrow the issue down to a possible vacuum leak. Sorry I thought you have a RPM meter. BTW your cold start idle sound can be high (means high in pitch because of high RPM) but not "rough". If so get a tune up done for you may have a dirty IACV etc which may be the cause for rough idle at low RPM.
When the engine is running running smooth on idle after fully warmed up and on "P" or "N" position, apply brakes and see if the idle beat changes. Also after a normal run, park the car, stop the engine and open the fuel cap while listening to the noise of air being sucked in to the tank. These are DIY things to identify if there is a vacuum leak from brake booster or fuel tank side of the EVAP control.

Thanks Rumesh88, I will check.

sampathapg · July 29, 2014

On 7/29/2014 at 8:07 AM, Rumesh88 said:
Assuming the things like TPS, IACV are OK, I just wanted to narrow the issue down to a possible vacuum leak. Sorry I thought you have a RPM meter. BTW your cold start idle sound can be high (means high in pitch because of high RPM) but not "rough". If so get a tune up done for you may have a dirty IACV etc which may be the cause for rough idle at low RPM.
When the engine is running running smooth on idle after fully warmed up and on "P" or "N" position, apply brakes and see if the idle beat changes. Also after a normal run, park the car, stop the engine and open the fuel cap while listening to the noise of air being sucked in to the tank. These are DIY things to identify if there is a vacuum leak from brake booster or fuel tank side of the EVAP control.

I opened fuel cap and listened the noise of air sucked in to the tank. It means there is no vacuum leak?

Rumesh88 · July 30, 2014

On 7/29/2014 at 2:43 PM, sampathapg said:
I opened fuel cap and listened the noise of air sucked in to the tank. It means there is no vacuum leak?

It means there is no vacuum leak on the "fuel tank side of the EVAP vacuum circuit", but there can be a leak in other parts of it. These are only a few DIY observations for a vacuum leak but beyond this point you need to seek the help of your mechanic. (The EVAP circuit begins with a vacuum port from inlet manifold and goes through a purge valve, a charcoal canister and ends up in the fuel tank).

Edit: You have asked for the same problem in another thread sometime back and I believe you can identify the VSV of the EVAP system (VSV Part No. 90910-12143 located near throttle). If you can identify the hose running from top of the throttle body to VSV, you can test if the problem is due to a vacuum leak in EVAP system by disconnecting the hose from the VSV and plugging the hose with a tight fitting stud temporarily to see if the idle picks up.

Edited July 30, 2014 by Rumesh88

The Don · July 30, 2014

Could it be that the breather valve in the fuel tank is blocked up?

sampathapg · July 30, 2014

On 7/30/2014 at 3:04 AM, Rumesh88 said:
It means there is no vacuum leak on the "fuel tank side of the EVAP vacuum circuit", but there can be a leak in other parts of it. These are only a few DIY observations for a vacuum leak but beyond this point you need to seek the help of your mechanic. (The EVAP circuit begins with a vacuum port from inlet manifold and goes through a purge valve, a charcoal canister and ends up in the fuel tank).
Edit: You have asked for the same problem in another thread sometime back and I believe you can identify the VSV of the EVAP system (VSV Part No. 90910-12143 located near throttle). If you can identify the hose running from top of the throttle body to VSV, you can test if the problem is due to a vacuum leak in EVAP system by disconnecting the hose from the VSV and plugging the hose with a tight fitting stud temporarily to see if the idle picks up.

Thanks friend, I will check it and get back to you.

sampathapg · July 31, 2014

On 7/30/2014 at 3:04 AM, Rumesh88 said:
It means there is no vacuum leak on the "fuel tank side of the EVAP vacuum circuit", but there can be a leak in other parts of it. These are only a few DIY observations for a vacuum leak but beyond this point you need to seek the help of your mechanic. (The EVAP circuit begins with a vacuum port from inlet manifold and goes through a purge valve, a charcoal canister and ends up in the fuel tank).
Edit: You have asked for the same problem in another thread sometime back and I believe you can identify the VSV of the EVAP system (VSV Part No. 90910-12143 located near throttle). If you can identify the hose running from top of the throttle body to VSV, you can test if the problem is due to a vacuum leak in EVAP system by disconnecting the hose from the VSV and plugging the hose with a tight fitting stud temporarily to see if the idle picks up.

I disconnected the hose from VSV and closed the hose end by finger; when I released the finger from hose end, idle RPM increased. when I closed the hose again, Idle RPM dropped to normal.

Then I fixed the hose again. After that I disconnected 2 wire connector from VSV. But there was no any change, I thought the VSV not functioning properly.

Edit: Please tell me how to check VSV? (I have a multimeter also)

Edited July 31, 2014 by sampathapg

Rumesh88 · July 31, 2014

Measure the resistance of two terminals. You should get a reading of around 30 ohms. However, it is not a complete test. What you can do is to keep the line plugged and take the car for a drive (no harm in plugging up this hose for observation. your ECU will not throw up a fault code either) to see if the idle still slows down. If the problem is still there then you need not worry about EVAP circuit parts.