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Old 17th September 2013, 13:04   #1
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Default Engine choking in petrol mode, works fine in CNG mode

BHPian,

Need help in this case.

Fiat Siena 2000 model 1.2 ltr petrol engine
Driven a total of 125,000 kms
Installed CNG kit in Sept last year.
Driving 35-50 km daily with a few 300-500 km runs abt 3-5 times a year.
Start on petrol, drive abt 1 km on petrol and switch to gas with rpm increase
While gas tank is empty drive on petrol mode for 10-20 kms.

With this back ground, the car has been CHOKING on petrol mode. ie While driving on petrol mode, I face a speed breaker, shift down to 3rd and after the breaker push the accelerator pedal down - the engine rpm DOES NOT go up but struggles to stay alive. Now - at a certain point on the pedal, a very sensitive positionI must say, the rpm surges and the car drives perfectly after the rpm is high. But that position on the pedal is difficult to find specially with traffic all around.

THIS CHOKING DOES NOT HAPPEN ON GAS. Vehicle is really powerful in cng mode. No idling vibrations in cng or petrol mode.

Have changed the fuel pump (in the petrol tank), serviced properly, cleaned throttle body & injectors. Driven for 7 days abt 250 kms on petrol only, still problem persists.

could someone point what to check or rectify. I am stumped. Same case with ac on or off.

regards
Soumo
Navi Mum
FIat Siena 1.2
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Old 17th September 2013, 13:19   #2
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Default re: Engine choking in petrol mode, works fine in CNG mode

Try and replace the ECU with another car and check if the issue happens still.
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Old 17th September 2013, 13:34   #3
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Default re: Engine choking in petrol mode, works fine in CNG mode

Quote:
Originally Posted by fast lane View Post

While driving on petrol mode, I face a speed breaker, shift down to 3rd and after the breaker push the accelerator pedal down - the engine rpm DOES NOT go up but struggles to stay alive. Now - at a certain point on the pedal, a very sensitive positionI must say, the rpm surges and the car drives perfectly after the rpm is high. But that position on the pedal is difficult to find specially with traffic all around.

THIS CHOKING DOES NOT HAPPEN ON GAS. Vehicle is really powerful in cng mode. No idling vibrations in cng or petrol mode.
With due respect, the problem does not seem to be engine related but due to your driving style. When you reduce your speed while approaching a speed-breaker and then accelerate in 3rd gear, there is a high chance that the inherent lack of torque obtained in 3rd gear (compared to 1st or 2nd) might be causing loss of rpm.

I would suggest you drive through the same speed-breaker in 2nd gear and see the difference. I understand that you might not have experienced the same before the conversion; but it is a well-known fact that engines start reducing less power and torque after gas conversion, even when driven in petrol mode only. Why does it happen is something I still don't know.

The other possible cause might be the malfunctioning of the throttle body. But since you seem to have got it checked, we can possibly rule it out.

Last edited by mohitk1993 : 17th September 2013 at 13:37.
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Old 17th September 2013, 14:24   #4
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Default re: Engine choking in petrol mode, works fine in CNG mode

When was the last time you had changed spark plugs ? Usually, when CNG is installed, sparks plugs are changed. Change your plugs to new Stock ones.
Also try resetting your ECU. Check the coil also.

I am also facing similar problem in my Baleno. In CNG mode, the car is sometimes better to respond at lower rpms. High rpms, any given day petrol is better, but then my plugs were changed to some plugs for CNG and also the ECU has to be reset. Prolonged use on CNG tends to make the ECU "learn" that mode.

Since a couple of months, I have stopped driving on CNG due to long lines and increasing CNG costs.
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Old 17th September 2013, 14:45   #5
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Default re: Engine choking in petrol mode, works fine in CNG mode

Quote:
With this back ground, the car has been CHOKING on petrol mode. ie While driving on petrol mode, I face a speed breaker, shift down to 3rd and after the breaker push the accelerator pedal down - the engine rpm DOES NOT go up but struggles to stay alive
I also guess the problem is with picking up after the speed breaker on 3rd gear. Ideally, I shift down to 2nd on concrete speedbreakers, and 1st on those new plastic road bumps. Try shifting it down on 2nd on the same bump and notice the difference.
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Old 17th September 2013, 16:11   #6
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Default re: Engine choking in petrol mode, works fine in CNG mode

Quote:
Originally Posted by furobiker View Post
I also guess the problem is with picking up after the speed breaker on 3rd gear. Ideally, I shift down to 2nd on concrete speedbreakers, and 1st on those new plastic road bumps. Try shifting it down on 2nd on the same bump and notice the difference.

What you say is true, downshifting helps. BUT the rpm issue remains. Even in 2nd gear the RPM does not go up unless i hit the sweet spot on the accelerator pedal.....and at the sweet spot 3rd is as good as a rocket.

My concern is the sweet spot where the engine responds by surge in the rpm. In other positions its like getting overwhelmed.

recently on my trip to harihareshwar, i asked for some directions and had to take an almost u-turn with the road ahead on a good incline. The engine choked on 1st gear !!! but did not get cut off. I had to find the sweet spot and it was like a rocket and went on to go up the hilly road on fire...

The sweet spot - what is causing that ......is there any issue with petrol flow? is it taking in too much fuel due to a bad air filter? or is it the reverse due to a bad fuel filter?

Maybe thats why is there no such sweet spot in cng?

HELP.........
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Old 17th September 2013, 16:12   #7
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Default re: Engine choking in petrol mode, works fine in CNG mode

Mod Note : There are several spelling & grammatical errors in your posts. This negatively affects the forum experience for other readers.

Kindly ensure that you proof-read your posts prior to submission. Also, it would be a good idea to use spell-checkers.

Last edited by GTO : 17th September 2013 at 17:42.
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Old 18th September 2013, 11:20   #8
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Default Re: Engine choking in petrol mode, works fine in CNG mode

get your petrol fuel injectors cleaned from a proper shop using a spray simulator machine. injectors current also can be checked, to make sure all the injectors are getting the same current/power.

when running on cng for a while, the petrol injectors are disable in cng mode (no power/current is sent to the injectors). they tend to get clogged, due to the lack of usage.

UM
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Old 18th September 2013, 14:12   #9
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Default Re: Engine choking in petrol mode, works fine in CNG mode

Its more of a fuel injector issue than anything else. The worst hit in a fiat car when using either the cng or lpg is the injectors. I had atleast four injector failures on my palio before I finally got rid of the LPG. Now for the past four years it rides on petrol and there has never once been a problem.
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Old 18th September 2013, 21:38   #10
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Default Re: Engine choking in petrol mode, works fine in CNG mode

Lolz! just clean the fuel lines and change petrol filter.

If not most likely your fuel pump is faulty.

PS: there is also a solenoid actuator that switched between petrol and CNG.

NO ECU or injector issue whatsoever! take my word for it.
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Old 27th September 2013, 20:24   #11
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Default Re: Engine choking in petrol mode, works fine in CNG mode

Any updates on this.?
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Old 26th August 2016, 09:50   #12
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Default Re: Engine choking in petrol mode, works fine in CNG mode

I have Sx4 and have been driving in CNG mode for about 6 yrs. No problem till about 7-8 months. I have this problem of smooth running while in CNG mode, but chokes when I shift it to petrol mode. I checked the injectors. All are working fine except one which it seems is not working at optimal level. Will replace this injector with another one (used) to see the difference.
Welcome any feedback on this. Will keep you posted about the difference or no difference after fitting my car with another injector (used). The next step could be to check the fuel pump.
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Old 28th August 2016, 13:04   #13
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Default Re: Engine choking in petrol mode, works fine in CNG mode

As said earlier, I have changed one of my injectors which was not functioning to its optimal level and look at the change. My car engine has become like new. No chokes and misfires. The only problem is the cost. A new fuel injector of Sx4 cost Rs 7,500/-. I decided to buy one used fuel injector from Mayapuri (Delhi) junk market with a guarantee from the seller. I paid Rs 2,200/- only for it. Lets see for how long it runs.
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