[TheMileCruncher]

To give a little background, in late 2016, my brothers and I were on my way back from Nashik to Mumbai after performing last rites for my deceased grandmother.

The car which usually runs on CNG had to be run on Petrol on our way back due to non-availability of CNG in Nashik. The car had done roughly around 30k kms on the odometer in the past 1.2 years of ownership.

Initially, the car ran just fine but as soon as I was about 100 kms away from Mumbai, I hit a huge pothole at around 50 kmph followed by a loud thud. My heart sank, I got down, inspected the belly of the car and could not find any traces of a hit to the oil sump.

Thanking god for saving me a heartache, I started the car and was on my way. As usual in Mumbai, I encountered a huge traffic jam as I was approaching Bhiwandi and out of nowhere, the engine RPM started dancing up and down between 500 - 900 rpm on its own at idle engine speeds. With the A/C on, I found it almost impossible to even move the car.

I turned off the A/C and the condition improved a bit but I still had inconsistent RPM on the console.

I somehow managed to reach Mumbai and stopped for a CNG refill. As soon as the car switched to CNG, the problem vanished and the car ran as it normally would.

Later that week, I dropped the car for service at Vitesse Prabhadevi (MASS) where I described the problem to the service manager and he added the same to the checklist.

When I went to pick up the car, I was advised that there was no problem with the car after re-calibrating the state of the engine tune-up.

The car worked fine for a few days but on occasions when I drove around on Petrol, I would encounter a similar issue intermittently but on a less severe scale. I got this checked again and MASS failed to find any issues.

Tired of my rounds to the service station, I avoided running the car on Petrol in traffic conditions unless there was no alternative.

Fast forward to January 2018, along with 4 friends, I went on a road trip from Mumbai to Kashmir in my Celerio and the car drove brilliantly for about 1200 kms and as soon as I reached Jaipur city entry toll gate (the cengine was running almost non-stop for the past 24 hours), the check engine light came up.

Me being myself, even though everyone was tired and were looking forward at our first halt in Delhi just about 200 kms ahead, I googled up the nearest service center and ended up at KTL Maruti (MASS) in Jaipur within 30 minutes.

It was around 5pm and I was hopeless about getting any help since it was almost closing time for them. The service manager listened to the problem and after finding out that we've been driving all the way from Mumbai, he agreed to get the car looked into and off went the car inside the workshop.

The car was hooked up to Tech 2 and 2 error codes came up. One for the knock sensor and the other was P0420.

It turned out that the knock sensor wire had broken off so that was fixed. For the second error code, I was told that the CAT CON has failed and will need to be replaced which will take days since the part was unavailable at that time.

After being assured that this should not hamper the engine or the drive in any way, we set-off for Delhi.

Just as what happened back in 2016, we encountered a 2 hour long truck jam near Delhi and the engine RPM started dancing again leaving the engine non-responsive to throttle inputs even with the A/C turned off.

Somehow we got the car off road, opened and cleaned up the air filter (I could not think of anything else as my heart had already sanked).

After a brief break of 5 minutes, we started the engine and the issue pursued but there was response to the throttle input this time.

Battling the traffic, we reached our first stop at Kailash in Delhi at 1am.

The next day, I looked up another MASS (Service Masters Pvt. Ltd.), dropped the car for a check-up and went on for our sightseeing tour.

This was one of my bad experiences with a MASS and after 30+ hours of the car being in the service center, all they did was tell me the same thing that the CAT CON needs to be replaced but the car will run fine even with in this condition.

Since I was on a tight schedule, I took delivery of the car and went on towards Amritsar. Thankfully, the car did not breakdown nor give much trouble for the next 4000 kms except for minor inconsistent idle RPM behavior.

The car is now back in Mumbai where the same issue was diagnosed and a replacement order under warranty has been placed.

For the time being, I'm running the car in CNG with no issues at all. The RPM is stable at all times even with the A/C on.

I'm not sure if the CAT CON has got anything to do with the RPM issue on petrol but I may be incorrect.

Has anyone encountered any such issues?

[greendream]

Was the throttle body cleaning done as per schedule.
This is one of the the culprits for erratic rpm.

[QuickLink]

Originally Posted by TheMileCruncher

The car which usually runs on CNG had to be run on Petrol on our way back due to non-availability of CNG in Nashik.

Are you sure about the quality of the petrol filled in the car? I know it's a wild guess but you should try changing the fuel filter.

[S.Marine]

I had a similar experience in my Wagon R CNG. The performance on petrol was jerky and unpredictable and the car would stall suddenly. The issue was a faulty throttle position sensor. In my opinion, ask a trusted MASS to run the engine diagnostics (preferably in your presence).

[Shreyans_Jain]

My old CNG WagonR used to exhibit similar behavior - it would run fine on gas but idle rpm would become inconsistent and throttle response dull on switching to petrol. However, since this was at the very end of the life of the car after having clocked 2.75L km, I didn't bother too much into it. Basic stuff like cleaning air filter, injectors and throttle body did not provide a solution though.

[Gordon]

Have you driven until the gas is completely run out? If yes, this is a common symptom.

[TheMileCruncher]

Originally Posted by greendream

Was the throttle body cleaning done as per schedule.
This is one of the the culprits for erratic rpm.

Yes, the throttle body was cleaned the first time I reported this issue. It helped for sometime but a few days later, the same issue reappeared.

Originally Posted by QuickLink

Are you sure about the quality of the petrol filled in the car? I know it's a wild guess but you should try changing the fuel filter.

Not sure about the fuel quality, but I mostly fuel up with fuels with additives like Power, Speed, etc. As for the fuel filter, I'm yet to try that. I'm waiting to see how the car performs with the new CAT CON which will be fitted in a few days.

Originally Posted by S.Marine

I had a similar experience in my Wagon R CNG. The performance on petrol was jerky and unpredictable and the car would stall suddenly. The issue was a faulty throttle position sensor. In my opinion, ask a trusted MASS to run the engine diagnostics (preferably in your presence).

Thanks for sharing this. I'll ask the MASS guys to check this when I leave the car for the CAT CON replacement in a few days. Engine diagnostics was run multiple times before but nothing came up except for the CAT error the recent couple of times.

Originally Posted by Shreyans_Jain

My old CNG WagonR used to exhibit similar behavior - it would run fine on gas but idle rpm would become inconsistent and throttle response dull on switching to petrol. However, since this was at the very end of the life of the car after having clocked 2.75L km, I didn't bother too much into it. Basic stuff like cleaning air filter, injectors and throttle body did not provide a solution though.

Yes, all of the above has been tried but nothing has helped me either so far.

Originally Posted by Gordon

Have you driven until the gas is completely run out? If yes, this is a common symptom.

Not sure how this should cause issues, but yes, I usually drive until the gas tank is empty. The car works fine when the gas runs out. It's only when the car is being driven in traffic snarls on Petrol, the erratic RPM issue crops up. Could you share your understanding on why running until the gas is empty might cause this?

[Jaggu]

Wait for the catcon replacement and if it works just let it be, modern day ECU being fed with wrong inputs from sensors can cause all sorts of issues, including such erratic idle RPM. As far as refuelling habits are concerned, do refill when petrol tank is at 1/4. CNG (am not sure) but even this would have condensation happening and you ideally don't want the water content collected being sucked at low fuel levels. But do check with CNG experts as to what is recommended, like I said I am not very sure.

[S.Marine]

Originally Posted by TheMileCruncher

Not sure how this should cause issues, but yes, I usually drive until the gas tank is empty. The car works fine when the gas runs out. It's only when the car is being driven in traffic snarls on Petrol, the erratic RPM issue crops up. Could you share your understanding on why running until the gas is empty might cause this?

One reason I've heard is that the oil from the worn out piston rings of poorly maintainted CNG compressors at some fuel stations tends to accumulate in your tank. This can get collected at the bottom of the tank and thus, it' essential to not drain the tank completely. Just sharing what i've heard. I've been emptying my tank on regular occassions and havent come across the problem of choked filter. Just ensure the 20,000 km routine of CNG filter renewal is carried out during servicing.

[TheMileCruncher]

Originally Posted by Jaggu

Wait for the catcon replacement and if it works just let it be, modern day ECU being fed with wrong inputs from sensors can cause all sorts of issues, including such erratic idle RPM. As far as refuelling habits are concerned, do refill when petrol tank is at 1/4. CNG (am not sure) but even this would have condensation happening and you ideally don't want the water content collected being sucked at low fuel levels. But do check with CNG experts as to what is recommended, like I said I am not very sure.

Thanks! Usually when within the city, I drive only on CNG (except when I run out of gas). When driving on highways, I refill as soon as the fuel gauge gets down to the last quarter tank. I'll wait for the replacement and see if that fixes the issue.

Originally Posted by S.Marine

One reason I've heard is that the oil from the worn out piston rings of poorly maintainted CNG compressors at some fuel stations tends to accumulate in your tank. This can get collected at the bottom of the tank and thus, it' essential to not drain the tank completely. Just sharing what i've heard. I've been emptying my tank on regular occassions and havent come across the problem of choked filter. Just ensure the 20,000 km routine of CNG filter renewal is carried out during servicing.

Yes, I think I've read about it on other threads here. I'll try to ensure to follow this advise. Thanks for the clarification. As for the CNG filter, I follow the service guide religiously and have replaced the rings and filters on time.

[Gordon]

Okay basically you should never drive until you run out of fuel (petrol, diesel or CNG).

We have had three CNG cars (externally fitted) prior to my Celerio, and faced this issue whenever the gas ran out. The only resolution was to visit the CNG service station where they had to reset a setting in the air intake manifold. Don't ask me what kind of settings exactly.

Well it's my suggestion, you can take a look. Make sure you never run out of gas or petrol. Refill before the last bar.

With this knowledge, I've ensured that I never ran out of fuel in my previous cars (Baheno/Altura/Swift) and never faced the issue again.

I've ensured this on the Celerio and not facing any concerns as of date.

[TheMileCruncher]

Long queues, shortage of pumps have made me a little lazy when it comes to refilling CNG. But point well noted, if the little extra effort saves me all this trouble, I'm going to ensure that I follow this advise. Hopefully with the new CAT CON and a new habit, I won't have to face this issue again. Thanks for the insight.

[SPIKE ARRESTOR]

Originally Posted by TheMileCruncher

The car was hooked up to Tech 2 and 2 error codes came up. One for the knock sensor and the other was P0420.

It turned out that the knock sensor wire had broken off so that was fixed. For the second error code, I was told that the CAT CON has failed and will need to be replaced which will take days since the part was unavailable at that time.

This rings some bells. What is the life recommended for your car's Cat Con? If it is higher than 55K, then I would say it will fail again at a premature age. I strongly believe that your cat failed because of this abnormal behavior.

Improper combustion -> resulting in damage to the converter. A Cat con not working properly throws up P0420.

Was it knock sensor or an O2 sensor?

Spike

[TheMileCruncher]

Originally Posted by SPIKE ARRESTOR

This rings some bells. What is the life recommended for your car's Cat Con? If it is higher than 55K, then I would say it will fail again at a premature age. I strongly believe that your cat failed because of this abnormal behavior.

Improper combustion -> resulting in damage to the converter. A Cat con not working properly throws up P0420.

Was it knock sensor or an O2 sensor?

Spike

As per the service manual, the CAT CON should ideally last the lifetime of the vehicle itself.

The error code (apart from P0420) was for the knock sensor. It turned out that the knock sensor connector cable had broken off and had to be reconnected by making a joint.

Surprisingly though, this same wire had broken off a few months ago but the Check Engine light did not come up then. I just happened to notice the broken wire dangling while I was cleaning the engine bay and I had it fixed from the service station.

Also, a day before leaving for my road trip, I had noticed knocking in the engine while in the 2nd and the 3rd gear at low RPM's. I called up my service manager at Maruti and he said that it might be due to dirt in the fuel lines and said that it's not something to worry about for now since it was too late to get the injectors cleaned.

To be on the safer side, I started adding System G (fuel additive) while tanking up everytime on my trip and the knocking seems to have disappeared.

[OSH]

Look what I found written in my '07 Honda City's manual. Running the car on extremely low fuel or completely dry does indeed damage the CAT-CON. Read the notice in the picture.