Team-BHP - Fiesta engine jerks and rpm varies while idling

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Originally Posted by Jeroen (Post 4141117)

But if you filled up more then say half a tank, I would have thought you would see fuel related problems soon after starting up.

Jeroen

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Originally Posted by vnabhi (Post 4141331)

How much petrol was in the car when you topped up? I suspect fuel adulteration in this case. Try to drain out the fuel and check again.

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Originally Posted by mayankk (Post 4141351)

Teh costs quoted by the ASC are horribly inflated. Get it done outside, it'll cost about a third as much. Apart from the mount, its not even a labor intensive job.

The car was having fuel of around 1/4th of the tank when I topped up it for another 1500 rs and drove for ~200 kms to Bangalore without any issues. For the next 2 days it was just parked at my home. On that day after issue propped up I filled full tank at ORR shell petrol bunk, thinking it was low on fuel. Now draining full tank would be a difficult task.

Today I went to ASC and met the SA. He took me to the mechanic who checked the car. The mechanic said they hooked up the car with their computer and based on measurements issue suspected in coil and HT cable. Asked him how both can go faulty at the same time, his explanation was actual culprit is coil but HT cable also very weak, so they are recommending to change it.

I asked him to change the cable and check. He immediately removed cables from a nearby Figo :Frustrati and swapped it with the one's in my car. We went for a test drive and still same issue. So I told them to change only the coil (Which will take 3~4 days due to spare unavailability) and if the issue is not resolved I am not going to pay for it.

Coil will cost around 6000 rs at ASC, I was thinking of taking the car with me but without paying some advance amount for the coil spare they would not release my car :deadhorse

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Originally Posted by sbalagru6 (Post 4141557)

Hi,

I think its a problem with the fuel only, have the same car and what i have noticed is when i fill it with Shell petrol the car jerks a lot, but once i shift back to normal fuel the jerkiness disappears. I understand from your update that you had put Shell petrol only after the jerking issue, but maybe it is a case of fuel adulteration exacerbated by the Shell petrol.

Just my 2c.

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Originally Posted by sbalagru6 (Post 4141557)

As I suspected earlier, I doubt very much it is the fuel. When you refill a tank (and you had only 1/4 left) the old and new fuel will mix very quickly. So any major problems with the fuel will show up pretty quickly and not after more than 200km’s. The exception could be dirt contamination as this could start clogging the filter. Hence my other suggestion to check fuel pressure.

But it looks like it’s the coil. I think you did the right thing by challenging the need to replace the HT cable. If the mechanic could show you why he thought the cable was weak that would be great. He should have checked visually and measured. In essence, in it looks good, with no visible wear and tear and or cracks then it should be measured to check compliance with specifications.

Somehow I doubt your mechanic did either on the cable. So lets see what the new coil brings!

Good luck

Jeroen

This may be complete OT, But I had faced this issue in My 2010 SXI petrol.

We had failing fuel pump relay, causing similar issue. Car would run properly, suddenly develops jerks and then corrects itself. Sometimes it even shutdown, doesn't switches on for long duration.

A good FNG did step by step diagnosis, concluded the relay is problem. He replaced relay, no issue since then.

Fuel pump relay is behind glove box. You need to remove glove box, you can access fuse and relay board.

Sharing my experience

I had a similar problem with my esteem and MASS as usual gave 10k plus estimate .
A road side mechanic did the diagnosis and changed one injector socket to solve the problem @rs150

His diagnosis and logic below,

One reason for jerk or erratic idling is 1 or more cylinder not firing to its full power.

Ex, a weak connection at one spark plug will reduce the strength of the spark for that cylinder causing it produce less power compared to others. engine vibrates when there is unequal power distribution

2. a weak connection at the fuel injector can also affect performance of one cylinder.

Diagnosis
At Idling, disconnect one injector cable at a time by removing the socket and observe how the engine behaves.
- If the engine hunts or tends to stop then that cylinder is working fine.
- if there is no change in the way engine idles then that cylinder has the problem

check all electrical connections related to that cylinder like injector cable, socket, spark plug cable, spark plug, distributor points..etc.

Connect back the socket and move on to the next cylinder.

Well my Fiesta seems bitten by the bug big time. I had this problem for sometime now but in a mild way.

Had gotten the timing belt changed at a FNG and had told them to have a look if possible big big mistake :( whatever they checked seemed to worsen the problem. Even after checking everything they were not able to revert it to at least the milder version of the issue.

Had a travel to chennai this weekend and couple of times in the city the car switched off due to RPM falling very low, was scary to lose steering and braking assist.

Left the car at a neighbourhood garage in chennai to check and they tried cleaning the plugs and then changed it but no solution, changed the ignition coil the usual suspect, again no change. They suggested it could be probably the injectors or something else (bottom line they don't know).

As I have to travel to bangalore for sure today asked them to put in the old ignition coil (as there was no change), now after being penny wise and pound foolish (had to pay 2000 for the new plugs and labor) planning to show the car to Metro Ford in Bangalore to diagnose the issue.

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Originally Posted by anandtheleo (Post 4195221)

They suggested it could be probably the injectors or something else (bottom line they don't know).

As I have to travel to bangalore for sure today asked them to put in the old ignition coil (as there was no change), now after being penny wise and pound foolish (had to pay 2000 for the new plugs and labor) planning to show the car to Metro Ford in Bangalore to diagnose the issue.

Get the fuel system checked; starting with the fuel pump for correct operation, fuel lines & fuel filter for choke points. You may have to replace the filter. After these parts check out OK, then proceed to the injectors.

I don't know how old your car is but if you have been filling fuel at random highway fuel pumps have the fuel tank checked for sediment.

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Originally Posted by R2D2 (Post 4195243)

So I traveled to Bangalore without any incident, the RPM fluctuation still persists and the ECU is struggling to keep the engine alive when the RPM drops below 500-600, the engine violently jerks at this time. When the foot is taken off the accelerator, the RPM drops to the normal idle RPM of 1000-1200 and as the car slows down, the RPM suddenly falls below this range and ECU compensates violently, sometimes rarely it cannot cope and the engine shuts down (with PS and brake assist gone :eek:)

The car is a 2007 manufactured one, i do stick to known bunks in the city and highway which have good repute here and with friends.

Just a doubt on your point on fuel system check, the problem manifests only at RPM below 1250, once it crosses this RPM everything is normal and revs till the red line normally and there is no hesitation at all and it drives smooth with no indication of any problem at all. If there is a problem with the fuel system shouldn't the problem manifest throughout the rev band?

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Originally Posted by anandtheleo (Post 4195524)

Get the throttle body cleaned and inspect the air filter. If it is time to replace it, please do that.

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Originally Posted by mayankk (Post 4195526)

Have you had the throttle body cleaned yet? Sounds like it is sticking.

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Originally Posted by ankurbhageria (Post 4195544)

Get the throttle body cleaned and inspect the air filter. If it is time to replace it, please do that.

Yes, the throttle body was cleaned and did not change the behavior/problem (sorry for not mentioning this point in my above replies).

The air filter was changed in the last service in January, but i will check its condition and replace if necessary.

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Originally Posted by anandtheleo (Post 4195524)

Just a doubt on your point on fuel system check, the problem manifests only at RPM below 1250, once it crosses this RPM everything is normal and revs till the red line normally and there is no hesitation at all and it drives smooth with no indication of any problem at all. If there is a problem with the fuel system shouldn't the problem manifest throughout the rev band?

Normally I'd have suspected the ignition system which you say has checked out ok. So let's leave that aside for now.

That leaves the fuel delivery system. At which point during the journey from the tank to the engine is the fuel flow being affected is frankly difficult to ascertain. Trouble shooting normally involves starting at the source and tracing the flow/route through different components to the destination. The same theory holds in electrical/electronics repair :)

As Jeroen has advised have someone attach an OBD scanner for the precise diagnosis without having to do much guesswork. Hopefully the OBD will pinpoint the fault.