RPM fluctuation when the engine warms up

Tiny
AMINSAMEIPOOR
  • MEMBER
  • 2014 HYUNDAI ACCENT
  • 1.6L
  • 4 CYL
  • 2WD
  • AUTOMATIC
  • 17,500 MILES
I have had rpm fluctuation for six months when the engine warms up for an hour and especially when I turns on lights or fan. I have done many thing like :
1- cleaning throttle body and TPS sensor
2- replace the alternator
3- cleaning MAP sensor
4-cleaning OCV or CVVT valve
5-Upgrading the ECU software

I am completely disappointed now.
Thank you for your help in advance.
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Wednesday, August 17th, 2016 AT 12:30 AM

16 Replies

Tiny
CARADIODOC
  • EXPERT
When does this occur? When idling in "park"? While driving at highway speed?
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Wednesday, August 17th, 2016 AT 1:15 AM
Tiny
AMINSAMEIPOOR
  • MEMBER
When engine at Park or Neutral.
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Wednesday, August 17th, 2016 AT 1:49 AM
Tiny
AMINSAMEIPOOR
  • MEMBER
Engine at N or P this issue appears.
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Wednesday, August 17th, 2016 AT 4:12 AM
Tiny
CARADIODOC
  • EXPERT
The first thing is to look for vacuum leaks. Pinch off various vacuum hoses while the engine is running and the problem is occurring. If you pinch the hose leading to a leak, the idle speed should go down and stabilize. You can also pour water over the top of the engine while it's still cold to see if it gets sucked in anywhere. As a last resort, your mechanic can use a smoke machine to inject a white, non-toxic smoke into the intake manifold, then watch to see if it sneaks out somewhere.

The next step is to use a scanner to view live data to see what the Engine Computer is seeing and responding to. It will show the desired engine speed and the actual engine speed. If the desired speed is fluctuating a lot, that is in response to a sensor reading or operating condition. Those will show up in the list of sensor data. If the desired speed is holding steady but actual speed is not, it's almost always a mechanical problem such as that vacuum leak that the computer doesn't monitor that is causing the unstable speed.
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Thursday, August 18th, 2016 AT 3:19 PM
Tiny
AMINSAMEIPOOR
  • MEMBER
Tnx.I checked the desired speed which was not stable. Someone told me it is because nock sensor problem. What is your idea?
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Thursday, August 18th, 2016 AT 9:29 PM
Tiny
AMINSAMEIPOOR
  • MEMBER
I should add additional information that " if I stay in idla state for about 5 min this fluctuation will be disappeared.
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Thursday, August 18th, 2016 AT 10:32 PM
Tiny
CARADIODOC
  • EXPERT
If the knock sensor is detecting what it thinks is spark knock, the Engine Computer will retard ignition timing in an attempt to stop it. Varying timing will cause idle speed to vary. I think I'd try a higher octane fuel first to see if that helps.

Loose and sloppy timing chains can slap against the cover and be detected by the knock sensor too. Your engine does use a chain, and not a timing belt, but the mileage you listed is much too low for this to be a valid suspect.
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Friday, August 19th, 2016 AT 12:24 AM
Tiny
AMINSAMEIPOOR
  • MEMBER
I found out a strange thing today. When I poured a Bottle of water on the throttle valve as engine is hot the fluctuation was smoothed. What's your idea about it?
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Saturday, August 20th, 2016 AT 9:51 AM
Tiny
CARADIODOC
  • EXPERT
Normally I would say there is wear in the bushings the throttle blade rotates on and the throttle body needs to be rebuilt, but there's two problems with that. First, this shows up due to very high mileage, as in over 300,000 miles. Yours does not have that much wear yet. The second problem is your car most likely uses the "throttle-by-wire" system that put Toyota in the news a few years ago. I've never autopsied one of those assemblies to see what's in them, but I do know they use a really strong motor to turn the throttle blade. I would start by looking for something that could have occurred on the assembly line, like a mis-positioned gasket, a cracked housing, loose mounting bolts, and things like that.

I know this can be hard to tell, but if it looks like there's something that could be a common or recurring problem on many cars, there may be a recall or service bulletin. Dealers will take care of recalls at no charge to you. Those are issued for safety and emissions concerns, and some manufacturers issue them for customer satisfaction concerns.

Service bulletins are issued to help mechanics find the causes of elusive, but common problems. They are only meant to save time. You have to pay for the repairs if the car is out-of-warranty. The bulletin will specify part numbers and repair procedures. I would leave anything related to throttle-by-wire systems to the dealer. Most independent mechanics don't want to work on them because anyone who touches it could become party to any future lawsuit, and it's only a matter of time before another malfunctioning computer sends a driver on a wild ride. That will probably change in the future when the independent shops learn more about these systems. There have been many instances where mechanics were baffled by something new, like Chrysler's "alternator", "Lean Burn" Engine Computer, anti-lock brakes, and air bags, and GM's "HEI" ignition systems, so they refused to work on them. Now those are very common and simple systems. This is the first one though where making a mistake can kill someone, hence the reluctance to work on them. This is also why I don't recommend you work on it unless you see something obvious and simple.
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Saturday, August 20th, 2016 AT 11:33 AM
Tiny
AMINSAMEIPOOR
  • MEMBER
Dear Sir
thanks for your previous comments.
My home town is placed at 1550 meter altitude. Recently I went to a place near the beach side at 0 meter altitude. So there was not any fluctuation. And engine was working smoothly at idle.
Then I went to a hyundai service center. They checked all parts of engine and there was not any error. So at the end technician increased the idle speed a little with G-scan device in CO emission control mode. But after driving I found out that the fuel consumption increased around 40 percent especially at traffic. While it has not changed at highways.
I am comppletely disappointed please guide me anything that you could.
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Monday, September 12th, 2016 AT 9:38 PM
Tiny
CARADIODOC
  • EXPERT
I'd be pursuing the throttle body. It's apparent that water blocked something and stabilized the idle speed. There may be a vacuum hose or port in there, or something else that is allowing vacuum to leak.
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Monday, September 12th, 2016 AT 10:13 PM
Tiny
AMINSAMEIPOOR
  • MEMBER
3 month ago I changed the tire size to bigger one. Is it a reason for overheating the engine?
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Tuesday, September 13th, 2016 AT 3:04 AM
Tiny
CARADIODOC
  • EXPERT
No. The Engine Computer doesn't know or care about tire side, but if you changed the outer circumference, you changed an alignment angle called "scrub radius" that is important for proper handling and braking. Lawyers and insurance investigators know all about scrub radius and will use it against you if you're involved in a crash caused by their client. They will say you were partly at fault because you were less able to avoid the crash, and they will be right.

Besides the legal ramifications, you also need to have the new tire size programmed into the Engine or Transmission Computer so the speedometer will read the correct speed, and if the vehicle has anti-lock brakes, all four tires must have the same circumference.
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Tuesday, September 13th, 2016 AT 10:37 PM
Tiny
AMINSAMEIPOOR
  • MEMBER
Finally I decided to replace throttle body completely. Because I checked all the things in my car. All the sensors act proper and send correct signals. I think electric throttle control valve has problem.
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Monday, September 26th, 2016 AT 10:42 PM
Tiny
CARADIODOC
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Dandy. I'll reserve my thumbs-up for a few days to be sure the problem is solved, but I have high hopes.
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Tuesday, September 27th, 2016 AT 4:13 PM
Tiny
AMINSAMEIPOOR
  • MEMBER
Not yet. Eventually I decided to replace the throttle body. I think the problem is related to electronic throttle valve.
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Wednesday, September 28th, 2016 AT 3:30 AM

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