Erratic and fast idling Lantra 1.6

Here are the symptoms: When the clutch is depressed, revs surge to 1500 rpm and then drop back to around 1000 to 1200 rpm. With the engine hot and idling the revs vary between 1000 and 1200
rpm over a 3 second period i.e. in 3 secs, revs go from 1000 to 1200 then in the next 3 seconds they fall back to 1000 again. I have checked the following :- 1) there is slack in the throttle cable 2) The throttle - stop screw just touches the plate; if I turn it out any further then it is the throttle flap that stops the throttle mechanism rather than this screw. According to my information, this is now adjusted as it should be. 3) Resistance measured on the TPS (throttle position sensor), 990 ohms to 3000 ohms in smooth progression. The manual says the range should be 700 ohms to 3000 ohms but I dont know if this slight difference is the problem. I'd have thought the only problem you'd get with a TPS is it not making contact at all due to wear; hard to see how resistance at the low end could increase unless its not going back far enough. I do not have an idle switch on this car.
Has anyone had these symptoms and found out what the answer is?
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1000 rpm is still too fast when warm. I'd suspect either a defective TPS (too high a reading when throttle closed) or excessive air entering the engine.
One of the failure modes of the TPS is that the contacts inside become dirty, causing excessive resistance. 1k Ohms vs. 700 Ohms is an error of about 40%, so I'd primarily suspect the TPS. Attach a voltmeter between the signal wire and ground. Then grab the connector and push it back and forth. If there's any significant change in TPS reading, replace the TPS.
I don't recall the specific reading to be expected from the TPS at closed throttle. On the Mitsu engines, it was to be set at about 500mV, but your TPS is nonadjustable. Unfortunately, it's probably necessary to have a scan tool that will read percentage of throttle opening to diagnose this properly.
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Thanks Hyundaitech. I will spend some more time on the TPS; I guess if the total resistance is 3K and I'm getting nearly 1K at idle then the voltage is going to be over a volt. Difficult to measure though with the connector on as you can't then get to the contacts but maybe I can backprobe or use thin wire. Its now idling at around 1200 rpm and fluctuating slightly since I set the throttle-stop to actually make contact with the throttle plate so something's not right !
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Let's pause for a minute. You adjusted the throttle stop screw? I thought you checked it and the throttle was closing fully (against the screw).
Put the throttle stop screw back where it was and then adjust your throttle cables so they allow the throttle plate to fully close. The reason that you're is high and fluctuating is probably that the throttle is held too far open (by the stop screw at this point).
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Hi, At first, the throttle was not contacting with the throttle stop screw and the throttle would close and stick a little in the bore. Then I adjusted the throttle stop screw so that now the throttle just contacts with it. There is plenty of slack in the cable. The throttle cannot be held open very much as the throttle only just contacts the throttle stop screw. If I unwind the screw anymore then the throttle would not touch it and becomes 'sticky' in the bore as the throttle flap closes fully (but even then the revs are high and fluctuating). I will try adding a resistor between the signal wire and ground on the throttle position sensor and see if that lowers the revs. If so I probably need a new TPS. If not maybe its the idle actuator.
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No, don't put a resistor inline. First, clean your throttle body to make sure the carbon buildup isn't preventing the throttle plate from closing properly.
Figuring out why the throttle mechanism doesn't touch the stop screw is the first and primary order of business. The stop screw should never be adjusted once it has been set from the factory. If no one moved the screw prior to the throttle not hitting the screw, then there's a problem that's preventing the throttle from fully closing.
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I would imagine the previous owner unscrewed the throttle stop in an attempt to bring the idle revs down. Anyway I have it sorted now. After the unpromising resistance readings from the TPS, I managed to read off the voltage from the TPS. I had to use thin wires poked into the plug and then push the plug and wires back on as it was not possible to 'back probe' the connector. It read 0.48v at idle and over 4v at full throttle so everything fine there (must be something more than just a potentiometer inside as my resistance readings and potential divider theory would have indicated a voltage of 1.5v or so). I had read elsewhere that some people had fixed this problem by resetting the ECU (ECM or PCM; anyway the car's computer !). So rather than start ripping things apart, I tried this. I took out the ECM fuse (labelled Burg Alarm) and left it for 30 mins (I think less time would be OK). Put it back and hey presto, runs like a dream. No more revving above 2000 rpm on cold start (cold start revs now around 1200), a nice warm idle at around 800 and no fluctuations ! I wonder how much Hyundai would have charged to resolve that one?
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Congratulations. Nice when a plan comes together, isn't it?

Nothing. They do this kind of work for free... when you can get them to do it. The Hyundai dealers on the other hand, probably would have charged a couple bucks.
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Wow Mike, you must live in a utopian society. I can't think I'd get much change out of 60 ($120) minimum here in the UK if I visited the Hyundai dealership and booked the car in for them to investigate idling problems. Never known anything free or a couple of bucks; only big bucks ! Seriously labour here is 100 ($200) per hour and minimum charge is 30 mins and on top of that add VAT at 17.5%. Anyone else in the UK care to comment?
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Wow Mike, you must live in a utopian society. I can't think I'd get much change out of 60 ($120) minimum here in the UK if I visited the Hyundai dealership and booked the car in for them to investigate idling problems. Never known anything free or a couple of bucks; only big bucks ! Seriously labour here is 100 ($200) per hour and minimum charge is 30 mins and on top of that add VAT at 17.5%. Anyone else in the UK care to comment?
I was just trying to be funny Ed, but talk about Wow - you guys are paying way too much in labor. Clearly the Artful Dodger is alive and well in the UK, huh?
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