Engine revving by itself

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Padraig 04
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Engine revving by itself

Post by Padraig 04 »

Hi all,
Been over a year and I still havent done anything to my car :crybaby: but when it stops snowing here I'm going to try start on her (YouTube made me think project cars r cheap :crybaby: oh how wrong I was) but I need some advice. Now I'm only 16 but in saying that Id consider my knowledge of cars and engines to be pretty good, no mechanic but at the same time I could probably do the basics and a bit more. The thing is though I don't have a eeefin clue what the hell is wrong with my car :laughing . The issue is when I start the car even a cold start, it will immediately start idling at 2k RPM (which sounds like s**t cause my manifold is cracked) and then it will after about 20 seconds it will start revving to 3k and drop to about 1500RPM and rev to 3k again and will keep doing that until you turn off the car. Sometimes after warming up it will barely idle about 800RPM and sound like its struggling to idle. If I unplug the Air flow meter (the one on the throttle body) The car will just idle at 2k instead of jumping up and down.
I have read its just the sensor but Also read about these screws that can adjust idle but the screws really shouldn't be touched unless u know what your doing.

I've also read that the cooling system could cause this and I am pretty sure my radiator is leaking but if it was that it would still idle weird even if I unplug the AFM right?

Since I on quite a tight budget and I mean tight, I dont wanna go spend money on sensors and stuff until I have a decent idea that they may cause that.

If you can help me figure out whats wrong with my poor MRpoo, I'd appreciate it sooooooo much. :D
Thank you lads (and ladies if their is any on this site :laughing )

Edit : changed the title after reading the big red box saying putting help in your title is bad :laughing
Last edited by Padraig 04 on 10/02/21 19:46, edited 1 time in total.


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jimi
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Re: HELP!! Engine problems

Post by jimi »

MK1. MK2 or MK3 ?
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Re: HELP!! Engine problems

Post by Padraig 04 »

MK2 rev 3 or 1994 and Its a uk spec GT T-BAR. Hope that helps :)

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Re: Engine revving by itself

Post by Bru_lynn »

I have got a similar problem in that cold start revs are way too high. However, when I first got the car the revs were as you describe, I then found out that if I turned the air con off at least the revs were constant. It rev'd higher when the air con compressor cut in, dropping back when it cut out.
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Re: Engine revving by itself

Post by jimi »

That's down to the aircon idle up solenoid, when the compressor runs it activates a small solenoid which adds extra air (after the throttle butterfly) this raises the idle speed by approx 200 rpm (it's adjustable) to compensate for the increased load on the engine at idle caused by the compressor. The bouncing idle described by the OP is something different, on a MK1 its commonly caused by a vacuum leak on then inlet or air in the cooling system, I'm not familiar enough with the MK2 to say if the same or similar faults apply.
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Re: Engine revving by itself

Post by Stevie2502 »

I once had a similar problem almost 20 year ago. I had a 1992 t-bar turbo at the time and the revs decided themselves up and down, wasn’t all the time tho. I took it to a specialist who pin pointed it to a temperature sensor. Think it cost a couple of quid but the hours trying to find it cost me a bit. I was lucky that was all that was wrong with mine.

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Re: Engine revving by itself

Post by Padraig 04 »

I’ve found a video where it is exactly the same thing that is happening me and what he did was replace the throttle position sensor and the air flow meter. He also replaced the intake gasket and it was grand then, but his car was a Hyundai Elantra which is a bit different than a mr2 :laughing: . Do you think that replacing these sensor may help? Also the gasket for the intake manifold is only 13 euro from MR2BEN so I’m thinking of just doing that anyway.
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Re: Engine revving by itself

Post by chrisE »

If you have a large enough vacuum leak, it will cause the revs to go up and down constantly (at least on my rev 3 turbo). Too much air entering the intake.
I didn't do it.

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Re: Engine revving by itself

Post by Padraig 04 »

So after waiting way too long for a new TPS sensor, I put it on and it has stopped bouncing up and down but its still revving at 3000 and turning the TPS doesn't change it so what else could cause the idle problem now? and if anyone is saying to reset the ecu my car battery has been dead for 2 months and has been unconnected so i'm pretty sure its reset by now :laughing:

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Re: Engine revving by itself

Post by Padraig 04 »

so I let the car run until it was at temp and the revs went down to 1300 and for now I'm happy enough with that. I also noticed that my radiator is leaking and I've seen somewhere before that the cooling system can affect the idle but I'm unsure on that.

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Re: Engine revving by itself

Post by MRHAPPY62 »

Air in the system will cause hunting .
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Re: Engine revving by itself

Post by Padraig 04 »

So when I get the new radiator and bleed the system theoretically it should idle normally? If anyone has tried the aluminum radiator from MR2Ben could you let me know if its any good? Just wondering why it costs the same as the normal unit.
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Re: Engine revving by itself

Post by MR2DI4 »

I would say the AFM is suspect, pretty easy to check it with a decent multimeter.

Your looking for smooth changes in resistance and the same resistance at the idle point.

Any sudden big changes in resistance or even an open circuit and its stuffed. Very common for problems to show themselves at idle.

Oooops sorry just read its a 1994 so yours doesn't have the old AFM, still worth checking this area out and that everything is clean. Even if the air filter is regularly replaced, eventually you can still have problems.

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Padraig 04
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Re: Engine revving by itself

Post by Padraig 04 »

Aye I've already replaced the air filter a few months ago, you see I'm only 16 so I don't necessarily drive it all that much 😂 it's just I run it for about a hour ever week just to charge my battery, it doesn't necessarily annoy me I'm just conscious of the neighborhood getting annoyed that their is car that's revving really loud, they're already fed up of all the is200s :laughing
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