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Mk1 Zetec SE self revving


BogdanBogdan
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Hello, recently i decided to clean my intake with a towel and some engine cleaning spray and so i cleaned the throttle body, the throttle flap and the IACV. 

Next day i took the car on the road and after about some time,  after warm-up, it began progressively to rev itself at stops, while on neutral. These revs became so bad that at a stop the car would rev itself to 3k rpm, hold it for a couple of seconds, then drop back to an abnormal idle of 1300-1400 rpm. As i mentioned, this phenomenon intensified with time. 

Another thing that i've found is that i can "trigger" this happening by just lightly touching the gas pedal. If i just "pinch" the pedal, the car would gain in rpms to about 2k, hold for 2-3 seconds, then drop down. Since then the idling has always been higher than normal, hovering above 1k. 

What i've done so far was check every posible screw of the intake body for false-air as you say that could be vacuumed and i could've find anything loose or indicating there are vacuum gaps. I did an obd test to check if the throttle position sensor has gone bad and when pinching the pedal it doesn't gain ginomously in value, i did a fault check and no errors popped up. 

I tried to diagnose if it's the IACV or the throttle body that's causing this weird happening. What i've done initially was to disconnect the iacv sensor that controls the valve (so the valve would stay closed). After doing so, the car still has the same issue but at a lower scale. If i reconnect it, the phenomenon amplifies. I also started the car without the intake filter box to take a closer look at the throttle flap during the self-revving happening. I considered the fact that maybe, after cleaning the flap, there would be an amplified vacuumed air flowing at the outer edge of the flap and this whole thing would make the flap slightly tilt and letting more air in the engine but it doesn't look like it's happening and also the throttle position sensor (taking information from the position of the flap) doesn't increase in value significantly after pinching the gas pedal.

 

Have you encountered this ever before? Do you have any suggestion? Thanks so much!

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Hi, When you say you used "engine cleaning spray" what exactly did you use?

If you used anything that doesn't evaporate and not leave a residue then that could make it perform worse than before.

I would suggest getting a can of carb and choke cleaner and a soft clean paintbrush, make sure it has a straw.

Spray the cleaner over an area, agitate it with a brush, then spray off the dirt. Repeat both sides.

Using the straw, spray cleaner down any little holes to clear the airways.

If it still plays up, after you have re-cleaned, you need to reset the throttle body.

Disconnect the battery for 10 minutes, pressing the brake pedal a couple of times whilst disconnected.

Reconnect the battery, keep your feet off the pedals and turn the ignition on - DO NOT START THE ENGINE. Leave ignition on for one minute ensure Air Con is turned off.

Turn ignition off, key out, then start the engine WITHOUT PRESSING THE THROTTLE PEDAL.

Leave engine to idle for 5 minutes - DO NOT TOUCH THE THROTTLE

Turn on Air Conditioning and leave for another 5 minutes.

Turn off the engine, key out, then restart and it should be OK

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Hello, thanks a lot for your time put into response. What i've used is this https://product.amtra.co/ktm/20-A57

 

I will follow each step you suggested and will come with a feedback. I've read multiple times this reset done by disconnecting the battery (which should reset the PCM unit). I will follow the steps and hopefully it will solve.

Coming back soon!

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Okay so i did all the above in the exact same order and time amount and it didnt work out. I screenrecorded some important values i could find using an obd scanner. At first, i pinched the gas pedal, revs go really up and stay for a couple of seconds, idling is also high. Then i pinched it again, then i pressed it a little bit more and held it there for a second or two, you can see the rpms hold but the amount i pushed the throttle was like 5-10%, abnormal to rev up to 4k rpm.

 

I also tried to look onto some fuel pressure values, i only found a fuel flow sensor that didnt go anywhere high in all this testing. Unfortunately i couldn t find any reading for fuel pressure, i also heard it slightly backfire when coming back to "idle" after those 3k and above rpms, i thought maybe there s a problem with the injection, but again, fuel flow value stood on minimum.

 

I don't really know how to correlate at first these values, there s the vacuum, the manifold absolute pressure and intake air temp, as far as i heard, the ecu calculates the amount of injected fuel and other things based on these three.

 

If anyone knows to tell me if they're in good range please help. Thanks!

 

Screenrecorder-2021-09-14-20-18-51-328_2.gif

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5 hours ago, BogdanBogdan said:

there s the vacuum, the manifold absolute pressure and intake air temp, as far as i heard, the ecu calculates the amount of injected fuel and other things based on these three.

Vacuum and MAP are the same thing in different units. Both come from the MAP sensor. I suspect fuel is mainly controlled by throttle position sensing, and by O2 sensing while idling with closed throttle. MAP, RPM & intake air temp will also go into the fuel injection look up table.

If the throttle position indicator moves smoothly (at least as well as the update rate of the scanner can show), and consistently comes back to the same value, then it is not likely to be throttle or its sensor. Fuel would not make it rev high: too much will be evident by smell or smoke, and will tend to make it run rough or even stall. Not enough will speed it up a little, but without air it can not rev much.

I suspect a sticky IACV. When you touch the throttle the IACV will close, then have to re-open as rpm drops back to idle, and can overshoot and stick open. I had a sticking IACV on a Vauxhall, which was a proportional solenoid. The drawings of the IACV for the Focus Mk1 look similar. Solenoids give a weak & springy force to the valve. I cleaned mine in situ, which did improve it a bit, enough to be fairly sure it was to blame. But it then rapidly got worse, and I had to remove it, and really clean it properly. I just used alcohol (meths will do, I used IPA), but I brushed it thoroughly inside, and moved the valve stem while doing it. This cured the sticking fully.

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18 hours ago, Tdci-Peter said:

Vacuum and MAP are the same thing in different units. Both come from the MAP sensor. I suspect fuel is mainly controlled by throttle position sensing, and by O2 sensing while idling with closed throttle. MAP, RPM & intake air temp will also go into the fuel injection look up table.

If the throttle position indicator moves smoothly (at least as well as the update rate of the scanner can show), and consistently comes back to the same value, then it is not likely to be throttle or its sensor. Fuel would not make it rev high: too much will be evident by smell or smoke, and will tend to make it run rough or even stall. Not enough will speed it up a little, but without air it can not rev much.

I suspect a sticky IACV. When you touch the throttle the IACV will close, then have to re-open as rpm drops back to idle, and can overshoot and stick open. I had a sticking IACV on a Vauxhall, which was a proportional solenoid. The drawings of the IACV for the Focus Mk1 look similar. Solenoids give a weak & springy force to the valve. I cleaned mine in situ, which did improve it a bit, enough to be fairly sure it was to blame. But it then rapidly got worse, and I had to remove it, and really clean it properly. I just used alcohol (meths will do, I used IPA), but I brushed it thoroughly inside, and moved the valve stem while doing it. This cured the sticking fully.

Hello and thank you for your reply. I took out the iacv and spray cleaned inside the valve with an electrical spray for potentiometers. This after i cleaned it in the first place with the engine cleaning spray. I let it overnight with this electrical spray cleaner and next day i tested it on the car.

I didnt put the bolts on and let it sit on the engine face-down and recorded the movement of the valve inside (when ignition is set the valve does a half to full to close motion test).

I did it for a couple of times (you can see in the gif attached below and also tried moving it by hand. The spring seems healthy and it doesn t stick. Im not sure though if it's as healthy as it should be but it looks fine. 

 

VID_20210907_152202000.gif

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