Baffling Throttle Problem

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My Name: pinvts

Contributor
Posts: 6
Joined: Sat Jul 27, 2019 8:59 pm
Model: Other
Year: 2005 (05)
Engine Size: 1.6 (16v)
Fuel Type: Petrol
Mileage: 80000
Trim Level: Other
Gearbox: Manual 5 speed
DPF: No
LHD or RHD: RHD (UK)
Engine name: TU5 16-valve (110 PS)

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Hi all,

I have a mk1 C2 VTS with a throttle problem that I am struggling with. I do realise this is a C3 forum, but I hope it is ok to post here - the most popular C2 forum is virtually dead and is no longer accepting new member registrations, so I cannot post over there. I haven't had any luck on Facebook forums either. Since the cars are extremely similar and share many parts, I was hoping someone here might still be able to help. (TU5JP4S, drive by wire throttle, Bosch throttle pedal)

The problem: Under approx. 2500rpm, rpm drops back to idle, despite the throttle pedal still being pressed. Above 2500rpm or so, revs will hold constant (as they should) without issue. Lexia confirms that the throttle position voltage is dropping despite the pedal position being held, thus the butterfly angle dropping back to idle position.

Steps taken so far:

- Another second hand pedal assembly bought, fitted and recalibration procedure done (ignition on 10s, press pedal 5s, switch off ignition 20s), but no change.
- Throttle body removed and cleaned thoroughly, no change.
- Another second hand throttle body bought and fitted, no change.
- MAP sensor cleaned thoroughly, no change. Problem still present even when MAP sensor disconnected.
- Pedal connected directly to original LHD position connector behind the glovebox, no change - I have read of the multiplug in this location causing throttle problems on C3s as well as other issues on C2s, but this doesn't seem to be the problem. I know of someone else who has a LHD mk1 C2 VTS who has the exact same issue as me; he hasn't been able to fix it yet, but this adds weight to my finding that the multiplug doesn't seem to be the cause, since the multiplug isn't even present on his LHD car.
- O2 Sensors checked on Lexia for function - pre-cat is 0.7-0.9v at idle (closer to 0.9v once running for a while) and post-cat just above 0.5v

The car has been fully serviced - new oil, oil filter, air filter, spark plugs, etc. Battery seems in good health, 12.0v+ and 14.2-14.4v when engine is running.

Diagnostics in Lexia don't reveal any codes. Well, that's a half-truth - I do have P0122 "Motorised throttle signal coherence" and P1157 "Motorised throttle signal short circuit to -", but I believe these codes to be a result of the time I tried to start the car when forgetting to plug the throttle body connector back into the throttle body. I also have codes for the MAP sensor, but again I'm certain those came about due to purposely disconnecting the MAP sensor to see if it made a difference, as mentioned above. I am confident these codes won't return after clearing, which I do need to do.

The question, then, is what is causing the throttle body to return back to idle position despite the pedal still being pressed when rpm is under 2500? I am wondering if I have simply been extremely unlucky in purchasing a used throttle pedal that happens to also be faulty - ozVTR has mentioned that early VTS pedals have been known to be dodgy, so even contemplating trying a third pedal.

Any help greatly appreciated which might help me fix this pesky issue. Thanks :D
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My Name: Arfur Dent

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pinvts wrote: Sun Jul 19, 2020 11:11 pm Diagnostics in Lexia don't reveal any codes. Well, that's a half-truth - I do have P0122 "Motorised throttle signal coherence" and P1157 "Motorised throttle signal short circuit to -", but I believe these codes to be a result of the time I tried to start the car when forgetting to plug the throttle body connector back into the throttle body.
Hi,

You need to put your faith in Lexia. It is the dealer tool,

If you have P0122 "Motorised throttle signal coherence" and P1157 "Motorised throttle signal short circuit to -", and you believe these to be due to disconnecting parts when the ECU was watching, then, reconnect them and do the Lexia scan again. Then delete the codes and run the engine. Then collect the codes again without disconnecting any parts this time.

This will then prove or disprove your theory that is is due to your disconnection or an actual fault detected by the ECU.

After you get the codes back, check the connector - the instructions are to actually 'delete the connector' - in the passenger footwell where the car was modified from a LHD to a RHD (well, you would with a C3, I don't know if it is the same for the C2, but please let us know :P )
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My Name: Arfur Dent

Guru
Posts: 3417
Joined: Sat Jul 23, 2011 3:47 pm
Model: C3 2002-2005, Original shape model
Year: 2002 (52)
Engine Size: 1.4 (16v)
Fuel Type: Diesel
Mileage: 100000
Trim Level: Exclusive
Gearbox: Manual 5 speed
DPF: No
LHD or RHD: RHD (UK)
Engine name: DV4 16-valve diesel (90 PS)
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pinvts wrote: Sun Jul 19, 2020 11:11 pm but this adds weight to my finding that the multiplug doesn't seem to be the cause, since the multiplug isn't even present on his LHD car.
This is troubling because if it doesn't have the connector, what does the accelerator pedal connect to?

Of course it is present in the LHD model. It connects to the TPS.
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My Name: Ozvtr

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Joined: Wed Jul 10, 2019 3:31 am
Model: C3 2002-2005, Original shape model
Year: 2003 (53)
Engine Size: 1.4 (8v)
Fuel Type: Petrol
Mileage: 80000
Trim Level: Other
Gearbox: Automatic PRND
DPF: No
LHD or RHD: RHD
Engine name: TU3 (75 PS)
Location: Brisbane, Australia.
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You have not mentioned about the "drivability" of the car. Does it affect how the car drives?

It is very normal for the engine revs to "die" when you just press the accelerator without the car moving. The engine ECU uses vehicle speed in it's calculations for air fuel mixture and if the car isn't moving this sometimes confuses it, particularly at low (or small)accelerator peddle positions and you may need to "pump" the peddle to get the revs up when the car is not moving. If you drive the car as it expects, it's usually not a problem. What can I say, a computer is in charge of your engine...not you.

If this is affecting the driveability of your car, do a throttle actuator test and monitor the butterfly valve position. If the butterfly valve returns to idle when it should continue up through the range, it might be the servo motor driver circuitry in the engine ECU. As you have changed the throttle body, it's unlikely that the servo motor is faulty in both, so it's unlikely to be that.
As you can monitor both the throttle and peddle positions it's not those or the interconnecting wiring.
If the actuator test is OK. then its probably one of the other engine or gearbox sensors or associated wiring.

Are you talking about the C2Club forum? TREK is the one to talk to there about engine and suspension matters.
My Name: pinvts

Contributor
Posts: 6
Joined: Sat Jul 27, 2019 8:59 pm
Model: Other
Year: 2005 (05)
Engine Size: 1.6 (16v)
Fuel Type: Petrol
Mileage: 80000
Trim Level: Other
Gearbox: Manual 5 speed
DPF: No
LHD or RHD: RHD (UK)
Engine name: TU5 16-valve (110 PS)

Post

Thank you both for your replies.
Arfur Dent wrote: Sun Jul 19, 2020 11:24 pm reconnect them and do the Lexia scan again. Then delete the codes and run the engine. Then collect the codes again without disconnecting any parts this time. This will then prove or disprove your theory that is is due to your disconnection or an actual fault detected by the ECU.
This is fair enough, but regardless, I understand these codes to relate to the throttle pedal and/or throttle body. Since I have changed both anyway, I don't think the codes can be of more use in this instance, but I do understand that clearing them and checking again would make this certain. I did forget to mention, my friend with the exact same issue has had his car on Lexia too, and the only code he is greeted with is apparently a crank position sensor fault.
Arfur Dent wrote: Sun Jul 19, 2020 11:29 pm This is troubling because if it doesn't have the connector, what does the accelerator pedal connect to? Of course it is present in the LHD model. It connects to the TPS.
Poor wording on my part, apologies. I have attached a couple of photos to clarify. The connector we speak of is of course two parts. 'Part 1' is the connector that connects directly to the pedal on LHD models, and 'Part 2' is found on RHD models. I meant that since his car is LHD, he lacks Part 2 (the additional long black connector which has been known to go high resistance) yet still has the same throttle issue. In his case, Part 1 connects directly to the throttle pedal, as you mention. As you know, Part 2 and wiring is just an extension so the RHD pedal can be reached. I simply replicated the LHD scenario by connecting the throttle pedal in situ directly to Part 1. I had to operate the pedal by hand in this position of course, since it was simply dangling in the glovebox area when connected like this, but the problem remained.
Ozvtr wrote: Mon Jul 20, 2020 4:04 am You have not mentioned about the "drivability" of the car. Does it affect how the car drives?
This is my first car, so I don't have much experience and it can be difficult to differentiate between abnormal judder/jerkiness and what might simply be poor clutch control.

What I can tell you is that since the revs always drop back to idle so quickly, it seems more difficult than it should be when trying to pull away smoothly. Any throttle/gas I give when moving off is rendered virtually useless, since it drops back to idle so quickly, and I am trying to clutch out and the revs are now back at idle and dropping even more, since the clutch is engaging at that point. Pressing the pedal further gives the revs another small burst, but it feels very cumbersome having to press the throttle and press further again just to move off in what is still only a half-smooth manner. Even after pressing the pedal further, revs still drop back to idle. It is almost impossible for me to hold 1500rpm, for example, since the engine keeps wanting to going back to idle. So half of the time I find myself having to press the pedal enough to just above 2500rpm to move off smoothly as revs will stay constant at this point, but of course this means you get going very (too) quickly. It just doesn't behave between idle and 2500rpm.

Otherwise, beyond 2500rpm, it seems ok. I haven't driven the car much at all though. It has juddered a few times after letting of the throttle and changing gear, but unsure if this is still down to not being used to the clutch. My idle varies between 750 and 800rpm - the needle can be seen moving about in this range. Not sure if this would be classed as a slightly rough idle. It's definitely not rock solid. There is some notable vibration of the car/engine at idle, felt when sitting inside.

This is what my friend with the same issue has to say:
(When asked if his car judders) Mostly 1st, don't reckon it does the same in 2nd, but it acts really stupid when I take my foot off the gas, not completely, in 4th gear, it kinda starts to slow down, then accelerates a bit, and proceeds to slow down. Also, I have some judgering from the engine, even can be felt putting hand near exhaust, as the gas flow is very uneven, and can be heard. (When asked about his idle) 800 roughly, diagnostic shows jumping between 760-800
Ozvtr wrote: Mon Jul 20, 2020 4:04 amIf this is affecting the driveability of your car, do a throttle actuator test and monitor the butterfly valve position.
My finding that the butterfly valve is returning to idle was via Lexia -> Diagnosis -> Global test -> Injection -> Parameter measurements -> Standard parameter measurements -> Air inlet (photo attached). When the pedal was pressed, throttle position voltage would increase, throttle butterfly angle would increase and throttle status changed from 'idling' to 'mid-throttle'. But in the next moment, voltage and angle had dropped back to idle figures and throttle status went back to 'idling', despite the pedal still being pressed.

But there is a separate actuator test somewhere else in Lexia for the throttle body? I will try and find that.
Ozvtr wrote: Mon Jul 20, 2020 4:04 amAre you talking about the C2Club forum? TREK is the one to talk to there about engine and suspension matters.
Yes. It would indeed be nice to be able to talk to trek given the amount of experience he has with all C2s, but since I can't sign up there, I have no way of contacting him sadly.
Attachments
Part 1.jpg
Part 2.jpg
Air inlet.jpg
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My Name: Arfur Dent

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Fuel Type: Diesel
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Gearbox: Manual 5 speed
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Engine name: DV4 16-valve diesel (90 PS)
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[
I understand these codes to relate to the throttle pedal and/or throttle body. Since I have changed both anyway, I don't think the codes can be of more use in this instance, but I do understand that clearing them and checking again would make this certain. I did forget to mention, my friend with the exact same issue has had his car on Lexia too, and the only code he is greeted with is apparently a crank position sensor fault.
The fault codes don't say 'change this part' or 'change that part'. They do tell you the ECU has detected a fault in the data or power they are giving or taking. There are wires and connectors between the ECU and the parts that are included in the calculation.

If lexia is giving you any permanent faults, then you should work on these first. They are errors that the ECU has detected and now it is aware of the faulty data it can alter the ECU MAP to help protect the engine or the environment. You can't really move forward with faults present.

If you are presented with a permanent crank sensor fault, you have a problem that needs investigating. It may not be a cause of a faulty crank sensor. But you should sort this out, that is why the ECU and BSI stores codes.


You can get problems with petrol engines where there is an unmetered air intake. A leak on the inlet side, runs lean and the poor ECU gets confused and doesn't store a code. Do you think you have something like that happening here?
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My Name: pinvts

Contributor
Posts: 6
Joined: Sat Jul 27, 2019 8:59 pm
Model: Other
Year: 2005 (05)
Engine Size: 1.6 (16v)
Fuel Type: Petrol
Mileage: 80000
Trim Level: Other
Gearbox: Manual 5 speed
DPF: No
LHD or RHD: RHD (UK)
Engine name: TU5 16-valve (110 PS)

Post

Arfur Dent wrote: Mon Jul 20, 2020 4:20 pmIf you are presented with a permanent crank sensor fault, you have a problem that needs investigating. It may not be a cause of a faulty crank sensor. But you should sort this out, that is why the ECU and BSI stores codes.
I have no permanent faults, and the crank sensor fault mentioned is one that my friend has. I don't have a crank sensor fault (according to Lexia). I keep mentioning my friend and his findings because his issue is exactly the same as mine. So it has been useful to compare faults and findings as we try and work on fixing the problem. I do wonder if the crank sensor may be the culprit, even though I don't have any codes relating to it. I have replaced/cleaned/checked all sensors bar the crank position sensor.
Arfur Dent wrote: Mon Jul 20, 2020 4:20 pmYou can get problems with petrol engines where there is an unmetered air intake. A leak on the inlet side, runs lean and the poor ECU gets confused and doesn't store a code. Do you think you have something like that happening here?


Leaking alloy intet manifolds on these cars is a well known issue. The nuts come loose and vibrate themselves off. I did indeed have one nut missing and one very loose about to fall off, so for a while I thought the inlet manifold was leaking and this was causing the problem. I recently removed the inlet manifold, cleaned everything up and fitted a new gasket + nuts, but this has changed nothing.
My Name: pinvts

Contributor
Posts: 6
Joined: Sat Jul 27, 2019 8:59 pm
Model: Other
Year: 2005 (05)
Engine Size: 1.6 (16v)
Fuel Type: Petrol
Mileage: 80000
Trim Level: Other
Gearbox: Manual 5 speed
DPF: No
LHD or RHD: RHD (UK)
Engine name: TU5 16-valve (110 PS)

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Update: Faults erased, engine ran, Lexia diagnosis reran. No more fault codes. The problem persists.

I tried looking for a throttle test under the 'Actuator Tests' page, but there doesn't seem to be one there. The options are only as folows: Coil 1/4, Coil 2/3, Injector 1 through 4, Fuel pump relay, High speed fan (chopper), Low speed fan, Canister purge elecrovalve and Air pump relay :|

I am wondering if it really is an ECU problem...
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My Name: Ozvtr

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Model: C3 2002-2005, Original shape model
Year: 2003 (53)
Engine Size: 1.4 (8v)
Fuel Type: Petrol
Mileage: 80000
Trim Level: Other
Gearbox: Automatic PRND
DPF: No
LHD or RHD: RHD
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As Arfur says you should first clear all the injection fault codes before you proceed. If they pop up again then it's a current problem.

In the LEXIA under injection, there should be an "actuator test" option. You should be able to move the throttle backwards and forwards without the engine running.

Edit: LOL! This post is redundant. You posted an answer before me.
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My Name: Ozvtr

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Joined: Wed Jul 10, 2019 3:31 am
Model: C3 2002-2005, Original shape model
Year: 2003 (53)
Engine Size: 1.4 (8v)
Fuel Type: Petrol
Mileage: 80000
Trim Level: Other
Gearbox: Automatic PRND
DPF: No
LHD or RHD: RHD
Engine name: TU3 (75 PS)
Location: Brisbane, Australia.
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I believe the crank angle sensor can "fail" and not throw up a code.
Have you cleared the codes on your friend's car? See if the crank angle sensor pops up again?
Sorry, I would have thought there was an actuator test for the throttle. I might have to have a look in the LEXIA.
The only failures I have seen with the throttles is that one of the set of the potentiometer tracks gets dirty and you get a "coherence" failure but that's not the problem here. Otherwise, the throttles seem to be reliable.
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