Sigma 1.6 (125 bhp) hesitating when moving from standstill

[40YOC-Neil]

I own a Caterham Seven Roadsport 1600 (Sigma engine) 125, registered 2014, mileage about 16,000.

The car is not modified except for a exhaust bypass pipe in lieu of the catalyst (Caterham supplied part).

The car has run very well for the last 5 years, except recently when I returned from a 800 mile round trip to South Wales, where the car experiences hesitation on starting from rest. This happens after a few minutes from ignition on and the engine warming up.

With the car at a standstill, engine on idle, when I accelerate away, the cars splutters and hesitates/kangaroos, and then cuts-out to idle. This also happens when I'm in second gear accelerating. I have to gently control the clutch and accelerator to keep the vehicle moving, and reduce the hesitation.

I have connected a MBE USB-CAN interface to the vehicle, and I notice 2 items;

a) There is a 'Unknown Fault' in the ECU status box.

b) The throttle site 1 voltage fluctuates between 0.0 to 0.4 V on idle of 920-950 rpm. (I see some people state the TPS voltage should be 1.05v, how correct this is I don't know)

c) On reving the engine, the throttle voltage rises smoothly.

Would you have any thoughts of where I should check, ie Throttle Position Sensor, EGR valve, ignition coil pack, Lambda sensor etc, and is there any known issue(s) associated with the above symptoms.

Many Thanks

Neil Claydon

[Tom_Arundel]

Check for air leaks/split pipes on intake manifold especially the rubber pipe to the MAP sensor (usually splits under the manifold)

Disconnect the lambda sensor under the driver`s foot box and drive without it as a first try.

TPS is very sensitive to idle setting adjust with caution but it does cause `KaNgArOoing`. Don`t overtighten the TPS screws as they have caused the throttle to jam open!!!

There is a balance between throttle opening and TPS reading which is hard to get right. Avoid changing the idle screw on the throttle body.

Did it get wet in Wales?

[40YOC-Neil]

Tom,

Thanks for the suggestions, I'll work my way through them.

Regarding Wales, I was there last week so missed most of the rain, the weather was rather hot, but good fun. A lot of admiring looks and people talking to me about the car.

Neil

[ChrisC]

Wet was my first thought, but it does depend if the MAP sensor is open to the air, which I don't think is the case for a 125.  The 1.05v is a 150 setting which has a mapping strategy based on TPS position.

However the balance between TPS / throttle site and throttle stop at idle is where I normally look for hesitation problems on pull-away. You are looking for a biggest throttle opening at idle, but with a throttle site of 0.  As soon as the throttle is touched, the throttle site should be greater than 0. If you make an adjustment, disconnect and reconnect your battery (easy with an isolator switch) before you retest. Tweaking the idle stop screw and the TPS position to achieve this is my normal approach, even if this means opening out the holes of the TPS sensor a bit. 

    

[nickhi]

I have the same spec car and had similar issue a year or so ago. The Tps voltage is set to 0.38 v for site 0.0.  It shouldn't vary at idle maybe the TPS is loose.  I found mine was a little loose and when applying throttle the TPS sensor would move slightly resulting in a momentary lower voltage reading and hence  non linear input to the ECU which I think caused mine to stutter.  I first set the idle then adjusted the TPS to give site 0.0. It was a bit of an iterative process.  Been rock solid ever since.

[40YOC-Neil]

Thanks to Tom_Arundel, ChrisC and nickhi for your replies.

a) I checked for air leaks/split pipes on intake manifold. Checked also the rubber pipe under the inlet manifold.

b) The MAP sensor is not connected to the manifold. A pipe is connected and bent facing downwards to prevent water ingress.

c)  I disconnected the lambda sensor and there was no change in the car's behaviour.

d) I noticed the symptoms only started showing after a couple of minutes from ignition on and driving the car.

e) The car has got progressively worst. Checked the TPS with MBS tool, and voltage fluctuates between 0 to around 3V

f)  Took of TPS to inspect it. The resistance varies from 5.5K at closed throttle to 0.5k and fully open throttle. I did notice at around closed throttle the resistance slowly goes down then increase very slightly as I slowly rotate the TPS.

g) I have not touched the idle screw on the throttle body.

I will order a new TPS, and see if this fixes the issue.

Thanks to everyone who contributed to this chat.

Regards,  Neil

    

 

[40YOC-Neil]

Thanks to Tom_Arundel, ChrisC and nickhi for your replies.

a) I checked for air leaks/split pipes on intake manifold. Checked also the rubber pipe under the inlet manifold.

b) The MAP sensor is not connected to the manifold. A pipe is connected and bent facing downwards to prevent water ingress.

c) I disconnected the lambda sensor and there was no change in the car's behaviour.

d) I noticed the symptoms only started showing after a couple of minutes from ignition on and driving the car. The car has got progressively worst.

e) Checked the TPS with MBS tool, and voltage fluctuates between 0 to around 3V. Took of TPS to inspect it. The resistance varies from 5.5K at closed throttle to 0.5k and fully open throttle. I did notice at around closed throttle the resistance slowly goes down then increase very slightly as I slowly rotate the TPS.  

f) I have not touched the idle screw on the throttle body.

I will order a new TPS, and see if this fixes the issue. Thanks to everyone who contributed to this chat.

Regards  Neil

 

[Tom_Arundel]

Maybe they changed it on later cars but the 125 MAP sensor was always connected to the manifold so check the connection on the manifold (stub pipe at rear of inlet manifold  underneath) is blanked off properly. (maybe you have)

Check for free return movement from WOT if you fit a new TPS (sticky throttles are quite exciting!!!)

[Tom_Arundel]

Did you get it sorted?

[40YOC-Neil]

Hi Tom,

Thanks for your comments/help.

a) The MAP sensor is not connected. Just a piece of plastic tube that bends over to prevent water ingress. I was told the MAP sensor is not used by the ECM (from Caterham). This has been like it for over 15k miles trouble free motoring.

b)  I checked the manifold/throttle body and the inlet stud is still blanked off and the plastic bung is ok with no damage.

c) I checked the free return movement of the WOT with the new TPS, and no issues.

d) The new TPS made no difference, symptoms still the same.

I'm trying the final idea from a friend, and to change the ignition coil pack as these have been know to go faulty and cause acceleration/under load issues. After this I will plan for it to be taken to a independent specialist to have a look at. I've had a good look at it and tried a few ideas.

Neil

[Tom_Arundel]

The TPS is really difficult to set against throttle opening at idle...it`s so sensitive that just tightening the screws can upset it.       It takes a lot of trial and error to get it right.  It needs to be done hot.