Rough running at light throttle

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DW199
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Rough running at light throttle

Sigma engine in 2019 Academy car.  The car runs very poorly on light throttle at around 3500 rpm.  It has done this ever since I bought it but during my last track day at Goodwood, in the wet the car was almost impossible to drive.  It feels as though the throttle is pressed and released once or twice a second. 

The traces above are from an OBD scanner and logged on my phone.  In the centre of the  trace the lambda sensor voltage goes low without any obvious fuel trim response.  The tps position seems quite noisy. Is this behaviour as expected when the rpm is betweem 3200 and 3800? 

There is a very slight exhaust leak at the 4-1 collector.  Could that account for the behaviour? 

Any suggestions welcome.

Chart below shows the traces with the car stationary  at different rpms

 

Tom_Arundel
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Sounds a bit like drive train shunt i.e. the rubber/flex in the drivetrain (engine mtgs..gearbox..diff. A frame etc) winding up and releasing.. .. sometimes making your right foot bounce on the pedal. Usual cause is TPS setting error....check voltage and position at closed throttle......but could be many other interesting things!

GulfSeven
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Could it be the immobiliser relays vibrating and causing the engine to cut out? I had this on my new 310R at 5,000 rpm. For me it felt like losing power briefly before continuing to accelerate. I think Williams changed how they were mounted or added some foam or something to fix it.

Tim.

Gulf Racing 310R

aerobod
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I would ensure that the 4 into 1 collector is properly sealed, as a continuous low voltage from the lambda sensor (shown in the graph) either indicates too much air in the exhaust giving a lean reading, too little fuel, or a faulty sensor as likely causes. In the mid range you could get the most relative air leakage with the exhaust starting to scavenge well, but not too much flow through it to make an air leak negligible.

James

GulfSeven
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Out of interest, how is the collector sealed? Mine seems to blow a bit when cold and I assume seals when it expands with heat? It doesn't seem to blow when hot.

Tim.

Gulf Racing 310R

StevehS3
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DW199
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Tom_Arundel - The excessive driveline shunt that afflicts these cars certainly exacerbates the fault but I don't think it is the root cause.  I was careful to brace firmly against the side  of the footwell to minimise that effect.  The tps has been reset several times by Caterham Gatwick which solved the poor idling and immediate off idle progression problem.  I think this is a different fault.

GulfSeven - That's interesting.  There is a slight hum from under the dash when the ignition is on.  I will investigate further.  The collector seems to be an interference fit.  I can't see any evidence of sealant.

aerobod - The thing that is puzzling me is that the short term fuel trim does not seem to have responded to the low lambda voltage.  Is that what you would expect with a faulty lambda sensor/exhaust leak? 

StevehS3 - I have a tube on order!

 

aerobod
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The lambda voltage stuck at the minimum voltage for a narrowband sensor of around 0.1V may indicate that it is not operating closed loop, even though the revs and throttle position would point to it being in a suitable state. The lambda control status would be needed to determine that. The trim would not be applied if it is open loop or if the correction value is outside the maximum allowed amount. If it is due to lack of fuel from a faulty injector the plugs should be mismatched from a colour perspective, although a TPS value that is too far out could cause an overall lean condition.

There may be an underlying condition that is not due to lambda control, but if the base map is always out when not under lambda control it could cause rough running under partial throttle, to eliminate lambda control as the problem I would ensure air flow wise the lambda sensor is receiving the right exhaust flow and there isn't an intermittent signal (although 0.1V would point to the sensor just retuning a maximum lean value).

James

John Vine
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#8: A couple of (dumb?) questions:

Can lambda control status be determined with an OBD scanner, or will the OP need to call on Easimap? 

The voltage oscillation rate in the second O2S1 trace looks very low (maybe 8 switches per 20 secs, rather than, say, 2/sec).  Is that simply down to the scanner sampling rate?

(As you can probably tell, I know nothing about OBD scanners!)

JV

aerobod
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Hi John, the Fuel System Status PID 0x03 should show Lambda control status, but it may depend on the scanner capabilities whether this can be displayed or not (https://en.wikipedia.org/wiki/OBD-II_PIDs#Service_01_PID_03)

I think the oscillation at 0.4Hz or so is on the low side, but not unusual from other vehicles I've seen and is going to be as much dependent on ECU response to the sensor input as anything else.

James

DW199
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Fuel system status can be displayed in real time on the phone.  I'm not sure whether the software I am using (Car Scanner ELM OBD2 for ios) allows this to be logged.

The right hand side of the lower trace where the revs are held constant at around 2500 shows an odd shaped trace for the O2 volts and trim.  I am reasonably confident this is a sampling frequency artefact.  

The car is going to be returned to Caterham Gatwick next week for investigation and fix under warranty.  Previously they couldn't replicate the fault and I didn't realise that so much useful information was available and could be logged with cheap OBD tools.  At least I now have some objective evidence of a fault rather than it being caused by my incompetent driving.