Sigma 150 - Engine bogging down at part throttle

[martyn]

Hi everyone!  This is my first post here but sadly it is of a technical nature trying to diagnose a problem.

I have an intermittent fault with my sigma 150 where occasionally at low RPM (<3000) and light throttle, it will hesitate, bog down and refuse to rev instead of picking up and accelerating.  This will be accompanied with increased burbling and popping from the exhaust.  Giving it full throttle will seem to push through this and the car will accelerate as expected.

I have experienced this 3 times during my 170 miles of ownership and each time the car will continue to do it until I switch the engine off for 5 minutes.  Upon restart it will be back to normal.  Its worth noting that 2 of these times were immediately after a period of spirited driving.

Am well aware of the many issues surrounding the sigma 150 engine and have done plenty of searching and reading up about it.  However most of the other issues are around lumpiness, drivability and cold running.  My car runs perfectly nearly all the time (apart from when it does the above) so I'm not sure if this is down to general 150 quirkiness or something different.

Thanks for your help.

Martyn.

 

[SM25T]

Welcome to BlatChat. Had something similar on a friend's Sigma. It was kept in an outbuilding and hadn't been driven for a while.

 

Unplugged all connectors around the top of the engine ..... onto sensors, etc. Drop of AC50 anti-corrosion fluid in each one and plugged them home. Seemed to solve it OK.

[OliW]

Is it misfiring?  I had a similar issue and after checking the leads I changed the coil pack, which resolved it.  Also may be worth checking the balance of the throttle bodies.

Oli

[martyn]

Misfire seems like a sensible way to describe what I'm experiencing.  Although without wanting to sound too stupid, I'm not exactly sure on what the technical definition of a misfire is.

Is there a guide anywhere to balancing the throttle bodies, or is that a job to get it into a garage for?

[ChrisC]

My Sigma 150 pulls clean from 1000 rpm in 5th.  No hesitation or flats spots.  So I don't agree its a quirk of the 150.   They are very susceptible to throttle body balance and TPS setting.  If it's off, even by a little bit, then you will get hesitation or rough running.    

Mine has only one minor problem, and that is the throttle pedal is to light, which can either be fixed by a heaver spring or bracing your right foot against the foot well.   It makes speed bumps a bit tricky to master, but not something that bothers me now. 

When was the last time the TBs where balanced and the TPS was setup / adjusted?  If you are local to me I can do it for you, or teach you how to do it.     

 

[martyn]

Thanks for the offer ChrisC, I'd love to take you up on that, but I'm based in Bristol so doubt I'll have my car in Essex any time soon.  Just to be clear the car runs perfectly almost all the time.  Its only the odd occasion when it starts playing up.  But when it does there is something clearly very wrong that you couldn't mistake for a quirk.  Just had a look at balancing TBs and that seems straight forward enough with the right equipment.  Whats involved in checking the TPS setting?

[ChrisC]

Hi Martyn 

You could purchase a MBE lead that will tell you whats going on when the problem occurs.  It will tell you if anything in "out of range", or disconnected.   Its also the lead I use to set the TPS, because the ECU tells me the voltage its seeing from the TPS.

You can purchase the lead from SBD Motorsport, for about £100.   I am sure you will be able to sell it on here once you have no need for it later.

 

[ChrisC]

PS The settings are (engine warm) 

You need to set the idle rpm to 950 + or - 50 

the TB's should be pulling between 4-5Kg/hr each of air (all balanced of course) 

and the TPS should be 1.04v.   

Its a bit of a balancing act, and each change effects the others, but with patience you will get it. 

[Jonathan Kay]

• I'd check the connections and wiring of all the sensors.
• Can you reproduce the fault while it's in the garage?
• How do you reset the TPS on this model?

Jonathan

[martyn]

Thanks, thats very interesting.  Out of interest where did you get those values from?  Are they specified in the build manual (I dont have one).

[ChrisC]

The TPS is a manual adjustment, but its reference is taken when the ignition is switched on.    The reason it's set at 1.04v, is at 1.05v the next mapping point is reached.   Therfore any throttle pedal applied, however lightly, pushes the ECU into the next mapping point.   Without that exact setting the 150 tends to be  very quirky on light trottle cruising. 

[ChrisC]

Blatchat and lots and lots or research before I converted my car last year to 150 spec (not 140 spec).

I keep posting the values, but I got them from here but not from an obvious thread, which is why I keep reposting them so all 150 owners can benefit. 

[martyn]

Hi Jonathan,  I haven't tried to reproduce it in the garage.  I've only ever experienced it while the engine is under load (accelerating).  Next time it happens to me I'll pull over and see how it behaves while revving in neutral.  Unless you can convince me otherwise, I'm reluctant to accept that it is an electrical fault.  This is because I can always use full throttle and the engine will pull out of it and accelerate normally.  Suggests to me that it is overfueling at this light to mid throttle position therefore causing the misfire then extra throttle (more air in) sorts it out.

[ChrisC]

One thing you can check.  Early 150s had a lots of mapping problems, this was generally sorted with the release of the "H" map.   The question is has your car slipped through the net, and not had the "H" map applied?    The way to tell is the MAP sensor, which is the small black sensor next to the ECU with a vacuum hose (or nipple for a vacuum hose).    If its connected via a vacuum hose to the TBs, then you have a earlier map than "H".   If you have no vacuum hose, or its open to the air them you have a later map (most probably "H") 

[martyn]

The vacuum hose has been cut, although it is still attached at both ends.  Guess that means that I  have map H

[ChrisC]

Sounds like map H me.  The end connected to the TBs should be capped off, worth checking.    The end connected to the MAP sensor should be open to the air.   A small bit of vacuum hose on the MAP sensor is a good idea to prevent water entering the sensor.    

[martyn]

I'm just ordering the lead from SBD now..   Do I need the one that connects via serial port, or the one that uses the CAN-BUS to USB interface?

[ChrisC]

The SBD Part number I have is 

MBE-MAP-KIT-3-CAN 

Its USB, and you can download the EasyMap software from the SBD website.    You won't be able to map using this software because Caterham has locked the ECU, but you will be able to see all the live data, and get settings while the engine is running, including when the fault occurs providing you have a friend with a laptop 

 

[martyn]

Nice one.  Lead is on order.  Hopefully it'll give me some clues to solve this misfire.  If not, it'll at least be interesting to see what the TPS is currently configured to.

[martyn]

Chris,  could you explain for me what you mean by "a reference is taken when the ignition is switched on".   I've seen this mentioned several times but don’t fully understand what’s going on here.  The TPS adjustment makes sense to me (unscrew slightly, twist to change the output voltage at idle, re-tighten).  But.. if the ECU re-calibrates to a new reference voltage when switched on then surely the actual voltage from the TPS is irrelevant?

[ChrisC]

To get the full story you are going to have to speak to the horses mouth "The two Steves or Road and Race" they did the maps and know that ECU inside out.

My understanding is 

When you connect ECU to the laptop you will see the the TPS voltage has a direct impact on the mapping position being used.  When the voltage is on or under 1.04 its position 0 and 1.05 see position 1.   When the engine is at position 1 the engine wants to rev, and on position 0 it doesn't.   The point to setting 1.04 is so as soon the pedel is touched the engine wants to pick up which helps with low speed pickup.   Without it trailing throttle become tricky.   I think the automatic adjustment is the ECU working out where its at idle.    The automatic idle doen't seem to effect the point where the map position changes.