Correcting the idle via the throttle stop

As I mentioned in my last blog post the 7 started to suffer idling issues, this situation started some weeks back, but it's only now with the aid of Easymap that I believe the problem has been resolved. In this blog post I'll cover what happened, what I looked for and how the issue was ultimately fixed.


Before proceeding I should give a massive shout out to the Lotus 7 Forum Blatchat regulars, as ever they were on hand to provide help and support diagnosing the issue. Without their knowledge and experience, I wouldn't have been able to tackle the problem. If you're a 7 owner and you've not joined the Lotus 7 club, I highly recommend it for this reason alone.


The idle problem started one evening during a drive around the TT course. I'd been out for over an hour and was on my way home having enjoyed the performance of the 420 R! I’d pulled up at a set of traffic lights and the car stalled, there were no prior warnings of it being unhappy. Upon starting the engine again the engine would fire easily but just wouldn’t run, it immediately stalled again. I eventually did manage to get the engine running but it required regular blipping of the throttle to keep the idle happy. Something had changed. Driving the remaining distance home the engine appeared to sound normal, so no worry there. Coolant temps and oil pressure were all good. The following evening I tried to start the car, it now being cold, but it refused to idle and again stalled. In response I changed the position of the throttle cable adjuster, increasing the throttle slightly, it helped but it wasn’t correct. The rubber sleeve which sits forward of the adjuster had moved forward off the adjuster threads, could this have allowed the adjuster cable to move due to vibration? Unlikely as the adjuster shouldn’t be in play when there is no throttle applied.


Given my inexperience I turned to Blatchat to seek advice, the general response provided implied that the throttle stop might have moved. At the time I didn’t have Easymap, thus accurately assessing the throttle stop position was impossible. I did try however, and moved the stop upwards, but this didn’t help and I believe it may have confused the situation. Now with visibility from Easymap I can better appreciate how the throttle stop requires the smallest of changes.


Having made changes to the throttle stop, albeit badly and without visibility, I concluded the issue must be elsewhere. Given the engine sounded fine when running I believed the issue was probably an electrical or sensor fault. Coupled with the fact the situation would slightly improve when the engine was at operating temperature I looked for a sensor which was related to temperature. I came up with the Lambda or O2 sensor. During research into O2 sensors, I identify a potential build problem. I’d placed electrical tape around the end of the O2 sensor cable sheath to help prevent it from fraying. For neatness the electrical tape was attached to the back of the sensor. Now this shouldn’t be done, an O2 sensor has a reference air chamber, and that that reference air flows into the rear of the sensor. Preventing air flow with electrical tape probably isn’t a clever idea. Once my research identified this potential mistake I immediately detached the tape from the rear of the sensor, but it didn’t correct the idle issue.  


O2 sensor following removal of tape from rear

O2 sensor operation is a topic I think justifies a dedicated blog.  Their construction and use are worth covering in detail, but it's best left for another time. In the context of sorting out the idle, I didn’t have a spare sensor, they’re around £70 and so without couldn’t progress this avenue of investigation. There was also a strong suspicion it actually wasn’t the sensor. When the exhaust is cold an O2 sensor causes the ECU to use a default map, which in theory would more easily sustain a steady idle. It is only when an O2 sensor is truly hot does it cause the ECU to switch operating modes and use varying fueling. If that hot state isn’t accurate due to a faulty sensor then that may cause the idle issue. Given the idle on the 7 was worse when cold, it stood to reason the fault wasn’t the O2 sensor.


During my time researching O2 sensors, I’d also ordered my Easymap cable. Thankfully the cable arrived shortly after I ran out of avenues investigating O2 sensor operation and so activity switched back to the Throttle stop.


The next section owes a huge amount of credit to members of the Lotus 7 Club, particularly Aerobod (James). I’d have been left guessing without his information relating to the operation of the Throttle Position sensor, site and voltage values.


Within Easymap there are two key values relating to the operation of the Throttle Position sensor, these are; 

  • Throttle Site 1; and

  • Throttle Angle Increasing. 

Values as shown in Easymap

Both are shown by default within Easymap when connected to the MBE ECU. These values are critical for ensuring a smooth idle, and it’s these values which are changed when re-positioning the throttle stop. Re-positioning the throttle stop is straightforward, simply slacken off the retaining nut underneath the throttle position sensor housing, you’ll need to ensure a screwdriver is inserted into the top side of the stop. Set to the desired height and then tighten via the nut. It is important to note it’s easy to move the stop position from the desired height when tightening, so a bit of trial and error is needed.  

Throttle stop, image taken from rear side. Image shows lower nut.

Note, after changing the throttle stop position you must reset the ECU via disconnecting the battery so that it learns the new stop height.


Initially, I misunderstood advice given via Blatchat and I attempted to set the Throttle Site 1 to 0.1. The stated recommendations were;

  • Throttle Site = 0.1 and;

  • Throttle Angle Increasing = 1.25 v to 1.3 v. 

Once changed I found the car would idle cold, but when hot started to hunt up and down between 950 rpm and 1300 rpm.  Not great and besides the car didn’t previously do this, so not a fix. The following video illustrates the described hunting.

Eventually I recognised there was a difference between the recommended values and that which I’d observed. Given this discrepancy, I changed the throttle stop height based on the Throttle Angle Increasing value only. I ensured the Throttle Angle Increase value was 1.21 v regardless of Throttle Site value. At such a Throttle Angle Increasing value, the Site value was 0.0, a value I’d previously assumed was too low. This configuration has shown to be a perfect set up. The engine starts and idles from cold with no stalling, it idles around 950 rpm and maintains operation at normal operating temperatures. I believe this to be a correct fix, job well done! A quick video showing the now stable idle.


Working with Easymap has shown the car is not achieving wide open throttle. This isn’t a fault, more tuning of the throttle cable and throw against the Throttle Position Sensor. I intend to correct this over the coming days and will post again once complete.


References


Previous blog

https://www.caterham7diaries.com/post/setting-up-easymap-mbe985


Lotus 7 Blatchat threads

https://www.lotus7.club/forum/techtalk/request-advice-car-wont-idle

https://www.lotus7.club/forum/techtalk/420r-wont-idle-when-cold

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Loves cars, light weight specials, jDM, Lotus, Caterham, Rallying and Trackdays