I've MIL too!

I’m running lean…P0171 My car on idle goes up and down continually from about 600 to 1200rpm. I thought this might be quite distinctive if anyone knows…otherwise I’ll go with the book recommendations


Based on P0171 and up/down idle it sounds like you have an induction leak - check induction side with soap solution to see if there is a leak.

Also, although this should give you a different fault code, as a quick check reset your Throttle Position Sensor (TPS) as follows:

Turn Ign on without starting, press full on/off throttle quickly for around 8 times, Ignition off, Ignition on start engine leave to idle for around 15 to 20 minutes. During this time, while ecu relearns, idle should be stable. If not I think you do have an induction leak.

If this sorts your idle and P0171 comes back, I would be looking at the Mass Air Flow (MAF) sensor.


I’d go along with an air leak, given you have swapped exhausts recently. Anything before the lambda, all the way back to the throttle body.

Either that or a duff lambda.

It can be the MAF also, and these can be cleaned if you are very careful. Did my GF’s Celica a few weeks back

The induction sounds allot louder than usual! Will chq but based on other info hope I it’s a straight forward case of relearning with new exhaust. Thanks guys will chq later

Hmm, it may be genuinely lean, but due to the idle problem i’d suspect it is a leak or a sensor.

As above I’d check the induction. My car used to run lean (if using crap supermarket fuel) with the exhaust and induction but before the new map, but since I’ve never had any issues (although haven’t had a reason to put supermarket petrol in it either).

How do I do a soap solution test? I guess it’s soapy water but where and what am I looking for? Thanks guys

Carry out soap solution test exactly as you would to find a puncture except that you will see the bubbles “disappear” in the event on a leak as your piston is sucking air into the combustion chamber.

Check all joints down stream of filter box. If your car is supercharged take off the intercooler and also check for bubble formation on the s/c mating joint. Do this while the engine is relatively cool or the solution will evaporate quickly.

Before you do any of this try to reset your TPS per above post as, if it is still not stable, it will give you a good initial indication that you do indeed have an induction leak (or a bad TPS, but you should have got P2121/2 fault code in this case).

Good luck,


Take a good look at the jubilee clips used if your car is SC, the ones supplied by Lotus are rubbish and won’t tighten correctly, I have now swopped out mine

Tar Ade, it’s a std Exige using an ITG Maxogen. Took it apart last night, nothing so far!

Found it, Lambda sensor wire to the ECU! Fixed just in time to have Ohlins and AP brakes fitted(thank you Will and tank you Hanger 111)…tomorrow! Bring on the soonest possible track day…it’s been 2 months already…!!!

Drove over to Horizon Motor sport and MIL came on again. Idle had the slightest of judders (is that a real word ) every now and then but it ran ok. Whilst Mike had it in he reset the idle did some other stuff and took it out. Ran ok. He’s turned the light off but the fault still registers. He said a sports cat would cause this problem, so my question is…(running 2ubular manifold and 8 GT3 exhaust)…Do I need a MIL eliminator kit?

Never had the issue with mine, what was the error code?

P0171 again & again & again

That is Bank1 too lean? are you running an induction kit? have you checked for air leaks? look at the jubilee clips used on the on the induction system Lotus use rubbish clips that don’t tighten properly. AFAIK that is a reading taken by the pre cat sensor so the cat will not be involved. Have you looked at the short and long term fuel trims?

ITG Maxogen, I’ll look at the fuel trims, changed MAF, lambda, tightened jubilee clips

Measure the internal diameter of the the tube the MAF is in and then the original tube on the standard airbox, if the ITG has a bigger size internal diameter MAF tube this might well be your issue, the MAF reads air volume based on a known diameter tube, if you increase the tube diameter it will be passing more air than the maf is reading, it will be able to adjust to a point, but in essence it will end up running lean.

As I’m fitting an ITG at the moment, just been and measured it. Standard airbox and ITG both at 65mm I/D.

Well that might not be it then I’m running out of ideas…


Your original post stated that your idle was fluctuating between 600-1200 rpm…and in later post you state it is stuttering?

Assuming you have checked/ruled out induction leak and reset TPS/ECU to learn?

Next job would be to take out MAF sensor and clean with non chloride solvent spray.

While you are in there remove the throttle body and thoroughly clean, including the air idle mechanism. (You’ll need a couple of new gaskets). The CAI (in my case Pipercross VIS) seems to result in a lot of carbon deposit around the throttle body valve. No reason for this to cause P0171 code but it could certainly upset the idle.

When you reinstall the MAF sensor it should be at 3 O’clock not 12 O’Clock.

If, after all that, you still get P0171 code coming up then it’s time to reinstall the stock air filter assy.

Take care,