MX-5 OC Forum

Hi all, I've read a few posts about high idles, but I wasn't sure if they were relevant to my problem.

When I come to a halt at a junction or whatever (engine hot) the revs sit at 1,500 rpm for about 10 seconds, sometimes less. The revs then quickly and abruptly drop down to normal. The car feels completely normal to drive, better than ever in fact.

I had a slightly erratic idle in the past, but I have since cleaned the intakes, changed the ignition leads and sparks, and the coolant and engine oil if that's relevant. This has made everything a lot smoother, definite improvement, but now I have this new problem. The only(?) bone-headed thing that I may have done is adjust the idle without shorting the relevant pins in the diagnostic connector.

Any pointers very much appeciated as always!

Cheers, Matt.

 Are there any fault codes showing?

Most high idle problems can be traced to a faulty coolant temperature sensor.

 Hi Matt,

I have exactly the same problem and haven't found a solution yet.  Would you let me know if you find the answer?  I will do likewise.

Regards

Mike

Taxieunos, apologies, I really should have checked for fault codes before I posted, I will try to do it this weekend.

MikeSmith, I will certainly keep you updated. You should do a diagnostic as well :) http://www.mx5oc.co.uk/forum/forums/t/204.aspx I'm going to make a list of things that it COULD be also, based on other posts that report idle problems, purely because I don't have a clue what it could be.

I'm fairly sure it is affecting the engine during normal operation now as well (or maybe I just didn't notice it before), it feels like there is a slight loss of power when pulling away sometimes, but that could be my imaginiation.

Cheers, Matt.

purps:

Hi all, I've read a few posts about high idles, but I wasn't sure if they were relevant to my problem.

When I come to a halt at a junction or whatever (engine hot) the revs sit at 1,500 rpm for about 10 seconds, sometimes less. The revs then quickly and abruptly drop down to normal. The car feels completely normal to drive, better than ever in fact.

I had a slightly erratic idle in the past, but I have since cleaned the intakes, changed the ignition leads and sparks, and the coolant and engine oil if that's relevant. This has made everything a lot smoother, definite improvement, but now I have this new problem. The only(?) bone-headed thing that I may have done is adjust the idle without shorting the relevant pins in the diagnostic connector.

Any pointers very much appeciated as always!

Cheers, Matt.

Have you re-adjusted with the pins shorted?
If so my money is still on coolant temp sensor.
If not then i'd give the re-adjustment a go.

OK, I will clear the fault codes and then re-adjust with the pins shorted, and try that for a few days. I will definitely do a diagnostic though whatever happens.

Interesting that are putting your money on the coolant temp sensor. I did change the coolant whilst the car was the off the road, which was when the problems started (maybe relevant). It's on the back of the engine isn't it? I don't know where exactly. I'm wondering if I have damaged it or upset it somehow.

 Perhaps you stirred up some crud in the cooling system which has got stuck on the sensor or perhaps it's just one of those things. The sensor is on the back by the bulk head. Coil pack off job to make access easy. Hope it works out.

Interesting - I could have damaged the coolant temperature sensor because I was removing the clutch and could have easily bashed it.

Mike 

Hiya we have a lot of problems with this at the garage it`s the thermo resister switch on the back of the cylinder head it feeds info to the ecu change it disconect the battery and it sould be fine.

Hope this helps [if not ring me 01785 252423]

Best regards.

Chris @ Broadspeed.

Thanks Chris, I carried out a diagnostic test but it didn't show up a fault. I'll certainly try the thermo-resister. 

Regards,

Mike 

Hi Chris,

Changed the thermo resistor but no improvement.  I had the exhaust system completely removed during work on the clutch so both the sensors in the manifold and the cat were removed.  Could it be anything to do with these?  I note that anything putting load on the engine increases revs to 1500 rpm for around 10seconds aswell.  Is that a clue?

Best Regards

Mike

When you cleaned the intake system did you use any solvent cleaner near the the throttle butterfly? If so it's possible that you stripped off the coating near the edge. If you did then you will need to adjust the throttle stop to bring the natural idle back to the 'normal position.

I shorted the magic pins in the diagnostic connector and re-adjusted my idle back down - whilst I have may have set the idle too low now (it struggles to start sometimes on these cold mornings we've been having recently), it does seem to have solved the high idle problem. I will report back if anything changes, but it seems OK now. I've just bough a hand-held tach, I will set the idle speed properly when it arrives.

ralphfcooke, correct me if I am wrong, but that wouldn't explain why the high idle is only temporary - it drops back down to normal levels if left for a few secounds.

Mike, you are describing exactly what I saw - put any load on the engine, and it would jump back up to the high level. If I was in slow moving traffic without my foot on the throttle, it would still sit at the high idle whilst pulling the car along obviously. Apply the brakes, forcing the revs down essentially, then declutch, they would jump back up again. I also tried (in the name of science) letting the clutch out a little with the hand brake on, to "force" the revs down this way, but they would always return to high again. Just to be clear, in every case, the revs would drop back down to normal again after 4 - 10 seconds. Do your revs still return to normal if left long enough?

Hi mike.

Sorry for late reply how do you check for fault codes, does the vehicle have A/C when you say putting load on the engine ie turning on electrics ?

Regards Chris.

http://www.mx5oc.co.uk/forum/forums/t/204.aspx