1993 2.0 prelude Auto choke cold start problem MOT

Totalfinlay · Post by **Totalfinlay** » Thu Oct 04, 2018 7:22 pm

Hi I have a problem with my prelude that's actually ended up being an mot failure !
When I start the car it does the standard cold start and revs up to roughly 2.2k rpm,
When you give it some throttle during this warm up period the engine revs nicely then usually drops below 2k rpm then the Auto choke kicks back in and brings it upto 2k again then drops to around 1.6 then back up and repeats itself untill it's warm when the idle drops below 2k and makes its way back down to 0.7k
I have a video of it doing this, it failed MOT for emissions on 2nd fast idle.

[youtube][/youtube]

Post by **vanzep** » Thu Oct 04, 2018 11:23 pm

Hi this sounds like a bouncing idle and is normally a problem with the iacv (idle air control valve) or fitv (fast idle thermo valve)
Standard start up revs should be between 1500-1800rpm
Should be easy to fix

I will see if i can find some guides on how to clean these valves

Post by **vanzep** » Thu Oct 04, 2018 11:27 pm

here you go have a look at this

http://www.ludegeneration.co.uk/viewtop ... tv#p335376

the 3rd screen shot is where you want to start

Totalfinlay · Post by **Totalfinlay** » Fri Oct 05, 2018 1:01 pm

Alright thanks ! I'll have a look and go through it and let you know if it works ! Managed to get the tester to do the emissions test again today for me after having warmed it up and it passed fine, still got the bouncing idle ! So will go through that ^^

89 UKDM BA4 4WS (resto job!) · Post by **wurlycorner** » Fri Oct 05, 2018 1:03 pm

Is this only from cold start?
If so and if the revs slowly rise while it warms up before then suddenly dropping back under control, it's likely to be the coolant temp sensor that's failed (not the thermo switch for the fan, they're different things).

ECU uses the coolant temp sensor to control mixture until the Oxygen sensor has warmed up, which it then swaps over and uses instead. Knackered coolant temp sensor results in the ECU not leaning the mixture out, so the engine idle revs go too high.

If you have bouncing idle, that's likely to be something else (as in the thread vanzep has linked)

Totalfinlay · Post by **Totalfinlay** » Fri Oct 05, 2018 1:38 pm

Yes it's only from cold start when I've given it a little rev, there's a YouTube link in the original post of what it's doing when cold

89 UKDM BA4 4WS (resto job!) · Post by **wurlycorner** » Mon Oct 15, 2018 1:45 pm

Ah, sorry I missed the original youtube link

Yep, that's not what I had in mind with the coolant temp sender.