Awful idle & stalling when hot

Hey guys,

Everytime I slow to a stop and depress the clutch (while the engine is at normal operating temp) my revs drop to between 250-500 and the engine struggles to cope. If I end up waiting more than a minute with the clutch depressed, the problem gets so bad that the engine actually stalls all together. Yet when the engine is cold or below standard operating temp I get no signs of the idle problem or threats of stalling.

I’ve given the throttle body a quick blast of carb cleaner but if anything it seems to have exaccerbated the problem.

Why is the engine having such a hard time when its hot? Do you think its being starved of air? Should I give the throttle body a good 5 minutes blast of carb cleaner or is there anything else I could do to diagnose the problem?

Cheers in advance for any info

Try this

Thanks very much Geoff,

I’ll give that a shot as soon as I get in from work. Also gonna try lubricating the ICV (mushroom shaped thing next to throttle body) to see if that helps.

Is it possible to say why the engine is only affected when its hot? Is it just to do with density of air in a cold engine or is something else afoot?


Hi Noddy, I’ve replied to your email. Get yasell up to mine this weekend

 Just a thought… Have you checked compression/valve clearances when hot ??

@Halli: Cheers bud, see you on Saurday.


@Sonarbell: Sorry to sound like a total newbie but how do I check the compression and valve clearances?


Using one of these in the spark plug holes (though I’m not 100% sure if that one is the right size)

It checks if the valves are leaking. Mine are leaking big style, that’s why I’m swapping them out this weekend.

 Yup… Thats the right piece of kit for the job…

As for checking valve clearances…This will be dependant on engine type and size…  Some engines have solid cam followers and others hydraulic…

Would the thread be correct for the MX5 spark plugs?

Do you mean the HLAs? (the metal discs that the cam lobes press down on)


 Im sure that the spark plug thread is 14mm but there are peeps with far greater knowledge than me on here who could answer that…

The HLA’s on some engines were solid whilst on others they were hydraulic…

Thanks for the advice gents. Found a good write up on what a compression test and a leakdown test is here.

I’ll try and borrow a calibrated compression tester and give it a whirl in the 5 since it sounds nice and easy. Has anyone an idea about how much leakdown tests run to at your average garage?


I’d say bring it on Saturday, but my engine will be in bits :(

Sorted then? It sounded much better after we had a tinker.


You should have stuck around to see the carbon that was behind my exhaust valves! If you thought the valves were black, behind them was frightening!I got all of the intake valves out in less than 15 mins after you left. Let’s hope the new ones go in easily too.

Pics to follow.

Halli you are a HERO. Thank you so much for the advice, the guidance and the show and tell with your engine. Its incredibly helpful to a n00b and also very interesting!

During the drive back the 5 was just faultless. The idle stammer seems to have completely disappeared, even after a punishing drive which was when it was at its worst before, solid as a rock now!

For any future thread searchers reading this, Halli knocked off the ECU and adujsted the idle control valve until the mk1 was idling at about 850, then he tested it under full load (heater maxed out, lights on etc) and then sync’d the non-ecu controlled idle and ecu-controlled idle with a couple more turns of the ICV. He also squirted a bit of carb cleaner directly into the throttle body through the little rubber-clad pipe atop the throttle housing.

Following a diagnostics check I’ve got an O2 sensor and cam sensor ‘O’ ring ordered from the usual place, so come Wednesday the car should be in rude health.

Are you still popping shiny new valves into your engine at the minute or are you chilling out? Its an epic undertaking like but totally worth it for the ‘nearly new’ purr shes gonna give on the second crank!

Again Halli, I can’t thank you enough, was good fun and you were hugely helpful so cheers!


Glad it’s behaving itself now. It was a nice suprise for you to find the car’s timing was already set to 14 degrees too. The new fuel filter that went in today might help with the running as well.

I knocked off at 6:00 for tea and to put the kids to bed and I’m going back out in a few minutes to get cracking with grinding the intake valves in. I should have them all done by tomorrow afternoon and I’ll start putting it all back together then.

I might see about giving the cam cover another squirt of yellow hammerite before bed tonight. That’ll give it until Monday to harden up.

Those valve stem seals pop of much more easily if you get a screwdriver under them and flick them up

It was good to see you again today. Thanks for the help

Just wanted to update this thread quickly for future reference.

  • Carb cleaner into the throttle body + Fuel filter change = eased poor idle problems a bit but after a few days of driving still seemed out of shape and idled poorly when hot.
Following Halli's diagnosis via fault code read out, I fitted a new O2 sensor last week and all my idle problems have disappeared!



Wish my problems were as easy to fix.


My engine is knackered :(


Sorry to read the above halli, are you going to fir a “new” engine from somewhere?

Going to try a new(er) head first