Hi all,

 

I’ve got a 1998 Ford Fiesta Mk4 1.25 Zetec-SE and I’ve been chasing an idle issue for a couple of years. The car starts and drives, but the idle has always been rough. (Also sometimes has a slight resistance about 45mph-55mph but seperate issue and not imperative) 

 

So I recently I went through a whole kerfuffle where the car would stall at every corner or junction. Sometimes 10-15 times on a short journey making it un drivable. . I decided to delve into the existing idle issue ontop of this serious fault.
 

so far I’ve done/checked/ changed the following:

🖤Changed the IACV (idle air control valve) and found any air leaks with a  smoke test and sealed thoroughly around the IACV and air filter box. 

The above fixed the idle issue but I then I had a rev hang issue which was even worse to drive (it was basically driving itself, tiny bit of gas and the revs kept up and up until peak THEN dropped to normal again  about 5-10 seconds of rev after releasing gas). 

 

🖤I have fault tested through a computer the only thing that came up was the fan setting which is either on or off - no in between which according to most sources isn't going to be or cause the current issue. 
 

🖤I’ve just Fitted a new MAF sensor, disconnected the battery for 15/20 mins, reconnected the let it idle for a while to reset its brains. The new MAF has solved the rev-hanging issue (which was only present after fixing the air system) but the idle problem has come back and I’ve rechecked the air system again - no leaks. 

🖤So back to square one?…. I thought it might be the IACV being faulty because various sources suggest aftermarket brands suck out of the box but If I unplug the IACV the engine stalls instantly, so it seems to be doing its job?

 

🖤I’ve taken it for a test drive and it’s drivable - as before. Stupid idle, sometimes stalls, but quite happy with gas albeit sometimes smelling like it’s running rich. 

. 

so now, what do I do? Have you experienced the above?

👉🏼tomorrow I’m looking at the Throttle Position Sensor (TPS) and the Coolant Temperature Sensor (CTS) as possible culprits.

 

My questions:

Has anyone had similar idle issues after fixing air leaks and swapping the IACV/MAF?

is there something I’m completely missing here? 

Am I on the right track looking at the TPS and CTS next?

 

im using a combination of google references and chat GPT pro to get me through this, as well as a friend who is a mechanic (though lives far away / we are both incredibly busy with our own businesses so getting him to the car or me to me to him is non viable) 

 

Any advice or first-hand experience would be massively appreciated as I use this car for work. 

 

Thanks in advance!

Jazz xx 

Id change Tps cps spark plugs and filters .. when that's all new ... do a smoke test... and check all vacuum lines... a small invisible crack can cause havoc 

We had a similar problem on a '98 Fiesta - there is a microswitch on the steering rack, which ups the revs when lock is applied.  The wire had been damaged and was making an intermittent connection, messing up the idle.

May be worth a look.

On 8/21/2025 at 5:19 PM, Paulkp said:

We had a similar problem on a '98 Fiesta - there is a microswitch on the steering rack, which ups the revs when lock is applied.  The wire had been damaged and was making an intermittent connection, messing up the idle.

May be worth a look.

Haven’t heard of this so I’ll take a look Thankyou 🙏🏼

On 8/21/2025 at 4:33 PM, festervas said:

Id change Tps cps spark plugs and filters .. when that's all new ... do a smoke test... and check all vacuum lines... a small invisible crack can cause havoc 

Yes I will do a smoke test again after I’ve changed the sensors. The rev hang is back but not as bad so it’s definitely feels like something sensory going on. 
 

thankyou 🙏🏼🙏🏼

Fun little update.

Still non driving. Having so much fun changing sensors.

Rev hang issue fixed

Air leak fixed

Spark plugs, leads and pack changed.

Test drive after new sparks was very reluctant to go. Gas would hold it back.

Runs (just about) but incredibly very rough idle. Sits at 200-500 revs.

Now there’s a clicking from the plugs since spark change so somewhere so assuming a cylinder misfire / arcing issue, or I’ve hooked it up wrong / it was wrong from when I had it?)

Or even perhaps wrong parts as I was advised incorrectly on the first spark plugs length (though plugs are definitely correct now) .

Any thoughts?

Back to the drawing board for me 🤷🏽‍♀️