hunting idle on ZX 16valve

marty_t3 · Post by **marty_t3** » 30 Nov 2004, 00:08

My newly purchased 93' 2.0 16v seems to have a rather erratic idle both when cold and when warm. When i blip the throttle the idle drops to about 750rpm, then jumps to about 1200... it does this 2 or 3 times before settling back to about 900rpm. It's also nearly impossible to hold the revs at a set level... take the revs to about 2000 and it stay there for just a second then drops to about 1500 (makes holding the car on a hill with just the clutch almost impossible).
There's also an intermittent (though most noticable when warm) power loss over 2500rpm ... it still accelerates above 2.5k but there is a distinct lack of power. Feels like a flat spot but lasts right up through the rev range.
Are these likely to be related? and if so, what is it likely to be? (bearing in mind that i only just bought the car and it's probably in desperate need of a good service)

VisaGTi16v · Post by **VisaGTi16v** » 30 Nov 2004, 00:30

could be the acav vacumns. If you look at the engine where the inlet manifold bolts onto the head, there are 2 round plastic things with a small pipe going into them, one each side. They have a little rod coming out of them at the bottom. When you rev it these should move and it opens up the inner track on the inlet manifold and over 5000 they shut. What often happens is they get splits in their diaphrams and leak air and so dont work right which will lead to random power loss. This will also give you a spongey brake pedal due to air leaking out. Other things could be the stepper motor I think although I dont know much about this, have a search on here.
Infact here you are, look where my amazingly well drawn yellow arrows point heh. http://homepages.nildram.co.uk/~visa/pi ... engine.jpg

marty_t3 · Post by **marty_t3** » 30 Nov 2004, 02:08

Cheers mate, much appreciated. I'll have a search around for stepper motor info.
To be honest i've only looked under the bonnet when i was checking the car out when i bought it. I take it these actuator things are controlled by the ecu? Are they a dealer only part and if so, any idea of a price?
cheers,

VisaGTi16v · Post by **VisaGTi16v** » 30 Nov 2004, 03:12

yer the ecu tells them when to open and they do so via a vacumn from the brake servo. I needed 2 on mine in 2.5 years and its a common problem. About £35 from citroen main deal, 3 screws to fit, 5 minute job. Might not be one of them thats the problem but its one you may get in the future

marty_t3 · Post by **marty_t3** » 30 Nov 2004, 05:32

I can't say i've noticed spongy feeling brakes but that's probably because they're already about a million times better than any of my other cars lol.
I'm gonna give the car a full service at the weekend so i'll clean the stepper and have a closer look at those acav things and see if they're doing what they're supposed to. You mention the valves close at 5000 rpm but when should they open (and i take it the actuator rod can be seen moving)?
The car hasn't realy been looked after by the last owner so there's about a million things i need to fix on it. I have a feeling just changing a few filters alone will make a world of difference.

gjb02 · Post by **gjb02** » 30 Nov 2004, 17:42

Welcome Marty_t3, my 16v does the same at idle. Specially when you're trying to park, it almost stalls then revs up then drops again. Have a good look at all the hoses that run from the throttle body to the idle by-pass valve. I have a couple of really soft ones that I have physically seen collapsing, when the idle by-pass valve is operating. Could be the cause, who knows.
As for the ACAV, here are a couple of picture that might help to explain. I didn't write it, a nice german man did, hence the pigeon english.

If the throttle is a little less than accurate think about the throttle switch (aka Throttle potentionmeter).

marty_t3 · Post by **marty_t3** » 30 Nov 2004, 21:41

Cheers for that mate... very useful [:)]

VisaGTi16v · Post by **VisaGTi16v** » 30 Nov 2004, 23:28

the haynes manual also says it doesnt operate until 1800rpm however on mine then move the moment you touch the throttle, ie 900rpm or so, they also did this on my friends mk2 mi16 which has the same engine but mine ran smooth the rest of the time and they did shut over 5000 as i tested it. nice diag though
im currently breaking my 16v marty so may have some bits you need, will post on here later or leave your email address

marty_t3 · Post by **marty_t3** » 01 Dec 2004, 01:58

I'll take you up on that offer. There's quite a few parts i reckon i'll need to get the car back to 100%.
Driver window mechanism (motor is ok though)
Passenger wing mirror
rear wiper motor
heater/blower "resistor pack"?
possibly some rear suspension bits once i find out where the knocking is coming from (probably just my subframe bushes that are shot though)
ACAV valves.
Cat (i'll need a donor part to bodge up a cat replacement pipe)
Clock
And maybe headlamps if i can't clean the rank euro yellow tint off them.
oh, and is there a glovebox? I just have a rather mangled piece of trim in front of the passenger seat... don't know if there's anything behind it or not [:D]
and i'm sure the list will have grown a bit more after i get my head under the bonnet at the weekend [:D]
Not a bad list for a car with just 76k on the clock lol

marty_t3 · Post by **marty_t3** » 01 Dec 2004, 02:00

Oh .. had a bit of a play with the car today to see just where the power loss was. It's between 2000 rpm and 4,100rpm.

VisaGTi16v · Post by **VisaGTi16v** » 01 Dec 2004, 02:14

Did the acav's move though? send me an email to darren AT visagti.com or post your address here as half of the above is available

marty_t3 · Post by **marty_t3** » 05 Dec 2004, 19:07

Well it turns out that the diaphram in the drivers side ACAV was leaking. I've tie-wrapped it open and just hooked up the working ACAV directly to the electrovalve in the meantime. Car feels much better and the erratic idle has gone.

peter_hirst1 · Post by **peter_hirst1** » 05 Dec 2004, 19:58

Re hunting at idle try adjusting the c/o content using a gunson exhaust analiser.If the c/o is wrong it causes surging at idle and missing at higher revs.