DaveK

Hi,
When started from cold, it takes about 10s for my idle to settle down. As the car starts (always instantly) it immediately revs up to about 1200rpm, drops right down to about 400rpm, hunts a few times then settles. On it's own, this is not a huge problem but occasionally it will drop enough to stall and if it does this more than once it means starting with my foot on the pedal. I've noticed that it's now doing this from warm, although to a lesser degree (never stalls from warm). Other than at this time the idle is rock steady and I have no idle hunting at all.

When I bought the car, I had "normal" idle hunting problems but this startup problem never happened. It always fired up and went straight to idle - only when I was driving did the hunting occur. The problem first appeared when I had a cat bypass fitted as part of the work to correct the "normal" idle hunting (it didn't) - ever since I got the car back after that I've had these startup problems.

Occasionally the car won't start when warm. It's pretty rare and again is cured by foot on the pedal but not sure if this could be due to the same problem.

Any ideas?

Dave
90 C4

Jeff Curtis

Ah, I wish I would've read this one before answering your other post...so you DID have a cat bypass installed - eh?

How long have you had the cat bypass? It may take awhile for your DME to "adapt" to it. It does cause less back pressure and may fool with your O2 sensor a bit...but then again, they may have "dinked" up your O2 sensor when making the changeout from your original cat.

Horst used silicone to hold his exhaust gaskets in place while reassembling his exhaust system and this really wreaked havoc on his O2 sensor.

So there's a few ideas to start!? Hope it helps. <img src="graemlins/beerchug.gif" border="0" alt="[cheers]" />

DaveK

Hi,
Cat bypass has been in for about 3 to 4 months and about 4K miles. O2 sensor has been removed, and DME knows it's now running without one.

Dave
90 C4

Jeff Curtis

Quote: "O2 sensor has been removed, and DME knows it's now running without one.

Why was the O2 sensor removed?? Are you saying you don't have an O2 sensor now? Please clarify. <img src="confused.gif" border="0">

DaveK

Hi,
Originally, cat bypass still had O2 sensor. I then had the car chipped by JZM and they told me that keeping O2 sensor was pointless with cat bypass. They told me that O2 sensor was mainly used to protect the cat so no cat meant no reason for O2 sensor. So the O2 sensor was removed and replaced by a blanking plug. They also do something to the connector and this tells the DME it doesn't have one. They fit another sensor - altitude or something, but can't remember what.

I've always assumed that 89 C4s probably don't have an O2 sensor since they don't have a cat?

So in summary - O2 sensor has been removed.

Dave
90 C4

Ade - C4 91

Hi Dave,

If your O2 Sensor is looped out, then this is not going to be the problem, plus the O2 Sensor will not effect the system until a few mins after warm up, as the DME bypasses this circuit until the sensor is warm enough.

Check your oil level isnt too low, this can have the same effect as discribed, don't overfill or you may start having surging problems when hot as well. (Reset DME afterwards).

Check that the vaccume hoses (the little ones on top of the intake mainfold) are holding pressure.
When the car is cold, disconnecting these should release a little hiss, meaning it hasn't any leaks.

Also the idle control switch may be positioned slightly out or the throttle may not be returning correctly and giving irratic readings on the idle switch. This needs a multi meter to check.

Also check the voltage on the battery.

That's all I can think of right now.

Good luck,

Ade 91 C4 Amazon Green

DaveK

Hi,
Battery voltage is OK (battery is 3 months old and alternator output has been checked OK). Oil level is correct and idle switch is definitely being activated - although I guess it may not be working but I assume this would cause hunting at other times.

I'll check the hoses, although again I would expect this to cause hunting the rest of the time.

It's odd - almost seems like the DME mis-judges something when the car fires up and then takes 10s or so to correct itself. After this, it idles perfectly.

Dave
90 C4

Jeff Curtis

I've heard that JZM does great work, so nothing against them. Let's see though, O2 sensor removed, cat bypass and a custom chip...you say you've had all of this for 3-4 months and 4k miles and it just started doing this recently?

Okay, guess I can't blame the upgrades this time, but they're still on the "suspect" list.

Ade has a nice list of suggestions to investigate for yourself. Beyond that, I think a visit to JMZ is in order? <img src="confused.gif" border="0">