open loop at idle

[kevsurf75]

my car is running in 'open loop' mode at idle and therefore fails the emissions test (I live in G.B). As soon as I increase revs above idle (i.e the throttle switch opens) it goes into closed loop. Strangly it seems to be in closed loop at idle whilst warming up but this is not definate.<BR>It can be bodged by moving the throttle switch so the contacts don't close at idle but then the idle valve is inoperative and the idle speed drifts with electrical load.<BR>Any ideas as to the cause?

[David Coleman]

How are you determining what closed loop and open loop is? If you could provide more information it might be easier to diagnose.

[ggmx3]

My '92 5 speed seems to be doing the same thing. Have you figured your's out yet? I've been told it's anything from the PCV valve to the O2 sensors to the air flow sensor! Mine will be in the shop on Wednesday. Hopefully the computer will tell me what's wrong.

[kevsurf75]

To check whether the ECU has control of the fuel mixture do the following - connect a voltmeter to the signal wire of one of the oxygen sensors and watch the voltage. If it is in open loop the reading will be steady (close to zero if the mixture is weak or close to 1 volt if rich).If it is closed loop i.e. the mixture is being altered by the ECU, it will be fluctuating between these two about once per second.

[Mike 94PGT]

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by kevin porter:<BR><STRONG>To check whether the ECU has control of the fuel mixture do the following - connect a voltmeter to the signal wire of one of the oxygen sensors and watch the voltage. If it is in open loop the reading will be steady (close to zero if the mixture is weak or close to 1 volt if rich).If it is closed loop i.e. the mixture is being altered by the ECU, it will be fluctuating between these two about once per second.</STRONG><HR></BLOCKQUOTE><P>This test doesn't necessarily confirm the system has gone open loop. If the O2 sensor signal is not crossing stoich (~450mV), it could mean that there's a fault somewhere causing it other than the PCM not modulating the IPW. For example, excessive fuel pressure or leaking injectors may cause an overly rich condition that the PCM cannot, despite it's best efforts, rectify by reducing the idle IPW. Lazy or contaminated O2 sensors can also cause what appears to be this "open loop" behavior by being extremely slow to react (this is typically what causes codes 15/17/23/24 while on the highway.)<P>The PCM has a comparatively narrow window of injector pulse widths it can operate within for a given operating condition. If your O2 sensors are reading lean or rich all the time at idle, check for vacuum leaks, VAF output, fuel pressure (PFR, return lines etc), EGR and even for leaking injector(s). If your O2 sensors are older (more than 25000KM), consider having them replaced before delving into deeper diagnostics.

[kevsurf75]

Reply to Mike -<P>Yes, I agree with what you say, however I have introduced a small vac leak at idle and the O2 sensor voltage drops immediatly to 200mV but the ECU does not react. This is what is leading me to think it is not O2 sensor related. I have seen too many people throw money away replacing sensors on the off chance of them being defective to go down that path. I have spent a lot of time researching the problem which I am sure is not unique - someone out ther must have had the same problem. It can't be obvious because my local Mazda dealer could not help. My next stop is the ECU.