Another bloody sensor issue...

Nickers · Postby **Nickers** » Wed Jun 06, 2007 11:37 am

While everyone is thinking about sensors I might as well raise my issue too. Since the onset of cold weather, I've been having this problem.

The car starts fine, but in the first 2-3 minutes of driving if I let it fall back to idle it'll stall. If I tickle the gas a little, the revs climb ok but drop really quickly and it sounds like its about to stall. It also struggles to reach 60km/h in 3rd, I think because its running rich.

However, 3 mins down the road the car performs flawlessly. It's appetite for fuel seems to be increasing, but this may be because I let it run for a few minutes before heading off in the morning.

I've checked all intake piping and seals to make sure there is no leaks. I've had the error codes checked, but no fault was reported. This was done when the car was warm and running ok; do you think it'd report a different response when cold and playing up? My mechanic has just replaced the coolant temp sensor, but my problems persist... any ideas?

JBT · Postby **JBT** » Wed Jun 06, 2007 11:46 am

Oxygen sensor :?:

Bevan · Postby **Bevan** » Wed Jun 06, 2007 12:10 pm

Definitely sounds like the O2 sensor. Taking ages to get up to temp, to is runs like sh*t, then runs OK, but really thirsty for fuel once operational.

Go to Slugoid's thread: http://www.aus-cartalk.com/viewtopic.php?t=25609
Or better still, go to Boags' thread: http://www.aus-cartalk.com/viewtopic.php?t=24837

Follow the step by step guide in that thread and you should be OK.

How long ago were the spark plugs and leads changed? Still sound like the O2 sensor though.

Boags · Postby **Boags** » Wed Jun 06, 2007 12:36 pm

Bevan wrote:How long ago were the spark plugs and leads changed? Still sound like the O2 sensor though.

Hmmmm, sounds like spark plugs or leads to me.

The ECU ignores the O2 sensor and runs off maps in open loop until the car is warmed up. So if the car runs poorly when cold then the problem is elsewhere, (unless the ECU is getting a bad temp signal and going into closed loop while the O2 sensor is cold (ie. reads lean) - which will make the ecu dump fuel in. It'll bog like crazy though, it's not something you can mistake for anything else.

I'd be starting with plugs and leads personally. As Bevan asked; when were they last changed?

Boags

Nickers · Postby **Nickers** » Wed Jun 06, 2007 12:41 pm

Cheers all for the comments.

Leads were done about 6 months ago, so I doubt it's them. I'll check if the plugs were done at the same time - I've been looking for an excuse to get iridiums! I'm keen to get the diagnostics read when cold, but of course by the time I get to the mechanics the bloody thing has warmed up and reports no fault. Maybe I should knock up the home made diagnostics LED to test when cold...

So the plugs may not be sparking correctly when cold, but rectify themselves when heated?

Fatty · Postby **Fatty** » Wed Jun 06, 2007 12:47 pm

it is my understanding that the fault code will be held in the ecu's memory until it is tested. it'll stay there until the unit is re-set. so, it shouldn;t matter if the car is hot or cold when yr mechanic tests it. at leat, that is my understanding but i could be wrong .

anyway my 2cs, yes it sounds like the o2 sensor to me. my car had similar symptoms but not as bad as you describe. new o2 sensor fixed it.

Garry · Postby **Garry** » Wed Jun 06, 2007 1:07 pm

I think the fault code goes away after a set number of restarts if the fault is no longer detected. I'm not sure how many restarts though. Definitely a fair number though.

Bevan · Postby **Bevan** » Thu Jun 07, 2007 10:47 am

Nickers wrote:I'll check if the plugs were done at the same time - I've been looking for an excuse to get iridiums!

Get them for eBay, much cheaper than anywhere else.

Actually, they were on sale from supercheap auto for $15 not too long ago, maybe keep an eye out there. Still cheaper on ebay though.

Nickers · Postby **Nickers** » Fri Jun 08, 2007 5:37 pm

I wish it was plugs and leads!! We can eliminate this as it is definately an air/fuel mixture problem, with too much of the latter being poured in before things get toasty.

Is it logical to assume that if no sensors are reporting a failure that the problem may be with the ECU? :cry:

If so, what is a replacement ECU worth?
Or is it possible for a stuffed sensor to not report a failure?

Postby **Okibi** » Sun Jun 10, 2007 2:26 am

Bevan wrote:....Actually, they were on sale from supercheap auto for $15 not too long ago, maybe keep an eye out there. Still cheaper on ebay though.

Group buy

My last lot cost $9 each.

yarnmx5 · Postby **yarnmx5** » Sun Jun 10, 2007 4:16 pm

My car has been doing the same thing. always got the hesitation just around 3000rpm but still ran ok other than that. Changed the O2 sensor (NGT sensor with connector E20?) after a bit of a struggle..tight in there with loch headers. But installed and now very happy....only did it today and so far so good. looks like the hesitation has gone.

Nickers · Postby **Nickers** » Tue Jun 12, 2007 5:53 pm

yarnmx5 wrote:My car has been doing the same thing. always got the hesitation just around 3000rpm but still ran ok other than that. Changed the O2 sensor

I don't see how it can be the O2 sensor... My problem only happens when the engine is stone cold, so the O2 sensor wouldn't be up to temp, therefore it wouldn't be transmitting a signal.

After doing some digging on miata.net, I have a few things to look at. I'll try and clean the IAC valve and check out the fuel pressure regulator vacuum control line to see if this has fuel in it, but I'm moving this weekend too so time is at a premium!

Hopefully I'll report a fix soon...

Nickers · Postby **Nickers** » Tue Jun 12, 2007 5:59 pm

That was my mechanic's guess too. We replaced it about two weeks ago, with no change.

He's now thinking that it may be the computer itself... :evil:

lowmiata · Postby **lowmiata** » Wed Jun 13, 2007 2:55 pm

The symptoms you post are very similar to when my O2 sensor died

I would just change it to factor in another point that has been changed to a known good unit.

Repco sell a universal single wire unit that will screw into the original hole
cost is about 45 buck IIRC

mine would hesitate and die coming into the roundabout approx 3 minutes from home would happen at the exact same time every day

(for a week or so i would drive upto the round a bout and pull over for two minutes then go cause there were a couple of hairy situiations where it died mid roundabout!)

could save alot of heart ache start with the cheap easy to get to stuff!

Nickers · Postby **Nickers** » Wed Jun 13, 2007 3:11 pm

Yeah, I see your point, but at the same time I don't want to replace parts for no reason.

Cheap or not, if it has no bearing on a cold engine it can't possibly be the cause can it?

I'm a newbie at all this so please tell me if I'm missing something...