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[grifter]

Hi

I've had this problem at MOT now for a couple of years. Luckily my tester is leniant on me and knows I look after the old girl well but for the life of me can't get to the bottom of the emissions problem.

The exhaust is leak free and in good condition, I replaced most if not all of the sections over the last 5 or so years with GM parts, it's had an aftermarket oxygen sensor and cat fitted a couple of years ago, had the usual fuel flushes, full service, head rebuilt with new tappets, valve grind in etc. new idle stepper motor, replaced a lot of the vac hoses in engine bay plus new inlet gasket, which are a common fault on these too. It runs really well, the plugs are normal colour, they don't soot up. Uses fuel frugally. Keeping in mind it is a 23 year old cavy, there is nothing to suggest a running problem or anything untoward, other than very occasionally the revs sit a little high after dipping clutch when approaching a stop, then will return to 800 rpm, it's not lumpy or jittery, they sit a little high then return to normal after say 5 seconds. It's always done that, and if anything is not as bad as it used to be.

I'm thinking that maybe I need to blank off the EGR valve, as many cavy owners do, but that seems to be more for improving tickover, as the 1.8 SPI is know to suffer strange/erratic tickover speeds, which mine really doesn't.

Other suggestions were CTS, but this has been replaced as well as stat, it heats up quickly at sits at 90oC on the gauge quite firmly.

I'm starting to think perhaps the single point injector or something, but again the performance and economy doesn't really lend itself to that being a problem.

Given that HC seem to be in range but lambda and CO are high, what might this point too being a problem? I've picked up this might be an air leak/exhaust leak, although these seem be ok and everything vac/manifold in engine seem to be fine and renewed relatively recently.

[Marks DTM Calib]

A lambda fault tells you it's either open loop control or there is an air leak into the exhaust

[grifter]

A lambda fault tells you it's either open loop control or there is an air leak into the exhaust

The clip on the lambda plug broke a few years back but put a tie wrap round it longways to pull the plugs together and it seems fine. Exhaust was rebuilt not that long ago as I had broken manifold studs to replace and a back box to replace.

[zirk]

If its the 8V engine Im thinking of, theres a Wire mesh breather Filter within the Rocker Cover that can clog up and then unblock now and then causing all sorts of intake / breather issues.

[johnnydog]

I've had several 8v and 16v Cavaliers, all the 2.0 versions, before I got my first Omega.
The symptoms you describe of the revs remaining high for a few seconds when you blip the throttle were the same on all my later model 16v engines, but no so on the 8v.
You weren't aware of it under normal driving, only when you revved the engine slightly whilst stationary and then let your foot completely off the throttle.  Can't remember specifically now about the Cavalier, but some cars (was it Fords maybe?) had a throttle damper to do exactly what you're describing.

[cam.in.head]

The revs can also sit high at first when throttle released and then fall back to normal if either the throttle plate isn't closing properly due to being gummed  up or adjusted wrong.or also if the throttle bypass screw is open too much.the idle air regulator valve then brings it down to the correct amount.idealy if you block off the idle valve completely by squeezing the hose revs should fall lower than idle.say 500 rpm ish

[Marks DTM Calib]

All, its a C18NZ engine with the single point Multec setup so no idle valve as such, what it does have is a stepper motor bolted to the side of the SPi throttle body which acts on a drilling in the TB.

But the bottom line is, the MOT test is saying there is oxygen still left in the exhaust gases so its running lean or there is air getting into the exhaust.

[grifter]

I've had several 8v and 16v Cavaliers, all the 2.0 versions, before I got my first Omega.
The symptoms you describe of the revs remaining high for a few seconds when you blip the throttle were the same on all my later model 16v engines, but no so on the 8v.
You weren't aware of it under normal driving, only when you revved the engine slightly whilst stationary and then let your foot completely off the throttle.  Can't remember specifically now about the Cavalier, but some cars (was it Fords maybe?) had a throttle damper to do exactly what you're describing.

It's actually happening when I pull up to stop, say In 2nd/3rd and I drop the clutch before coming to a halt, the revs sit about 1.5k then strangely or not, stay there until the car is coming to a stop, say 5mph and under. I've also had this when driving along, you take your foot off the gas and it's still tanking along, you drop the clutch and the revs are sitting up at 1.5 - 2k. It doesn't seem to do that now though after replacing stepper motor, but still get the high idle when in neutral coming to a stop sometime.

[grifter]

If its the 8V engine Im thinking of, theres a Wire mesh breather Filter within the Rocker Cover that can clog up and then unblock now and then causing all sorts of intake / breather issues.

Cleaned all that out recently when replacing rocker gasket.

[grifter]

All, its a C18NZ engine with the single point Multec setup so no idle valve as such, what it does have is a stepper motor bolted to the side of the SPi throttle body which acts on a drilling in the TB.

But the bottom line is, the MOT test is saying there is oxygen still left in the exhaust gases so its running lean or there is air getting into the exhaust.

That's it, the C18NZ. That stepper was replaced recently. It did help with the idle somewhat. Other than things I described above about high tickover it would also sometimes drop revs way down to 500 or below then shoot back up to about 1.2k then settle. Doesn't do that now.

The only things that haven't been replaced or tinkered with are the SPI injector or the internals of the carb body, except that stepper of course.

I keep wondering if something happened when I converted it to manual. There is a procedure for shorting out a part of the ECU that is basically the bit that tells it the car is running an auto, then you unplug the auto loom and take out the auto ECU, although strictly speaking I don't think you have to do that. Some of the Earth wires on the back of the block got stretched and one snapped off, when the engine got dropped to drop the box out. Those got repaired right enough.