Strange idle on 2.0 8V

[dorsetdave]

The symptoms on my 94 2.0 8v CD auto.
It has 101k miles on it, I have owned it since last January and it was professionally serviced when I took delivery.
It runs well with good m.p.g. uses no oil but as I bought it with a new M.O.T. I did not see the emissions readings.
Somewhere I have seen a post about the fault on the Forum but can't find it.
When the engine temp rises (stuck in traffic for example)and the fan starts to cut in the revs go up to about 1500 on tickover and then it starts to fluctuate between normal tickover speed and the 1500. When I bought the car it had the wrong octane select plug ( probably from new) but this was corrected earlier in the year thanks to help from the Forum. With the old plug the tickover was about 600 and it was pretty close to stalling in drive. It now ticks over at about 850, the same as my other 2.0 8v car. Once the cooling fan has done it's job everything reverts to normal.
Could someone suggest what I should start checking.
Thanks for looking.

[Cavalier342]

Just a random thought, if it idles fine when the fan isn't running, then it could be a faulty sensor giving the ECU some bad information (sending the wrong signal), I've no idea why it would affect the engine RPM so much, but it sounds like something electronic to do with a temperature sensor. Probably the large fat one bolted into the radiator half way down on the offside. Either that or the Idle Control Valve doesn't like heat for some reason or other and that's screwing up the idle, maybe worth checking the plug wiring on the ICV to check for any obvious signs of damage/loose connections.

Not really sure what else to look at based on your message.

[dorsetdave]

Thanks for that quick response. Just a thought. I can't see that it would make a difference but this car has an automatic transmission cooler, original fitted with the original Vauxhall towbar.

[Cavalier342]

As far as I can remember, the transmission cooling system is just a couple of pipes which go from the transmission to the radiator, snaking under or around the battery tray area. Doubt it's anything related to your issue.

[Robsey]

It does sound electronic to me.
Almost like a "load compensation" function over-reacting.

Unless the extra pull on the electrical system by the fan is causing the engine to almost stall, and so it is upping the revs to prevent cutting out.

Obvious stuff is check all electrical connections,
clean and dry them,
if necessary spray with moisture repellant / wd40 type stuff.

Also, at this time of year, check the charge of the battery.
Weak, partially discharged or dying batteries can cause all sorts of odd running behaviours.

Just another thought - if it is an automatic, don't cars with auto boxes often lift the revs a little when you put them into gear?
(Move out of P or N).
Apologies if I am talking garbage.

[Cavalier342]

Yes Robsey, the tickover rises slightly when in gear, but only very slightly, 100-200RPM or so, no more than that.

[dorsetdave]

Before pulling anything apart I took the car out where I knew I would get stuck in traffic. The erratic tickover started about 30 seconds before the fan cut in. I did not have this fault when the car was fitted with the wrong octane plug. As far as low voltage goes the car was misbehaving in the height of summer and the battery is pretty new. My other 2.0 8v does not have this problem. I was wondering whether to refit the original octake plug (if I can find it) or perhaps the one from the good Cavalier. Could it be related?

[Robsey]

If you suspect the octane plug, you have three choices...

1 - Flip the plug over to see if you have the wrong setting plugged in.
(All plugs have two values / settings).

2 - Refit the original plug if you can find it.

3 - Unplug the plug altogether as this will give another setting. (In some cases, it is the same as No1 above).

The ideal would be to have the same plug and setting as your working car.
(In simple terms, the plug is just a glorified resistor - the value of which sets the engine configuration in the ECU).

But -

I still can't help thinking that it is a sensor or idle compensation setting.

So next question is... how old is the coolant temperature sensor at the back of the thermostat housing body ? - (the two-pin square-bodied connector near to the alternator).

Most of the time, when they are failing, these give high revs when cold, but sods law says that yours could do this when the car is at it's hottest, triggering a choke-on like fuel enrichment and thus ramp up the revs.

Oh - and don't forget to check that the connector pins are clean and the connector is securely connected and in good condition.

Finally - if all else checks out, have a look at the engine ECU in the driver's kick panel.
Check for any water damage, corrosion of the pins etc.

[dorsetdave]

Had to help a friend whose Mercedes had a hissy fit, so didn't get time to work on the Cav. However I did check the octane plug on the problematic car and it's yellow. It was suggested earlier this year that the original one in the car was wrong and was setting the tick over to 600 which was too slow. I think the one that came with that car was black.The one on the LS is white and I am sure that it's the original. Any thoughts as to whether the CD has the wrong plug?
Both cars are the same spec. but built six months apart.

[Cavalier342]

Never seen a white plug before, I thought it was just Brown, Black or Yellow.

[dorsetdave]

I have re-checked the non faulty 2.0 LS and it definitely has a white plug, 95 and 98 on it. It's running 95. The car was UK supplied, Moorlands, main Vauxhall Dealers in Blackpool. The car was always used in the UK. I have the original invoice and all of the documentation from new. I cannot see any reference to the plug being changed, or indeed any complaints about it running badly. I understand that there were some cars that left the Factory with incorrect plugs. Perhaps the Dealer just swapped it and said nothing. The first owner does appear to have been quite pedantic in his dealings with Moorlands.

The faulty car, CD was also UK supplied, Foleys, Vauxhall Dealers Swanage. It was originally running on a black plug when I bought it. 91 and 95 are marked on it. It has been changed to a brown one apparently part number 90241963, 91 and 95, running 95. This one was supplied in an original sealed Vauxhall bag but I have no idea if the white one is even a Vauxhall one. I can't see any numbers on either plug.

[Cavalier342]

I remember someone suggesting that they should be set to 98, as back in the 90's the fuel wasn't as high octane as it is nowadays, maybe worth swapping it to 98 setting and see if anything improves. It's possible that the painted numbers on the plug have been rubbed off if it's been swapped around a few times.

[dorsetdave]

Sorry I should have made it clear, the numbers that I can't see are part numbers. The octane ratings are in perfect condition. The only part number that I have for sure is the brown one that is now in the CD Cavalier. That was shown on the parts bag.
Apologies again.

[Cavalier342]

Ah ok, no dramas.

[dorsetdave]

Bit of an update.
The battery is showing plenty of volts, even today when it's cold. Car starts very easily even though it was left out last night.
The temp. sensor looks to be the original.
Contacts all look good and clean.
Have now put the original black plug back to give it a try.
Will keep you posted. Thanks, everyone for your interest.

[Robsey]

Just to give a run down of the possible plugs that can be used on a C20NE.
(There are a lot more than you would expect).
There are some that are not applicable to the C20NE, so these say n/a.

Black = 91 Ron (0 ohms) = Basic Setting.
Black = 95 Ron (220 ohms) = Basic Setting
--------------------------------------------------
Brown = 95 Ron (220 ohms) = Basic Setting
Brown = 98 Ron (470 ohms) = n/a
---------------------------------------------
Red = 95 Ron (Infinity) = n/a
Red = 98 Ron (4700 ohms) = n/a
---------------------------------------------
Orange = 91 Ron (750 ohms) =
Idle speed increase by 100rpm.
+5% fuel acceleration enrichment.
Ignition retarded by 5deg throughout rev/load range.
Orange = 95 Ron (Infinity) = n/a
----------------------------------------------
Yellow = 95 Ron (2200 ohms) =
Idle speed increase by 100rpm.
Yellow = 98 Ron (Infinity) = n/a
----------------------------------------------
White = 95 Ron (2200 ohms) =
Idle speed increase by 100rpm.
White = 98 Ron (750 ohms) = n/a
---------------------------------------------
Blue = 91 Ron (Infinity) =
Idle speed increase by 100rpm.
Blue = 95 Ron (0 ohms) = n/a
--------------------------------------------
Violet = 95 Ron (Infinity) = n/a
Violet = 98 Ron (2200 ohms) = n/a
----------------------------------------------
Green = 95 (1200 ohms) (part No. 90276398) =
+5% fuel acceleration enrichment.
Green = 98 (750 ohms) = n/a
------------------------------------------------
The Non-Cat versions are completely different. So if you have a 20NE, 20SEH then the plugs should be different to those listed.

[dorsetdave]

Thanks Rob. There are far more plugs than I thought. Looks as if it should be yellow or white set to 95. This gives the 100 rpm extra, presumably because it's an auto. I've put a yellow one in on 95 as the black plug tickover plug is just too slow. It doesn't quite stall but it's getting very close on tickover in drive.
Will report back.

[Robsey]

To narrow the field down.

From my Octane Coding Plug thread.
Here are all the possible plugs and settings.

95 RON
220 ohms = Basic Settings

1200 ohms = Fuel Enrichment 5% higher than basic setting when accelerating.

2200 ohms = Idle is 100 rpm above basic setting

4700 ohms = Fuel Enrichment 5% higher when accelerating and Idle is 100 rpm above basic setting.

-------------------------------------------------
So taking that into account -
All plug options for Basic Settings are:-

BLACK = Coding "A" - (220 ohms)
Part No = 90276397 

BROWN (ALSO SERVICES YELLOW) = 98/95 RON - (220 ohms)
Part No = 90276401 

BROWN (ALSO SERVICES YELLOW) = 98/95 RON - (220 ohms)
Part No = 90276401 

BROWN (ALSO SERVICES YELLOW) (NLS.) = Coding "A/B" - (220 ohms)
Part No = (90307657) 

All plug options for 100 rpm above idle basic settings are:-

WHITE = 98/95 RON 
(2200 ohms)
Part No = 90241962 

YELLOW = 91/95 RON
(2200 ohms)
Part No = 90241963 

Noting that it is unlikely that you will come across 91 RON.
and there was never a 98 RON / 'B' option on the C20NE.

So all settings are for "A" or "95".
And because the resistances are correct, we can ignore EPCs engine listings.

[dorsetdave]

Thanks for the information. Car was in for servicing yesterday. The air hose from the air cleaner to the throttle body was split. I have ordered replacement part number 90324550 which is due to arrive by Tuesday ot Wednesday. Could this be the cause?

[Cavalier342]

That may not help as it would suck in hot air and probably cause odd idling or some loss of power. Those thick rubber air hoses tend to split at the ribbed sections for some reason, they just get that way after years of heat/cold.

[Robsey]

Yes it could play a big part in causing your issues.

Any leaks in the induction system will cause an imbalance to the mixture control and therefore to the smooth and efficient running of the engine.

I suppose you could wrap some cling film around the hose a few times, as a quick check to see if you are on the right track.

[Cavalier342]

You can try patching the cracks and splits up with glue as well just to fix it temporarily.

[Robsey]

Now is when a bicycle puncture repair kit may be useful - or at least the rubber solution.

[dorsetdave]

Thanks for your thoughts everyone. I will report back when the hose is fitted.

[cavalier1990]

Yep, on 2.0, or any of the muli-point inj. engines, any splits in air hose make huge difference to running as it uses air flow meter. On the 1.8/1.6 it doesn't as it is only a carb effectively.