ECU causing idling problem?

Hi everyone,
This is my first posting, not just here, but to any forum! Let's see what you make of this.
My 1998 Corolla has been messing me around since March. The symptoms are that it sometimes won't keep to a steady idling speed - I think it should be around 800 rpm; it will drop to about 200 then surge to 1200 and then back again. On occasion it will drop so low it stalls. This does not always happen, though it tends to behave when cold. I have take it to a Toyota dealers who suggested the idle speed control valve was the problem, which I have replaced, also to another garage who changed my throttle positioning sensor. I have put in new plugs and changed the fuel filter and the entire throttle assembly. I have had the exhaust tested and it looks like the O2 sensor is working fine. I have disconnected the battery and left it for 1/2 hour and reconnected, this seems to improve matters for a few days, but the idling surging eventually returns. Plugging it in to the garage's diagnostic computer does not show up any error codes. I am guessing that the problem is something to do with the ecu unit itself. I am unable to find Haynes manual for this age of Corolla in my town, the only ones available on the net seem to be in the USA.
Does anyone out there agree with my diagnosis?
Can anyone tell me where to find the ecu so I can send it off to be diagnosed?
On a Celica it is supposed to be behind the glove box, which I have removed on my car but no sign of an ecu there.
Help would be appreciated.
Thanks.

I have a similar car with a 2E engine which is behaving in a manner identical to yours. I have browsed the Internet but have no answer. It probably has to do with the carburettor settings, but I have not been able to get any info about setting the carburettor.

I don;t think a 1998 has a carb - it should have fuel injectors. Under Troubleshooting, Section 8 (Engine lopes while idling or idles erratically), it could be:
1 - Vacuum leakage
2 - Fuel or electrical systems
3 - Leaking EGR
4 - Air filter
5 - Fuel pump
6 - Carb out of adj.
7 - Leaking head gasket
8 - Camshaft lobes worn

Now, I would think that the head gasket may be the problem because of the intermittitness of the problem.

First - Shiob, thanks for replying, let's hope a solution comes along which will sort both of our vehicles out!

Now, bcopeland. You are correct, the 1998 is fuel injected, which is why I considered the ecu to possibly be the cause. Of those 8 options the only one I hadn't really thought about was your favourite, the leaky head gasket. About 18 months ago I took the car in to have its cambelt changed, as it had hit 60000 miles. When the garage did this they told me that there was a little discharge from the head gasket, nothing serious but they said they felt obligated to let me know (and even provided a digital picture of it), so I am very keen to pursue this line of investigation. Do you know if there is a "quick fix" - such as putting some substance in the water or oil which would plug the leak - rather than having the head gasket replaced, which I guess would be rather expensive? Not being particularly clued up on mechanics, I don't see how a leaky head gasket would cause the symptoms I am getting, but never mind, maybe we are getting somewhere.

Thank you very much, I'm glad I joined this forum now, cheers guys!

I have since flushed the coolant from the engine and replaced it with new antifreeze and a substance which is supposed to seal small leaks in the system. After running it around for 40 miles the surging had returned.
I am still inclined to think that it is the ecu - does anyone out there know where I can find the little blighter on a 1998 Corolla?

What is a "blighter"? It very well could be the head gasket. You can replace it with the engine still in the car. As for Shoib's problems, you really need the Haynes manual. Try http://www.autozone.com or http://www.amazon.com

I have the same problem with my 97 corolla. No luck finding the problem yet however. I hope its not a leaky headgasket. :shakehead

either way, those manual. say something like ECU malfunction or faulty EFI... so your best bet, is go to a shop or a dealer and get the car plugged and then come back with a diagnosis..from there if it's a sensor( and it should be...MAP or wiring or something) we can tell where to look for putting a new one in.
it sux man to go and pay a dealer for that but sometimes, and certainly on a car thats OBD2, you have no choice.


Deadring

bcopeland; apologies for using colloquialisms, I was forgetting that most of the folks on this website are American. In England a “blighter” is an annoying person, a gremlin. I cannot find my ECU and it is annoying me.
I have just ordered a Haynes manual from Amazon, maybe that’ll be able to help me locate the ECU.

rob1005; I too hope it is not a leaky head gasket, the stuff I put in the coolant doesn’t seem to have affected the problem and its been in there now for the last 1300 miles.

deadring; I have had the car on a diagnostic rig at the Toyota dealers, which gave a code saying it was a faulty idle speed control valve. I then had it on another rig which said it was the throttle positioning sensor. I then replaced both and had the codes checked again – this time nothing, but I still have the surging. The garage said that the ECU is good at diagnosing faults except for when it is the ECU itself which is faulty!
When I finally locate the ECU I may have to send it away to be analysed

I'm pretty sure the ECU is behind the steering column.

Hi everyone - good news!
It is now two years later and the problem is resolved.
My car failed its yearly MOT check - on emissions. The garage said the timing was out and reset it, they also replaced the spark plugs (which I did two years ago) but still the idling was oscillating wildly. Then (for an extra £130 ($260) they REPLACED THE LAMBDA (O2) SENSOR. Job done. It idles perfectly now, I have done 400 miles since and no problems whatsoever.
At first they were confident that it was going to be a leaky air hose, a frequent problem apparently on the Ford Focus but then dismissed that idea.
So, for anyone else with this problem - it could be worth considering.
I'm glad I didn't shell out on a new ECU!
Thanks again for all your replies.