Whats The problem guys?

[zshahz]

I have a corolla AE92 '87 manual..

sometimes when its on nuetral, the revs go up and down. What could be the issue here?

[CozmoNz]

Does it have a idle stepper motor?

also could be a vac leak...

[Skidman]

How quickly?
When cold is it more noticable?

[fangsport]

does it have a full compliment of coolant??

[zshahz]

whats a idol stepper motor? ill try to see if it happens when its cold and get back to you guys.

[Dragger_Dan]

does it have a full compliment of coolant??

This is the most likely explanation. Try burping the cooling system.

[fangsport]

does it have a full compliment of coolant??

This is the most likely explanation. Try burping the cooling system.

don't you mean 'purging'??

if the 2 temp sensors arn't constantly on coolant, they will give dodgy signals to the ECU.

[Dragger_Dan]

I ment burping as in getting all the air out, if there are air bubbles in the system.

[Skidman]

Im not getting smart, only curoius, how would lack of coolent effect the engine in this way? Why would the ecu's failsafe do this for high temp? wouldnt the temp gague in the dash be reading way high beacause of the steam?

[fangsport]

Im not getting smart, only curoius, how would lack of coolent effect the engine in this way? Why would the ecu's failsafe do this for high temp? wouldnt the temp gague in the dash be reading way high beacause of the steam?

the sensors give the ecu certain values at consistant levels/ temperatures, if the coolant level is a bit low, the signal will be intermittant , as it won't be consistantly immersed, it will only be getting washed by the passing coolant.

steam, not being a liquid, usually won't give a reading on a temperature gauge, unless it has super-heated the passing water(if the water level is high enough ) i.e blown H/Gasket.

[Skidman]

K, so your sayin that it thinks that its going between cold start and normal running temp?
Wouldnt the ecu have parameters that have a time worked into them, so the ecu can tell that its a fault?
Also the temp sensors dont care about being in water or not, they just go off heat. I didnt think that they had a level sensor either?
Im still lost (call me stupid if you like, but this just dosnt make any sense to me at all)

[fangsport]

here is an example of a failure.

my old van was getting a wee bit hot (according to the guage), it then went back to normal (no worries i thought, stuck thermostat), it then went to nothing and was really hot ( as in in the cabin).

the H/G went, pressureised/heated the cooling system(high guage reading).the top radiator hose opened up like a zip chucking most of the water out( low reading). what was left in the system quickly turned to steam (high cabin temp).

most temp sensors will only work in water, until the surrounding metal gets so friggin hot, that the sensor itself cooks.

[Skidman]

Yep but thats when all the water is lost not just an air pocket...
All of the cars that have had an air pocket that i have had to purge out have shown it by the temp gague sailling right up to the big 'H' and i have never seen a car that has the described problem that has been fixed by simply purging the system.
Plus if the system was doing that, reading cold, then hot, seem as the ecu temp sensor is right next to the temp gague sensor wouldnt you see the temp gague going in-sync with the high/low idle???

[Skidman]

Check the coolent just incase. Nobodies perfect i could be wrong.

Carring on with the diognosis, dose it have a digital or an analog TPS?
If it has an digital TPS check that its not been tampered with, cause they usualy only sensor idle and wide open throttle, and it might be mal-adjusted and not telling the ECU to control idle, (thinking that its not idling when it is.)

They run a vein type AFM aye? (so thats probably alright, they dont usually cause too much hassle, check voltages anyway if in dought)

I take it that its got an oxy sensor...check it on a scope to see how fast its cycling (should be around 8 herts) if you dont have a scope just unplug it and it will go open loop and run ritch.

Check Battery voltage just incase (this has caused problems in the past)

[fangsport]

Yep but thats when all the water is lost not just an air pocket...
All of the cars that have had an air pocket that i have had to purge out have shown it by the temp gague sailling right up to the big 'H' and i have never seen a car that has the described problem that has been fixed by simply purging the system.
Plus if the system was doing that, reading cold, then hot, seem as the ecu temp sensor is right next to the temp gague sensor wouldnt you see the temp gague going in-sync with the high/low idle???

not big on the inner workings of EFI systems, but my old AE92 would get erratic idle, fixed by topping up the coolant. my diagnosis was to do with the 2 sensors on the back of the head, as they are amlost at the highest point of the cooling system. also, top-up has been documented on here before many times before as a cure.

[spencer]

the sensors give the ecu certain values at consistant levels/ temperatures, if the coolant level is a bit low, the signal will be intermittant , as it won't be consistantly immersed, it will only be getting washed by the passing coolant.

steam, not being a liquid, usually won't give a reading on a temperature gauge, unless it has super-heated the passing water(if the water level is high enough ) i.e blown H/Gasket.

The sensor is a basic thermistor a metal that changes resistance with heat. The response time of these things in cars is relatively slow they dont have to react fast. The heat in the alluminium housing they are bolted into will keep them from fluctuating if there is no water flowing past.

[Frosty]

How much is the idle changing? 1 or 2 hundred rpm?
It may just be a Faulty idle up solinoid for the air-con, charging or other sub system.
Also blockages in the small air gallerys in the throtle body can affect idle speed.
To see if the idle-up solinoids are playing up just un plug them one at a time to see if the idle stabilises.
P.S dont forget to plug them back in or the next time you turn on your lights or air-con at idle the engine could stall (could suck)

[zshahz]

wow.. alot of stuff to take in here fellas!!! i dunno its weird, it happens sometimes then stops, car doesnt have A/C as previous owner took it out. Is there anyone here who is a mechanic and can tell me what the actual problem is if i took it to them? This would be greatly appreciated.

Thanks

[Skidman]

Yup, where bouts do you live tho?

[zshahz]

fangsport your a bloody legend. Today it was revving up and down hardcore.. was on the northwestern motorway when i saw temperature guage going up.. i took the rosebank exit, pulled up at petrol station and yes water had almost fully run out.. quite lucky here. I re-filled it and since then problem has been fixed.

But i dont understand how this made the revvs flick up and down.. why didnt it happened to my other old car?

skidman im in west auckland... but its all good at the moment.