Fluctuating Idle

[Lonnie]

Hello all, first post here. I have a '90 celica with a 2200 16 valve EFI engine. The idle fluctuates from about 500 to 800 rpm up and down. There is also some black smoke from the exhaust. The car runs great out on the highway and accelerates great but this idle problem is killing me at stoplights. I unplugged a sensor mounted at the center of the firewall that I have identified as a vacuum sensor and the idle smooths out but is too high and the presents of black smoke is still there. this sensor has a vacuum hose connected to it that connects to something that threads into the plenum.

Could this vacuum sensor be bad?

[daktari]

Hi Lonnie. Welcome to TOC. :D Do you have Aircon fitted? If so, is it working correctly? And does it make any difference to the idle when you turn the A/C on or off?

[Lonnie]

Hi Lonnie. Welcome to TOC. :D  Do you have Aircon fitted? If so, is it working correctly?  And does it make any difference to the idle when you turn the A/C on or off? 

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Yes, the car has very cold A/C. The problem is not as bad with the A/C on. With the A/C off the car almost stalls

I can disconnect and plug the vacuum line from the vacuum sensor (with the electrical connector still plugged in)and the idle smooths out but is too high. I can hook a hose up the the vacuum sensor and apply air pressure and the idle will drop pretty close to where it is suppose to be If you put too much vacuum on this port, the car will stall Oh, BTW this car has a manual tranny.

[Lonnie]

I plugged the vacuum hose and connector back up to the vacuum senser(which cured the black smoke) and started jiggling the throttle position sensor wiring connector. The fluctuating idle has disapeared but the idle is rough. Drivability is better now but I need to get my idle smoothed out. I think i am going to buy a TPS. What do y'all think?

[daktari]

It`s a good possibility that thats whats causing the problem. Might be worth seeing if you can borrow one thats known to be ok. I dont` suppose they`ll be cheap. And if it`s not the problem it`ll be money down the drain.

Incidentally .. I had a similar problem with my GT4 when i first got it. But affected the whole rev range. Turned out to be the aircon idle up valve. Might be worth investigation.

[Lonnie]

Does the '90 celica GTS have an idle speed control valve? There is a gold colored screw on the throttle body that must control the air, Any idea how this needs to be set, as in how many turns out from fully seated?

Thanks!

[mikeb]

Unplugging the vacuum (MAP) sensor leaves an open pipe into the inlet manifold. It sucks in extra air, and brings your idle speed up. It's nothing to do with the cause of the problem though.

If you have a multimeter you can check the tps:

The screw on the throttle body is for setting the position of the TPS. You cannot adjust idle speed mechanically, it is under closed loop control of the ECU. Which leads us on to the idle control valve, which could well be the cause of your problem. They often get gunked up, and sometimes go faulty.

Having said all this, the black smoke is as pretty strong indication of an internal engine problem, which could well be !Removed! up your idle. I have seen lots of cars with idle problems, and the owner blames the electronics, idle speed settings etc, when 9 times out of 10 it is a mechanical failure. Get a compression test done, it will highlight any obvious engine mechanical problem.

[Lonnie]

Thank you very much for the info! I have a multimeter so I will check the TPS out. Compression checks out good on all cylinders. It only black smoked when I unhooked the MAP. Car runs great on the road, Idle is rough and when you flog it the rev's are kinda slow to come up.Timing is at 10 degrees with terminals E1 and TE1 jumped. I do not see a schrader valve to test fuel pressure.

Where is the Idle control valve?

Thanks!

[Lonnie]

I jumped the correct terminals at the diagnostics port and got:

two flashes pause two flashes

two flashes pause four flashes

three flashes pause one flash

Can anyone translate these codes?

[Lonnie]

Ok, I found a site on the net that gives a discription of the codes, one of the codes has to do with the coolant temp sensor for the ECM. If this were bad, could this be causing the rough idle?

TPS checks out OK but I have my doubts about the connector.

[mikeb]

Code 22 is Coolant temp sensor (THW)

Code 24 is Intake air temp sensor (AIT)- probably won't cause your idling problem.

Code 31 is the vacuum sensor (MAP) - hardly surprising since you pulled the pipe off!

Reset the ECU (pull the EFI fuse for a minute, or disconnect the battery), to clear these codes. Then run the engine again, and any sensors that are faulty now will cause more diagnostic codes.

Does your idle problem occur when the engine is hot or cold or all the time? When the engine is cold the ECU raises idle speed to ~1800 rpm, and slowly lowers it as the engine comes up to temp. If the temp sensor is failed open circuit (and possibly closed circuit as well), then it won't attempt to fast idle when cold, and will have the symptoms you describe.

Check the connector to the THW sensor before writing it off as faulty, but the sensors themselves are not expensive or difficult to fit anyway.

[Lonnie]

The idle problem is present all the time. The engine never goes to fast idle even when dead cold.I take that back,when dead cold it will idle up for about 1 second and then fall back to a 800-900rpm rough idle. Strange, but I cleared all codes and drove the car about 15 miles, rechecked for codes and none present. I think i will buy a coolant temp sensor anyway since the fast idle for cold startup is not there. Odd thing is,you could not ask this car to run any better on the road. You can be in 5th gear at a low rpm lugging it and mash the pedal down and it never hiccups.

What is the THW sensor?

Thanks again!

[Lonnie]

I am going to try a set of new set of NGK plugs and a set of plug wires. It is getting time for new ones anyway. I will let y'all know.

I appreciate everyones help so far

[mikeb]

The idle problem is present all the time. The engine never goes to fast idle even when dead cold.I take that back,when dead cold it will idle up for about 1 second and then fall back to a 800-900rpm rough idle. Strange, but I cleared all codes and drove the car about 15 miles, rechecked for codes and none present. I think i will buy a coolant temp sensor anyway since the fast idle for cold startup is not there. Odd thing is,you could not ask this car to run any better on the road. You can be in 5th gear at a low rpm lugging it and mash the pedal down and it never hiccups.

What is the THW sensor?

Thanks again!

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THW, aka ECT - engine coolant temp, is your water temp sensor. This will deffo cause your idle problem when cold, without really affecting engine performance when you open the throttle. Does it idle correctly when fully warmed up? By then the fast idle will have disappeared and the engine should idle comfortably around 900 rpm. If not, then there could be a problem with the idle control valve.

[Lonnie]

Same rough idle cold or fully warmed up. Someone told me the distributor could be the problem. I am going to do the plugs and wires in the morning.

I appreciate everyones help so far

[Lonnie]

Installed new NGK plugs and a new set of wires. Same crappy idle when cold and no fast idle when cold. Idle is much better now when engine is fully warmed up but still not what it should be. Since the engine does not want to idle up when cold, I am thinking it has something to do with the Throttle body or the IAC valve.

What do y'all think?

[mikeb]

It won't be the throttle body itself. Idle speed is regulated by the ISC valve (near the throttle body). Remove it and give it a thorough cleaning. Also check the connector pins are not corroded etc, and pipes are not gunked up as well. It could be faulty, and I don't know if the ISCV generates a fault code.

Have you fixed the coolant temp sensor because that could be affecting things?

[Lonnie]

Have you fixed the coolant temp sensor because that could be affecting things?

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I plan to concentrate on this next. I think there is a way to check the CTS with a multimeter. It has not throwed a code since I cleared them the other day.

[mikeb]

I think you will find this technical document rather useful:

http://www.autoshop101.com/forms/h24.pdf

Not all of it will apply to your car, but pages 16,17,18 cover the coolant temp sensor and air temp sensor. Both would be worth checking with a multimeter (it should be obvious how to do it from those pages). However, temp sensors are unlikely to fail intermittently, and you say they are not throwing the errors now. In my experience they either work or they're faulty full stop. I would check the connectors very carefully, corroded contacts could well cause intermittent problems.

[Lonnie]

Very good info there Mike, I really appreciate you taking time to post that. I am trying to fique out the color of the wires going to the Coolant temp sensor. There are two sensors at the thermostat housing that look simular, one has red and brown wires and the other green and black.If the red and brown wire sensor is the CTS then it checks out OK with the OHM meter. The car is running fairly decent now when fully warmed up but not like it should. I wish I knew if this were a fuel or fire problem.

[pink24]

ecu temp sensor and that either work.or they dont.i just repaired my mates gt4.i think your looking in the wrong area.check fuel pressure.dizzy cap.coil pack lead.get ur self a compression tester.and also check for vacume leaks.these toyotas are a pain!!!!