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Discussion Starter #1 (Edited)
SOLVED! Idle goes up/down/up/down/up/down/up/down

Car is a 2000 GT-S 6-speed.

The idle goes up and down over and over from 2000 to 1000RPM and back and forth and back and forth over and over several times, say anywhere from 5 times to 20 times before "leveling off" usually around 1000-1500RPMs and holding put for sometimes a long while and sometimes a short while before going "rhythmic" again. This happens every time the engine idles such as coasting to a stop light, idling at the light, etc.... This can make shifting and engaging the clutch fun. Note sarcasm.

The rhythmic up/down/up/down/up/down is faster when the engine is cold (it is about 35F degrees here right now) and takes about 1/2 a second to go up from 1000 to 2000 and 1/2 second from 2000 back down to 1000, and the rhythm is slower when the engine is warm and maybe takes a full second to go all the way up or down between 2000 and 1000RPM.

Now, before I get flamed for anything, I already removed the whole throttle body, removed the IAC and TPS, and cleaned the throttle body clean with carb cleaner (I did not clean the electrical sensors themselves as not to damage them). The IAC seems to spin pretty freely, but there is a very faint bit of binding (not sure if it's enough to cause a problem).

Next, I removed the MAF and cleaned it good with CRF MAF cleaner. I shined an LED light down the tube and made sure the resistors were nice and clean when I got done. One of the resistors looks brand new, the other one seems to have a faint light tan crust on it that does not come off like the black "intake goo" did. No, I didn't touch the resistors with anything or jamb a rag in there.

Yes, I have an aftermarket short ram intake with some K&N ripoff (but is in new shape), unsure of what brand because a previous owner installed it. That being said, I was about to do the highly recommended BB mod when I realized my SRI aluminum tubing is already the same size as stock anyway (so the BB mod is pointless). The aluminum intake tube is already 2.5" in diameter along it's entire length, so there's no need for me to do the BB mod.

One other comment that may or may not help. If I am driving along at 55mph and put the transmission in neutral and start slowing down (to eventually come to a complete stop), the idle goes up/down/up/down as previously stated that is, until I get to about 5mph. IF I stop quickly or if I stop slowly, the idle always seems to smooth out as I hit about 5mph. Once I fully come to a stop, the idle may hold steady for a while, or it might start going bizzerk again after 20 seconds of holding steady.

In older Toyotas I've owned, this was a sign that the coolant is low and the coolant temp sensor has an air pocket, but with this '00 Celica, the coolant level seems to be fine.

My car does not stumble, hesitate, or miss or have any other of the problems people normally associate with a dirty MAF etc.... The only symptom is an idle speed that rhythmically goes up/down/up/down/up/down.

I don't want to just start throwing parts at it (some of these sensors are expensive!), so any help would be great.
 

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Discussion Starter #6
a friends car used to do that, it ended up being a bad 02sensor, u have a CEL?
That poses a good question, and I forgot to mention this detail in my first post. There is no cat. The previous owner installed a straight pipe. Yet strangely, it takes a couple hundred miles to trigger the light.
 

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Discussion Starter #7
After doing more searching, my idle problem is likely the IACV even though I already cleaned it well. I am going to start a new post because I think the problem is the bearing(s) that the shaft rotates on. The shaft spins freely but binds up slightly when first turning it. There isn't any more gunk in there, but it feels/sounds more like bearing problem. So now I'm wondering--has anyone tried dismantling an IAC to replace the bearings (will start a new thread about this).
 

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i also have a 2000 gt-s 6spd, mine does the same thing, but very rarely.. i've noticed it in cold or rainy weather immediately after i start it.. i would clean the MAF sensor after that and it wouldn't happen again.. but recently i've noticed the idle will drop too much after i put in the clutch and brake real hard, but both have only happened a couple times... keep us updated..and good luck
 

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Yes I read your post. Initially, with the idle problems on the stock ECU it's either a bad IACV or a vaccum leak somewhere. Hope the new IACV works out for you.
 

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Discussion Starter #11
Unfortunately, the replacment IACV didn't fix the problem. It wasn't a new one, but it is functional and made me realize my old one was working fine too. So, my problem is elsewhere. The problem may be related to no cat (just a test pipe in place), so I am actively searching for a replacement (stock) cat as well. Surprisingly, this isn't tripping the check engine light (meaning the missing cat).

I'm starting to wonder if it really is the MAF simply because one of the resistors had a light tan crust on it that would not clean off. Everything else cleaned off the MAF just fine with CRC MAF cleaner. I don't think this tan crust is supposed to be there and am wondering if it's causing the problem.

After driving about 1200 miles on the car and now that the air temps are dipping down to near 0F, the failure is pretty specific and is worst when the engine is midway between warm and cold. When the engine is 100% cold (as in it it 9F degrees here right now), it will start fine, idle around 2000, then slowly start dropping to about 1500 and after about 2 minutes of idling, it will start the up/down/up/down/up/down. It will continue to do this until the engine is fully warmed up and the problem is somewhat gone. If I let the car idle once the engine is warmed up, the idle might bounce once or twice as it hunts where to go, but then it's fine (but still idles high around 1500). However, if I am driving along at, say 55mph, put the transmission in neutral and coast in neutral to a stop (with brakes or without), the idle will bounce up/down/up/down etc... until I get down to about 10mph, then the idle will smooth out around 1500. The idle will be fine as long as I continue to just sit there, but once I start driving again and put the transmission in neutral above 10mph, the idle will bounce again until I slow down enough.

Very annoying.

Regarding the ECU, is there an updated revision #, part #, or date code I should be looking for?
 

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My 07 1zz Matrix does the exact same thing, and it is extremely annoying (as everyone who's posted has included also). I replaced my o2 sensor for another reason, and the problem persists. I'm also running an aftermarket intake system, mine's an Injen, and I suspect that it's related to that. I've wondered about it being from a vacuum leak. At one point I had also thought it could be an exhaust leak, but I've gone over that and i have no exhaust leaks, so that's clearly not the cause. I don't imagine it's your ecu, because I have an 07 Toyota ecu and it still has this problem. I'm very interested to hear how you fix this. Please continue posting here
 

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Discussion Starter #13
Well, after a month, the up/down/up/down still exists. As previously stated, I replaced the IAC valve with another one (used one). The new used IACV operates much smoother than my old one, but I'm thinking the old one still worked ok.

But I wanted to update the post today because I had a new Magnaflow high flow cat. A previous owner had removed the cat altogether and welded in a straight pipe. I wanted to put a cat back in to make it right, and a small part of me was hoping the cat would fix it. Nope.

I'm running out of ideas. I hate to be swapping parts for no reason and throwing money at it needlessly. In the 2500 miles I've been driving the car, the check engine light only came on once, and that was about 2200 miles ago. Even without a cat for all 2500 of these miles, it only tripped the light once. And, the idle problem still exists.

In my very first post above, I mentioned the MAF had one clean resistor and one with some tan "stuff" on it that looked like a crust. I am wondering if this has anything to do with it. I already thoroughly cleaned it with CRC MAF cleaner, so I'm not going to bother cleaning it again. The local parts store can get a new remanufactured cardone MAF for $80 which is tempting, but before I do that, I wanted to see if anyone else can confirm if the "tan crust" is normal or if both resistors are supposed to be clean.
 

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Either the replacement IACV is also no good (new or used?) or you have an intermittent vacuum leak somewhere -- anything open to atmosphere after the MAF but before (or just past) the primary O2 will cause a vacuum leak.

It's not the absence of the cat, although you should do the double defouler mod trick to keep the ECU happy for that.

As to the MAF, if you mean the light brown bulb, that's the air temp sensor. It's possilbe that your MAF is bad, so that's another item to try replacing.
 

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I would replace your intake manifold gasket and throttle body gasket as they are the cheapest to cross out.
 

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Discussion Starter #19
Either the replacement IACV is also no good (new or used?) or you have an intermittent vacuum leak somewhere -- anything open to atmosphere after the MAF but before (or just past) the primary O2 will cause a vacuum leak.

It's not the absence of the cat, although you should do the double defouler mod trick to keep the ECU happy for that.

As to the MAF, if you mean the light brown bulb, that's the air temp sensor. It's possilbe that your MAF is bad, so that's another item to try replacing.
The replacement IACV is used but low mileage. The seller assured me it worked fine when removed, and there was no binding. It hadn't been cleaned, but it wasn't very dirty. I cleaned it before installation. I can spin the shaft, and the magnet has no problem putting it back to it's resting position.

I have a cat installed now, so no need to do the defouler mod. Strangely, even without the cat, it only tripped the engine light once in 2500 miles. Regarding the MAF, I do not mean the intake air temp sensor. I am referring to the two coiled resistors down the "tube." One looks fine, the other has a tan crust on it. I may try to get pictures, though considering I don't know the history of the MAF or when it was last replaced, I should probably replace it anyway.

I did find an exhaust leak yesterday between the exhaust manifold and mid pipe. Actually, the shop that installed the magnaflow cat found the leak. I will get a new gasket for it, and maybe some new springs if needed. For a temporary fix though, I am just going to seal it up with RTV to see if it actually fixes the problem.

When I did a search on "idle" on this board, I found a thread buried around page 20 of the search results that mentioned there was a TSB of some sort for year 2000 ECU's only in which the diagnosis is a "searching idle." I didn't find (much less read) the TSB myself, but it said something to the effect of replacing the IACV, MAF, cleaning the TB, and if that didn't work, replace the ECU.

I am going to try to try and search for some vacuum leaks using carb cleaner, check the MAF with a multi-meter, check the IACV with a multimeter if I can, check the actual connection pins of the IACV plug for corrosion, check the throttle cable tension and stop screw, seal that exhaust leak with RTV for a temporary fix, unplug IACV during idle to see if anything changes, check TPS if possible, and unhook battery for 10 minutes, then start car and let idle for 15 minutes without touching gas pedal, shut off, then try again. These were all possibilities I found while searching extensively on dialup last night.

Some how some way, the idle problem is temperature related. In the summer time, it hardly does it at all. Now that it's -12F out in winter time, the engine is running much colder. This might make sense for a vacuum leak somewhere I guess, but I'm not getting any "lean" codes. If anything, the car was running rich without a cat.
 

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if your idle is supposed to be at 1000 rpm and jumps up to 2 or so, try holding the gas down just off idle. say 1100 or 1200. see if it messes with anything. If its temp related it could be the maf since it detects temp, but it could be anything mechanical as well since the colder something is, the harder it is to move. was the IAC you replaced a brand new part or used/remanu?
 
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