Truck is a standard 1999 2 wheel drive 3 litre (Mazda B3000).
Starts fine from cold and idles fast as it should (just above freezing outside)
The idle starts to slow too soon before the engine has a chance to warm up.
The ECM appears to detect this and the increases the RPM's.

It goes through this slow and speed up routine until the engine gets warm enough where the computer stops increasing the idle.

New IAC valve, but still using the original coolant temp sensor _ it tested OK.
I also cleaned the Air Intake Charge Temp sensor _ wasn't really dirty.

No codes...

Is it supposed to behave this way, other fuel injected vehicles don't ?
On other vehicles the idle starts up fast when cold and gradually decreases until the engine is fully warmed up.

New faulty IAC valve ??? I know it does hold the idle for quite along time when I stop at a light, so maybe the IAC valve is sticking a little ???

 

Was not expecting to see you post this one, Jeff :)

FI should do as you experienced on other vehicles, it's in open loop, it takes info from coolant sens, MAF and apparently takes info from the O2 - well, that last part is when the system starts to get the varying signal from the O2 sensor, meaning it's up to temp and supposedly ready to help run the engine.

 

Still shouldn't speed up and slow down like that though.
New oxygen sensors all around too.

Maybe it's just the way it is, but I don't remember my bosses 4-Runner doing that.

 

Oh, no, not that part, that speed up/down, that's all wrong. It should do as you experienced on other vehicles.


If you connect a scanner, what does the primary o2 state when it's doing this?

Also, as it takes the water temp into account, what is the water temp reading whilst it's doing this hunting?

 

My scanner isn't that fancy, all it does is pull codes.

I did mention that in my first post.
It idles fast as it should, but then it starts to slow down when it's still too cold to idle slow.
The ECM appears to check that it's slowing and increases the idle.

This goes on until it's fully warmed up.

Speeds up and slows down about 3 times _ the morning where it was just above freezing.

I don't normally let it warm up in the mornings _ I drive it gently until the temp gauge shows some movement.
I drive it right away for the most part, but I was cold that morning so I started it and did a few things while the engine was warming up.

 

Out of curiosity, what is the instant cold idle in these conditions?

I keep re-reading your post, from what i can tell, it's just dropping the revs too soon? or too low?
How low does it go before it picks up again?


Possible fan clutch locked up/not releasing? extra load on engine in those specific conditions might be enough to do this.

 

About 1200 to 1300 rpm _ wasn't paying much attention.

It's dropping the revs too soon. (not too low)

It almost wants to stall when it tries to slow the idle, but it never gets that far, so we're talking almost instantly.

It's just like in the old days with a carb the way it's acting.

Someone inside the vehicle tries to turn the choke off too soon, and then realizes that it's still too cold.
The idle decreases for a very slight moment, until the choke and fast idle circuit is put back in play.

Don't see it being the fan clutch.
I don't think it has that much influence on engine even it was faulty.

 

Hmm, you were right, that is too low to start, should be nearly 2k.

It still points to one of the circuits that are looked at before going into closed loop, so we're talking temp sensor and MAF.

 

I have a new temp sensor, I'll put that in and see if that helps.

Fuel economy is OK, tail pipe is nice and clean too.

I could try cleaning the MAF again, it was done last year when I first bought the truck.
If the MAF sensor was faulty, there would be a code, wouldn't there ?

I'll check and see tomorrow what the RPM is when it's cold.
It's kind of mild right now, so not the best time to check it though.

 

I don't think the MAF will throw a code unless it's actually faulty, dirt etc will cause it to mis-read and alter it's feedback, which i think is the more important part.

 

I put my old IAC valve back in, and it still acts the same, but on a more gradual basis.
Which would simply indicate that the old one is worn and or dirty.

The initial idle at -3 degrees this morning was 1500, and then it slowly drops and increases off and on until it's warm enough to idle at 1000 rpm.
Only when it's really warm does it idle at around 750/800, but that's normal I think.

I'll put in the other coolant temp sensor and see what happens.

 

Interesting, and yeah, that initial idle speed at that temp is way to low in my opinion, should be about 2k.

Try cleaning that MAF again, just in case.

 

I was hoping others would post as to their warm up and idle speeds when cold.
1500 seems fine, it doesn't stall at that speed and runs fast enough for a cold idle.
I think 2000 would be a bit much.

Do you have a tach on yours, is yours 2000 for sure ?

 

Mine is nearly 2k when it's no where near that cold, but i do know my tach is a little off at hot idle.

I'll start it tomorrow morning, whilst it won't be that cold out, i will have my scan tool plugged in and let you know the *actual* RPM.

 

I put the other coolant temp sensor in this morning (-5 out), but I screwed up my experiment and forgot to plug the MAF sensor in, so of course it ran like ****...

However it does run smoother over-all.

The truck always had this slight jerkiness to it, it was especially noticeable on a flat smooth road in 3rd gear if you held the rpm's at 2000.

I believe that may be gone now.

The old sensor tested OK, but maybe it didn't react quick enough.
Just because a sensor tests OK on the stove, doesn't mean to say it will work properly in a real world satiation.
It obviously was functioning with in parameters, but not enough to throw a code, assuming the ECM monitors this particular sensor.
But the age of it and the slight jerkiness of the truck would indicate that it was functioning poorly.

Should get a little better mileage now, although slight.

It's one of those things that should be replaced, just because it's old.
It's inexpensive too.

I'll probably replace the Air Intake Charge Temp sensor too, but I need to read up on it's exact function though.

 

I'd be interested in an update when you next start up in full cold.

It was 60 out when i started mine up, so initial rpm was more 1500, so not a good test.

 

60...
That's 16 Celsius, that's quite warm.

 

Yeah, exactly, useless comparison, sorry :(

 

No need to apologize.
Us Canadians can't help it if you guys have trouble with the Metric system...

 

But then again, you're British (if I remember correctly), so that makes things even more complicated.

 

I grew up with metric temps, have converted to imperial since emigrating.

 

Minus 5 again this morning...

Truck starts right away and now revs to around 1800 as apposed to its 1500 before.

The idle still comes down too fast and tries to stay at 1000 rpm, until the ECM decides that it's still too cold to idle at that speed and then bumps it up to around 1200.

I let it do this a couple of times and then drove away.

The other sensor involved here is the "Air Intake Charge Temp Sensor"
I suppose if it's not working properly it's giving false info to the PCM, making it believe that the air is warmer then it is.
If that were so, then what is causing the PCM to bump up the idle from 1000 to 1200/1300 rpm.
Unless there is something wrong with it, and it's just sluggish _ it's probably the original one, so 18 years old...

It's not that expensive so I may just replace it.
It may be still operating with in specs, but not good enough, however still with in parameters so I don't get a CEL.

 

Something has to be really wrong a lot of the time for CEL's to throw, i know what you mean about being in spec, but it would annoy me too.

I can't say go ahead and replace that sensor, but i probably would if it wasn't expensive. Seems like you're heading in the right direction if the fast idle is now much higher :)

Did you re-blast the MAF again?

 

Yes, cleaned the MAF sensor.
I would be surprised if that was the problem, or even part of it.
It was cleaned when I first bought the truck and it really hasn't been driven that much since the engine rebuild.
New air filter too.

All new oxygen sensors as well.

 

Installed a new "Air Intake Charge Temp Sensor" and still acts the same, although it's warm up period is more responsive.

Only when it's cold does it keep going through the high and low idle sequence.
It tries to idle at 1000, realizes it's a bit too cold and then bumps it up to around 1200.
It will continue to do this until the engine warms up enough that breaks this cycle _ after which it idles normally.

When it's not too cold (10 degrees ore more), this doesn't happen, the engine settles down to 1000 and stays there until it warms up enough to its normal idle.

I'm getting descent fuel economy and it seems to be running well enough, so I guess that's the way my truck is.

The only sensor that's left that would effect this would be the MAF sensor, but I'm not going to replace that, it's too expensive, and honestly, I don't think that would help anyway.