3.4L High Idle / low coolant temp *Solved*

Hi all,
I have been chasing a high idle for a while now, I was pretty sure I had it pinned down to the throttle body needing a rebore, as I can see light through the butterfly. This is what symptoms the truck is showing:

The truck starts at around 2k rpm cold, and may drop to 1750 warm. When coming to a stop in neutral from speed, the RPMs will bounce or search between 1200-1750rpm until settling at 1750 by the time I come to a complete stop, it may bounce around 5 times during this. The idle never bounces while at a stop, I have only seen it while decelerating. It will sometimes start at a more 'regular' idle of around 1100 (still higher than it should be) if already warm, for example after running into a store quickly. However, while driving it will return to the higher 1750 idle.

I was also playing with my ultragage and threw on the coolant temp monitor and noticed I am running significantly colder than any post I can find. My coolant temp is being shown at between 155-160F. Every post I can find states normal operating temperature as between 170-190F, and I appear well below that. My temp needle sits very slightly below middle.
I feel like I can remember the needle used to sit slightly above middle in previous years, and I did have an incident a few months back where I let the coolant run low (hadn't been checked since it got flushed around 2 years ago), and when I noticed the needle rise I checked the coolant and replenished it and the needle went right back down. On that note, is it normal to have to top off your coolant over longer time periods? I frequently check for any drips under the truck and haven't noticed anything. Would that mean I could have a small leak in the head gasket?
Could a high idle be a symptom of a failing thermostat, due to the engine not reaching operating temperature? Or an unrelated problem to the high idle, but still a problem that needs to be addressed?

I don't mind throwing parts/solutions at it until something sticks, as I have neglected this truck for years and have been working to get everything back in order and maintenance up to date.

I am already planning to send the TB out to MaxBore to get the air gaps fixed soon, but the temperature seems really low and seems like it could effect the idle.

In chasing the high idle, and in catching up on general maintenance, I have cleaned the throttle body (I think it got worse because I cleaned out the gunk sealing the butterfly valve to the bore), cleaned IACV, new gaskets for both, new intake hose, new plugs and wires, new fuel filter, new fluids everywhere except coolant, and new silicon vacuum lines on all the easily accessed lines.

 

A vac leak somewhere will cause high idle and fluctuations......no1 plague in these old girls,,,,
and the leaks can be in surprising places.....like failing brake booster....
pcv valve hose is rotten.....there's fuel vapors going thru that to help it come to its demise ...
egr valve could be stuck open or closed...

An exhaust leak from the headers to 1sr O2 sensor.....would mimic same as vac leak symptoms...

Next would be that thermostat is bad....maybe prevent it from going into closed loop mode...

Ect temp sensor, they wear out eventually...if you run low on coolant, you might get scale deposits built up on it..
overtime they get cooked alive....the tip disintegrates...

A new thermostat and Ect might be a good maintenance prevention....even though the oem ones aint cheap...

there's vac or electric control pot on the TB that can keep the butterfly from completely closing.....right there where the cable attaches...

As I said to the last guy....if the problem isnt obvious....stupid simple....get a scanner and drive around using live data and look for anomalies from the different engine sensors.......air flow numbers and fuel trims can tell u alot about whats going on...

I've got one of those 'Blue drivers' dongles....used for years......before I got me a ThinkScan plus
which reminds me I need to top charge it off.......thankyou...

 

Thank you for all of these tips. I went ahead and swapped out the PCV valve and threw a new hose on it tonight to check that off the list, didn’t seem to make a difference though.
I am pretty sure my truck does not have an EGR valve which seems to leave a random leak somewhere as the last cause of a vacuum issue.
I believe the brake booster is original, and at almost 300k it definitely wouldn’t hurt. The brakes on my friends Tacoma do feel a lot better than mine so that might be a good thing to replace, vacuum leak or not… that will probably come after a boring the throttle body.
I also was messing around with the dash pot and idle screw to see if those needed adjustment. However, while lowering the dash pot screw and idle screw did temporarily adjust the idle down, it ended up returning to the high idle after reving a little. I ended up putting them back to the original location. That behavior makes me feel like it is all just the throttle body.

 

I’ve been gathering some more information but am struggling to interpret what it means. My short fuel trim at idle is between -5 and -9, with the long term solidly 0. The timing advance sits at 20-21, with MAF showing 8-10 g/s.

Does this information shed any light, if you understand what they mean?

 

Well, for 3.4L engine the maf at idle.....that normally should be~800 rpm.... should be 4~5 g/s...
...timing advance should be 12~15,,,,

So at the higher idle....those numbers are not that far off.

so that in itself does not reveal the cause of the high idle...

it could be a number of things.......it could very well be TB....
highly doubt it is the thermostat......but wouldnt hurt/kill you to replace...
same on the ECt......those are like the LBJ's....if very old....may not be a bad ideal to replace...



short term FT being that that large neg means ....its trying to cut some of the fuel....
but that's not way too far off.....I have a cold air intake....and mine gets almost the numbers trying to compensate for that,
but thats at hwy speed on ....at 800 rpm idle mine is -2 or-3


The 1st O2 sensor could be off/going bad...
You should be able to voltage of it off live data....on that O2 data...

wonder what your timing advance and short trim is at hwy...
I'm a 2rz/3rz....4 cylinder person......maybe a 5vz 6cyl can chime in...

Really hate for you to throw a bunch of parts at it.....and get nowhere...

but there is a number of stuff it wouldn't hurt replace as mention above...

I got me a little smoke machine for hunting down problems in the future....
vacuum stuff around the engine......and then the fuel system and vapor canister can develop leaks as it ages..

The tacos in the 95~97 use a that distributor...and thats a special sub class that I dont know much about..
maybe one of those could chime in if they notice something...

Like I said before ....the no1 reason for high idle is air incursion somewhere along the intake or vac system...
and the vapor canister can contribute to that.......

Wish I were more help....

 

The timing advance cruising at 65mph is ~30-33, short term -5->+5, long term 0->-5.

I’ve been checking everywhere for a vacuum leak and have used the starting fluid technique to no avail.


I did find there is some play in the butterfly valve, with the spring pushing it towards the firewall and wearing down the bore and valve on the firewall side. I threw a couple little shims on the butterfly valve shaft to center it and that lowered the idle down to ~1450. Feels worlds better than the 1750-2000 it was before. This is telling me it is really just the throttle body and that the best thing for me to do is stop screwing with everything and get through the next couple weeks until I can take the taco out of commission for a while to send the throttle body out to maxbore, before I break something else.

Thank you for the tips on the thermostat and ect, I will keep that in mind in case boring the throttle body isn’t effective. Otherwise it will go on the to-do list when I get around to the timing belt.

 

Maybe not the same issue but in a similar vein, my truck was having idle issues where it was bouncing around (usually in the opposite way where it almost stalls). I got a check engine code that pointed towards one of the o2 sensors, replaced both the front and back sensors with densos and now my idle is smooth. When's the last time you looked at your o2 sensors?

 

I haven’t taken a look at them really ever. I’m learning they do a lot more than just monitor emissions as I had thought.. I’ll have to take a look at the live O2 data in the morning, looks like healthy voltages are ~3.3V for the front and ~.5V for the rear?

 

It turned out to be the thermostat all along.
I was keeping a real close eye on the coolant temp and noticed that on hot-starts the coolant temp would be up at 180F, and thats when the idle would be normal.
Swapped out the thermostat today with a new 180F thermostat and she is idling amazingly under 900. Completely forgot how quiet these trucks are at idle...

So, if anyone in the future stumbles upon this thread; I had these symptoms which are commonly attributed to a vacuum leak, IACV, or a worn butterfly valve and it turned out to be none of those common problems:
High idle of in the range of 1300-1800
RPM bouncing between 1200-1750 when braking in neutral from high speed (commonly attributed to a brake booster vacuum leak)
Lower than normal coolant temp (not really associated with other problems, but could be over looked if you don't know the correct operating temperature)

If you dont have an Ultragauge or other way to view live coolant temp, I noticed that the dashboard temp gauge used to be just a hair below middle, and now it is a hair above the middle. Its a change you'd probably only notice after you've been driving your truck for some years but it was barely visible.

I noticed when buying the thermostat, you can opt for a 170F thermostat. As an Arizona resident, before this experience I would have definitely opted for the cooler running thermostat thinking it would be better to keep the engine cooler. However, my idle didn't drop below 1000rpm during warm up until it was at 180F. Not sure what the idle coolant temp would be with a 170F thermostat, but I would be concerned, after all I've experienced, that a colder operating thermostat would cause a slightly higher idle. It seems like the ECU has a linear idle chart that brings the idle down from ~1500 -> ~750RPM as the engine heats up from ~150 -> ~180F.

 

Good find, and good on you for posting the solution.

 

Very interesting… I am looking at the 3.4’s coolant flow and I can see where coolant flows through the IAC from what looks like coolant in the lower intake. I am not 100% sure why. If the PCM reads its engine coolant temp (ECT) from the 2 wire ECT sensor and modifies the IAC “duty cycle” for cold engine rpm what does running coolant through the IAC actually do? In other manufacturers IAC’s there is a wax pellet that expands/contracts for cold start rpm so I can see why they run coolant through theirs.

 

From what I’ve found it’s to prevent the throttle body from freezing in cold temperatures. I would think that with the coolant running directly through the iac, that Toyota would utilize some sort of thermomechanical valve directly inside the iac, even something as simple as the pellet you mentioned. It does seem to reference the ect instead though.
Nonetheless, the temperature dependent idle adjustment function of the iac seems to be pretty overlooked when diagnosing an idle issue.

 

Thanks, that makes sense.