UpInSm0kE

Hi all,

This is like beating a dead horse since there are a lot of posts on here about the IAC vlave for the 3.4L engine. My problem is getting really frustrating. I have cleaned and even replaced the IAC valve twice now due to high idle (2K) and hard starting when warm. I am really confused this does not seem that difficult. The problem seems to be worse now that winter is here. I puchased this truck about 2 months ago with no problems except this one. The first time I cleaned it the idle dropped to 1200 rpm consistently and now that I replaced the IAC completely now (Used one that was tested) the idle is back to 2K and will not go down it is running at 2K all the time. I have checked the slack in the throttle cable and nothing is sticking. All wires and connections have been double checked there are no vacum leaks. I have reset the computer every time I have messed with anything with no change. When I unplug the IAC while the engine is running it will drop the Idle to 1400 or so. There is also no CEL coming on. Do I need to get a new one from Toyota for 280 bucks? Please help I am getting really frustrated any advice is welcome. Thanks in advance

brian2sun

Did it ever idle at 650-750 once it warmed up? If not, I would move on to the TPS.

UpInSm0kE

Brian thanks for the reply. Yes it use to idle just fine around 750-800 when I bought it but now it will only idle at 2K after it warms up. When it first fires when the engine is cold it is only idling around 900 rpm and then it gradually ramps up to 2K and it stays at that after the engine heats up. One other question that is kinda off topic is there a temp sensor in the overflow bottle? The PO put a tupperware homemade overflow bottle in the truck the PO must have cracked the original. I need the truck to run good I use it for work and the Idle is driving me crazy should I spend the money on a new IAC valve and a new TPS?

brian2sun

No temp sensor in the overflow, but there is one on the top of the engine. I would get a new overflow tank since it sounds pretty hokey.

Did all of your problems arise right after you cleaned the IAC the 1st time?

cpizzle42

I had same problem. I turned the truck on, unpluged TPS plug on TB, (check engine light will come on) then plug back in. Your 3.4 should idle where it should.

The CEL should turn off, but I had to take my positive cable off the batter in order to make it turn off.

Hope this helps, I know how you feel with the high idle problem.

vr4boy

possibly the temp sensor?
from your description couple post up,the prev owner macgyverred it to the overflow bottle

maybe this thread would help....
https://www.yotatech.com/forums/f2/9...-start-194444/

UpInSm0kE

Guys thanks again for the replies. cpizzle I tried this yesterday unplugging the TPS while the truck was running and the idle didnt change at all. This is driving me crazy lol. Now the dang thing is loping when i hit the brakes until i get almost to a complete stop and quits loping and resumes the 2k high idle. I am desperate at this point I am not sure what to do. I am ready to just start replacing sensors but they are sooooo expensive. Brian the problem was first noticed right after I bought the truck but it was only idling up for maybe 5 sec and then would return to normal. It has just recently gotten bad with the cold weather (0-15 degrees).

okie81

You may have done this already, but check the voltmeter readings on the TPS and the throttle control motor. If you're readings are off, especially on the throttle control motor, it can cause a high idle.

brian2sun

Check for vacuum leaks too.

UpInSm0kE

I have been doing a lot of reading today on the internet and Im wondering do you guys think the Engine Coolant Temperature sensor is the culprit like vr4boy mentioned? One thing that makes me suspicious of this is when I start the engine when its cold you would think that it would idle up (1200K or so) as soon as it starts like most cars do. My truck will idle around 700-900 right off the bat and gradually ramp up to 2k from there. What are your thoughts?

brian2sun

The coolant temp sensor does have an affect on the idle and seeing as though yours seems to be messed with, it sounds like a good possibility to me. IIRC, the sensors are only like $20 so it might be worth a try to replace.

hross14

There are a couple other things to check out also. considering the PO messed with the coolant system...

1. Make sure the Radiator is topped off--look out for bad Head gasket symptoms--"Just saying" and make sure the cooling system is burped. Aks if you are not sure how to do this.

2. Sweep the TPS and Coolant Temp sensors like the others said.

3.Brake clean check for vacuum leaks--Remember the 3.4 has Air Assisted Fuel injection whereas air is bleed right to the injector tip via passages in the Lowest intake manifold. Make sure the vacuum hoses and injectors are seated properly. The vacuum hose comes off the IAC and splits at the rear of the manifold--going into either side.

3a. AND check to make sure your TB coolant passages are clear and flowing. Its possible you could be freezing the TB

4. check the routing on the vacuum hoses coming from the carbon canister

5. Clean the MAF with Electronics cleaner--it does have an integrated Air temperature sensor

6. Disconnect battery cables while you are doing all this and get back to use.

UpInSm0kE

Brian Thanks for the reply again. I will get one tonight and replace it. Just to give you the best info possible here is a breakdown of how this all started.

1. Bought truck 2 months ago
2. got stuck in a parking lot one day right after I bought the truck with the warm start problem wasnt aware of a high idle yet.
3. Went hunting and let the truck warm up from cold and for about 15 sec the truck would idle high (2k warm engine) and then immediately drop back to 750.
4. Gradually got worse from there until a week ago where it would constantly stay at 2k.
5. Cleaned TB and IAC valve last weekend and the idle dropped to 1200 constantly but still has hard start with warm engine.
6. Replaced IAC with a used OEM guaranteed, tested IAC from the wrecking yard that looked really clean and now back to 2K hard starting and engine loping when you hit the brakes.

I am leaning more so at the IAC because I would think if the IAC was good I should have the same results whether I changed it out or not correct?

UpInSm0kE

Hross thanks I was not aware that the 3.4 had a air assist FI. I will double check the hoses at the back of the engine coming off of the IAC.

hross14

Do check the coolant level though and make sure the system is free of bubbles/air. This causes problems also--including hard starting...

UpInSm0kE

Ok guys Here is what I did last night.

1. Bought and replace the Engine Coolant Temp Sensor (No Change)
2. Cleaned MAF Sensor (No Change)
3. Re-Checked all vacuum lines and double checked all connections. (No Change)
4. Put old IAC back on TB. (This changed the Engine idle speed back to 1200 instead of 2k when warm and engine loping is gone)

I am thinking it is the IAC valve that is bad because whenever I replace or mess with it its the only thing that will change the symptoms. What are your thoughts? Thanks again guys for all replies.

hross14

"97' Toyota Tacoma SR5 Ext Cab 4x4 3.4L 5spd 225K".

I assume this is the truck you are working on??

HERE IS WHAT YOU ARE MISSING!!!! PLEASE, do these things and get back to us--we cant help if you dont take our advice

1. SWEEP THE TPS--do it slowly so you can see if there are any out of spec readings!!!--or just replace--they usually bite the dust around 130-160k ($40)--This is a very common problem for higher mileage trucks

2. Check the cooling system for Air--if there is air in the system it doesnt contact the sensor which varies the OHMs depending on temperature. The computer needs this information. These rarely go bad if ever.

3. Highly doubt the IAC is bad--check the coolant passages to the Throttle body and make sure they are clear and flowing. Because port fuel injection does not require heat to vaporize the fuel in the runners--they heat the TB to keep it from freezing.

When its cold outside what happens to the water suspended in the air that was sucked into your TB? It freezes....this is also rare but check it out--Basic high pressure to low pressure scenario. Even on a decent day the TB can freeze if not heated.

4. Think about also replacing the O2's/AFR's. URD sales them for cheap. They are probably bad (100k mile replacement schedule).

We have all been "Here" done that--check out these items and then we can move forward. sound good?

UpInSm0kE

Hross My signature is the truck that I am working on. I will try to check all of these things tonight and reply back with results. Just out of curiosity though if the TPS or o2 sensors were bad wouldn't this make the CE light come on? I am not getting a CE light. Also the one thing that is going to make the coolant thing a problem as far as air is the jerry rig setup from the PO with the Tupperware bottle it definitely isn't sealed like it should be and it will make it hard to know if there is enough coolant in the system. I am in the process of getting a new one but I wont have it until later this week. I will go over the TPS sensor tonight and check the resistance. Thanks for all the advice I appreciate it.

hross14

Ok--no problem--

Q: Wouldnt a bad TPS/O2/AFR/Sensor throw a code
A; Not always and it wouldnt be the first time.......Common for TPS's and O2's to do this.

**Coolant in the system** To see if there is air in the system Park truck on level ground and take off the radiator cap with the engine cool. there should be coolant to the very top of the radiator with no space for air. If it wasnt full (Add distelled water and coolant)

OR you want to make sure there is no air in the system-->Start truck with cap off-->let it come to operating temperature and it will start to spill/burp coolant out of the radiator top. If there are bubbles in it then let it continue to burp. The coolant level might drop suddenly due to thermostate opening--top off coolant. Continue to do this till there are no bubbles coming out of the radiator

The overflow bottle just catches the coolant after the truck heats up....and then after the truck cools down it is sucked back in.

hross14

Forgot to tell you to turn the heater on high but leave the heater fan off when burping the system.