High Idle, No Check Engine Light

1998 3.4 5-speed 196,000 miles.

Yesterday I noticed a high idle upon start up. Usually it idles around 1500 or so, then starts to drop down to 700-750 (dead horizontal pointing at temp gauge) after about 1 or 2 mins.

During the high idle I also noticed a very faint whining/whiring (hard to describe) noise that I don't normally hear.

I drove home, no problems and when fully warmed up and at idle, it was set at 1000 rpms. About 250 to 300 rpms higher than normal.

Last time this happened it was a bad TPS sensor. But I had a check engine light/code.

So I searched on TW and the normal suggestions come to light:

Clean TB, clean IAC, clean MAF, plugs, etc.

I cleaned TB, cleaned IAC, cleaned MAF, changed plugs and wires 45,000 miles ago.

Is it time to do it again? Obviously I will start with those easy maintenance items.

However in my search I read a multitude of posts that suggest a vacuum leak, cracked vacuum hose, etc.

My question is where do I begin my search for vacuum lines/hoses?

Thank you in advance.

 

Could be a vacuum leak possibly. I was having this same issue about two weeks or so ago. I disconnected the battery cables, took out the MAF and cleaned the crap out of that. I then re-seated all the vac lines as well. Problem went away.

Maybe this will help ya.

 

Where are all the vac lines? They are the smaller diameter black hoses, correct?

 

Check the molded rubber line that exits the TB/ intake and goes to the rocker cover.
As I have the 2.7, you'll see it here on the RT, 2nd hose from the top. It's the dull hose below the ProArmor sticker. I had loose connections and simply used zipties on each end to seal it up and have had zero problems since.

Hope this helps you.

 

My wife's Rav 4 did this and it was a line that came off the air box when I pulled the filter out bug it threw a code almost instantly. Sounds like a vacuum issue somewhere.

 

There should be a vacuum diagram I think on the bottom of the hood near your brake booster.

There really isn't much to our vacuum systems. Here's a pic of something that is really close to our setup.

The black tubes represent what vacuum lines there are. and then there's one from the booster to the intake.

 

Your truck should startup and idle between 1000-1500 normally by the way. It does this until the cats and engine are properly heated/treated. Then it drops back down to 700 after reaching boiling point.

Also, you should get a scanner. I honestly believe EVERYONE in the world should learn about vehicles enough to be able to have one and know what it means. I personally use an ELM327 with Torque (paid) for android. Reading and knowing the LTFT and STFT tells you a LOT about your engine health.

Regarding your "not having a CEL" yet. your engine's fuel trim can be off by as much as 25% (nominal is +-5% or so) before the CEL kicks on. I had a bad booster that was passing a lot of air and messing with my computer, but was only at 15% lean fuel trim reading.

 

I had the firewall break where the cable comes through causing some interesting high idle issues that did not trip the MIL

I also lube the throttle body linkage every oil change or more often.

 

Not a bad idea, but also check the dashpot while you're at it. They don't usually go bad, but it might be pushing on your throttle a little.

 

I envy your bay so much........

 

My 99 tacoma is high idle when engine gets hot and idle ramp up and down when slow down and on neutral every time it do that 3-4 times before idle settle at 1800 1900 rpm, need helps or idea what need to be done,

 

It's now November 2022, ***1997 Tacoma DLX 4x4 3.4L 319K -- I'm going through this high idle event too. Very 1st check engine light ever. I chased the codes, MAF fail. P0171 - too lean, P0130, P0131. I have replaced the MAF, the upstream and downstream O2 sensors, cleaned the throttle body with vengeance to purge my frustration, replaced the IAC, the Throttle Positioning Sensor, sprayed throttle body cleaner on all the vacuum hoses and replaced the ones that caused a response, but upgraded most of the others. New Plugs, New Wires. Replaced the fuel filter. The air filter is fresh; I replace it every other 3k oil change.

The thing starts fast, no lag. The live view shows it's in OPEN Loop but when it hits temp while sitting or when it's being driven it goes to CLOSED Loop. I have been looking everywhere for information on the Short term Fuel Trim and the Long term. and O2 B1S1 Short Term ... which today after plugs and wires was at 0.0 - 3.1.

DTC-CNT 0
Fuel Sys Closed
Load 14.5
ETC 177.8f
Short FT -0.8 / 1.6 / 2.8 % (flipping about) [neg 0.8]
Long FT 21.1%
RPM 1085
Spark Adv 11
IAT 84f ...warm day in fl.
MAF g/s 5.25
TP 10.6 --- the resting position of the sensor -- with the throttle closed is 10.2
O2B1S1 0.130 -- 2.45 v
Short FT O2B1S1 0.0 -- 3.1
O2B1S2 0.60 v

I pulled the new MAF and cleaned it... I got the old one and cleaned it again too. I installed the new one and ran Live Data on the cold start:

DCT_CNT 0
Fuel Sys CL
Load 18%
ECT 117f
Shrt FT1 -7.0% ~ -3.9%
Long FT1 21.1%
RPM 1084
SPK Adv 12
IAT 95f
MAF g/s 5.29
TP 10.2
O2B1S1 0.115 ~ 0.820
Shrt Ft O2B1S1 8.6%
O2B1S2 0.055

WARM START

DCT_CNT 0
Fuel Sys CL
Load 12.9%
ECT 190f
Shrt FT 8.6 ~ 10.7%
Long FT 21.1%
RPM 1144
Spk Adv 10
IAT 93f
MAF g/s 4.65
O2b1s1 0.0215v
Shrt FT O2b1s1 9.4%
O2 B1S2 0.070v

I installed the old MAF and instantly got P0171 and P0100. P0171 was Too Lean Bank 1. P0100 is MAF circuit A fail. So the old MAF is for sure toast.

Open Fuel Sys -- RPM 944. Cl Fuel Sys 1k plus.

I'm lost for lack of skill and information.

PEACE

 

What's your LTFT at ~4krpm?

 

I just did the valve cover gasket service on mine. I had a high pitched whine for a while. Ended up replacing the vacuum lines one by one with carb fuel lines from O'Reilleys. I clamped them down to and got rid of it. My idle was high for a while and I suspected my IAC was a little dirty. Cleaned it once before. Took a while for it to clear up. Idles between 700-900 now when warmed up. I also suspect the throttle plate in the TB as well on mine. Probably worn a little. I have cleaned it up twice already, pulled the EFI fuse and reset it that way. That also seemed to help.

 

@CodeSeven

After reaching temp; at 4021rpm the LT FT is -1.6% - negative one point six percent -
at idle 14.8%, fourteen point eight percent.

The truck has been sitting since the Friday before Thanksgiving week. When it started today the fuel system was in closed loop. That's a good sign. The initial rpm was 820 to 900. Lower than the 11-hundreds it has been... when warm at idle it was 984-992rpm. This data means nothing unless the 4k LT FT data means something.

Thanks for the help.