high idle problem

with 260k on my 98 tacoma with a 3.4 my valve covers were leaking. Ever since replacing them I have had a high idle (1750-1250). in the process of replacing the valve cover gaskets I broke the dash pot (very brittle). I cleaned the throttle body while I had everything apart and some gunk may have drained into the IAC valve. I took the IAC valve apart and gave it a deep clean (2x). I replaced the dash pot. Still a high idle!
When I press the plunger of the dash pot in manually the idle goes down. I have adjusted the depth of the plunger screw and loosened the stopper screw that limits the plunger depth.
I tried a new mass air sensor-no change. When the mass air sensor is unplugged the truck starts hard but maintains a low idle.
Very confused and frustrated. Any ideas?

 

I'm having the exact same problem with high idle....

 

Get a cheap OBD-2 Bluetooth or Wifi dongle and a phone app to connect it to like Torque for Android or InCarDoc for iOS. Then see what your throttle position sensor is reading at idle. If it’s out of spec, you know where the problem is or isn’t.

I’ve used this one for 7 years with no problems. There’s better rated ones for more on Amazon.

$22 Foseal Wifi OBD2 Scanner, Car Code Reader Check Engine Light Diagnostic Scan Tool for iOS and Android

https://www.amazon.com/Foseal-Scann...18&keywords=foseal+obd2&qid=1723252533&sr=8-1

 

Dash pot plunger should be held in (at op temp) by vac pressure right? I’d be checking for vac leaks, specifically those on the throttle body. Good luck.

 

the large "hose" that goes from the MAS to the intake has some cracks that could leak air. Could this be a problem? going after those leaks with duct tape tomorrow. If not successful I am removing the throttle body (I'm getting real handy at that) and shining a flash light looking for gaps in the butterfly valve. Maybe all the crud I cleaned out of there was sealing the gaps.

 

For what it's worth I replace the throttle position sensor several months ago and it ran great before the recent valve cover gasket repair.

 

There’s probably a vacuum leak if it ran great before your or someone else opened up the intake to do the valve covers. I’d go through the whole upper intake manifold including the IAC vacuum line under the TB and check for cracked vacuum lines - there’s a small one going to the fuel pressure regulator in some that’s easy to miss. At least even if you see nothing wrong, you can move on to other lines of inquiry.

 

I duct taped the crack in the intake hose and the idle dropped to 1200. At least it's drivable now. I'll investigate those other vacuum lines tomorrow.

 

update...I am now getting "chugging" at low RPM. The high idle is still present. I'm thinking I need to redo the steps of the valve gasket replacement in an attempt to find a disconnected vacuum hose somewhere. Any ideas?
I have a new IAC valve but I have little hope that it is the source of the high idle and chugging.

 

another update...I replaced the throttle body gaskets (cuz I have throttle body spacer). Got a whistling sound but the idle was lower. Redid the gasket, no whistle but the idle is back up. I replaced the IAC valve and nothing changed. Still a high idle at about 2000 rpm. When I press on the dash pot plunger the idle drops. I took a pic of the intake. Kinda looks like a large gap around the butterfly valve. Do I need a reboring or a new throttle body?

 

So you were at 1200 idle and replaced the TB gasket(s) and are back at 2000? A throttle position setting readout at idle would be helpful at this point to account for what if anything is wrong with where your valve is at idle rather than throwing the parts cannon air it.

 

Do you have an OBD2 reader?

 

I was at 1800 idle, added throttle body gaskets and could hear a hissing, but idle dropped to 1000. Sprayed brake cleaner around TB and revs went up. Redid the TB gasket, now no hissing but the idle is at 1800 again. If I force the throttle closed with the dash pot mechanism the idle drops to 900. I'll try to include the pic of the gap around the throttle plate. It's big.

 

I would suggest replacing all your vacuum lines and getting a TPS readout with OBD2 as mentioned above.

 

Put a smoker on it and marvel at all the vacuum leaks. These days you can get a great smoker for under $100 and they are invaluable as a diagnostic tool.

 

Smoker is a good tool. But if this problem presented itself when you tore into the engine for the valve cover gaskets, it shouldn't be much work to check all the vacuum lines that were removed and replaced. Also, if you want to make removing the throttle body easier for problem solving, you can bypass the coolant hose. Just plug one end of the hoses back into the lower manifold where the other one comes out of.

Replacing any cracked vacuum lines with silicone ones is pretty nice as they're really flexible.

LTI Universal High Performance 4 Size 5 feet Length 6mm/8mm/10mm/12mm Inner Diameter Silicone Air Vacuum Hose Line Tubing (5ft Black)

https://www.amazon.com/dp/B08L1KP3R...p_csd=d2lkZ2V0TmFtZT1zcF9kZXRhaWxfdGhlbWF0aWM

 

That gap could definitely be the cause of your high idle. Maxbore would have fixed that right up.

 

So the dash pot plunger should be held IN by vacuum pressure at idle when at op temp, fully warmed up. If it isn’t then you have a vac leak somewhere still. Also, if that picture was taken with the dash pot unhooked and plunger out then it’s gonna show daylight there. I read all over here about the need for the butterfly to be completely closed on the throttle body. Guess what, on my 2.7 there is daylight around it and it idles at 700rpm perfectly once I fixed vac leaks, cleaned intake, replaced faulty IAC etc. The point is, fix the other stuff before you start messing with the throttle body.

 

Ok, I like the vacuum leak hypothesis. While driving I get some lurching when engine is under load at low RPM. That doesn't sound like throttle gap problem.
When I originally removed the throttle body to replace the valve cover gasket I tore the hose under the throttle body. I went to the parts store, showed it to the dude behind the counter and he says "that's just a fuel line hose, right?"
Now I am reading and see that there is a big difference between fuel hoses and vacuum hoses! I'll put a vacuum hose on it tonight and let you know..
Thanks for all your help!

 

Also, the lurching, chugging gets worse as the engine heats up. Sounds like the vacuum line (that I used fuel line material to replace) could be sucking shut.