Another high idle question 2001 3.4 5VZFE

1st a little history.... replaced plugs, wires and valve cover gaskets awhile back. Truck ran fine after the job was completed. Fast forward a few months. Started to pick up a high idle problem (~1700) after start & warm up but it would calm down to about 1000~1200 after awhile. Then it started to miss and cough on extended up hill pulls. It would like to throw a code sometimes P0171 (vacuum leak) and sometimes P042 (Cat problem) Replaced the downstream O2 sensor (didn't help) spent a lot of time trying to find a vacuum leak but couldn't. After a lot of research spent the big $ on the Denso upstream sensor and wow! the truck has never run this smooth and has gained a lot of power but it's got a high idle problem (~2000 rpm).

Now, there are a lot of write ups on high idle but this problem seems to be different.
Start the truck up cold it idles about 1200, then as it warms up the idle goes up slowly to 1500 start to drive and it idles at 2000 and is stuck there if you are parked. Drive it, and between shifts it drops down to 1200 then up to 2000 (or take your foot off the gas and and step on the clutch) it will drop down the 1200 and sometimes hunt a few times between 1200 and 1700 and return to 2000. If you take a drive on a deserted level road, put it in 3rd gear, take your foot off the gas and it will roll at 1500, step on the clutch it jumps to 2000 (do to drivetrain and rolling resistance.) Now if you just leave it in gear with your foot off the (gas while idling down the road) you can feel and see it drop down to 900 then back up to 1200. It will do this about every minute (if you have enough clear road.)

If it just had a high idle I would be looking at a stuck/dirty throttle body (it's clean), IAC, vacuum leak, etc. but with it dropping down in RPM then back up without any foot action input when driving but then being stuck at 2000 when parked....... I'm a little confused.

More info:
This is my son's truck the only time I drive it is to analyze a problem.
2001 Tacoma 3.4 5VZFE
240,000 mi
285/70R17
Cleared old codes, disconnected the battery over night, I ran it for 3 -20 minute drives no CEL
A in cab fuel rail pressure gauge shows a steady 46 all the time
Throttle cable has slack in it
Dashpot works correctly
Can't drop the idle by pushing/pulling on throttle plate linkage

Any ideas?

 

Did you replace your vacuum lines or clean your MAF, TB, or IAC when you did the plugs/wires/vcg?

Replacing the valve cover gaskets is a great time to replace all the hard to get vacuum lines. There are some that hide under the primary wire loom that runs across the back of the motor.

I'd start with the vacuum lines. Pay close attention to the brake booster line. And then also check the brake booster itself - it can have a leak in it.

Then turn your attention to the TB and IAC. Get yourself some actual JIS screw drivers before you try to remove the MAF or IAC. The screws are prone to stripping with normal Philips. Remove the IAC before trying to clean the TB (you don't want the dirty residue getting down into the IAC). Only use MAF safe cleaner on the MAF, and EFI safe cleaner on the TB (IE, not brake parts cleaner).

 

Thanks for the input, can't work on the truck till this weekend. Posted early so I can get ahead on some new ideas. I know the TB and MAF are clean and the IAC is one of the top things on my list. Vacuum lines all looked good when we did the VC gaskets and have sprayed a lot of Starting fluid around looking for any leaks, but if the booster diaphragm it self is leaking, spray wouldn't get in there. Will pick up some EFI cleaner.
Thanks

 

Spraying starter fluid could have made a potentially bad line worse. An unlit propane torch is the suggested method, so you're not soaking old rubber and plastic in a high octane starter product.

I'd also get your hands on a FSM and check out the TPS.

 

Unless the vacuum leak is really substantial I've found spraying carb cleaner or whatever to generally be ineffective at finding vacuum leaks. Your best bet is to use a scan tool in conjuction with propane. It helps if the scan tool has graphing abilities then you can see small variations in the fuel trims & O2's which may not result in an obvious audible increase of RPM via the traditional method. Either that or smoke test but if you have a scan tool it should be pretty straight forward to detect vacuum leaks just with the scanner by looking at fuel trims at idle then rev to around 2500 rpm and see if the fuel trims normalize. That would indicate a vacuum leak.

 

A scanner would be nice. I have an older one. I will see if it will communicate with a Toyota.

 

Did you ever come to a conclusion with this problem. If like to hear what the fix was.