Hello,
"Dug the Trug" is a 2001 Tacoma, 3.4L 5ZVFE with 200,000 miles, automatic transmission. No check engine light, no pending codes. I change the (synthetic) oil every 3,000 miles religiously.

I am trying to find the cause of an engine shudder at idle, occasionally it leads to a stall when sitting at a red light. I would say that the shudder happens every other day and a stall happens once every two months.
What I have tried so far:
-Cleaning the Idle Air Control (IAC)
-Replaced two ignition coils- the idea was to swap them one at a time with a new one, but then I broke one taking it out so I had to get another. I have concluded that coils are not the problem.
-Replaced a couple of suspicious vacuum lines and ran a vacuum test which seems normal. It idles with -20.5 pressure and nothing suspicious when revving.
-New fuel filter
-New plugs and wires- I noticed oil on the threads of plug #4. The valve cover gaskets need replacing but I don't think this would cause a shudder at idle, maybe I am wrong?
-I Recently noticed that the coolant was low (the temperature gauge on the dash never went above center, but the hood was warm to the touch) I had the coolant system checked for leaks and none were found, mechanic advised me to get a new radiator cap and speculated that it could be a head gasket leak.
Concerned about the possibility of a blown head gasket I performed a leak down test at 90 PSI the results were:
1: 86/90
2: 84/90
3: 82.5/90
4: 82/90
5: 82/90
6: 83/90
The results seem to indicate that the engine is healthy, a total of 7.5% leak, the only significant noise that I heard was from the dipstick on cylinder #3, but no bubbles in coolant.
It seems to me like I do not have a head gasket leak and that it was the radiator cap... time will tell.

the problem might still be in the IAC even though I cleaned it thoroughly, perhaps it is mechanically failed?

Any ideas?

Thanks in advance!!

 

Try testing & adjusting the TPS, or bump up your idle, should be about 800Rpm and bump up to 900-1000RPM if you run the AC or turn the wheel.

 

Thanks Malcom99. I don't believe that the TPS on this particular year and engine is adjustable but I DID replace the original TPS about a year ago (it had a TPS code at the time) All I could find was Duralast brand so that's what I put in it. I will consider ordering on OEM one, it seems like an important and relatively cheap sensor to get OEM.