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1998 Tacoma 3.4 V6 Replacement Engine Soaring Idle

  • Dalandser
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04 Jul 2019 01:24 - 04 Jul 2019 01:27 #31373 by Dalandser

A couple years ago I decided I wanted a fresh engine since I was planning on building this into a reliable exploring vehicle. The truck was running ok with no known issues, but I found a good deal on an engine and trans so I decided to swap it in.

Vehicle ran fine before removing engine - no CELs, etc. Swap took awhile, but eventually I got it running with a 2nd hand supercharger.

The engine ran smooth but the only problem was the engine would idle at 1100-1200 rpm when cold as expected for a little less than 30 seconds, but then it would increase and slowly make it's way higher and higher.

About 6-9 months ago it was even reaching 2400 rpm after the engine had reached 184* which was where it plateaued and I'm assuming the thermostat opened up. This is when I began my thus-far unsuccessful diagnosis for what is causing this high idle. I'll give a brief timeline and any info I think can be important.

I used a Foseal Wifi OBD2 Scanner and Car Doctor to check ECU data and manage any CEL's

The truck doesn't have any brakes (well rear brakes) as I'm planning an axle swap and needs to be smogged so it can't be driven. It also has no cat or exhaust sensors (installed headers with engine swap and need to limp it over to a muffler shop to get the exhaust hooked up).

I was also wondering if the fact that I didn't hook up the power steering pump had anything to do with my high idle since I had broken the PS pressure sensor switch and didn't replace it or run the PS belt until the second test. As far as I can tell it didn't make any difference.

*** TLDR ***

Truck 1998 4x4 manual 3.4 V6 5vz

Donor: 2000 4x4 manual 3.4 V6 5vz

What was swapped over: lower plenum, long block (fuel rails, injectors)

What was re-used: PS Pump, AC System

What was new: OEM Knock Sensors, PCV Valve, Injector O-rings, OEM Coolant Temperature Sensor, Alternator, H20 Pump & Timing Belt Kit, SC w/ stock pulley & no fuel mods, IAC Check Valve, 170* Thermostat

What was removed: Removed cruise control module from engine bay

*** First Test ***

- Ran new engine and idle went from 1100-1200 to 2400 after a number of minutes

- No check engine lights

- ECU temp planes off around 184* with Champion 3 row radiator and 170* thermostat (bought it with my supercharger), Temp gauge reads half way on dash

- Throttle Position 10.5%

- Fuel system on OBD 2 says "Open Loop: Insufficient Engine Temperature"

*** Second Test ***

- Swapped in donor throttle body after cleaning it and the IAC valve

- Idle started at 1100-1200 to 1800 after a number of minutes

- No check engine lights

*** Third Test ***

- Hooked up PS belt, no change in idle

*** Fourth Test ***

- Read up on IAC valve Diagnosis - found this:


- Pretty much followed it step by step and got this:

- TPS 11.6%

- Resistance test was good ~22.5 ohms

- KOEO middle wire got battery voltage

- This is where things became different. Engine @ operating temp (184* steady) top wire received 0 V and lowest wire received 12.4 V

- Unplugged IAC and rpm dropped to 1050

- Grounded out upper IAC wire and no change

- Grounded out lower IAC wire and rpm dropped to 1050

- Duct taped over IAC opening on MAF side and engine barely ran at like 2-300 rpm until my cheap harbor freight duct tape tore and rpm went back up to what I assumed would be 1600+ but I got scared my engine ate the duct tape and I cut the ignition. Luckily it just tore a hole / flap and nothing went down into the IAC valve port.

- Since then I messed with a couple other things w/ no change in symptoms. I did a compression test and things seemed healthy. This is a thread I previously made on it with a little more info:

Original Tacomaworld Thread

Any help will really be appreciated. I plan on removing the supercharger and checking if the symptoms persist with the NA intake manifold soon. It may have nothing to do with it, but maybe it'll lead to a mistake I made being uncovered or at least rule out that it's something to do with the SC.
Last edit: 04 Jul 2019 01:27 by Dalandser.

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