[Solved] High Idle After Engine Replacement & Supercharger Install Diagnosis 3.4 5vz

History

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:



https://www.yotatech.com/forums/f4/5vzfe-how-diagnose-high-idle-tests-iac-etc-295444/

- 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.

*** Summary ***

So what I assume the case is, is that the ECU is telling the IAC to be wide open since one of the wires is basically getting all the voltage and the other is getting none. If I'm right, I need what can tell the computer to make that happen.

Comments, questions, concerns are welcome

 

I suspect the engine coolant temp sensor, the two wire one on the front of the lower intake manifold.

 

Just to double check - would the sensor possibly be faulty even though it's sending real time data to the ecu that I'm reading with my scan tool and which looks normal and accurate?

 

maybe, tough thing to figure out. As a test of sorts, disconnect the battery and take the plugs of the PCM one at a time and reinstall them a few times to "clean the terminals". Dont try to wire brush them or spray contact cleaner, just disconnect and reconnect a few times.
The fact that the "duct tape test" over the IAC and the donor throttle body swap show some changes I would next suspect that you are on the right track there. does your scan tool show fuel trims? those are a good indication of if there is a vacuum leak.

 

@Glamisman I did what you said with the ECU connectors and when it ran there was no change. My scan tool is reading 0% for both long and short term fuel trims. I read a thread on another forum here where another guy with similar issues ended up having a blown headgasket and cracked heads. Both times purging the coolant (initial fill up and after replacing the throttle body) I've gotten bubbles after a good amount of time has gone by and I squeeze the top radiator hose and I have a spill-proof funnel above the radiator. I am going to test compression and try to find a cheap leak down tester that is acceptable quality - for some reason all the parts stores in my area didn't rent leak down testers.

This is something I obviously certainly should have done when the engine was out of donor the truck, but they had had it driving and running the day I picked up the engine - I am guessing they may have overheated it driving without coolant or something. It is what it is now, so I'll see if I can find a compression issue as soon as I get a test tool.

 

with the fuel trim results this is looking like like a flaky IAC or a throttle body. If you were to manually shut the throttle or push back and forth on the bell crank does the idle change?

 

So if I hit the gas pedal / manually give it throttle the rpm changes as would be expected - I'm not sure how I would close it any more than it already is when at idle since the throttle cable has slack in it and I don't think the "bell crank" (if that's what the throttle cable's connected to which I think is a bell crank so I'm guessing that's what you are referring to) can turn in the other direction any more to close the butterfly valve, however I never explicitly tried it. TPS reads 11.7% consistently whenever I check it out when the engine's idling. I haven't watched the live data when I give it throttle.

As far as the flaky IAC - wouldn't that seem to not be likely since I've used two units that were good prior to me installing them? More and more I'm hoping it is a weird glitch. I'm going to try to test the compression / do a leak down test this week. I've got three weeks before I get busy with work and I would like to figure this out (and drop in another engine if I absolutely need to - probably JDM this time).

Before I got on the headgasket hunch, I wanted to know exactly what informs the computer of what voltage to send to the IAC. I am guessing coolant temp and maybe power steering pressure sensor, and ????

Thanks for the help

 

3rd times the charm IAC.

 

as the throttle shaft rotates in the bore it wears and the hole becomes out of round. The fit between the blade of the throttle and the interior bore of the body is not the same every time or is slightly larger. If you have the intake hose off of the throttle body and wiggle the bell crank and plate assembly and that distance changes it is wore out, we are talking a few thousandths of an inch here.

The fine threaded Allen screw with paint on it is the base idle adjustment. The throttle bodies are allowed to close completely then this screw is turned in just enough so that the blade does not stick in the bore. The IAC handles all additional idle air inputs the motor requires up to the point that it can... the orifice is only so big. False air, or un-metered air, is air that is used by the engine and your 10% STFT shows that. With it idling pinch off the brake booster hose and watch the fuel trims and/or see if the motor reacts. Do the same for the PCV valve hose. One by one do this to all vacuum lines. The front O2 sensor reacts pretty quick. It doesnt take that large of a vacuum leak to cause issues... a 1/16" diameter vacuum hose leaking will create havoc. Things like this are notoriously difficult to track down. If after all of your hose squeezing efforts and you find nothing, then it is time to try a propane test. Get a propane torch and do not light it... open the valve and introduce some propane into the brake booster vacuum line to see how the motor reacts and what the front O2 sensor does. Do this a few time so that you recognize what the scanner is showing you and drift the propane over anywhere you think the motor might take the propane in. If after all of this has been done and she still idles high and she didn't do this before the SC install then the issue is with the SC.

I went out and data logged my 1998 3.4 from cold start to warmed up with my Blue Driver scanner as an example of what you should be seeing. I never touched the throttle.

 

Second what @Glamisman said. I'll add that the throttle stop screw also sets the throttle plate angle (engine running, at idle). Yours looks a tad high but likely with in a range the IAC can compensate for.
Fiddle with it and see. TPS should read "around" 10°, mine is 9°.

A vac leak in the throttle opener plumbing will not only leak air into the intake but not let the throttle plate close all the way to the stop screw.
Of course swapping t-bodies would "tend" to rule all this out.

Double check all the vac lines including the "idle air assist" line from the IAC back to a tee and finally to each side of the rear intake. These guys could easily get abused in the engine install process.

Following

 

I decided I would compression check the engine since I didn't before. I found all cylinders to be 200-210 psi with cylinder 4 being the lowest and basically the rest having 207-210. It took a few more cranks (like 7-10 to get cylinder 4 past 190 where all of the other cylinders reached their max pressure after 5-7 cranks. Compression check was done with the engine cold, all spark plugs out. I used a rental OEM tool from Autozone.

@Dirty Pool it's interesting you mentioned the vacuum line from the IAC back to the tee that goes to the crank case. I followed this video to learn the supercharger installation and followed their suggestion that I put a one-way check valve in that line.

https://youtu.be/7RmHXvEQwXs?t=3056

I bought a metal one from Amazon since the Toyota part was like $30+. I never liked a couple things about the valve - one is that the size of it made it hard to run the vacuum line beneath the wiring harness / fuel rail and the other was that it took a little pressure to get it to open when I'd blow into it to make sure it was installed the right way. If this thing won't open with the pressure provided by the IAC to open the valve, would it be able to cause what I'm experiencing do you think? I already ordered another one that I saw on the supercharger thread on t4r.org.

https://www.ebay.com/itm/392308340561

I figured since I was messing with the engine I'd swap it for the one I didn't like in there already.

I have to work tomorrow and didn't get a chance to thoroughly read your info on the throttle body @Glamisman but I'll read it tomorrow. I took the bell crank off of the throttle body that's on there now and completely re-assembled the parts after painting the outside. I'll mess around with the throttle screw and let you know what happens - I remember my other throttle body showed 10.x% on my scan tool at idle. Once again thank you for sharing the information I really appreciate the help!

 

Keep in mind the "air assist" vac line is a sort of an air injection system controlled by the IAC. It allows manifold vacuum to suck air from the IAC and direct it thru lines and a small sub manifold in the intake all the way to just ahead of each injector. I would think the intent with the check valve would be to prevent boost from flowing backwards in the system. The IAC regulates both this flow and the passage that simply bypasses the throttle plate.
I always thought this was strictly an idle or just off idle thing and the IAC would close the port long before boost, but have never investigated further.

Picture worth a thousand words.
Complete with the standard Toyota typo. The ninth word in the body text should be "from" not "to".
Carry on.

 

Thank you for the explanation - I had watched a video where a guy said that was supposed to drain oil sucked into the intake back into the crank case so I am more enlightened now lol. I removed the check valve and replaced it with a section of silicone hose and unfortunately no change from what it was like before. It started at 1100 rpm and in a matter of minutes was around 1640. I stepped on the gas a couple times and took a couple screen shots from when I hit the gas.



I guess one more time going over my vacuum connections wouldn't hurt, but I was meticulous when I followed this diagram:



I just didn't have the T for the 4x4 because I converted over from ADD to manual hubs.

 

Still spitballing.
Check the igniter connector. Among other things the igniter handles some RPM signaling and other feedback to the ECU. It's on the fender well below the MAF with around 10 wires in a row. The igniter itself is spendy and tough to test without some pretty advanced equipment and they usually set "circuit problem" codes. Couldn't hurt to give the connector a look see. That connector/harness can also see abuse with major under hood work.

 

@Dirty Pool and @Glamisman thank you for all the help. I did douche out the igniter with contact cleaner. It seemed clean and from reading most guys with no start conditions and a CEL were ones that had to deal with their igniters.

I wasn’t 100% sure about that 0 volts I was getting on the RSC wire. I was almost going to take off the supercharger and go through the task of re-installing the stock intake in case something was going on with the sc I didn't know about - it being pretty unfamiliar to me. Then I decided to check for continuity and thanks to my good friend @BartMaster1234 I had a nice copy of the FSM wiring diagram. Here's the pics I took for continuity testing.




Meanwhile another good friend @mechanicjon told me to put the two exhaust sensors in. Tonight I found in the continuity test that my back probe on the RSC wire wasn't pushed in enough and I had to push it in just a bit to make connection. Good news is that there was continuity so I started her up and this is what I got:

With the upper sensor hooked up and plugged into the down pipe I at least got a short term fuel trim - long term was still reading 0. The lower sensor just was hanging under the truck.



RPM hovered around 1400 which it would come back down to after many stabs on the throttle and not go up on its own to 1800 + like it used to.



Fuel System read this which apparently means "Open Loop, insufficient coolant temperature" is Japanese for "Plug in the O2 sensors."



Also watched throttle position sensor while hitting the gas just to see and it looks like while it could possibly use some adjusting, at least it's working.



I'm really grateful that all you guys stepped in to help. Knowing my engine replacement wasn't an epic failure feels pretty good.