1KZ-TE Idle Issue, Stalling and No Throttle Response (1 Viewer)

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Joined
Jan 13, 2022
Threads
3
Messages
200
Location
Apex NC
Hello, friends... I'm hoping someone can help me solve a head scratcher. I'm trying to get my 90 ready for the 70 Series meetup in TN next week, and it's having a idle issue that I can't seem to solve. If I can't get it sorted, I'll have to skip the event and that's going to suck.

Details on the vehicle:
1996 Prado 90
1KZ-TE automatic
196k kms

Here's a summary:

Intermittently, but with increasing frequency the warmer the engine gets, idle speed will drop <600 rpm and it will not respond to throttle inputs. Occasionally idle will drop as low as 400 rpm, voltage drops will cause dash lights to flicker and there will be an audible miss. If I can get the truck moving with gravity, RPMs will pick up with vehicle speed and as soon as I get over ~2k rpm, boost comes up and the truck drives fine. If I constantly feed it throttle and never let it drop to idle, it's fine. Also occasionally, and I assume related, under load at low RPMs it will not respond to throttle inputs until it gets in boost (~10psi or so).

EGTs are fine- 300 F or so at idle, ~500 F around town and around 900 F under full load.

However, if I put it in Neutral or Park, idle jumps back up to ~900 and it responds to throttle inputs. Put it back in Drive or Reverse, idle drops under 600, it misses and nearly stalls, and I get the behavior above. This makes me think it's not a physical fueling issue... it would persist no matter what gear it was in if I had a bad lift pump or something. Also, I assume that a true fueling issue would be reflected in the EGTs.

What I've done:
-Rebuilt and re-tipped injectors
-Cleaned fuel lines
-Replaced fuel filter
-Replaced glow plugs
-Replaced timing belt
-Replace throttle position sensor
-Replaced leaking capacitor on ECU board
-lots of other things that I don't think are related

And the behavior is unchanged. Could it be fuel spill valve? Are there two different idle circuits for the auto 1KZ-TE, based on whether it's in gear or not? Any ideas?
 
I may have a solution. According to the FSM, the resistance across the spill control valve is supposed to be 1-2 ohms. From what I've read, actual target for the sensor is 1.4 ohms. I'm reading 17.5 ohms, so it looks like the coil in the valve has gone bad.

Now I just need to see if I can source a new one quickly... anyone have any ideas where I can locate one in the states?
 
Copy-pasta'ing here from my build thread, in the hope that someone has some ideas on the electronic diesel control on the 1KZ-TE. Does the Toyota ECD work in a closed-loop/open-loop mode, or is the fuel metering and timing always open-loop? It would help me immensely to diagnose here.

Well, I'm very glad I didn't wait for the spill valve to arrive before I left for the 70-series meetup. That would have made for a frustrating day and a very late night. We took the GX instead and had a blast- thanks again @Gun Runner 5 and everyone else for such a great event!

The fuel spill valve arrived while we were at Windrock, and I got it installed yesterday afternoon. The new unit tested 1.6 ohms resistance out of the box and by all indications was a genuine Denso unit.

IMG_5001.jpeg



Cut down a fan clutch wrench for the install, got it installed and hooked up in less than 30 mins. With the new unit to compare to, it's obvious the old FSV has been handled in the past. There's tool marks on the body, the cap on the adjustment screw is missing, and one of the tabs on the electrical connector is broken.

Got the new unit in and buttoned up, ...and then the truck wouldn't start. At all. I was getting a dribble of fuel at the injectors, but nowhere near enough to actually crank the truck. Adjusted the new valve, but still got just a dribble at the injector. Put the old valve back in and she fired right up. I used a heat gun and heated the old valve up to ~190 degrees F, to see if the coil was functioning correctly when cool but not when the engine is fully heated up, but even at that temp the behavior didn't return.

So either I got a bad unit out of the box, or it's back to the ECU as the culprit and somehow the bad ECU and the bad FSV are cooperating in a way that the new unit can't.

When I pulled the ECU the first time, there was a wire soldered onto the top side of the board. I thought it looked funny, but there wasn't really any indication that it had been tampered with or that the unit had been removed from the vehicle previously. I couldn't find a picture of the ECU from a 90 series with an A/T, so I shrugged and went on. Here's the pic from earlier in the thread, for reference... you can see the red wire in the bottom left hand corner.

IMG_4647.jpeg



Well, lo and behold last night some guy in Latvia had 2 1KZ-TE ECUs for a 90 series A/T up on eBay, complete with pictures. Here's the picture from his listing...

s-l1600.jpg



The numbers on the circuit board match, but there's no wire between those two diodes. Needless to say, I ordered one of the used ECUs the guy has post-haste.

Still not sure what conclusions to draw here. This must be the reason the truck was sent to auction in the first place, and I'm re-tracing some Japanese mechanic's steps in diagnosing the intermittent fuel starvation issue. I'm guessing someone jumpered between the two diodes to bypass part of the ECU board, and maybe made similar modifications to the FSV? If this was a gas engine, I'd be concluding that there was an issue somewhere in the closed-loop fueling that's not happening in open-loop mode. When the engine is cold, or when it's in park or neutral, I'm assuming it's in open-loop fueling and giving the engine a pre-calculated amount of fuel. Once it's good and hot it seems to switch to another mode that struggles to give the engine enough fuel? But for a gas engine, the "loop" is completed with O2 sensor data, which this motor doesn't have. So basically I'm lost.

But I do know the resistance reading on the new FSV is correct, and that red wire, whatever it's for, is not supposed to be on the ECU. Used ECU is on order.
 
26 year old electrolytic capacitors - these have certainly caused problems on other 1KZ-TEs, most notably with transmission shifts in the Toyota Surfs. Not saying these are your issues, just pointing out a possibility they may be involved.
 
26 year old electrolytic capacitors - these have certainly caused problems on other 1KZ-TEs, most notably with transmission shifts in the Toyota Surfs. Not saying these are your issues, just pointing out a possibility they may be involved.
Thanks, Duncan. I did replace one leaking capacitor on the ECU, but it did not correct the issue. Did not go through the process to replace them all.

Sounds like the ECU is more and more likely as the suspect?
 
Thanks, Duncan. I did replace one leaking capacitor on the ECU, but it did not correct the issue. Did not go through the process to replace them all.

Sounds like the ECU is more and more likely as the suspect?

I'm really not sure. All of my experience is with much simpler engines with no significant electronics involved in the diesel injection.

Maybe a mechanical injection conversion for your 1KZ-TE is in your future. Its not a well trodden path, but its not entirely new ground either. Hopefully the replacement ECU just sorts it right out, if the vehicle is a long term proposition I'd consider the conversion because even your replacement ECU may be on its last few years of service.
 
I'm really not sure. All of my experience is with much simpler engines with no significant electronics involved in the diesel injection.

Maybe a mechanical injection conversion for your 1KZ-TE is in your future. Its not a well trodden path, but its not entirely new ground either. Hopefully the replacement ECU just sorts it right out, if the vehicle is a long term proposition I'd consider the conversion because even your replacement ECU may be on its last few years of service.

That's a fair point. It would certainly simplify things to go totally mechanical...
 
i did a 1kzte swap on my lj78 few months ago, i have given up the dream of keeping it electronically controlled after so many breakdown and expensive repairs for possible faulty sensor and mainly the injection pump itself. with no error codes, the engine just refused to start.
swapped working injection pump
new tcv (timing control)
rebuilt ecu (new transistors/diode)
i ended up installing 4m40 pump and never had fear of sudden engine stall again. i really tried keeping the EFI as it was efficient and powerful stock.
 
i did a 1kzte swap on my lj78 few months ago, i have given up the dream of keeping it electronically controlled after so many breakdown and expensive repairs for possible faulty sensor and mainly the injection pump itself. with no error codes, the engine just refused to start.
swapped working injection pump
new tcv (timing control)
rebuilt ecu (new transistors/diode)
i ended up installing 4m40 pump and never had fear of sudden engine stall again. i really tried keeping the EFI as it was efficient and powerful stock.
Thanks for the feedback... I think this is the next step for me. We're up to ~$1k in sensors, ECU and other electronically controlled bits and it's not yet drivable. If the new (used) ECU doesn't fix it, that may be the next option.

Where did you go for guidance on the project?
 
Thanks for the feedback... I think this is the next step for me. We're up to ~$1k in sensors, ECU and other electronically controlled bits and it's not yet drivable. If the new (used) ECU doesn't fix it, that may be the next option.

Where did you go for guidance on the project?
i did few search here and watched others who posted videos of their swap then jumped on to doing it myself after having last resort already just to make my rig drivable. i just gave up on the electronic chaos and fear of breakdown over and over. it has become a good money pit to save the EFI. not worth it at all. the 4m40 pump is very cheap and u can spend a little more to make the rotorhead/plunger 12mm. (standard is 11m)
if im not mistaken, the standard 1kzte is 12mm. u can consider it if your new ecu doesnt work, it is an easy swap and still retain AT function. i will try to post pics of my conversion as soon as i can recover from my broken phone.
 
the downside is that your RPM (tachometer) wont work. i have not figured it out yet as of now, so im getting inaccurate reading from the alternator to make the tach work
 
Wanted to close this thread out, in the event that someone else has the same issues.

It was a bad ECU. I wish I had a good diagnostic protocol other than just throwing parts at it and hoping, but it worked in my case. I picked up a used ECU that looked good and clean off eBay, and it immediately resolved all the issues. Power is up, startups are quick, idle is smooth and all the issues are resolved.
 
Fantastic outcome, well done. Hopefully you get some reasonable life out of the replacement ECU.
 

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