1uz swap, limp mode?


Rock Crawler
Hi all. So i took a 1uz, believe it to be a 1994. non vvti.
completely rebuilt the engine, all new timing, gaskets, sensors, etc... (also all stock)
Installed said engine into a full tube rock crawler with a modified a340 trans.
Harness was previously cut down from another crawler so some of this I'm trying to fix from another person, some of it was myself.

Auto trans runs from an electronic shifter to control the solenoids.
fuel pump runs off of a relay from the ignition switch panel so the ecu does not control that, fuel pressure is controlled from a standalone regulator set at about 45 psi.
Diagnostic plug on engine was removed with all wires so i havent been able to pull codes. need ecu pinout help for that.
02's are hooked up to ecu as pre and post, didnt know which ecu wires were which.
emissions have been deleted other than the sensor in the intake manifold on drivers side.
IAC on front of engine, along with all wires were removed.

Basically it has crank, 2 cam sensors, knock sensors, 2 02's, igniters, coils, injectors, afm, tps, coolant temp.
tps was measured and set per spec, coolant temp sensor was tested good.
afm appears to be working correctly, tried 2 of them with same result.
tested spark and it does not seem to be dropping off.

Engine starts right up, idles fine, and will rev up smoothly until a couple thousand rpm and then it starts sounding like a cylinder is dropping fuel/spark and gets worse with throttle until its to the point where it is cutting almost all cylinders and dropping rpm and then back up again.

sounds like limp mode or the ecu thinks its in neutral.
I've tried finding this mysterious blue wire for the ecu to think the trans is in gear.
And all sorts of things that I have read on the interwebs to try and diagnose this.

I can post a link to a google drive that i used to wire the ecm. some of them i do not know where or how to wire and that may be the problem.

ecu is a 34,22, 16,28. part number 89661,50-224
also checked ecu for leaking capacitors and found nothing.


Rock Crawler
link to ecu wiring i tried to follow

things i do not know about:

16 pin connector
pin 1 "do i need this" what is custom plug for?
pin 6 do i need to wire this to something even though i'm using a stand alone trans controller
pin 12-14 more auto trans pins it says to use

22 pin connector
pin 9 speed sensor, what should i hook this to. I have no speed sensors, might could find a way to hook to trans?

28 pin connector
pin 24 i believe this is powered key on but i have no efi relay

34 pin connector
13 and 14 do i need these and what should they go to?
17 more auto stuff
20-23 deleted, idles fine though


Well-Known Member
Very cool truck.

Its great you included so much info as it's easier to work out issues with good info.

Did the ECU/loom run fine in the previous truck? If so what did you remove or change?

If I was wiring this I would run it like a manual so no trans stuff at all.

I would have the ecu controlling the fuel pump as the 20 series ECU have a condition for a fault code for fuel pump which causes them to lose power. This might be able to be tricked out but I always wire them correctly.

Pin A24 (Mrel) is an output to the main relay from the ECU. Not an input.

I would leave NSW out but put STA in and ensure it works correctly. I would be running the fuel pump from the ECU too though. STA locks timing during cranking and triggers fuel pump.

The ECU can look fine but still be at fault. There could be lots of different things causing your fault. A diagnostic plug and scan tool is a good start. Or an aftermarket ECU.


Rock Crawler
I’m not sure if it ever ran well in the other vehicle. It did run though.

Correct running it like a manual would be great but if it was an auto ecu do i need to do anything to trick it?

Pin a-24 the output to a main relay gotcha. I have none of that. The computer controls no relays at all so what do i do here?

The sta is for running the fuel pump? If i can find out what ecu pins are suppose to run the fuel pump i can wire it to run the relay i have installed now.

I would love a diag port if i knew where to wire it up but a scan tool is unlikely being it is obd1


Well-Known Member
Slight misunderstanding.

STA is the start input which once received by the ECU the ecu turns on the fuel pump and locks base timing during cranking - ie - when STA is received.

Tricking is just part of wiring correctly.


Rock Crawler
Okay so sta gets power while cranking and then turns off while running correct? so basically i can hook it up to my control side of relay for starter. And then figure out which ecu pins to run the fuel pump off of


Rock Crawler
Thanks, any idea on which relay gives fuel pump full voltage as I do not want to run my pump under that.


Rock Crawler
is it a relay/resistor version or fuel pump computer?
Not quite sure what you are asking. I run a fuel cell with an inline high pressure pump, it needs 12v constant, not the 1/2 voltage that i have read the lexus computer gives the pump under no load or cruising speed.


Rock Crawler
still not quite sure how an output of the ecu can cut rpm or go into limp mode when it has no way of knowing if said output “fuel pump” has no feedback to the ecu


Rock Crawler
is it a relay/resistor version or fuel pump computer?

If i’m reeading this right your talking about jumping the resistor to keep full voltage?

Thats where i think i am running into problems. I do not have any of so called factory wiring or relays. Unless they are internal to computer. I literally have the ecu and what little sensors i have left and thats it. The only relays i have, i installed for fuel pump, lights, etc


Rock Crawler
another thing i was thinking about, oil pressure. Does the computer need to know that it is making pressure to stay out of limp mode? what about the oil level sensor in the pan?