LQ4 stuck in limp mode

[Grand_Wag_85]

Did an LS swap on an old IH truck a few months back, ran great! I Hydro locked the motor so I swapped a new one in. Ever since I got it back on the road with the new motor the truck is practically undrivable. It keeps going in and out of some kind of limp mode where the horsepower is cut back about 50%. While it does this there are no codes, live monitors and graphs show a healthy engine and sensors. Idle is also absurdly low, so low the alternator won’t charge.

It goes in and out at will and usually starts to do it after being parked and especially when shut off. Sometimes it goes away after a restart, but that doesn’t always work. Sometimes it will go away if you rev the piss out of the motor. There is some heavy soot in the tailpipe.

I put about 1000 miles on the truck over the weekend. It went into the limp mode at least a hundred times. About 10 miles from home it lost nearly all power and I had to use four-wheel-drive low to get it home at the blistering speed of 3 miles an hour. I cannot get it out of my driveway even on level ground.

Grounds are good! Battery is grounded straight to the block. Main harness ground on back of cylinder head is good and the body ground has a dedicated loop back to the cylinder head.

Fuel pressure is 58psi

Injectors only have about 2500 miles on them, if that.

Coils, wires and plugs are all new.

Live data shows a healthy engine.

I am absolutely positively beyond stumped.

Swapping ECM’s produces the same results.

[Yeller]

That’s odd. Normally there is some kind of something showing up. Only things that comes to mind is did you accidentally swap sides with the O2 sensor plugs? That will cause exactly why you describe. Other is since you said you hydro locked the motor is an issue with the MAF or maybe an O2 sending erroneous readings.

[Grand_Wag_85]

Replaced the MAF which did help slightly.

Never unplugged the 02 sensors

[Grand_Wag_85]

Just checked it all with a NOID light. Passenger side injectors are not firing. Pending code P0200 but it’s not active. Whatever that means. The code does not show up on the first scan until you dig for it.

EDIT: on my harness, half of the injectors are on one fuse, and half are on another. It was a fuse.

One issue resolved lol

[Yeller]

Always the simple stuff that is buried

[Stuka]

Is the fuse under rated, or maybe one of the injectors is pulling too much current?

[Grand_Wag_85]

Is the fuse under rated, or maybe one of the injectors is pulling too much current?

Good question. I’m not sure.

The injector connectors that came with my harness, pretty much suck. I think I’m going to try and splice in the connectors from the donor vehicle.

[Grand_Wag_85]

Replaced all 8 injector connectors. That seemed to help out a lot. Still started off in limp mode then came out of it. Went into it again when I ran into the store.

This time around I was getting knock sensor codes while it was acting up. Not sure if this is indicative of bad knock sensors, or if the code is because of it going into this limp mode.

Will bad knock sensors cut the HP back?

[Yeller]

Yes it will it keeps pulling timing out, but if an injector is malfunctioning it could be causing the knock. Have you watched to see if the O2’s are having trimming up or down events when it happens? You can do the same with timing. Could just be getting enough timing pulled out to make it feel like limp. Too bad the knock sensors are under the intake….. unless you got a very late LQ4 that was built from a genIV block, then they are on the sides of the block

[Grand_Wag_85]

Knock sensors: done

New harness: done

Ran like a raped for approximately 3 seconds and back to limp mode now.

Battery was disconnected while I made the repairs.

Live monitors and data logging show an absolutely healthy truck and no OBD2 codes.

I am beyond stumped and ready to throw this off a cliff. It’s to the point now where it’s undrivable and just a stationary noise maker.

[Yeller]

Just poking ideas….. sure you have the correct MAF? Lq4’s used a few different ones, but typically only the correct one will plug on, may or may not make a code. Sure fuel pressure is correct? Sure you don’t have a DBW computer and DBC motor or vice versa? Sure the transmission isn’t faulting, will cause limp as well.

Dunno, just ideas that were thrown at me by my harness builder and computer guy when I had a similar issue with mine. Turned out he put a DBW program in my DBC motor. In his defense it is a DBW computer with the IAC circuits added for a DBC motor from the factory. All my pieces came from the same vehicle

[cmaje72]

I had similar problems until I found the bad pins in the coil harnesses on both sides. I would intermittently be running on 4 only the left or right bank. I did use a thinned factory harness though so they were old.
Do you have the ability to read/flash the tune on the ECU? What are you using for data logging?