How To Troubleshoot your LS1 Swap

How To Troubleshoot your LS1 Swap


So you’re nearing the end of your epic LS1 build, only to find out that when you insert your ignition key and turn it, your LS1 isn’t starting up or running right. With so many new manufacturers bringing so many new products to the table for LS1 swappers, it’s a small wonder than a true turnkey solution is still available.

No matter what the budget of your LS1 swap or build, or even if you are building or swapping a different GM engine, you have to know how to troubleshoot your LS1 swap.

How To Troubleshoot Your LS1 Swap –  No Fuel condition

When you are swapping a LS1 into your donor car, you’ll need the right fuel upgrades to ensure your chassis can deliver the fuel your LS1 needs. Whether you’ve installed a Walbro or another brand fuel pump, you need to check the following :

  • Fuel pump primes when key is set to “ON” position
  • Fuel pump receives power and ground
  • There’s 56 psi of fuel pressure at your rail.

If the following things have been checked on your LS1 swap, but it’s still not starting you’ll need to check the following.

How To Install LS1 Fuel Injectors


Fuel Pressure – The stock pressure at the LS1 fuel rail is 56 psi worth of fuel. You can achieve this a number of ways.

  1. You’ve got an aftermarket fuel pressure regulator and fuel system. Click here to learn how to install LS1 injectors and rail.
  2. You’ve got stock LS1 pump and stock rail – You will need a LS1 fuel rail adapter (shown below) to tap into a fuel pressure gauge.

If you have an aftermarket fuel pump with the stock fuel rail and returnless setup, chances are your fuel pressure is sky high. You can cheat this by installing a Bosch fuel filter, part number is 0756161.

How To Install LS1 Fuel Rails


This aftermarket fuel pressure regulator is used in a Chevy Express with the 5.3 liter engine, and automatically sets your fuel pressure to 56 psi.

You can also adapt a Corvette pressure regulator, GM part number 10299146 or 19239926.

How To Troubleshoot Your LS1 Swap – No Injectors

If you have sufficient fuel pressure at your fuel rail, but one or more of your fuel injectors are not firing correctly, you’ve got a big problem. To troubleshoot the fuel injector system in your LS1 swap, you should always check your MIL status first for any stored check engine lights.

If just one of your fuel injectors are not firing :

You can test your injector and the harness using a fuel injector noid light. The part number we’re going to be using for our 5.7 liter LS1 swap, is BOSCH 510597 or 510587.

How To Wire LS1 in a 240SX


Install this noid light and then attempt to crank over your LS1 swap to see if there’s a reaction. If your noid light does not respond, check your engine grounds. If the noid light does respond but your injector is still not firing and causing a misfire condition, you must replace the fuel injector itself.

LS1 Sensor Guide- What you need to swap a LS1


If none of your fuel injectors are firing :

Already check your ECU and it’s clear of any OBDII trouble codes? Then the next thing to check is the real time data in your ECU from the engine. Look for engine inputs that are stuck at 100% or values that your ECU should not be seeing.

For example if your TPS wiring has an issue, or is wired incorrectly you’ll see 100% value for the throttle or 0%, depending on what wire is mispinned. If there’s a vital engine input that’s miswired, or there’s a problem with the wiring no part of your LS1 EFI system will work correctly. Check your ECU values first.

How To Install LS2 Ignition Coils


How To Troubleshoot Your LS1 Swap –  No Spark condition

When your LS1 swap does not turn over or start, you’ve got to start with the basics. Testing your LS1 swap for spark is easy, simply unplug one of your ignition coils and plug in a spare spark plug into the spark plug wire. With someone inside the car and cranking the engine over, you should see spark at the test spark plug.

If you don’t have a spare spark plug, you can use an inline spark tester to determine which coil and cylinder is the problem.

How To Test LS1 MAF


Here’s a shortlist of things to check on your ECU when you are trying to learn how to troubleshoot your LS1 swap.

How To Test a LS1 MAF


ECU – Check your ECU terminal for power at these following pins.

LS1 Blue Connector

  • PIN 2 LT GRN 1867 12 Volt Reference- CKP Pin “C”
  • PIN 3 PNK/BLK 1746 Fuel Injector 3 Control
  • PIN 8 GRA 596 5 Volt Reference TPS Pin “A”
  • PIN 48 GRA 416 5 Volt Reference (MAP Sensor) Pin “C”
  • PIN 57 ORN 340 Battery Positive Voltage

LS1 Red Connector

  • PIN 15 RED 225 Generator Turn ON Signal
  • PIN 39 RED 631 12 Volt Reference CMP Pin “C”
  • PIN 66 PPL/WHT 2128 IC 8 Control
  • PIN 67 RED/WHT 2122 IC 2 Control
  • PIN 68 DK GRN 2125 IC 5 Control
  • PIN 69 LT BLU 2123 IC 3 Control

ECU – Check your ECU terminal for Ground or low reference at these following pins.

LS1 Blue Connector

  • PIN 1 BLK 451 PCM Ground
  • PIN 21 YEL/BLK 1868 Low Reference CKP Pin “B”
  • PIN 40 BLK 451 Ground
  • PIN 53 BLK 407 Low Reference (Transmission Temperature Sensor) Pin “M”
  • PIN 54 BLK 407 Low Reference (MAP Sensor Ground) Pin “A”
  • PIN 60 BLK 452 Low Reference TPS Pin “B”
  • PIN 61 PNK/BLK 632 Low Reference (Camshaft Position Sensor) Pin “B”
  • PIN 80 BLK 407 Low Reference ECT Pin “A”

LS1 Red Connector

  • PIN 1 BLK 451 Ground
  • PIN 57 PPL 719 Low Reference IAT Pin “A”
  • PIN 60 BRN 2129 Low Reference Ignition Coils bank 1 (1-3-5-7)
  • PIN 61 BRN/WHT 2130 Low Reference Ignition Coils Bank 2 (2-4-6-8)62 — — Not Used

That does it for our How To Troubleshoot your LS1 swap, if you have any questions or a problem with your LS1 project, leave a comment for us below!




  1. In need of some help with my swap cause ive run out of ideas to look at. Got Ls swap in my 54 bel air. Got fuel psi, spark, compression, and the injectors are spraying. It hits like it wants to start and then just goes back to cranking. Ive sent my PCM back to verify that vats was turned off. The PCM works on another swap so I know its not the computer. Any help would be VERY grateful.

    • Hi Josh, connect a datalogger and check your sensors. TPS / MAP or AIT being off, miswired can cause this issue. Any values real time on your ECU or PCM look off?

      Thanks for commenting, let us know and we can assist you further

    • Hi Jamie, so everything is working but you have no trigger signal at your GEN III LS1 coils? Do you have a datalogger? can you check to make sure that the vital sensors in your engine are reading correctly? If you have a bad signal at TPS, MAF or your crank wiring your coils won’t fire.

      • Hey I’m having trouble with my ls swap in my 71 nova I have every thing hook up but my o2 sensor when I turn my key on my check engine light come on but my fuel pump won’t come on and car won’t start

        • Hi Darrell, thanks for reading!

          Have you connected an OBDII port or do you have one in your swap? I would start there… sounds like your NS condition may be caused by a lack or incorrect signal. Connect the scan tool and read what your LS1 ECU is seeing. Make sure things like TPS or MAP signal aren’t showing 100% or anything weird like that.

          Let us know how it turns out, it sounds like an awesome swap! Thanks for commenting

  2. Hey John I was hoping you could lend a hand here. I just installed a 2004 5.3 Vortec truck engine into my C10. The truck starts great runs great however have a few problems.after I start the truck the idle will continue to raise on its own until it reaches about 3000 RPM if I give it any kind of gas the idle will drop to about 850 and then it the idle ssures to 1100. Runs like it had a cam. if I give it any gas the throat has no response the body is a drive-by cable on a 2004 Drive by wire intake I converted it when I bought it.the tps iac and maf are all new. I checked the iac and wiring all check out. Any suggestions? Clogged injectors? I spread everywhere with carburetor cleaner to look for vacuum leaks and no luck.

    • Hi Eugene, cool swap and thanks for reading.

      Couple of questions, do you have the OBDII port intact on your swap? When you converted the TPS did you also switch to a manual throttle ECU? Which MAF are you using? the one from the 04 Vortec right?

      Let me know and I can try to point the way, thanks for commenting!

  3. Hi John, swapped a lq4 with ls3 top end into a 2006 silverado (4.8). Went to start it, nothing happened.
    1st problem I found was a blown fuse for the starter, changed it out. Now it cranks but will not start. Checked grounds, fuel injectors, even made sure the timing on the cam and crank were set properly. Checked wire harness for signals at crank sensor, cam sensor and so on. Checked fuel and spark all are good, next thing to look at will be compression. I’m also concerned about the cam, it is GM’s ASA cam. I’ll clear the DTC codes and check the ECU values. If you have any other suggestions let me know. Thanks CA.

    • Hi Cedric, thanks for reading our LS1 swap guide.. So when you say you have DTC codes what are they? Also have you checked the live values of the TPS, MAF or MAP sensors? And you do have spark and fuel right ?

      Let us know and we can help you with your GEN III GM engine swap

  4. Hello,

    I’m having two problems with my 5.3L swap in my 65 Nova.
    1. Car tries to start with MAF plugged in but will not start; I unplug the MAF and it starts.
    2. Car starts and runs but #2 Coil don’t Sparta. I swapped out the coil with another coil from a different location and still the same problem on #2. Car runs, however the #2 header tube don’t get warm.
    Your assist is greatly appreciated.

    • Hi Charles, thanks for reading.. Couple of questions for you

      1. what ECU are you running? more specifically what 5.3 liter ECU part number? Are you sure you have the correct engine computer?
      2. what part number of MAF are you running?
      3. Have you checked the continuity of the wiring from coil 2 on your ignition coil wiring? Are you using the stock cluster harness for your ignition coils?

      Let me know and I’ll try to help you in the right direction. Thank you for commenting on your GEN III GM swap!

  5. Hey there,
    Having trouble with my LS swap 5.3 car turns over runs fine for about 10-15mns then begins to spit sputter cough and backfires also, sometimes it dies but it starts right back up if I get on it will do it immediately if I drive it conservatively it will start doing it in about 10-15mns… Any and all suggestions will be helpful..thanks

    • Hi Juan, couple of questions for your LS swap there.

      have you connected a scan tool? Do you see good values for TPS, IAC? MAP sensor? If you have a malfunctioning IAC it could definitely be the issue. what ECU are you running? Let us know and we can help troubleshoot your LS swap for good! Thanks for commenting!

  6. I’m having trouble with my ls1 swap it starts runs for 10 to 15 seconds then idles way down and eveny dies if you try to give it throttle it sounds like a big dead spot and dies if you feather the throttle real fast you can get some rpm out of it but when you stop it dies?

    • Hi Randy, quick question do you have your LS1 swap with the OBD reader connected? if so can you see what the real time values of your ECU are? Did you wire up the LS1 swap harness yourself? Often times a mismatched wire can lead to this kind of problem. Can you tell me what the TPS readings are before the engine shuts off? Do you have fuel pressure?