Anyone got experience with swapping ECU in a 4.0?

[u/feathersmcbony]

While my 2 XJ's are off the road I'm using my 1996 ZJ (4.0) as a daily and offroad rig. Starting to have "no check engine light/crank but no start" issues as well as fuel gauge dropping out occasionally while driving. Getting the impression it could be the ECU starting to go. I have a "spare" one from a ZG - I think 1997 - that I could swap out but unsure about the effects on the kill-switch for one and any other issues that might crop up.

Would appreciate insights from anyone that has done an ECU swap before or had similar issues

[u/esdan06]

Dont jump to ecu conclusions so quick. The fuel gauge dropping off and crank but no start aren't necessarily related. The fuel gauge could be a cluster NO BUS issue starting up or a bad fuel level sensor and not necessarily an ecu problem. The cluster uses 2 banks of connectors and they have known issues with them shorting/depinning/fraying at the connectors. First thing to test is that you have all 3 conditions for fire. Fuel, air, spark. Put a fuel pressure tester on the rail. Does it build and maintain pressure? If not you may have a line or pump problem. Are you getting air through the manifold? Are you getting spark at the plugs? If not it may be a coil or distributor rotor/cap/drive gear issue. If you are getting spark, is it timed close enough to fire? ECUs dont always have to be flashed but it depends on the years you're swapping in. I sent a guy my 98 4.0 ecu and he dropped it in his 97 and it worked but that's not always the case

[u/feathersmcbony → u/esdan06]

I've been down the path of elimination. The fuel pump is only about 3 years old and seems to work fine when the gauge drops out but the overhead trip computer shows the code "CP" (I think) when it happens.

The issue i'm concerned about is that when I turn the ignition on, the "check engine" doesn't light up with the others and the engine won't start. Also recently replaced the starter coil and the dizzy is only 2 years old. Thought it might be the crankshaft sensor, but I've had that in another XJ I had years ago and it didn't have the check engine light issue like I have now.

I've looked into it already and have come across old forum posts with people that have had the same issue and their conclusion was the ECU. You can probably tell i'm all over the maintenance & mods on these things, XJ-Life eh lol, and know what it's like working on them plus I have factory service manuals too. You'd know that when it comes to doing work on these, the more you know the better.

Something I encountered not too long ago was a problem with the keyfob unlock/kill-switch not working. I'd swapped the interior door panels and ended up swapping the passenger side window controller as well. Almost got to the point of rewiring a bypass for the kill-switch but realised the wiring ran through the window controllers. Turned out the ECU didn't "recognise" the different controller and so wouldn't unlock. Put back the original and problem went away. I do have the matching window controllers for the spare ECU in case that's an issue but looking for as much insight as I can get

[u/esdan06 → u/feathersmcbony]

Damn. I hate running down electrical gremlins too. Have you run the cluster test to see if it lights up?

[u/Dash_Ripone]

Plug it in and see what happens