Just replaced the 4.0 liter in my 98 discovery le with a used motor from a wrecked one. Got it all in and hooked up but it won't stay running. Starts, idles rough, backfires then dies. Have replaced all the plugs, plug wires, coil pack, checked for vacuum leaks, checked the fuel pressure and replaced the sensor above the harmonic balancer. Any ideas?

 

Did you use the ECU from the donor truck?
No?
Thats your problem, your ECU has "learned" your engine, it knows how "your" engine runs.
Until it relearns the "new" engine it wont run right.
The quick way to fix it is to have the dealer erase all of the old info, the ECU will then go into "default" mode and it will run "default" until it relearns, which takes about 400 miles.
If you can keep it running well enough to drive you can reset it just by driving it.

 

Can a normal mechanic with the a generic code reader erase the data in the ecu or does it have to be a special piece of equipment?

 

Spike, what about him installing his old IAC in his new engine, his ECU is used to that one?

 

What is an IAC?

 

Idle control unit.

 

You need the test book dealer software. You can't cannot even program the ECM with the hawkeye scanner. If it is drivable (sounds like no) just keep driving and your truck will adapt.

 

Good point Mike, I did not think about that.
If you can still get part off of your old engine then pull the IACV (idle air control valve) off of it and swap it into the "new" engine.
It is located on the front of the throttle body, closest to the air intake tubing.
Remove it, clean it with carb cleaner making sure you do not move the plunger, take special care with the one on the new engine too.
Put it into your "new" engine and you should be well enough to drive it.

 

So, if the IAC is replaced by the dealer, what do they have to do after they replace it? Reprogram anything?