Hello, I have a 2003 Land Rover discovery. It originally came with a 4.6 v8 but I swapped in a rebuilt 4.0 v8. I am at the point where I am testing it and working through issues. It has been running great except for two issues.

Firstly- The car runs great at idle but I noticed once it warms up and I give it a little gas in park and then take my foot off or when I go to brake at a stop with my foot completely off the gas the revs drop very low like it wants to die and then it immediately rebounds itself and sits right where it normally should around 700 rpm. This problem hasn’t been too concerning but is very strange considering I have installed a new IACV, and I have also looked for vacuum leaks to no avail.

Secondly- Like previously stated the car sounds like it is running great and drives around nicely from 0-35mph. The exhaust smells a little rich but not bad. The problem comes in when I am above 35 mph and it starts to misfire. Not a terrible misfire but definitely noticeable and the SES light flashes. I checked my code reader and I have codes for SAI, the SAI is completely taken out on this truck and I am planning to get the ecu reflashed. Based on what I have read this should not be causing the problem though. The other codes include a random misfire code and specific misfire codes for all cylinders on the passenger side 2-8. After driving the car again at about 45mph to see if it would happen again it did and I received a P0139 and P0159 code for a slow response to both back oxygen sensors. The oxygen sensors are brand new and have been replaced but still could be faulty. I also have replaced spark plugs, coil packs, plug wires, and the crankshaft position sensor when I rebuilt the motor. I also had codes P1117 and P1481 which I have read could be o2 sensor codes which probably means my o2 sensors need to be replaced all though they are brand new. I also have a P1300 code which I have read is a random misfire code. I am thinking I am going to replace my o2 sensors and see if that makes a difference but not sure. Other things I have read is the throttle position sensor and camshaft position sensor but don’t believe it is those because the misfires only read on the passenger side, which leads me back to o2 sensors. I also am led to believe that these Bosch ecu’s are able to learn as they go so the engine swap shouldn’t be the issue.

I know this is a lot to read but I am working through this issue and just wanted to see if anyone else had something to say that is helpful. Thank you!

 

Can your reader get your trims/freeze frame? I'd first compare banks. Sometimes O2 codes (not heater codes) are the sensors doing their job.

Did your check for vacuum leaks to no avail mean you weren't able to check or that you didn't find any (just want to cover bases)

Misfiring sounds like it's under load only? Sometimes that's a slow or drooling injector, although I'm learning injectors on these are pretty reliable. Under load misfires can also point at coil.

On my own Discovery, my misfire under load was cracked injector wire.

But I'd eliminate the things you can check with scanner and go from there, before parts or tearing into it.

 

Maybe check exhaust back pressure to rule out clogged cat (or cats). Measure pressure at the upstream O2 ports. Pressure should be well under 3 psi even at high revs. Especially check the Bank 2 side, since that is where your codes are. If pressure is high, that could indicate plugging that could cause misfires. Fast and easy to check but you will need a gauge and some way to adapt to the O2 port, and a helper to blip the throttle a couple times.

 

I will look into the reader asap. I did look at o2 sensors on my reader but seemed like they were running rich around .9 volts. I looked for vacuum leaks but was unable to find any using a can of throttle cleaner spraying around possible leak points. I had the injectors sent out and rebuilt, so I think it is possible but unlikely it is them. The coils are both brand new and the misfires are all on the passenger side so it would be both coils if the coils were broken, which I have also found unlikely. I greatly appreciate your info and I am definitely going to test my reader out this weekend and see what I can find. Thank you!

 

I am definitely going to look into this asap, thank you for your help!

 

Start with the easiest troubleshooting first. Your description is misfire under load, which is normally ignition related, especially on these engines. Start by swapping all spark plugs left to right. If the misfire codes don't move with the plugs, then swap the oxygen sensors left to right. If those don't cause the misfires to move swap the plug wires left to right. If those don't move the codes, you can swap injectors left to right.

Lean conditions rarely cause misfires above 35 because as engine load and rpms rise the vacuum leak becomes very small relative to the airflow through the MAF.

Suggest you get an elm327 and the free torque app so you can watch the 02 sensor voltage in an analog graph cycle up and down, digital scan tools do not tell the full story. If they are cycling normally then the misfire is not due to fueling.

 

Will do, thank you!