Dear Gentlemen,

This is my second year owning this 2001 XJ8 (4l). I live in the Baltics, and as some of you may know, there are countries where people install biogas supply units to run their cars on biogas. I acquired this car with an engine gasket that had exploded due to such modifications, and the previous owner had replaced the engine with a donor engine. This is where all the problems began.

I am quite desperate to find a solution to these electronic issues, but I still want to give it a try. If the gentlemen of this forum cannot help, I am sure no one can.

My car's VIN is SAJDA14CX1LF24348 (originally a US car), and as I mentioned before, the previous owner swapped the blown engine with a donor engine from another car with VIN SAJJHALD4CR829747. The issue, as discovered later by my electrician, is that the engine was the only part taken from the donor. After the engine was replaced, electrical issues appeared immediately—most notably the well-known engine check and "restricted performance" errors.

The previous owner replaced the engine coils and later the engine control unit (ECU), but that didn't solve the problem. He also purchased another throttle from the UK, supposedly programmed for the swapped engine. Despite these changes, the engine check light and dashboard errors persisted. The car does not go faster than 130 km/h, and if you accelerate aggressively, it starts to "jump," as if it accelerates quickly and then suddenly restricts the acceleration.

When I got this car, it had not been used for around two years. I also noticed that the transmission behaved poorly due to a lack of maintenance. There were other issues like the central lock and a few other minor problems. The major repair I undertook was replacing the transmission (from another donor car) and changing the tensioners and chains, as the engine sounded bad and there was an error indicating a damaged chain.

As the electrical issues persisted, I found an electrician to diagnose the problem. After waiting six months for him, he was quite surprised by what he discovered. To sum up, he told me the following: with parts from different donors (engine, throttle, coils, engine control unit, and transmission), it is impossible to set everything up correctly. All these parts are so heavily electronically programmed that they need to come from the same donor to work properly.


2001 XJ8 with swapped engine


Therefore, I received the verdict that it's impossible to repair my car. Indeed, it doesn't make sense to replace all these parts, especially since the car's body isn't in great condition either.

Currently, the car runs as described and appears to have a major throttle issue. The throttle doesn't work properly—it opens too fast (scanner shows this). When I put it in reverse and accelerate, it jerks back so hard that it's frightening. If I accelerate too much, the rear wheels spin as if the car were chip-tuned for more power (though this only happens when starting to move; continuous acceleration leads to the "jumps" I mentioned earlier).

I've shared my predicament with you, gentlemen, and if any of you could offer some advice, I would really appreciate it. The long XJ has been my childhood dream, and I don't want to give up on it so easily. Deep down, I understand my electrician's point, but perhaps there is still some hope and light at the end of this tunnel?
Max

 

Welcome Max,

the donor engine number could confirm it is the correct one for the car?

The trans sounds like it needs a rebuild or at least a trans-go valve kit, as you're suggesting the classic 'hard shifts' into reverse - the bang into gear at higher revs is a valve sticking in the trans, against a wear ridge. This leads to a pressure spike and failure of a circlip holding the A drum, once that goes it's a new trans or rebuild.

Your electrician is correct in that the ECU's can only be swapped with the correct software - a Canbus system that can read and rewrite to the cars memory, high end OBD readers have this capability, you'll either need dealer level WDS/IDS or very good aftermarket for reprogramming. Thing is, why was the original ECU swapped and can you get it back in the car? The donor engine doesn't need a donor ECU, just all the sensors plugged in.

A simple Bluetooth OBD reader like the ELM unit plugged in under the dash will give us codes that can help pinpoint engine faults, the software mentioned above will give a lot more info.

There is a throttle adaptation procedure, and a throttle cable adjustment guide within the forum, these 2 things you can carry out now - they are fly by wire throttles, firstly confirm the throttle cable has little slack, then perform a 'hard reset' - leave the battery disconnected and go for a coffee. Battery (12.5V+) back on. Start the car and let it perform the recalibration, high/low idle and settle, leave it to idle for 5 mins.

A few suggestions to get on with. I'm sure other members will come in with advice being a US supplied car.

 

The VIN 829747 is a 1998 XJ8. That is an AJ26 engine.
The 2001 is an AJ27 engine and they are NOT directly compatible.

I have done the swap but many things need to be changed from one engine to the other.

I have even used an S-Type 4.0 (AJ28) cylinder block with XJ8 (AJ27) heads and oil pan to work in my 2000MY XJ8.

All the V8 blocks are similar and can be modified to work, you just need to know what parts go where!!!!!!

 

Thanks Bob, saved me the time.

 

I agree highly with Bob and Sean. The flexplate is different and the lack of a camshaft sensor and trigger wheel on bank 2 along with a different bank 1 cam trigger wheel that the sensor reads. I’m going to assume that the original engine is long gone?

 

You will need aj26 sensors and probably throttle body
​​​​​good electrician can solve this with the right amount of $$$
However the transmission also is another problem
I think the better choice is keep it as car parts and get xjr or another low mileage xj8 in good condition but it's all your choice