Hi all

I have an issue with my beloved XJR. I have spent a small fortune looking into it. The car miss fires at 3000rpm and isn't as smooth as it should be. I had it in at a specialist and they fixed a small air leak and changed the crank shaft sensor.

Just wondering if anyone has experienced anything simular or has any ideas on what the problem could be?

Thanks in advance

Richard

 

Any "check engine light" or OBD codes? What are the fuel trims doing at that speed? The fuel pressure? Spark Plug age/?
"Doctor I feel sick in the afternoon. What's wrong with me?"

 

Hey, thanks for the reply. No codes and no engine light. Full service with plugs done recently, they could be cheap plugs Im not to sure. Not to sure about the other bits but will find out

 

On the XJR of this vintage, there are two fuel pumps and fuel pump circuits. The secondary is used at startup and above 3500'ish rpm. Each circuit has its own fuse and relay.

If the engine cranks a bit long on a cold start, it could point at the secondary going out.

The pumps can be tested individually by jumpering across the proper blade sockets at the relay with key off. Listen for each pump. Should be a quiet "whir". Extended gurgling could be the hose loose in the tank.

After the pumps have been verified to work, swap relays in case the relays are bad.

If you are able to clear codes, or willing to run a couple of times with "restricted performance" showing before it clears, there is another test. The system is designed to work when one relay is bad by using the remaining circuit. The ECM detects a bad relay signal circuit not a bad pump or relay. Pull one of the relays, engine should still start and run well at idle. Replace relay and try same test on the other circuit. Make sure that you run for a few minutes to ensure that it is not just running on residual fuel in the lines. If either test fails, then start sniffing around but it is likely the pump itself.

 

is the misfire in neutral parked or driving?

The fuel pumps are an obvious suspect, the secondary cuts at 3000rpm run the checks plums suggests.