Hi have an XJ40 1989 sov 4.2l on start up car revs to 2000/2500 and stay there then on restart will clear or surge between 1200 to 1500 rpm if you let it surge a few minuets and restart runs fine for rest of day the fault code at present is 42 [electrical] but im sure it was a different code when the problem first started the fault is intermitant I am afraid it has been a long time since I have been on the forum so have forgotten the rules and etiquete hoping for some help HAZ

 

Hi Paul, been a while since you have been on. We met at the NSW/Ballina meet a couple of years ago. Glad to see you still have the XJ40.

A quick repost of your question in the XJ40 forum will get a better response than here. Click on the link below.

XJ40 ( XJ81 ) - Jaguar Forums - Jaguar Enthusiasts Forum

 

Paul,

I've moved your question from General Tech Help to XJ40 forum. Members here with the same model will be able to help.

Graham

 

Hi Haz,

Welcome back!

First of all, I assume the engine in your 1989 XJ40 is a 3.6L rather than a 4.2L (which was used in the earlier Series III XJ6)?

Also, as far as I can tell, 42 is not a defined fault code for any of the XJ40 versions. Is it possible you have a 4 code (Oxygen sensor full rich) and a 2 code (Mass Air Flow sensor not in range)? Have you tried clearing the code to see if it recurs?

I'm sorry for this random shopping list of ideas but it's late and my brain is tired:

It sounds like the engine is behaving as though there is an air or vacuum leak, but the fact that it will run fine after a warm restart suggests the "leak" is not constant and that must be a clue.

My first thought is that there could be a problem with one of the emissions-related components that allows air or fuel vapors into the intake at certain times might be misbehaving. For example, if the EGR valve is sticking open. If your car is equipped with a supplementary air valve or carbon canister purge valve, those would be possible suspects. The supplementary air valve is opened for cold starts and the Engine Control Module (ECM) decides when to open it based on the signal from the Coolant Temperature Sensor (CTS).

That reminds me that on the early XJ40s a common problem was the electrical connector on the CTS shorting (to the hood?), and all XJ40s are susceptible to damage of the electrical connector and wiring insulation, which harden over time and crumble. The sensors themselves fail, too, but it's easy to do a rough check by comparing the resistance across the sensor's terminals when the engine is cold and again when the engine is hot, and comparing the readings to the chart in the AJ6 Engine Management Systems manual which is available at jagrepair.com courtesy of our forum member Gus.

Another component you could suspect would be the Idle Air Control Valve stepper motor, but again, the fact that the engine runs fine after a warm restart doesn't seem consistent with an IACV problem. It would still be worth removing the valve, disconnecting the stepper motor and cleaning out all the carbon buildup inside. Also clean the electrical connector with zero-residue electrical contact cleaner. See my photo album on the IACV at the link in my signature.

Other things worth checking: Ensure that the Throttle Position Sensor (TPS) is secure on the underside of the Throttle Body (TB) and hasn't worked loose. Also check the throttle cable for any signs that it is binding and preventing the TB butterfly valve from closing properly.

I suppose another possibility is that there is an air leak at the intake manifold gasket or head gasket when the engine is cold, but it is sealing up when the engine gets hot. It's easy to check the torque of the intake manifold screws & nuts. The torque spec is low and more than once I've discovered that a few of the fasteners on our intake manifold have worked loose.

Oh, and I should mention that sometimes this sort of unusual behavior is caused by multiple issues, so consider giving the car a good tuneup (plugs, air & fuel filters, distributor cap & rotor, and maybe plug cables if yours are several years old). This will at least rule out these common items as contributors to the problem.

Also clean the engine bay ground points, especially the ones used by the ECM (the two on the intake manifold and the one on the firewall/bulkhead behind the cylinder head).

Hopefully others will have some better ideas. Please keep us informed.

Cheers,

Don