For almost 2 years I have been trying to get my 1986 XJ6 tuned in so it will start easy and idle at 800rpm. When I got the car the pickup and ignition module were bad. I replaced both of them and it runs ok now but if I set the timing correctly it is near impossible to start. I also have to have the idle set at 950rpm so in gear with the A/C on it doesn't stall. I checked the compression and all cylinders are good. I have put around 8,000 miles on it since I have had it and my gas mileage is almost 16mpgs. I had a little preignition (spark knock) but switched to 93 octane and that went away.

Any advice as to what I should check would help me greatly because I'm trying to convince my wife that we need an XJS convertible too but if I can't fix the Jaguar we have now and make it so she can drive it than the XJS is probably out of the question.

Thank you

 

Are you having difficulty starting the engine when cold or after a hot-soak? If you're having a problem with cold starting, you may have a clogged/disconnected cold start injector. If the problem is after a hot soak on restart, you may have leaking injectors, a leaking cold start injector or a bad fuel pump check valve.

What is the engine idle speed with the air conditioner in the off position? If the engine is stalling check for vacuum leaks as unmetered air will cause the engine to run very lean, something the XK engine doesn't like to do! Also, check the auxiliary air valve, which is designed to allow additional air to enter the system when the engine is still cold or warming up. Sometimes the valve will become stuck open or one of the hoses will develop a crack given the age of these vehicles, anything is possible!

NBCat

 

Here's an easy possible fix:

Clean the throttle body and make sure that the throttle blade is set at the correct .002" gap.

Cheers
DD

 

That's right....plus many symptoms are not "single-fault" fixes; that is, a problem may be the result of a "stack up" of problems

Cheers
DD

 

I will clean and check the gap of the throttle plate this weekend along with a few other repairs I need to do. I just replaced the fuel pump and it holds pressure for quite some time so leaking injectors or check valve is out.

Cold start is harder than when the engine is warm but almost every time I start it I have to push the accelerator up and down fast while I'm cranking it.

Right now I have the idle set at about 1050 with the A/C off and in park but with the A/C on in gear it drops down to between 500 and 600. If you turn the steering wheel in gear with the A/C on it will some times stall.

 

There is also a thermo-time switch that operates with a bi-metallic spring, which over time, can cease functioning. From your detailed description, it appears the engine is trying to run on a very lean mixture, something the XK isn't very fond of, thus the stalling under additional load. In addition to checking the throttle position at idle, check for any possible vacuum leaks as unmetered air will create an over-lean condition. Bear in mind also that with the Bosch L-Jetronic system, as installed in the Series III XJ6, virtually all fuel delivery and performance problems can be traced to the air-flow meter.

Here is a link to a technical manual for the L-Jetronic and while it's for Alfa-Romeo, the system basics and components are similar on the XJ6: L-jetronic Fuel Injection Technical Troubleshooting Article

NBCat

 

Here is an article by Roger Bywater of AJ6 Engineering in England that discusses the Bosch/Lucas EFI system specific to the XJ6:

Fuel injection and the Jaguar XJ6 4.2 Series 3 / AJ6 Engineering

NBCat

 

Is there a way to test the air flow meter it probably says somewhere in the links but I haven't had a chance to check yet. I may have a leek between the air flow meter and the throttle body the one hose looks a little rough.

This weekend I will sit down with my car and computer and hope I can make things better.

 

Yes, there are several tests that can be run using a volt/ohmmeter. Both links have that information as well as schematic diagrams of the systems and some components.

NBCat

 

After looking at the location of the cold start injector I will bet it is plugged up with stuff. When I got the car it had been sitting and the gas was old in it and there was a lot of debris in the fuel system. The port off of the fuel rail for the cold start injector is on the bottom so any crap in the fuel system would probably settle in the cold start injector.

Hopefully by tonight I will be able post my findings.

I have been buying parts from Motorcars LTD. Is there someplace better?

 

I have had good experiences with AutohausAZ (Auto Parts at AutohausAZ - OEM Auto Parts - Discount Replacement Parts, Resources and Car Care Tips) when it comes to reasonable prices and fast shipping service.

NBCat

 

I checked the air bypass valve, air flow meter and the cold start injector and they are all working. I should say the parts aren't bad. I had no power to the cold start injector when I cranked the engine over and it was cold. Also the contacts in the air flow meter for the fuel pump relay do not turn on the fuel pump relay. A long time ago I put a jumper wire on the fuel pump relay to help with the hard starting problem.

Where does the cold start injector get it's power from? Is it possible that I have a fuse or bad connection to the fuel pump contacts and the cold start injector?

 

Check this forum and JTIS for the wiring diagram. The cold start injector is energised by the thermo-time switch located on the water jacket that runs across the inlet manifold. It's been some time since I've worked on an XK engine, but if I recall, it's the device with the black connector. The switch should allow the cold-start injector to work only when the engine is cold, however, over time, the bi-metallic spring can separate and the switch will no longer work.

A good source of information is also to be found in an article written by Roger Bywater of AJ Engineering in England, which describes in detail the design and function of the Bosch/Lucas EFI L-Jetronic version fitted to the XJ6: Fuel injection and the Jaguar XJ6 4.2 Series 3 / AJ6 Engineering.

NBCat

 

I have read that article a few times now and it seems so simple on the computer but when I get out to the garage it's not as a simple on the car. It's a very good article and I'm going to save a copy so I can refer to it in future.

 

The cold start injector gets voltage from the starter circuit...the starter relay specifically. The the circuit ground comes from Thermotime (TT) switch

Cheers
DD

 

Thanks Doug and NBcat

I finally found a problem the Thermotime switch is bad. Surprisingly enough the maintenance handbook that comes with the car has some good information on testing various parts.

Now all I have to do is find one and get it ordered. I found one on one website and put it and some other miscellaneous I need in the cart and printed it to a PDF. I'm going to email it to another parts supplier and see if they can beat the price.

 

I'm only familiar with the thermotime switches and cold start injectors in older VWs but I'm sure they're similar are they're both Bosch designs. That disclaimer aside I don't believe the thermotime switch will have any impact on your concern about it stalling while in gear at idle. It will likely help correct hard starting problems, however. It's sole function is to fire the the extra cold start injector for starting enrichment. Once the engine's running the cold start injector is no longer used.

 

Your probably right but at least it will be one less thing wrong with my car. If the car will start easier I can tune it in to run better without trying to adjust for the starting problem. One of the things is the ignition timing is way off so it will start better.

Thanks for the information.

I haven't checked my fuel pressure yet but I did just replace the pump. If the fuel pressure regulator was weak how likely is that the fuel pressure could be good enough to run but not good enough to run good.

 

The best way to tell is just put an external gauge on the fuel rail and see what the pressure is during idle. Did you also change the fuel filter along with the fuel pump?

You will also need to reset the ignition timing to specification while also checking the throttle position at idle for correct setting of the throttle plate and making sure there are no vacuum leaks weakening the air/fuel mixture further.

NBCat

 

Hi
Does anybody know whether the AJ6 on my 1994 XJS 4.0 Litre shares the same idiosyncrasies as on the XJ6 engine, with specific regard to starting and running from cold.
I have just started to have a problem with this- specifically that when I start the car from cold (either in Winter or Summer) it starts first time on 6 cylinder and idles at about 950.
However, if I attempt to drive it (normally) it stalls and shudders.
It makes no difference whether or not the car is left to idle and 'warm up'.
Also, if it is left to idle for about 4 mins, it begins to hunt - which can be rectified by 'blipping' the accelerator.
Once the car has been driven for about 10 mins consecutively, it is fine.
It pulls and accelerates well and idles OK when hot.
Any suggestions??
Your help would be appreciated.