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Jaguar Repair/1984 XJ6 poor idle



My XJ6 hasn't really been out of the garage since passing TN emissions a couple of weeks ago, but tonight I started it to find it had a low 5-600 RPM, loping idle. I tried a few things right quick - pulling on the accelerator results in a strong, healthy sounding engine, reaching in to the MAF to actuate that flap while running causes it to quickly die, although it starts right back up. I look the large rear plug out of the exhaust manifold and though I did not test back pressure there, it being removed offered no change in idle or revving performance, indicating no extreme exhaust blockage at least. What do I need to look at next?

ANSWER: Hi Michael,

Pushing in the flap and not getting a good responce is normally an indication that the problem is not a lean mixture because pushing in the flap riches the mixture. The open plug on the maifold only tells you it is not a stopped up exhaust system. The fact that it reves up ok is an indication that the air flow meter is working (not a 100% test but a good indicator)

Do this, Fire it up and let it warm up to operating temp. Then take a volt meter and set it on a low volt scale and on DC. Remove the wire off of the O-2 sensor (an 84 should have a one wire sensor) check the voltage out put from the one wire to ground and while idling it should read .5v and if it reads above .5v on up to 1 volt the mixture is too rich for some reason and if it reads less then .5v then it is too lean for some reason. More then likely a vacuum leak. (start pinching off vacuum hoses with a pair of needle nose pliers to look for a change in RPM)

Next check the ignition timing to see that nothing has went wrong there. If all this fails you need to run a compression test of all cylinders with the throttle open. Put a fuel pressure gauge in a "T" between the fuel pump and the fuel rail. should read about 32 PSI at idle and go up to about 42 PSI on a quick throttle opening (or by removing the vacuum hose off of the fuel pressure regulator on the fuel rail)

If you did ALL of the above and it all is correct then use a Allen wrench and try to adjust the Idle speed screw (counter clockwise to increase idle speed)

Let me know what you find.


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Voltage at the O2 sensor was actually over one volt and I did not have quick access to a timing light today, so I had to skip to compression test. Plugs all had buildup typical of rich mixture and I cleaned those with wire brush wheel on bench grinder. Compression 1-6 was 135,130,130,145,135,135. With the plugs back in, I removed and checked the bellows between the MAF and elbow and the rubber sleeve between elbow and throttle body and found no cracks or tears and so reinstalled those and made sure clamps were tight. I skipped putting fuel gauge in fuel line as the issue appears to be too much fuel, not enough. I can hear the flow in the line with key on, and opening flap as that was one of the first things I tried the other night. That is when I decided to, on a hunch, clamp the fuel supply line to the cold start injector. With that in place, she started first try and settled right in to a 750 RPM idle. Ok, so it seems I have a leaky cold start injector. I'm done for tonight, but I'll definitely test that before I fork over the money to replace.

Let me know any additional thoughts you have on this, and thank you.


Yes the O-2 reading and the flat black plugs confirmed a rich mixture and you pinching off the cold start injector confirms it to be the fault. The cold start injector is only operated in the Key position of "Start" and will only operate if the coolant temp is cold. It also has a "time" factor. The Thermo-time switch mounted in the coolant rail over the intake manifold grounds the injector if the coolant is cold but only the first couple of tries then it shuts down so as not to flood the engine.

The injector should not be operating while the engine is running as the key is released from the "Start" position. But you still can't condemn the injector until you do the same test without pinching the hose but removing the electric plug on it to see that it is not still held open by an electrical problem. If it is still rich with the plug removed it is the injector at fault. Keep in mind that the injector can me just stuck due to varnish inside. You can remove it and make up a simple test stand and soak it in straight injector cleaner in a pan. Blow it out with air and it can be operated by straight 12 volts. It is not like the engine injectors that operate on a lower voltage by a resistor pack.


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Howard M. Fitzcharles III


Jaguar from the XK 120 to XJ-6 ser. 3 1987


Dealership line mechanic on MG, Triumph, Jaguar for 15 years, Instructor in commercial mechanics school 2 yr. Product information manager for piston and valve manufacture, Instructor & hotline answer man for import car parts importer 15 yrs.

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