XJ6 & XJ12 Series I, II & III 1968-1992

Idle stumbles while warming up...

 
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Old 01-02-2012, 12:36 AM
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Default Idle stumbles while warming up...

Not sure I gave the problem the correct description but I just replaced the thermostat on my Series III . Engine temp is now right at 90 degrees when it warms up and it now idles just fine.

Only problem left is during warm up, when I'm at a stop light, the engine almost dies when I press the accelerator to go. It then quickly recovers and get the car moving.

Just wondering if anyone has seen this? Not sure if it is a vacuum leak or perhaps the mixture is off.
 
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Old 01-02-2012, 12:49 AM
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The coolant temp sensor controls enrichment during engine warm-up and could be the cause of your problem. If you have a multi-meter I can give you the specs so you can test it.

Or, at about $20.00 or so, it's a fairly painless guess.

Cheers
DD
 
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Old 01-02-2012, 05:16 AM
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There is a very detailed description of the EFI system in the official workshop manual for the Series 3. In it, it states that the two main inputs to the ECU for determining injector pulse width are Mass Air Flow, and Engine Temperature (coolant). The rest of the inputs are trims only.

So as Doug says, engine temperature transmitter sounds like it needs checking out or a new temperature sensor. Check for a good earth (ground), and all the connections from it to the ECU first before whacking out your $20 !

If its OK when hot, the mass airflow meter seems to be OK.

Have you got the Workshop Manual ?
 
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Old 01-02-2012, 03:54 PM
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I don't have a workshop manual unfortunately. Sounds like a wealth of information. Can it be downloaded?

I am on a tight budget but fortunately I do have a multi-meter handy.

Will I be measuring resistance or voltage or perhaps current flow if I don't want to interrupt the circuit while I'm measuring. Is there a link where I can find the specs for the measurements?

ps. The plastic cap on the air flow meter is loose which leads me to believe someone may have removed it to tweak it internally. Is there any way to tell if the meter is still at its default setting?

Thanks,
 
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Old 01-02-2012, 04:40 PM
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The plastic cover on the MAF meter has probably been removed to fix the two little contacts that keep the whole fuel supply shebang running after the engine has started. These are the WORST aspect of the system, a real and dangerous disgrace, IMHO. Yet these contacts are supposed to be a safety feature, by detecting whether the engine is running or not, and stopping the fuel pump when the ignition is on but engine is not running. Starting bypasses these contacts, by the way, or one would never be able to start the engine. My own experience is that they put danger into the car, rather than removing it.

The contacts maintain supply to the fuel pump relay, so if there are any problems with the contacts passing volts and a small current, the fuel pump stops running and your engine dies. This engine-dying can be extremely alarming, like happening just when you are overtaking a car on a single carriageway road, or rushing up a 1 in 15 hill on 3-lane dual carriageway in the outside lane with several other fast cars behind you, and halfway up with loads of traffic in the other two lanes !! Yes this all happened to me a few years ago with a Series 3. So whilst the cover is loose have a quick look inside and see what these contacts look like. You can test closure by moving the air meter flap when the contacts should close. I believe some people solder these contacts together, and arrange for an alternative means of detecting engine run for keeping the fuel pump switched on.

Once you've fixed the contacts and checked out the track on the meter for integrity, the cover needs to be resecured, preferably with a glue that is easy to overcome if it needs to come off again.

The workshop manual is an 1" thick book and still available on the internet. The contact problem above is NOT in it.
 
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Old 01-03-2012, 04:05 AM
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I snapped a pic of MAF internals with the cover off. For those in the know, does this look stock or has it been tampered with? I would like to reset it to the original settings but am not sure what to look for.



https://docs.google.com/leaf?id=0B9P...0MTQ0&hl=en_US
 
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Old 01-03-2012, 05:20 AM
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Yes, that looks as it should, (as I remember it, it was a long time ago now !!). You can see the arm made of a thick wire that opens the contacts I mentioned when the flap returns to rest. When the flap moves this arm moves out of the way to allow the contacts to close. There is no contact closing spring only the springiness of the contact arms themselves.. A possible mod would be to arrange an insulated closing spring so that closure is more positive. The springiness of the contact arms reduces over time until they cease to pass volts, or fail to do so at inconvenient times as I said earlier. Roger Bywater of AJ6 Engineering takes you through the EFI system in much detail on his website (menu item in "The Jaguar XK")

WELCOME TO AJ6 ENGINEERING / AJ6 Engineering

Adjustments can be made to the resistance track setting - you can see the clamping wire bracket and the teeth on the edge of the track, but there are no instructions anywhere that I know of to check and reset if out of limits. The track is one side of a Wheatstone Bridge in the electronic circuit, but apart from that, I know no more.
 
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Old 01-15-2012, 12:39 AM
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Default Update: Passed the smog test

I spoke to the local Jag shop who repaired the XJ6 and got it though the dreaded California smog test.

Turns out the engine temp sensor was faulty. After it was replaced the engine ran extremely rich. Why? The air flow meter had some missing parts and had been tampered with. Once the shop replaced the A/F meter with a used one they had, the car ran smoothly and even passed the smog test with near zero emissions. Not bad for a car with 166K miles.

I also learned from the shop that the XJ6 has a high altitude compensator sensor in the trunk that can sometime cause emission problems but not in my case. Also, they said my ignition module was original which is not common and that I might need to on the look out for an new one or possibly and aftermarket one.
 
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Old 01-15-2012, 12:45 AM
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Ignition module is a common failure point. It's a GM part, AC Delco # D1906, GM # 10482820

(Actually the "2820" number has bee replaced but I can't remember the newest number. Just Google the 10482820)

Cheers
DD
 
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Old 01-15-2012, 05:18 AM
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Congratulations on getting your car operational !

My, I did a lot of work on an '80. I spent more on it than Jaguar did making it ! Had it 14 years. My daughter cam home from the maternity hospital in it, so always it will be in my heart.
 
 
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