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

Mystery of two pipes...

 
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Old 07-20-2014, 06:11 AM
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Default Mystery of two pipes...

Was chasing an exhaust leak in my Series III XJ6 that I found in the front cat.


When I pulled the front cat, I found something unexpected. The inside of the front pipe of Y in the cat was full of soot while the back pipe was very clean with a light tan whitish inside.




I got underneath and looked up inside the exhaust manifold and saw the same thing -- soot in the front manifold and a clean rear manifold.




I recently had the head rebuilt with all the intake, exhaust and fuel system gone through.


Motor seems to be running well but wondering what I might look at to correct the soot problem?
 

Last edited by e21pilot; 07-20-2014 at 06:13 AM.
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Old 07-20-2014, 08:36 AM
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Interesting.

How do the front spark plugs look?

Cheers
DD
 
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Old 07-24-2014, 01:49 AM
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I pulled out the front three plugs (#6, #5 and #4). They seemed a little carbon fouled but not too bad. What was interesting were the gaps. I believe they were all 0.035 when installed. When measured #6 was about 0.040; #5 was 0.050+ and #4 was 0.050.




Seems like these NGKs opened up a lot in the first 1500 miles since the rebuild. It could be that my relatively stock ignition system couldn't handle the large gaps in the #5 and #4 plugs.

If large gaps are the issue, this reminds me about the Jag mechanic that always recommended iridium plugs for the older cars -- mostly because the gap was stable and the idle quality was best when the gaps were bang on.
 
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Old 07-24-2014, 09:06 AM
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number 5 looks sooty. I'd do a compression test. If the compress test on #5 is same as the others, then I'd tighten up the gap a bit and try a new fuel injector for #5. In this case a non contact IR thermometer could be very helpful to see if #5 is running cooler than the rest.
 
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Old 07-24-2014, 09:22 AM
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Odd, I was thinking about it. My idea was that the sooty pipe was the normal one and that the clean pipe indicated a lean mixture. But, a leak at the manifold to pipe jpoint on the clean pipe might account for it's color. Modern fuels do not react in the same way as old stuff, making color reads more probleimatical.

Look in he open end of a tail pipe in almost any car and it will be sooty black. At one time, we desired the grey look.

As to the plug gaps. Wacky gapping at installation? but, even so, the smooth running engine seems to say that the wide gaps were within the coils' ability to fire them, each and every time.

The IR readings would be possibly interesting.

I've got my antique Lauson small engine apart at the mag side. WICO, an old and respected name in magnetos. Crummy with wet and caked oil. I can't quite tell if the poits are even opening and closing. If so, the gap is infentesible. That gap thing. I and feeler guages have never gotten along well.

I'm mentaly cooking up a method to check them via dial indicator or with my VOM set on conductivity.
 
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Old 07-24-2014, 12:09 PM
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Using a non contact IR thermometer is an interesting idea. I have one but am not sure where to point it to get the best measurement or even when to do so during the warm up cycle. Any suggestions?
 
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Old 08-09-2014, 04:19 AM
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This mystery was solved. Took the car into the shop today and checked the mixture on the front three and back three exhaust ports. Found out that the front was too rich and the back too lean. The mixtures were adjusted to put these two back in balance.

Wondering if someone could explain how the mixture is adjusted in this way. I was told the two groups were never adjusted for exactly the same mixture. One was slightly richer than the other.
 
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Old 08-09-2014, 07:31 AM
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Originally Posted by e21pilot View Post
This mystery was solved. Took the car into the shop today and checked the mixture on the front three and back three exhaust ports. Found out that the front was too rich and the back too lean. The mixtures were adjusted to put these two back in balance.

Wondering if someone could explain how the mixture is adjusted in this way. I was told the two groups were never adjusted for exactly the same mixture. One was slightly richer than the other.



I, too, would love to hear how this adjustment was accomplished. I'm not aware of any way to adjust mixture on individual cylinders, or groups of cylinders.

Cheers
DD
 
The following 2 users liked this post by Doug:
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Old 08-09-2014, 11:04 AM
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Me too! I double checked as the engine in question could've been a carb'd engine.
If so, possible.


But, FI. I don't get it??


My 1932 Lauson Fired up! Helped a bit by an infusion of Ether. Noisy at first, but settled down. Idles nicely, Rev's up when butterfly opened. Neat little one lunger.


Oh, and fit an air cleaner of some type.


Next adapt back to the Roto Hoe. throttle cable needed. Belt and a tire and it's ready to kill my shoulders!!


Original green paint with patina to be kept. Quite decent.


Carl
 
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Old 08-09-2014, 03:03 PM
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Yes, I'm not clear on this either. What are the caps on on the exhaust manifolds used for on fuel injected cars? Is there some Jag procedure for measuring exhaust gases that uses these ports?
 
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Old 08-09-2014, 03:32 PM
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I think those are plugs for EGR pipes.

Jag used the same manifolds for ages and when EGR wasn't called for they just plugged off the holes.

Cheers
DD
 
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Old 08-23-2014, 05:27 PM
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Picking up this thread again as I have learned a little more.

Those EGR ports on the exhaust manifold were just used to measure the exhaust gases to see if the engine was in balance. Adjustments were made to the dial of the AFM and the air bypass valve I believe.

This was all well and good initially but over time, the front cylinders started to run much richer than the rear cylinders (see the first pic in this post).

This is strange because there is nothing setting wise that could make this happen. My understanding of the fuel injection system is limited but what I believe is that for 1987, it is a Bosch L-Jetronic system at heart and that the L-Jetronic is a batch type injection system meaning the injectors for all 6 cylinders fire at the same time, for the same duration and the air/fuel mixture just waits for the intake valve to open. If this is correct, and the injectors are fine, that means the rich/lean imbalance is coming from somewhere else.

My current guess is a vacuum leak somewhere on the intake side of cylinders 1,2 and 3. A lean condition detected by the O2 sensor would cause the L-Jetronic ECU to enrich the mixture on all 6 cylinders, This would bring the lean front cylinders back to normal condition but make the front three cylinders run a little on the rich side.

Just a theory but wondering what others think.

Also, it would be interesting to monitor some of the signals going to the ECU which I believe is in the trunk. Wondering if there is any kind of connector out there that makes it easy to tap into the signal at the ECU. I see write ups on the net about the signals I should see and it would be nice to monitor them if possible.
 
 
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