76 Jag XJ12L Runs Rich - Chevelle Tech
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post #1 of 9 (permalink) Old Jan 1st, 19, 10:58 PM Thread Starter
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76 Jag XJ12L Runs Rich

So the kid next door asked me to help him figure out what's going on with this car. He's 21, a nice young man. I've known him since he was about 7. His parents are recently divorced, and this project was his dad's, someone I could call a friend.

I'm really hoping someone on here knows these things. It's a 12 cyl fuel injected model. Everything is present, but I can't find a breakdown on how it's intended to work. It has 2 tanks, one in each rear quarter, a single pump, and 2 "solenoids" that are supposed to open/close to direct fuel flow and return to each tank.

A it is, it will start and run, but it's so rich, it's like a diesel "rolling coal". Absolutely terrible. After a few minutes it gets so rich it won't run anymore. Basically, I'm trying to find out what is supposed to regulate this system and would appreciate any knowledgable input.

Thanks,

Devin

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post #2 of 9 (permalink) Old Jan 2nd, 19, 9:13 AM
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Re: 76 Jag XJ12L Runs Rich

If it has O2 sensors, start there.
Also, start by disconnecting injectors and see if it clears up. Those old injectors can clog/stick/wear and just flow fuel.
^ (All just W.A.G. on my part)

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post #3 of 9 (permalink) Old Jan 2nd, 19, 3:19 PM
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Re: 76 Jag XJ12L Runs Rich

Iím not a jag mechanic but when an engine first starts the O2 sensors are not controling the fuel mixture. I googled this info. It talks about a ground problem. Electronic Fuel Injection
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post #4 of 9 (permalink) Old Jan 2nd, 19, 11:29 PM Thread Starter
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Re: 76 Jag XJ12L Runs Rich

Quote:
Originally Posted by 1967 chevelle malibu View Post
Iím not a jag mechanic but when an engine first starts the O2 sensors are not controling the fuel mixture. I googled this info. It talks about a ground problem. Electronic Fuel Injection
Interesting article. I had done some searching but hadn't found that. It describes his symptoms to a T.

Thanks,

Devin

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post #5 of 9 (permalink) Old Jan 3rd, 19, 3:39 AM
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Re: 76 Jag XJ12L Runs Rich

the fuel injection used on that thing is a Bosch designed lucas system. it pulses the injectors from a constant pressure rail system but uses an old analog contoller with transisters. 1st thing i would do with that is remove it and adapt a new efi system from holley etc or better yet a new gm ls engine. theres people out there that rebuild and produce new components for the factory system but i would guess none of its cheap
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post #6 of 9 (permalink) Old Jan 10th, 19, 2:21 PM
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Re: 76 Jag XJ12L Runs Rich

If this is a 1976 model year vehicle, you can rule out O2 sensors and a "feedback" style computer. There may be an electronic box, but it won't rely on O2 feedback.

I'd be looking at fuel pressure, and put an inductive ammeter "low amps" clamp on the injector wires to see how much current they're drawing. If you find some that draw more current than others, those will probably have shorted windings. An amp-clamp connected to an oscilloscope is best for this because then you can compare the oscilloscope patterns of the injectors to see if there are one or more that look substantially different from the others.

If the system won't hold fuel pressure, you've potentially got stuck-open injectors. If the pressure regulator was stuck open, fuel pressure would be low, it's unlikely that the engine would be so rich. A failed diaphragm in the pressure regulator could allow un-metered fuel into the intake manifold(s), though. Pull the vacuum hose on the regulator to see if there's fuel on the vacuum side of the diaphragm.

I wonder if you can find an on-line service manual for this cat.


As with anything electronic, MAKE SURE THE SYSTEM VOLTAGE IS PROPER. Low--or high--voltage can do crazy things with electronic systems.


Find out what the kid and his dad have dicked with...you'll want to assure they didn't play pin-the-tail-on-the-distributor, or some such.

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post #7 of 9 (permalink) Old Jan 10th, 19, 2:43 PM
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Re: 76 Jag XJ12L Runs Rich

Follow the air intake tubing if its an early Bosch type system it may lead to a box looking aparatus with a 5 or 6 wire plug atached to it.This "air box" contains the air valve with a plate that rises and falls as air flows through the box and that is what regulates the amount of fuel entering the engine.If it has this Bosch (kind of mechanical/electronic injection) it is very important that there is no unmeasured air entering the engine's intake after the air valve.As more air enters the air box the air valve opens further and then more fuel is metered in.
First thing you have to do is open up the air filter (should be directly above the air valve) and make sure nothing is out of place and that none of the rubber intake hoses between the box and the throttle plate are ripped.Make sure no debris has fallen in there and is holding the air valve open or not allowing it to close.There is also a base idle adjustment that can be adjusted that some people try screwing with and that may be over-adjusted.
If it is an early Bosch type injection show some pictures as I may have an old repair manual for working on those systems.

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post #8 of 9 (permalink) Old Jan 25th, 19, 4:35 PM
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Re: 76 Jag XJ12L Runs Rich

How'd this turn out?

I have a soft spot in my head for Jags, and V-12s in particular.

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post #9 of 9 (permalink) Old Jan 26th, 19, 1:20 AM Thread Starter
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Re: 76 Jag XJ12L Runs Rich

I havenít had a chance to get back to it since I posted. Iíll report back if I figure anything out.

Devin

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