Hey all been asking for help with this problem on the xjs board
and thought I would see if anybody here can help. I just purchased
a 84 Jag XJS and it ran perfect coming home except for a bad AAV. I
replace it and it started and idled find for a minute.

Something happened the idle began to jump from 700 to 1200 and
will not keep running. When you begin to press the gas it will
stumble and have a small intake backfire and will not rev. There is
a bad smell of raw fuel also.

We have checked all vac. leaks. The fuel PSI is good and the vac
line is on the ECM in the trunk. This problem came out of nowhere.

Do you have any ideas? Could this be a bad ignition box (lucas) if
so how do I test it? At one point this evening the mechanic said it
ran perfect for 30 minutes when I went to bring it home......the
same problem all over again.

Thanks for any input or ideas, I really want to get this car on the
road.

Brian Felts

--
Brian Felts 1984 XJS V12.....X300 (sold)
Portsmouth Ohio, United States

 

Hi Brian,

The most common causes of intake backfires are ignition timing (too much advance at idle) or an overly lean mixture.

So check the distributor advance mechanism and make sure it is working correctly. Then have the timing checked.

Check the mixtures to ensure it is not running too lean at idle. This could be caused by a manifold leak.

I had a case with my boat where the port engine would backfire and stall (350 Chev Mercruisers). The compression was down in a couple of cylinders so I had the engine rebuilt at considerable cost. The backfire and stall was still there. After hours of diagnosing it ended up being the ALTERNATOR. YES go figure after installing a new ALT the problem vanished. The moral of the story is you might need to think outside the box.

cheers W

 

Thanks for the ideas..........I just finished restoring a 86 IROC for a fellow and I am back to the Jag on Monday. The crazy thing is that this all happened when I changed the AAV and there is really nothing in the left rear of the motor to disturb. This is melting my brain.



Brian

 

Whenever I work on a Jag V12 and I get an unexpected result or new fault, I immediately go back to the area I was just working in and make sure nothing has been inadvertantly broken or disturbed. There's lots of "stuff" in there, most of it heat baked and fragile. I've broken a few things without even realizing it.

As for the up-down idle, if you're 100% sure....make that 110% sure.... there are no vacuum leaks, and since the AAV is new, I'd check that both throttle blades are set at the proper .002" gap. Clean the throttle bodies while you're there....just on general princples. It's also a good time to synch the throttle linakges, also on GP.

The ignition amp won't cause an oscillating idle.

The raw fuel smell needs to be fixed. Assuming you've already checked for a leaky hose or fitting I'd start by pulling the vacuum hoses off each pressure regulator and look for signs of raw fuel.

Cheers
DD

 

Thanks Doug...........

I can't find a good vac routing diagram for a 84 v12 for nothing. I do need to check the throttle blades in case they have been mess with hiding another problem like maybe a high idle in the past.

My big problem is that there is nothing down around the AAV to mess with but I am going back over it all again. Both of my FPRs are dry but can they have a internal break?


Brian

 

Brian, check out this link:

http://www.jag-lovers.org/xj-s/book/AAV.html

The "overrun cutoff" may be working in your case.

and this link:
http://wiki.answers.com/Q/1991_Jagua...ny_suggestions

I have a similar problem, a vacuum leak. It isn't big enough to oscillate the idle, but it never goes below 1,000 rpm. One weekend I'll fix it.

 

Brian, I found this in my files.Hope it will help.Don't remember what yr its from

 

Brian,

That 90deg elbow on top of the AAV is reknown for splitting when disturbed, as it is a pain to R&R at any time, and a "just fits" type of item.

I agree, there aint much in that particular area, but as Doug said, some things are just waiting for fickle fingers, and then they go AWOL in sympathy.

The fact you drove it home fine, then had AAV issues, indicates, maybe, that as you said, the discs have been messed with to hide the AAV issue??.

Just for fun, check the TPS voltage, red and yellow from memory (that aint the best), coz if these do not read 0.32v-0.036v at idle the ECU WILL NOT drop into idle fuel map, and hunting idle is one of the results.

 

Thank you for the vac diagram is there a diffrent one for the left side of the motor or does that cover it? I Really needed that the ones under my hood are so old and beaten you cant see them.


GRANT:

You are right about the AAV elbow and I did replace it, it went to pieces. I am feeling its going to be something like a vac leak or a sensor like the TPS. I have been storing it at a buddies garage while I finish a couple of paint jobs. I have replaced the Ign Module in the box,coolant temp sensor, AAV

The only thing I did while I waited for the AAV to come in was to change the stereo.


Brian Felts
www.southernohioauto.com

 

The TPS voltage is just as you said it should be. I am checking the CTS resistance as the car cools for its correct numbers. Its warm number .26 is correct I will keep you posted on the cold numbers.


The vac. line diagram is great but there seems to be more lines on my car than in the diagram. At one point I had 18" of Vac as the car was warming up and running good, however when it hit RUNNING temp the car just sputtered a couple times lost all power and died.

I have covered the holes in the air filter housing for the over run and it didn't seem to do anything for or against the car.


As it was warming up it ran great.....not too rich and took gas fairly well No idle surge.


IDEAS??

Thanks again!!
Brian Felts

 

I keep looking but just as the temp gage hits the normal running
temp the car quickly chugs 2 to 3 times and shuts off. Runs fine
with just a high idle until it gets to this point. I have not been
able to set the new AAV idle screw.

I was seeing 10'' of vac. at idle and now I am up to 16 to 18'' I
also ordered a new CTS for the left side of the motor the Right
side has a new one in it.

Was there a Vac Line to the Cruise Control? It has been removed on
my Jag.

Thanks for anymore input...

Brian Felts

 

Brian, my speed control vacuum hose originates on the front of the right hand intake manifold next to the vacuum connection for the fuel regulator.

 

Sandune: Cool thank you. I think I have that one plugged and I am betting it was a leak I fixed and didn't know it.


HERE IS MY UPDATE>>>Problem still not fixed!>>>>>>>>>>>>>>>>

I changed the CTS and nothing changed with the problem. I ordered
the Pertonix single coil mentioned from Kirby's book and I will see
if this coil may be the problem.

I just don't know what else could be wrong. When it runs, it runs
too darn good. When it goes back to the very low idle..chugging and
not taking gas noting bring it back. When the problem comes on the
Dist. Cap is showing me spark at all terminals some maybe a little
weaker than others but my tester is cheap also.

THIS IS ALL NEW
pickup coil (prev. owner)
HEI Module
Left/Right cts sensors
Vac. 18"
New AAV (did fix initle idle problem)
Have Vac at computer in trunk
Throttle blades .002
tape tested overruns
Air Sil. on Passenger side air box working correctly
Good vac to vac advace and it looks new.


Don't be easy on me LOL let me know what ya think.


Thanks
Brian Felts

 

Brian,

That is an extensive list, and I share the frustration somewhat.

Had a '85 here recently that had an engine fire, yep they are still out there, and when we got it back to "mobile" it had this annoying cutting out, and we also thought it to be temp related, dunno. Our list reads almost the same as yours.

Just for fun I checked the spark, it was weak, coil change, better, but nowhere near right, so I removed the ignition amp, opened it up, and unplugged the capacitor thingy, tucked the wire out of the way, sip some alcohol, why not, put it back together, fired it up, waited for the s^&t to hit the fan (as we say), it DID NOT, bugga, we fixed it.

Not convinced, we drove it round and round the block to much amazement of the neighbours, and it kept running, and as strong as you could ask.

That was 2 months ago, and this is his every day car, and he does some serious kms every week, and I just called him prior to typing this, and he said "it has NOT missed a beat and has done 5500kms since that fix".

Being "brutal" as you asked, remove that capacitor, give it a go, at this stage you have NOTHING to loose.

 

How is your wiring on the engine? Does it look brittle from heat? My car ran great when I first got it but I let it set for a couple of years. I would periodically start it up and let it get warm. One day it just wouldn't run. This spring I dug into the problem and found fried wires near the distributor. There was a connector on one wire that looked thicker than the others in the bundle. I discovered this wire was a coax cable. Only the center wire is connected through and the shield wires float. When the insulation is brittle and is moved it breaks away and allows the inner wire to short to the outer wire and your car will not run. I cut the coax back to fresh insulation and soldered in a new coax section.

It could be while fixing the previous problem you moved this bundle of wires and broke the brittle insulation. The problem may only show when the engine warms up.

 

Grant, I have the new coil ordered and it should be here today. I also pulle the plug on the silver thingy. It still did it after I unplugged it but I know it can cause this so I guess if I don't have any radio noise I will keep it off.


My wiring looks good Sandune but I will go back and check once again for that bundle and see where it takes me.


Thanks
Brian

 

Yes the car will still die with the fuel pump jumper on, didn't change any symptoms.

I can hear the injectors clicking as its running. Seems to be spark at the coil. New coil on. New HEI module. Pickup had new wires. Is there any wires on the igninion system the previous owner could have crossed and caused this? like from the pickup or etc.

This morning it would not run, then she fired up and ran perfect then 30 seconds in it just died. Then would refire and start run fine 30 seconds die. Now it will start and chung a few times and die and not run good at all.

I have a continuous 12 volts from the comp. to the coil and does not change when the car dies. When it begins to shut down it just slowly does it. When running good it will rev...like its new then just start cutting out. I was getting this when it got good and warm not it is about 30 seconds in to a run.


Thanks guys. I don't have any Jag mechanics in Southern Ohio to lean on. I am two hours from Columbus or Cincinnati. and can't thank you enough.


Brian Felts

 

http://www.youtube.com/watch?v=SYDqZxT3cyo
Video of it running bad..



http://www.youtube.com/watch?v=9hbZoBxb24I

Video of running good.

could i have a bad ecm? Bad map sensor on the ecm?


Brian


Brian

 

Since it stumbles when it warms up, you might check the water temperature resistor on the left bank. If it is out of spec, it may cause the computer to lean or rich the fuel mixture beyond operating range.

I do know if the connector is off, the computer thinks the temperature is near absolute zero and will dump extra fuel. It will not run. The Jaguar engineers did not believe in 'limp home mode'.

 

I did replace the CTS then even jumped it to make the computer this its HOT HOT. No change, pulled the plug and it would chug when it ran good. No change when it ran bad. I am starting to think its a bad ecm. I even chased the CTS wire back to the ecm with all the same readings.


Any ideas on signs of a bad ecm?


Brian