85 XJ12 runs perfectly.....sometimes????

Hi all

Sorry for bringing this post back, but I just thought I would mention the final outcome (long!) and thank everyone for all their help as well.

After replacing too many parts and rebuilding the wiring harnesses ect. I was left with only one possibility for the poor running.. the main ECU.

I sent it off to England for a go over by Roger Bywater at AJ6 and he found some bad solders, failed parts ect. and also upgraded it. After the return the car went like a scared cat, acceleration was incredible, what a great improvement!!

Then very shortly after I was driving down the highway with family, and at speed I felt a slight bump...almost as if I had lifted the gas slightly, but I hadn't...still doing about 50 mph I noticed the car required slightly more and more gas pedal to maintain the speed. This wasn't right so I pulled over and the car would barely idle, it was very rough yet it still ran. I walked around the car as it sputtered and noticed alot of water coming out the drivers tailpipe. Odd..
Getting back in I noticed the gas guage reading nearly empty which was strange as I thought I had about a 1/4 tank in the left side.
We drove a few minutes to a gas station and noticed the car ran smooth above idle but had hardly any power anymore.
At the gas station I pulled in and my kid jumped out before I shut it off and yelled to me to turn it off as gas was literally streaming from the drivers tailpipe!!
Turned it off, added a 1/4 tank of gas and restarted it up and it seemed to run smooth even though we noticed the drivers side exhaust manifold and pipe were red hot! and also the tailpipe was now just dripping.
Thinking it might of fixed itself and the drip was normal condensation we drove home a mile or so and the car still needed the gas pedal pushed down more than normal.
We make it home and I'm floored when I look at the gas guage..the 1/4 tank or $20 dollars is GONE! the tank is empty.
I immediately email Roger and after much testing and emails he asks me to send the computer back and he finds that an output transistor had failed and left all 6 drivers side injectors stuck wide open!!
It seems we are very lucky to not have had a serious accident or fire and I must publically THANK Roger Bywater for quickly finding a repairing the fault at no charge even though there was NO WAY he could of anticipated the transistor failure. He had thoroughly tested the ECU, sent it back and the computer just decided that was it's time to burn out. (20 years old!)
Thanks again Roger!
The car went to a man who has the time and money to restore her properly and now I have another cat a 1987 XJ40, aparently one of the first to hit N.A. or Canada
Randy

Thanks Doug!

Randy

The GM ignition module part number is 10482820. In AC Delco Jargon it's a D1906

Cheers

Doug Dwyer
Longview Washington USA
1987 XJ6 Ser III
1988 XJS V12 Coupe

Edited on 2005-10-22 1:20:28

Thanks for the reply
In Park the engine will just cut out completely (down to idle) with a quick turn of the throttle plate, it will rev normally if reved slowly however. The quicker you try to rev it whether driving on the road or manually in Park, the more it will cut out.
No clouds of black smoke at all, yet the tailpipes are sooty.
ALL "huffing" is gone, that it seems was the two plug wires the previous owner had swapped. Idles very smooth now when it's not hunting.

I picked up a new primary Echlin coil and cleaned, oiled and freed up the AAV, and will set the throttle plates to .02 as right now they are shut tight it appears...and then I'll give it a try tommorrow, weather permitting!
I hope not to bore anyone, just hope "WHEN" it's fixed, someone may be helped by my struggle!
BTW
I'll get the part # for the module ASAP, I understand it's a very common, relatively inexpensive General Motors HEI module.
Randy

When the transmission is in park, does the engine still accelerate poorly?
Do you get fairly large clouds of black smoke when the RPMs are increased
Is the huffing even in both tailpipes?
I also would like the part # for the module...
GDK

Thanks for the help people and an update.

I have not checked the cats yet, can they be clogged even though (at random times) the car will go like a rocket???
So far I have renewed the connections to the coil as they were hanging by a strand, replaced plugs with NGK's at .027 gap (all the old plugs were in good similar shape), tested coil and found between the +/- terminals, 1.2 ohms (sound good?), cap and rotor were nearly perfect, just scraped the contacts slightly and reinstalled.
Rotor rotated a few degrees then quickly returned, does this mean the weights are free??,all Magnacor wires tested at similar low ohms,replaced the CTS.
Biggest thing I found was that the previous owner had two plug wires swapped, and when I started the car ALL the "huffing" (or missing) was gone, engine runs very smooth now, except....
The hole, or hesitation when you quickly hit the gas is still there and at random times the idle will still rise and fall. I removed the AAV and the pot of water test showed the valve to smoothly open, I cleaned and lubed it anyway but have yet to install it as I'm waiting for a new top elbow hose.(had a crack in it) Funny thing was that the idle bolt in the side was cranked in tight...odd (maybe a vacuum leak has been keeping it running??? I did look at the throttle plates and they appear to be shut tight, I will be adjusting them to .02 before I try again to start it.
Fuel filter was claimed to of been changed by the PO and maybe my next target will be a new fuel pump or possibly the GM module (anyone have a part #) that is in the amplifier.
If it was a carb'd engine I would immediately think accelerator pump was bad but with fuel injection.....who knows.
Any more thoughts all you smart people out there?...even random ones gladly accepted!
Randy

Have you considered the possibility of clogged catalytic converters? If your car is a Canadian specification V12, it has converters. If it is a "grey-market" import, it may not. While this may not be the whole answer, you should also make sure that the distributor has been serviced and that the vacuum advance is working properly. Another cause of poor running can be the GM module in the ignition amplifier (atop the left side manifold).

Gregory Andrachuk
1992 Series III V12 Vanden Plas
1966 Mark 2 3.8
2002 X-Type

Thanks for the response.
I have adjusted the positioner to about .32V and no change, odd thing though, I stopped for gas and the passenger side gas cap when opened went woosh and the drivers did nothing....also it did not get better today, the idle hunted up and down and the exhaust from the back is "huffing" if that makes sense. Absolutely no power except if you barely push on the gas slowly and I can hear a preignition type of rattle like timing....

Adjust the TPS to get a base reading (throttle at idle position) of apprx .35 volts.

Remove the four nuts which hold the top of the throttle turret in place and lift off the throttle wheel, after disconencting the t/cable. The TPS will come with it. You'll see three tiny screws which can be loosened so as to allow the TPS to be rotated/adjusted. This is a bit frustrating as the adjustment is minute and merely tightening the screws will alter the setting. Be patient :-)

The coolant sensor can be checked be measuring resistance between terminals. There's a chart in the ROM (repair operations manual) which gives temp vs. resistance values. If you can't find it I can look up the spec.

Cheers
Doug Dwyer
Longview Washington USA
1987 XJ6 Ser III
1988 XJS V12 Coupe

J.A.M.

Yes, thanks for catching that :-). Your interpretation is correct. This is what comes of replying while under the influence of pain medication !

Doug Dwyer
Longview Washington USA
1987 XJ6 Ser III
1988 XJS V12 Coupe

Thanks everyone

I tested the throttle switch as decribed and results were as follows:
Key on,no throttle--.48V
then smoothly stepping to .99/1.2/1.5/2.05/2.5/3.0/and up to 4.6V
Is there a way to test the CTS, I pulled a sparkplug and it (along with the tailpipes) was powdery black.

P.S.
I do have the original VdP manual and electrical diagram, but by any chance does anyone happen to have a vacuum diagram for a 85XJ12 that was originally sold in Alberta Canada?

Quote: "The 6 cylinder cars don't use a throttle position *switch*, not a throttle position sensor".

Doug, I'm sure you meant that the 6-cyl. cars DO use a switch instead of a sensor, right? ok, I got it.

Good points. Checking the cleanliness and security of relevant connections is always important and the coolant temp sensor can indeed cause some weird symptoms. The CTS can be checked with an ohm meter, by the way. The specs are available in the ROM.

The V12s, however, do not use an air flow meter so that item, and the associated ducting, would be a bit of a wild goose chase for Randy.

If you are thinking of saving the throttle position sensor info for use on your Ser III XJ6, don't bother. The 6 cylinder cars don't use a throttle position *switch*, not a throttle position sensor.

Cheers

Doug Dwyer
Longview Washington USA
1987 XJ6 Ser III
1988 XJS V12 Coupe

I'm not familiar with the '85 XJ-12, but in my '84 XJ-6, a similar surging can be caused by a loose "bellows" hose, (the large-diameter, black rubber, "accordion" hose sections going from the air filter housing to the AFM, then to the air intake). Especially the last section, the "J-shaped" bellows hose section, if not seated and tightened just right, can be a source of strange symptoms.

the Air Flow Meter's (AFM) connector plug can produce the same surging. Remove the plug very carefully, (looks like a Printer cable Serial or Parallel plug with a locking wire across it), and with a soft-Brass "toothbrush-style" brush, clean the plug's contacts of any corrosion or film until they are shiny. Some people suggest applying Olive oil to the plug's terminals, (Extra Virgin?), but I have never taken that seriously. In any case I would apply DiElectric grease, and I have never done that either, too much trouble.

the same plug cleaning should be done at the Electronic Control Unit,
(ECU) also known as "the computer", located in the trunk/boot behind an upholstery panel.

If none of the above checks cure the surging, at least you will become an expert at the process of elimination. Then the next check would be the Water Temperature Sensor, a component that produces weird symptoms too when they go bad or "kaput". Based on how you describe the surging, this sensor could be causing it too, but you will know only by substituting it,
unlike the TPS, which you can check with a Ohmeter.

I'm going to save Doug's troubleshooting instructions for the Throttle Position Sensor, since my old Service Manual does not have that procedure.

good luck.

from J.A.M.
1984 XJ-6
1965 S type

The throttle sensor is directlt under the throttle turntable and has red, green, and yellow wires.

You want to probe the red and yellow wires while leaving them connected....so you'll have to tuck the probes under the wiring insulators.

With the key "on" you should show .32-.36 volts with the throttle closed. When the throttle is *slowly* opened you should see a very *smooth* increase in voltage, all the way to about 5 volts or so at full throttle. If the voltage "falls out" or is erratic, then the sensor is kapoot and/or the connector terminals are dirty/loose.

Doug Dwyer
Longview Washington USA
1987 XJ6 Ser III
1988 XJS V12 Coupe

I do, any help is appreciated!

Quite a few "could be" possibilities here but the first thing that popped into my mind is the throttle position sensor.

If you have a digital voltmeter we can give you instructions on checking it

Doug Dwyer
Longview Washington USA
1987 XJ6 Ser III
1988 XJS V12 Coupe