76 XJ12C D-Jetronic Hard Start Issue when Cold

V12-Engine
1

For 12 years I’ve owned this ‘76 12C but the car is rarely driven. I’ve been giving it some attention the last few weeks and it occurs to me it has slowly (over years) become more difficult to get running when cold. The engine cranks fine, fires a little and then quits. Next turn of the key fires a few more times than before, but won’t stay running. About the 4 or 5th try, the engine keeps running, but it is best described as, “slowly comes alive to a 700-800 rpm.’ The idle isn’t too rough, but not smooth either. The car starts quicker when fully warmed up.
The first items investigated on the list include the components which add enrichment and air needed when cold (cold, in Dallas, the air temperature in April/May is anywhere from 50F - 80F.)
AAV - I pulled the AAV thinking it was stuck closed. Nope, fully open. Tested while in hand in water and it fully closed rather easily.
Water Temperature Sensor - I removed the ECU connection clip in the trunk to test the harness wiring and the temperature sensor. At 68F, the ohms reported was 2.8K, fairly close to expected per chart. Certainly a close enough signal for the ECU to add fuel.
Fuel Pressure - I have a permanently mounted fuel pressure gauge which tees off one of the cold start injectors. Turning on the ignition, the pump runs for a few seconds, turns off waiting for cranking. The gauge reads 29-30 psi which is correct.
So what’s next? While the ECU connector is acceptable, I ran down the list of devices and various pins to check for correct signals. Everything reported properly.
I did vacuum simple test also the MAP Sensor to ensure it holds a vacuum and it does.
So now it is time to turn to ignition.

I pulled the 12 spark plugs and they all looked very similar - tarnish to dark brown. All gaps still at .035”. Not many miles on the plugs and the plug leads (wires). I cleaned up the plugs, chased the threads, a little anti seize, and put the plugs back in.

As I stood with a beer and gazed at the engine deep in thought. It occurs to me that this engine doesn’t “jump” to life like other engines. Makes me think I’ve had some kind of ignition retardation creep over the years. (As if the distributor isn’t tight to the engine)
Even when it started better a few years ago, it didn’t run at 1,100 rpm while the AAV was open and then calm down to 750 as it warmed up. Makes me think I have retarded timing.
So I pulled off the distributor cap to check the rotor and cap. A little corrosion but I sure have seen a lot worse. (Years ago I converted to the Lumenition system)
I tested the vacuum advance, and it wasn’t responding. No vacuum pull at all. So I’ll order a new one.
I’m thinking I have to pull the distributor to replace the vacuum advance, but stopped for a break. I removed the trigger board. It looks rather corroded or something on the downward facing side. See photo - anyone seen this before? Is it a problem?

Any chance of replacing the vacuum advance without pulling the distributor? I see about 1/8” of a split pin sticking up - holding it in place?

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IMG_45804032×3024 2.94 MB

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2

If you can find a way to remove the split pin, yes you can do it without puling the distributor.
But it will be much easier to do it in the bench. Just make sure you mark the position of the rotor.

3

That does all look very tired. OPUS ignition was never very reliable to begin with, and then add many decades! Faced with a similar situation in my 76 XJ12C, I went for SNG Barratts REOPUS kit DAB113HEF which is basically a later Lucas CEI smushed into an OPUS case. It’s not cheap, but includes all new electronics, vac advance, rebuilt distributor, etc. No more trigger board. Very good product!

I ended up going this route after destroying the distributor body trying to remove the vac advance. On my car it had corroded into one solid mass.

4

Thank you for the replies so far.
Another theory question - How much advance does the vacuum advance really add when the vacuum port is just outside of the butterfly in the throttle body? The port is literally as close to the closed butterfly as it can be without overlapping the closed butterfly. This is obviously not part of my cold start trouble since the engine is started with the butterflies closed (foot off the accelerator pedal).

5

I don’t think the vacuum advance would be a start issue; have you checked coil? Easiest way is to pull a plug lead and plug in a known good spark plug, ground the shell of the plug and have an assistant crank/start the engine. Should have a regular blue spark at crank and running.

6

No, didn’t check coil. Good idea though, thanks. Car runs extremely strong when fully warm, just didn’t think it was spark related (coil). Worth the easy check though.

7

My experience on bikes is they show up on cold starting.

8

My parents had a 1969 Volvo that would refuse to start on cold mornings. Ran great otherwise. Frustrated, they loaned me the car for a few weeks to see if I could work it out. One cold morning it stubbornly refused to start, so I pulled all the plugs and gapped them down to .025". It started right up and started easily every morning from then on. I concluded the coil was weak but the fix was easy.

Well, my parents insisted on taking the car to the Volvo dealer every few months for a “tune up”, and every time they’d gap the plugs back out to spec, and the next cold morning the car would refuse to start. And the next time I was there I’d gap the plugs back to .025" and the problem would go away.

Finally decided to fix it once and for all. You’d think it’d be a simple matter of buying a new coil, but on a Volvo that job is problematic. The coil and the ignition switch are one piece. The coil has a flange and mounts in a giant hole in the firewall, and the ignition switch is coupled to it with an armored cable. Buying an entire new coil/ignition switch from Volvo would be stupid expensive. So I bought a generic ignition switch with keys, fashioned an aluminum blank-off plate for that giant hole in the firewall, and installed a generic high-performance ignition coil. Worked great.

9

Now that is interesting.
I’m going to pull the distributor, clean the weights, etc., and then install the hall trigger board upgrade just as a precaution against my old reed switch trigger board.

11

Update: I FINALLY received my new vacuum advance after weeks of waiting. It was the final part to install the ignition system back into the car. What I changed:
Rebuild & tested AAV with kit from John in Lithuania (old one still worked, but suspect)
Spark plug leads (plugs pulled, they were nice tannish and gap was .035" which is spec)
Distributor cap
Vacuum advance
D-Jet trigger board (switched from reed to hall technology)
O-ring on distributor shaft (not performance related, but needed)

Remember, I started this project because the engine didn’t “come to life” like it should when it is cold and then idle speed drop as engine warms up. I basically had one idle speed, cold or hot, and the throttle response when cold was not good.

One of the items I noticed incorrect was my throttle shaft bushing was gone on the driver’s side so my butterflies were not synched at all, which of course makes horrible low throttle cold engine operation. That has been rectified and butterflies are spot-on synched. So yesterday, when engine was first started and after just a few seconds, the engine revved easily and smoothly.

I wish I had checked the timing via the strobe light on the crank pulley prior to removing the distributor, but I did not. I have checked it now, and I’m about 10-12 btdc at tach indicated 800 rpm with vacuum advance disconnected. With engine fully warm after driving, I did not notice much throttle response difference or starting difference with distributor between 8 degrees and 15 degrees BTDC. I left it about 12 for now and did not hear any knocking/pinging under load.

Remember, I also have checked at the ECU, thus the harness tested too, all the components such as temperature sensors, MAP sensor, throttle pedestal position switches, etc. and all are within specification.

Throttle response now when cold and hot is excellent. Car goes like a scalded ape. (700r4 tranny with its more efficient torque converter and lower first gear is a lot of fun)

So why don’t I have an idle drop from cold to hot as engine warms up? The only item can be the AAV, right? When removed, the coolant was at the top of the head, so I know the bulb is in the coolant. I tested my new rebuilt unit and it closes as should per temperature spec.

As I write this, I think about the idle speed AAV control bolt. Maybe the idle speed bolt is so fully open the AAV’s sliding door air volume doesn’t change anything? I’m going to turn the idle speed nut in to close off and drop idle air volume down and see if the AAV then can do its job the next time it is cold.
What makes me think about this is the amount of “slop” in idle speed control on my 81 Porsche CSI fuel injection system - When engine is hot and the a/c is on, if the idle speed screw is in too far, I can have good idle speed, but putting in the clutch from high rpm as I come to a stop at an intersection can cause the engine to drop so quickly, it dies. But if I open the idle screw just a little, hardly enough to register an RPM change, the engine doesn’t die under the same circumstance.

I’ll play with the AAV idle speed screw and let you know what I learn.

12

My bike suffered for a very long time with hard cold starts.
New plug leads somewhat helped, and gapping the plugs down to .025" made cold starts better but made hot running worse.

Interesting enough, changing the starter brushes was temporarily fixing the problem…
It took me a while to find the connection between the two. It was all about voltage drop.

The +12V cables for the coils go from the battery to the fuse box under the seat, then all the way to the key switch at the front and then back again to the coils.
And the ground cables go all the way from the under the fuel tank to the kill switch on the steering handle in the front and back to the coils… Combined that’s about 4m of cable! They would almost never see pure 12V, and while cranking maybe half of that.
Connecting the coils directly to the battery via a relay cured the problem once and for all.

13

Maybe that means that the AAV works as intended?
What is your idle RPM?

14

Hi,

Yeah it failed on our 1972 V12 E-type already after 46 1/2 years and just a bit over 105.000 kms. :laughing:

A swiss expert also said it may have failed due to being over stressed with a couple of dirty connections on the BR9 ballast resistor. It’s now been rebuilt, the same transistors and autonotive capasitors are still available. Works perfectly again. YMMV.

Cheers!

15

Gents,
Several posts back you see my long post from June '21 regarding my V12 idle troubles. I did rebuild the AAV and it tested well. I haven’t driven the car much since then as I’m working on an old Bentley and other cars.
This morning, I decided to drive the Jag to the office. I did drive it for 30 minutes on the highway on Tuesday, two days ago.
It was about 38F where the car was parked. It took quite a bit of cranking to keep the engine running, but the idle was only about 800rpm. The engine is in top-notch tune with timing, new trigger board, butterflies, wiring harness, sensors, etc. all in tested, configured form. When warm, idles shockingly smoothly as the V12 should, about 800 in neutral, 700 in gear.
But why doesn’t it idle higher when cold, and then settle down to when-fully-warmed idle? What am I missing?

16

if your car runs fine when hot, and you want higher iddle when cold, you need to adjust the AAV screw to allow more air to get in when cold, by unscrewing the adjuster

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

17

No, the adjustment screw on the AAV is the warm/hot idle control screw. It idles properly when hot. If I open the adjustment screw, it will increase the warm/hot idle rpm.

18

If you want a higher idle when cold, maybe you can tweak that little air valve that comes open when the AC compressor is on. Maybe hook a relay up to the temp switch and have that thing open. I had an XJ12C that I drove all winter and I recall fiddling with the cold start injector. I don’t remember what was wrong

19

“If you want a higher…little air valve that comes open when AC Compressor it on…”
I don’t have that kind of valve.
THis is the D-Jetronic system.
The Cold Start injector only functions for a short time during very cold cranking, so it doesn’t help after engine is running. - my engine idles rather well when cold, just doesn’t have higher rpms.
Sure makes me want to check my AAV, again, for function during a range of temperatures. I know the moving piston can be adjusted by pushing up/down in the cylinder. Maybe that needs tweaking?

20

And the spec idle is for a warm engine at normal operating temp.
Cold engine should idle higher and start coming lower as the engine warms up (cause the thermal bulb is working correctly)

21

my bad : sorry as I mixed up

could you increase the idle inlet air flow at the throttle butterflies, and compensate by reducing the AAV hot setting ?
this should help to get the higher cold idle, but let the hot the same ?

my car had the opposite problem (rpm too high which I couldn’t get down enough, because of air leak)