Ok, gang, here’s one that we can have some fun with

Car: 1985 Series III V12

Symptoms: Very poor running when cold. Engine starts easily and idles well when cold but will not take any throttle. It’ll buck, stumble, and nearly stall out. You can hear misfire/thumping in the intake tract. This bad behavior will occur until engine reaches about 2000 rpm, at which point it smooths out and runs great.

The symptoms gradually decrease as the engine warms up. At full normal operating temperature the problem is almost entirely gone. Almost. I can still feel a tiny roughness and hesitation below about 2000 RPM...although I’m quite sure a passenger would never notice. In normal driving you would typically ‘drive through’ such momentary aberration without any notice. But...it’s there.

Background: I wanna say this began developing a couple months ago. In the course of routine checks I discovered my vacuum advance was not working...despite the capsule being replaced a few months ago. To their credit British Vacuum Unit https://www.britishvacuumunit.com/ made good on their guarantee without question or argument. I took advantage of this opportunity to replace the spark plugs (old ones were virtually prefect).

Coincidentally and accidentally I found in my stash a red TPS that I had apparently forgotten about. Some here will recall my recent problems with a new-but-defective TPS and my switching back-n-forth between the red style TPS and my Mustang TPS.

What I’ve checked and know so far:

TPS set at .34 volts and voltage progresses smoothly. (Note 1)

Throttles set at .002” and linkages synchronized; throttle bodies clean.

Pick-up coil air gap at .010”

Fuel pressure at 34-36 PSI (Note 2)

Coolant temp sensor in spec and connector in good condition (Note 3)

Air temp sensor in spec and connector in good condition (Note 4)

Mechanical and vacuum advance both working

Swapped ECUs, no difference.

Slightly advancing and retarding the base timing makes no difference (didn’t think it would)

Spark plug wires ohm’d out OK. Distributor cap and rotor look good; nearly new

Injector harness (4-5 years old?) passed jiggle test OK

Connections at coil and amp clean and tight



Note 1: no difference with or without the 1mm wedge to hold throttle open. No difference with Mustang TPS vs Red TPS

Note 2: for giggle swapped in a spare regulator. No difference

Note 3: checked both CTS wires back to the ECU and both show .5 OHM resistance. I am not sure if .5 OHM is significant here

Note 4: specs for the air temp sensor are wrong in all my manuals. They are pre-HE specs. I think this has come up before. Pre-He specs are in OHMS. HE specs are kilo ohms.


What I suspect: I think I have an injector problem. Not sure if it’s with the injectors themselves or something else. The misfiring thru the intake suggests lack of fuel; the roughness below 2000 RPM could be a lean misfire.

Using a stethoscope I heard all the injectors are clicking evenly but I realize this is not a conclusive test. Also, the jiggle test of the injector harness is not conclusive.

I’m thinking that partially clogged injectors might not respond well to light throttle/light load. “Lazy” injectors might do the same, even if clean.

Before removing the injectors, though, I plan to clean the terminals at the injector resistor pack, check the resistance of each injector, and properly check the injector harness.


Other info: I do not have an AAV. Also, I have a non-feedback ECU so oxygen sensors and closed loop-versus-open loop operation are not part of the equation. I have also eliminated all the vacuum advance timers and delays. No cat converters. All these changes pre-date the problem I’m having.



Thanks for reading. Any ideas welcome !



Cheers

DD

 

Doug,

Some thoughts, at random a bit.

The Vac supply TO the ECU is holding Vac I trust. Not normally a cold engine issue, but easy enough to check.

The 0.5 Ohm in the wires to the ECU from the CTS are possibly of no concern. I doubt the P Digital is that fussy, but, maybe it is?

I have had the solder "mess" where the ATS and CTS wires travel along the B side back to the splices at the TPS chassis connector. Usually a dead engine, but worth a look.

The vac hoses and caps on the A Bank rear end plate are all OK?, especially the blanking cap.

Resistor Pack plug cleaning, always a good place to start and tick off.

I will keep thinking.

 

Doug, I agree that fuelling/mixture is the most likely, given the symptoms. A question: does it misfire if you are not in gear and under load (ie if you open the thottles, engine cold, using the capstan)? If not testing is far more difficult., and some of the random thoughts below assume you can test it car stationary.

Have you tightened up the intake manifold nuts and throttle body bolts? etc This sounds like classic intake manifold failure symptoms.

So, try a bit of ether while it is doing it and see if it evens out, which would confirm mixture, I think.

Is the misfire one side only or both? What happens if you remove each injector plug, one at a time, engine running?

In any event, I would try a new rotor, and if you have them, another set of leads, another cap, and another CTS, befoe anything else.

 

Yup, checked.

(Heh heh, after checking I forgot to reconnect the hose. Running the engine with the hose off perfectly demonstrates the expression "she's running pig rich" )

I was expecting zero. Half an ohm is pretty small....but I've never been sure how/when/if a small resistance like this might be important

Vac hoses and caps OK, will check the solder mess

Thanks Grant !

Cheers
DD

 

It happens with the car stationary....both in gear and out of gear.




Checked intake bolts but not throttle body.

I can't tell...yet. I have an X-pipe in the exhaust so the classic/easy way of checking by listening at the tail pipes won't work

Meant to do that, forgot. Will do!

Forgot to mention that I already tried a spare CTS just for giggles; no change.

It just doesn't seem like an ignition misfire to me....but I've been wrong before !

Cheers and thanks
DD

 

It might be worth spicing in a new wire for the CTS temporarily to see if it fixes the problem. Run it outside the car even, see if lower resistance makes a difference. If it does, then rewire it properly.

 

Doug,

My knowledge of the V12 is very limited compared to yourself and the other gurus here.

But re-reading your first post implies to me that you start by looking at things that change with engine temperature. So, therefore, the native capability of the injectors doesn't seem to be in question if they can operate, even at idle, when the engine is warm.

And the only things that could seem to be are:
- components that have air leaks which might then seal up as the heat expands them. So potentially inlet manifolds or, less likely, vacuum pipes feeding ecu etc.
- operation of components that receive different signals when cold compared to hot eg injectors

So, my limited knowledge would be that once you've checked air leakage components, you'd then fully revisit that CTS as I assume it's the only influencing factor on how the injectors run when cold compared to hot?.

Apart from that, I have nothing....sorry!

Good luck

Paul

 

I thought about that...and haven't ruled out the idea.

The CTS operation is measured is thousands of ohms so, on one hand, I'm not sure how .5 ohm would matter. On the other hand, is that .5 ohm degrading the signal to the ECU? need to wrap my head around these things; not always easy for me

Cheers and thanks
DD

 

Thanks for chiming in, Paul.

In the next few days I'll be digging in again and will recheck for vacuum leaks.

What I'm beginning to think, though, is that the fault itself doesn't change according to temperature. What changes is how the engines reacts to the fault.

The symptom exists no matter if the engine is cold or warm. If there's a lean condition a warm engine can tolerate much more easily than a cold engine.

Some thinking out loud here.....

I think I have an injector problem. This could be that:

A) The injectors are being incorrectly commanded by the ECU. The ECU might be receiving bad information (sensor inputs, for example) and is commanding the injectors based on that bad information. Or, it might be receiving correct information but is giving bad commands due to an unknown fault. The symptom didn't change by swapping ECUs so I've pretty much excluded a faulty ECU. What remains uncertain is whether or not it is receiving correct information.

Or.....

B) Everything upstream of the injectors is OK and there is a problem with the injectors themselves. A few are not responding well to low pulse width commands, or are partially clogged....something of that nature.

Interesting problem. We'll have fun with this!

Cheers
DD

 

That's an interesting observation, Doug, and one I hadn't appreciated.

Just thinking about your point on sensor information to the ecu. The CTS seems to be the biggest influence on cold running compared to hot running. If the CTS was failing (or a fault in it's wiring), it could still be causing a influencing problem when hot, as per your comment that the problem seems to be slightly there at idle when hot. Therefore, perhaps once the engine is hot, maybe you could remove / bridge the CTS to completely remove its signal and see if the hot engine idle improves?

Paul

 

Ahh, yes. An old trick that I totally forgot to try. Gah !

Will do. I'll report back.

Thanks
DD

 

Doug

have you checked timing lately, especially during these conditions, and then as it warms up? I would even start with a timing light on your main lead from the coil to the distributor just to see if it's flashing all the time while it's running. Then check timing under the car on the balancer / pulley. I thought there was some temperature related vacuum control for the vacuum advance. It sounds like the timing is off when it's cold so bad that it misfires and then when it warms up it goes away.
I might even disconnect the injectors one at a time and see if one of them shows a dead cylinder by plugged injector.

Jeff

 

I forgot the CTS bridge also, age is a wonderful thing.

Is that Condensor "thing" still inside the Ign Amp?. I remove them, as you know, and those I have removed them from ran so much sweeter.

That -5 Ohms could be as simple as a bad connection, at the CTS or as I mentioned, the Solder Mess along that path to the TPS splices.

I know that RICH running well., Had a HE here, post engine fire, 6 weeks later, running, RICH, damn that thing could lay a smoke screen. Eventually found a wad of fire extinguisher foam stuff inside the pipe from the balance pipe (that hose was burned at the pipe connection). NO vac at the ECU. Once blasted out, sweet as.

 

Yup. Binned it some time ago.

Removed and opened up the injector harness today. No faults found, visually or with ohm meter.

Remove the injector resistor pack. Terminals not corroded ...or even dirty, to speak of. I made 'em shiny and bright but I don't think they were a problem. I don't know the value of the resistors but each of them ohm'd the same so I'm guessing they're OK

Moving on to injectors next.

Cheers
DD

 

Injectors out and being sent to Jaguar Fuel Injector Service.

With any luck I'll be reporting back with good news in a couple weeks ! Till then there's nothin' to do but wait.

Cheers
DD

 

How do the injector pintle caps and O rings feel?

 

Meant to reply to this but forgot

I didn't check the timing, no. For giggles I did try advancing and retarding the base timing a few degrees and it didn't make any difference.

All the vacuum advance controls are long gone on this car.




Yeah, but it doesn't go away. It's still there even when warm.


Good idea but I decided to just pull the injectors and send 'em off for servicing. I am hoping to get a call saying that a concrete fault was found with a few of them

Cheers and thanks
DD

 

I didn't feel them...and am not sure what I'd be feeling !

Visually they looked fine.

Cheers and thanks
DD

 

UPDATE

Car is back on the road ! It's always so nice to get back in the Jag.

I sent the injectors to Jaguar Fuel Injector Service

As always, excellent service from this company. I've sent many sets of injectors to Dave and have always been happy with the results and customer service.

I was hoping that a couple injectors would be revealed as clearly faulty in some way. Concrete faults are easy to deal with. It's the borderline faults that are frustrating. And such was the case with all 12 of these injectors: none were outright faulty and none of them, technically, were operating out-of-spec. However, several were right on the cusp and, importantly, the deviation between them was quite significant. After cleaning all of them are much closer to each other in operation.

End result?

Significant improvement. The cold running problem remains but is much less severe. Still have some coughs but they clear up much sooner. The misfire/roughness on light acceleration, which was occurring hot or cold, is 100% eliminated. The engine acceleration is totally smooth from 600 RPM to 6000 RPM no matter if very gently applying throttle or pouring on the coals !

The coughing-when-cold obviously requires more research and checking but, with the symptoms now being much less severe, I'll live with it....at least for the time being. The car drives too nicely to be sidelined for such a minor fault plus I have some other projects that need attention.

I must say that this has been a very eventful year for this old Jag! As I've mentioned before, I must've tempted the fates with all my boasting about how trouble-free the car had been all these years !

Oh well. When you run a 36 year old car as a daily-driver this is what you get

Cheers and thanks
DD

 

have you checked fuel pressure at the rail?