Thick Black Smoky Exhaust then back to normal
 

On my last run (86 XJ6 4.2), engine warm and at 50 mph, suddenly thick black smoke from the exhaust and the car could not maintain speed. Then clean exhaust again, then black. I parked and the engine idled very rough and hesitant to rev when I moved the throttle. I really thought she was done for.

But then I drove home (30 mins) and there was no problem evident most of the way. I have since driven the car up and down my long driveway, no problem.

Other details:

-normally idles at 1000 hot or cold
-coolant temp sensor brand new but gauge on the dash still reads 55-70 C (120-155 F) but that can't be accurate
-it cranks forever before firing cold, not so bad hot.
-there's a leak in the exhaust well downstream of the catalytic converter
-30,000 miles from new.
-*******PO drove the last 4000 miles on 87 octane and this happened on the second tank of Shell 91.

I looked at the spark plugs this morning and they look like combustion has been good (normal few thousand mile Champions) except two that had black soot deposited around the face of the body of the plug (the threaded part).

Do you know what could cause intermittent black smoke and loss of power? A bit worried about driving it at the moment.

back in 1989 when I bought my '84 XJ-6 the same thing happened, the car had been sitting when I bought it, then I started driving it and a cloud of black smoke would suddenly happen and it would lose power.

if I remember correctly, it "fixed" itself, i.e., it stopped happening, otherwise I don't remember what was done if anything. I do remember the Cold Start Injector (a Bosch part), had to be replaced because it was acting intermittently. Otherwise it has never happened again.

could it be accumulated carbon on the intake or exhaust valves?, bad gasoline? not sure of that either.

Maybe Doug has experienced this problem.

Thanks again Jose. You've been very helpful to me lately.

The car has travelled about 50 miles since the smoky scare, and it is running fine. Now I have to examine the cold-start problem.

you can visually check the Cold Start Injector by removing the hex/allen bolt, pulling the injector out; (NOTE: easy while pulling, you want to preserve the paper gasket under the injector).

while someone else cranks the engine, you should see a gasoline spurt/spray with each crank. Engine needs to be cold for this check. If no gasoline spray, the CSI is bad.

Once I've seen something done, I can do it. But diagnosis is not my strong point. I have been wondering about the thermo time switch, which I've never heard of before, and wondered whether it can be tested with an ohmmeter or similar.

Also, there appear to be two water temperature sensors on the water rail. The rearmost one is actually marked "water" and I thought it sent a signal to the ECU for fuel injection reasons, and to the dashboard gauge. Am I correct there? Then there's another sensor about three inches forward of this. What's that for?

Problem sounded very much like the problem I had with my 1984 XJ6 back in 2000. At the time we never did find the problem and sold the car. Recently I read and article about this very problem and for the life of me I cannot find it. Anyhow I do remember that the guy mentioned the temperature sendors fault coud be the cause sending the wrong message to the computer and them opening the cold start injector thus flooding the system with fuel. Black smoke and loss of power. Just a mention but I could be wrong. Best luck.

[QUOTE=Mkii250;988813]On my last run (86 XJ6 4.2), engine warm and at 50 mph, suddenly thick black smoke from the exhaust and the car could not maintain speed. Then clean exhaust again, then black.




Black smoke = over-fueling.

Here's a bit of reading.
XJ6 Series - EFI Rich Mixture

The come-n-go nature of the problem is your case will make diagnosis a bit more tricky. My gut feeling is that yu have a fuel pressure regulator problem. Just a hunch.


Cheers
DD

Starting from the front:

- Thermotime switch
- Sender for temp gauge on dashboard
- Coolant temp sensor for fuel injection
- Temp switch for air injection and cannister purge

All Ser III 4.2 cars will have the first three. Not all will have the last one, though.

Cheers
DD

A faulty coolant temp sensor can cause over-fueling, yes! It's fairly common.

A cold engine needs more fuel than a warm engine. If the engine is warm but the CTS sends a 'cold' reading to the computer the system will over-fuel the engine.

For our older Jags a CTS in only about $20 and takes but 10 minutes to replace so, if a person feels like taking a guess, it isn't too hateful if things don't pan out.

A loose/dirty connection at the CTS will fool a person into thing the CTS is on the fritz so, as always, check the connections first!

However, to clarify, the cold start injector on a Ser III XJ6 doesn't talk to or listen to the computer. It gets 12 volts from the starter relay and is grounded/earthed by the Thermotime switch. Unless something is very seriously fouled up with the wiring the cold start injector can't operate unless the key is in the 'start' position.

Cheers
DD

You could do what I do when my car smokes. Don't look in the mirror and pretend it isn't your smoke. And just let somebody else deal with the smoke.
Attachment 87259


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Just did this, engine cold. The end of the cold start injector was plugged with a black oily substance so I cleaned that up but no fuel emerged when cranking. The two-wire connector to the CSI was giving 10.4V while cranking.

If 10.4V is enough, it must be a bad CSI.

Thanks for all that, Doug. The spark plugs look perfect to me, so I cannot believe there is persistent over-fueling. I replaced the CTS last week. Also I just went for a 20 minute drive to get her hot, and she ran perfectly: no hesitation or stumbling, just the reluctant cold start. I have not seen anything come from the exhaust since that one horrible event.

Well there was a bit of new-looking soot on two of my plugs right after the foul blackness (but not on the electrodes), and that made me think there was a temporary problem at those two injectors. How that could happen is beyond me. But you did mention the fuel pressure regualtor...could that be implicated in this scenario?

Doug can you please help me follow this. Are you saying that the Thermotime switch is closed-circuit (zero ohms) when cold? then increases resistance as the engine warms? Or does it go from closed- to open-circuit at once because of its bi-metallic nature?

This morning, engine cold, my thermotime switch had 51 ohms across its two terminals. Then when hot I got odd readings--it would read around 150 ohms for a sec then go to infinity.

I really tried to pretend but traffic behind me was getting very annoyed. It was like a Supermarine Spitfire going down with a shot up engine .

I have however learned from you the importance of carrying a fire extinguisher :)

Yes. With emphasis on 'cold'.




It'll go from closed to open very fast....you gotta be quick. At about 65ºF coolant temp the TT switch is closed for approx. 1 second. At -4ºF coolant it'll stayed closed for 8 seconds, which is the max duration.



Hmmm. I'll have to mull that over.

The key thing, though, is taht the CSI gives a short squirt of raw fuel on cold start. Does it? And not give the squirt once the coolant temp is above 95ºF or so or after the engine has cranked for more than a few seconds

This might help:

Cold Start Circuit Checklist, Jaguar XJ6 Series III

Cheers
DD

As I see it, the clue is the plugs. If it was the cold start side playing up, I'd expect all plugs to be black, but you only have a couple sooted up. It might be a good idea to clean all the injectors, and to also check each one is clicking away merrily using a "mechanic's stethoscope", i.e. a long blade screw driver against each injector and listen at the handle end.

Don't think your car has a modern digital computer for its ECU, it is an analogue device to convert sensor inputs into injector pulses, using analogue techniques; there is nothing digital in there at all.

It could also be on the ignition side. Dirty distributor cap contacts, two faulty or loose leads etc.

Well chaps, I really do appreciate your input and thoughts on this. Here's where things stand today:

She cranks forever when cold so I have ordered a new cold start injector (David Manners 50 quid!). I had tested the current CSI and it was not squirting at all, not a dribble.

Thermotime switch still has 51 ohms cold instead of zero, but that's a moot point until the new CSI is in.

Had the exhaust fixed this week; new intermediate pipe that splits into two fabricated from mild steel by an absolute artist, AND as I was leaving the shop the owner said "bring it back for a tune up. You need cap, rotor, plugs, leads, fuel filter." Kudos to anjum on that one.

I'm putting the one-time smoky exhaust down to bad gas or the Shell 91 cleaning something out.*

I'm surprised to read in the handbook and Haynes manual that 87 octane is recommended and up to 10% ethanol not discouraged. It just seems wrong to me but should I just go with 87?

* re: using 91 octane in an 87-recommended car. I once bought a '78 Merc 280CE that had over 100K miles on it but had sat unused for a year or so. Under my ownership she got nothing but Shell91, but on the fourth or fifth tank the catalytic converter was plugged solid wlth soot. The good gas must have cleaned up the innards quite quickly. That wonderful M110 engine ran perfectly once I'd removed the exhaust and let it exhale.

By the way, if you haven't had one of those W123 Mercedes, I can't say enough good about them. They're wonderfully comfortable cruisers with all the mod cons of the day.