I'm not new to Jaguars, or to this forum, but I suspect I'll be spending a lot more time here in the future. I've owned my '96 XJS since '99, but I've had hardly any problems with her. But now...

Sunday I bought an '85 Vanden Plas. I was actually looking for a '95-'97, but this car was just SO gorgeous... "Esme" is near perfect inside and out. She has two ~1/4 inch wide rust bubbles, one small dent, one 3 inch seam split on the driver's seat and some varnish crazing on the glove box. Anything not mentioned is pristine. The previous owner used to keep her covered AND in the garage.

One small problem, she won't (I'm told) pass emissions. (Colorado is one of those enlightened places where anything after 1975 still needs smog checks.) The P.O. was not mechanically inclined, so I'm hoping it might not be a huge problem. And I'm much better at fixing mechanical things than cosmetic, so I took the chance.

This car has not been driven much, especially lately. It has only 85,000 total miles, and an oil change sticker makes me think she's covered fewer than 10,000 miles since 2002. And I was told that before Sunday she had not been run in over a year. Before starting her I took out the plugs (rather black) and poured some Marvel Mystery oil in each cylinder. That was last Wednesday, but she wouldn't start then. I told the guy he should put some new gas in her, which he did, and he called me Friday to say he had gotten her started.

On Sunday she started right up, but idled a bit rough. She also backfired (I think through the intake) several times as I started driving her. She did seem to run better the more I drove her though. I had about a 50 mile drive to get her home, and she handled it without drama. Along the way I ran into a horrific traffic jam on the freeway, the kind were you crawl less than a mile in 45 minutes, but her temperature never rose above 90. Fortunately it was a cool evening. She felt maybe a little loose in the front end on the freeway, though I didn't notice that on side streets, and it might just be due to the flat-spotted tires wobbling.

But getting her to pass emissions is the first priority. I figured I would do some simple things that might work before I even try to get her tested. Definitely new plugs, cap and rotor. And I'll replace the O2 sensor, and at least test the coolant temperature sensor in case that's the problem. I will also drive through at least one tank (the left one) of new premium gas with a bottle of Lucas injector cleaner in it before I go for a test. Any other "might as well" ideas I should try that don't take too much money or thought?

I think there's a good chance one or more fuel injectors is clogged. She does idle rough once she warms up, like at least one cylinder is missing. I've read various things about how to clean fuel injectors, ranging from plugging some sort of aerosol can into the fuel rail to taking it to a shop for ultrasonic cleaning. There is even a video of a guy to using a syringe and a AA battery to squirt brake cleaner through each one, which seems a little scary, but kinda cool. Any thoughts or memories about how well these methods might work?

I'm sure I will have many more questions as I start driving and working on her more. All advice is appreciated. Thank you,

-mB

 

Congratulations on your find. I have a 1984 and it still passes inspections.

One thing I learned early enough was to drive the car for at least 2 hours before taking it to the inspection station. Let the thermostat go through all its open and close cycles before the temperature stabilizes and the thermostat remains open.

Adding Lucas or STP to the fuel tank, and filling up with premium gasoline will not improve emissions or even performance, in any case the opposite is true. These engines were designed to run on 87 octane fine.

you would do better at an inspection with fresh engine oil and filter.

 

Welcome Mike:


1. I agree with Jose, save your money, higher octane fuel will get you no more than 87. But, fresh fuel is a good idea. For test purposes, I like Chevron fuel. Their Techron additive is first rate. I only want about 5 gallons in the "test' tank. And, I add a big can of Techron. Super loaded fuel!! Lucas stuff may be as good or even better. No real proof that this works, but it seems to be a way to get the injectors to work as well as thee can.


2. Yup, almost anyone agrees that the car must be well warmed up. To make sure the Cat/s are optimum. Loafing on the freeway isn't enough. Gotta rev it up. I go direct from the run to the test station. stationary the engine and cats will get even hotter, that is good. And, you want the coolant temp at least at 90 C.


3. The O2 sensor does little. Not anything that will effect engine fuel mapping.


4. But, as long as the engine is running rough, a failure is guaranteed.


5. I would start with basics. If the engine has serious mechanical ills, it will never run well enough to pass SMOG!!! Wet and dry compression test.


6. Black sooty pugs denote an over rich mixture. Guaranteed SMOG failure. The answer might be a really dirty air filter!!


7. But, are the plugs oily as well as sooty?


8. Look up top on the forum. 100.00 miles/ A great primer on plug reading.


9. a strobe on each plug will denote spark to each cylinder. A regular beat. Not erratic.


10. run the engine in the dark. do you get a light show denoting failing insulation on the HT leads?


11. Check the condition of the wires to the + post on the coil. Same for wire from the - post to the amplifier and on to the distributor. Poor wires and connectors lead to erratic engine running, if it runs at all. wiggle them, carefully as the engine runs. Does it falter as you wiggle 'em? I foud this on a past critter. it fixed an erratic stumble.


12. Get under the car on the right side. You will fid a ground strap from the transmission to the car body. It must be intact and the connections clean. Grounds are a lot on cars.


Enjoy.


Carl

 

Carl and Mike,
another issue with the single wire O2 Sensors, is rusted exhaust manifolds and valve cover oil leaks over/under the manifolds preventing a good Ground to the O2 Sensor. (repeat: SINGLE WIRE O2 sensors, not the later 2 wire sensors).

try adding a ground strap to the O2 Sensor if yours is a single wire sensor. (I have no clue when Jaguar went with the 2-wire sensors but it might have been sometime in late 1985).

this "remedy" comes from the 1990's, it was recommended back then in other Jag forums, one of those "lost tips".

I fabricated my own strap per the picture below, the result is a stabler idling and a more "positive" signal from the O2 sensor to the ECU.

Snake Oil some people conclude? Try it, you have nothing to lose. It will not cause a nuclear explosion!!, I have had a strap in my O2 sensor since around 1991, the engine idles great, starts right up, and mine is older, first sold in 1984 and built in Sept. 1983.

wonder why Jaguar added a second wire to the O2 sensor later on??

 

Have a read -

Fuel injection and the Jaguar XJ6 4.2 Series 3 / AJ6 Engineering

As was said in the Jaguar Workshop manual, the two main inputs to the EFI are (1) engine temperature, and (2) mass air flow. Everything else just trims the effect of these two.

 

Status report:

Since last week I replaced the air filter, plugs, cap, rotor and O2 sensor. That didn't seem to help.

I tested the coolant sensor for the EFI and it seems to be working right. Also tested the throttle switch, ditto.

Today I did a lot of work. I cleaned the throttle body, cleaned the grounds at the back of the intake, the cylinder head just next to that and under the car from the bellhousing to the chassis. And I went all out to try to eliminate any vacuum leaks. I bypassed everything I didn't really need and now have just the brakes (tested and found leak-free) the fuel pressure regulator, and a new hose directly from the throttle body port to the distributor advance.

This made a difference. For one thing the car, which started well enough before, now starts at the first hint of key turning. For another I seem to have gained about 300 rpm on idle speed (it's now at 1100 in park, too high). And the car seems to idle much smoother, but maybe that's just because it's idling faster.

Unfortunately I still backfire on acceleration from a stop, engine warm or cold. At first I thought it was through the intake, but now I'm not so sure.

I haven't yet checked compression or fuel pressure (I need to get a fuel pressure gauge). And I suppose I can replace the fuel filter.

A couple questions (aside from the obvious, "How the hell do I fix this?")...

Does it hurt anything to drive (temporarily) with the fuel evap vacuum hose off?

I checked the timing. With the vacuum advance connected to the throttle body and idling at about 1100 rpm I get about 13 degrees BTDC. I know this is a bit retarded, but is it enough to cause backfiring?

Thanks for your help,

-mB

 

exactly which fuel evap vacuum hose off ?

backfiring:
have you double-triple checked the wire connections at the Coil ?
Are you sure the spark plugs wires were reinstalled correctly?
have you checked the air hose connections from the air filter to the intake manifold?

here's what the manual says:

A backfire occurs when there is an imbalance in the air to fuel ratio required for the engine to operate properly. If the fuel mixture is too lean (not enough fuel) will backfire through the intake, or too rich (too much fuel) will backfire out of the exhaust system.

 

Mike:


Fuel evap hose off? At the intake manifold is a vacuum leak. engine lean and high idle.


You can plug it at the manifold and leave open to the tanks. It should run OK.
But, dependent on Colorado test, it my flunk. Evap system inoperative.


Carl

 

Just read as much Jag lore as possible as recreational education.


Lottsa fun and great commaderie.


Carl

 

Thanks for the replies.

I am sure about the plug wires being in the right order, and about the ducting from the AFM to the throttle body being tight. I should recheck the coil connections including actually taking them off and cleaning them.

Right now the evap hose is off the manifold, but the manifold source is plugged. Any other car I would assume that everything would be fine except for maybe a slight gas odor. But this is a Jaguar so I wanted to make sure nothing weird would happen. I know I'll need to have that hose connected properly before The Test but in the meantime I wanted to disconnect as much of the vacuum system as I could to try and eliminate vacuum leaks as a possible source of problems. I probably found at least one, considering the 300 rpm jump in idle speed, but she still backfires.

-mB

 

Engine timing!!!


In manual spark days, we would "rev: up the engine and get the car going at a good pace, then retard the spark and close the throttle. Pretty loud explosions, almost like gun fire.


That is the result of combustion in the exhaust pipe or manifold, not ion the combustion chamber!!


Popping out the intake is another matter. That is ignition with the intake valve open.


That timing thing again.


Valve clearances can also do this kinda stuff. Intake or exhaust valve not closing tight. Just a little leak can cause havoc.


Carl

 

I've done a little more work. Today I got out the vacuum pump / gauge and checked the distributor vacuum advance. It holds vacuum, and I even heard a little sound of motion from the distributor when I released the vacuum, so it must be moving.

I also checked the fuel pressure regulator. That did NOT hold vacuum. I traced the problem to the thermal vacuum switch on the fuel rail. With this out of the system the rest DOES hold vacuum. Correct me if I'm wrong, but that switch is supposed to be open to release vacuum only if the fuel is quite hot, right? The car was cold (and off) and I think that means that thermal vacuum switch is bad, right? So I bypassed that and plugged the regulator hose straight into the manifold vacuum port. Now there are only three things connected to engine vacuum- brakes, distributor, and pressure regulator (minus the thermal switch) and all of those have been tested and shown NOT to leak. So no more vacuum leaks then. I didn't have the time to test drive her after this improvement.

Getting back to the fuel pressure regulator, it doesn't leak vacuum, but it doesn't show any sign of doing anything, by which I mean when I raise and lower vacuum on it I can't hear or feel anything moving. Should I be able to? I know I should test the fuel pressure (got to get a fuel pressure gauge) but I was wondering it there's some other way to find if the regulator is doing anything, or if it's just stuck.

Thanks,

-mB

 

as far as I know the fuel pressure regulator does not make any noise.

 

Right. When hot, the switch opens to vent vacuum to atmosphere. No vacuum to regulator = higher fuel pressure




If it doesn't leak fuel or vacuum the only other test is a fuel pressure test

Jose is correct---no noise when it operates.

Cheers
DD

 

12. Get under the car on the right side. You will fid a ground strap from the transmission to the car body. It must be intact and the connections clean. Grounds are a lot on cars.
Carl[/QUOTE]

This ground strap is critical in fuel injector operation. Make sure the contacts are spotless. I had a no start on an 1985 XJ6, and it turned out the injectors weren't being grounded properly, due to oily connections. I also added a seperate ground strap from the body directly to the coolant rail. No irratic idle since.

 

Back to ignition timing:


1. Remove the distributor cap. Twist the rotor CW and CCW. it should turn freely. If not, the mechanical advance is stuck, retarded or advanced. either not good.


2. Remove the vacuum hose form the advance diaphragm. Plug it. if you are a golfer, a Tee is perfect. In my opinion, the only useful thing about that 'sport". Hook up your timing light. Run the engine. Some where around 14 to 17 degrees BTDC should result in a nice running engine. It can be fine tuned in another way. Rev it up bit and see if the strobe indicates more advance. You would like to see about 34 or so degrees.


Carl

 

I changed the thread title because I'm narrowing down the problem. A little more progress today. I finally did a compression test, and it came out pretty good. From back to front I got 115 - 110 - 107.5 - 110 - 112.5 - 110. Seems reasonable for a worn but healthy engine. If any cylinder is suspect it would be #3 (107.5) because all the other cylinders reached their peak compression after 3-4 cranks. #3 took 6 (after 3-4 cranks it showed only 95-100 psi). Still it seems to me that's good enough. Correct me if I'm wrong. I checked the timing properly - idle at 850 rpm, vacuum advance plugged. I still get about 13-14 BTDC. I revved it up a little (I didn't have a helper today so I stuck a screwdriver behind the throttle spring and the tach read 1800) and the timing was 20 BTDC. It seems to run a little better, but it still idles rough and backfires on gentle acceleration from a stop. Flooring it from a stop it DOESN'T backfire. The car doesn't feel very fast, but informally timing 0-60 (counting out loud) seems to come out to about 12 seconds. No where near my '96 XJS, but it was the '80's, so maybe that's about right. To recap the car now has new air filter, plugs, cap, rotor and O2 sensor. All ground wires, including under the car have been cleaned and reinstalled. I did the test with grounding the O2 sensor to the engine with no change. The throttle body has been cleaned. The engine coolant sensor and throttle switch test good. I can feel little clicks from all 6 injectors. Compression and timing test good. Every vacuum hose has been disconnected except for distributor advance, brakes, and fuel pressure regulator. All these have been tested and show no leaks. My remaining ideas all involve the fuel system. Next up I will test the fuel pressure. After that I'd suspect either the injectors or the air flow meter. Any other ideas? Thanks again, -mB

 

I may have missed it but I don't see where you narrowed things down between an intake backfire or an exhaust backfire. The former suggests too little fuel, the latter suggests too much

Most commonly an intake backfire is a bit of a pop where an exhaust backfire is an almighty boom


Cheers
DD

 

Doug:


Yeah, I composed a post along that line. Many too many words. Lost it, somehow????


And, 20 degrees total advance seems puny. Cause of pop or boom, probably not, but may have a lot to do with slow zero to sixty numbers!!
Carl

 

On the 20 degree advance I did only have her at 1800 rpm. Maybe today I can check timing again at a higher rpm. Should this check be with vacuum advance connected or not?

The backfire definitely is on the little pop side. I was driving with my wife yesterday and we sometimes had to debate whether it had popped or not at a given moment.

If it is too lean, does that change the diagnosis? I had been thinking stuck pressure regulator, but that would make it run too rich, right?

Thanks again. Now that I know the basic internals are all right I know we'll get this cat purring eventually.

-mB