Mike:


You are getting there.


Connect the vacuum advance and rev it some more to get total advance.


So, 14 BTDC sans vacuum and 20 BTDC on a rev means 6 mechanical advance degrees at the 1800 rpm.


You might construct an RPM/degree scale to see the rise. Or lack there of.
Folks have messed with that curve from the beginning of IC engines.


The occasional; pop suggests an intermitant anomaly. I'd look to wires and connectors.


Carl

 

Thanks, Carl.

I wouldn't call the pop occasional, because it happens every time under a very specific circumstance: whenever I accelerate gently from a stop. If I floor it no backfire happens, but if I ease the throttle down, like when starting from a red light in traffic, it always happens.

I didn't get to recheck the timing at speed today, but I did check the fuel pressure. Comes out about perfect: 36 psi with the engine off and fuel pump running, 32 psi at idle. I took the vacuum hose off the regulator and it jumped to 36 psi, put the hose back on and back down to 32. I'm assuming that clears not just the regulator valve but the fuel pump too, right?

On one hand it's nice to find so much stuff working right, but on the other hand I'm starting to run out of possible fixes. Now I'm thinking either AFM or a clogged injector.

By the way, did I mention that the MPG (by the dash computer) seems to be in the mid-12's? That seems low to me, but I'm not sure. And I think it gets driven down by a lot of time idling while I fiddle with stuff.

-mB

 

Has this still got the factory air injection setup? the pump can fail on them, as well as the valve.


Have you changed the fuel filter or inspected the fuel tanks, a very common issue is a rusted out fuel tank, I've had to strip my fuel system to remove all the rust particles that destroyed the fuel pump, clogged the filter and clogged all the injectors, just to get the old girl running.


Looks like you've checked just about everything, also the EGR valve will most likely not be working and rusted shut, but make sure its not rusted open allowing burnt exhaust gases into the intake.


While these aren't really beneficial to the outcome of passing emissions, deleting the air pump setup and plugging the 6 ports (1/8th bpt) on the cylinder head, along with removing the egr setup and fitting a blank plate to the rear of the inlet manifold both eliminate potential failure points that can contaminate the air quality leading to worse emissions if they are faulty.


Plus the engine looks a whole lot neater without all that crap on it.

 

Thanks, Stoney. I'm afraid removing any emissions equipment isn't an option for me, thanks to Colorado's cursed emissions laws which say anything 1976 and after has to be smog checked, forever.

I hadn't thought of the EGR valve. I will put that on my list to check.

Progress report: Not much because the weather has been horrible. But in a lull between storms I finally checked and adjusted the throttle stop (I didn't have a 0.002 feeler gauge before). I had to open it up a bit. Then the idle was too fast again. To get it back below 900 rpm (in Park) I had to basically turn the idle screw down to the stop. It does idle a bit smoother now. But that can't be right, that idle screw, so I guess it's getting extra air from somewhere.

But it still backfires.

I had noticed that it doesn't backfire when I floor it, only when I accelerate gradually. I was thinking about what the difference was, and it occurred to me that when you open the throttle more than about 1/2 way the throttle switch engages. So for an experiment I hooked up a pushbutton to that switch's wires. And yes, if I push that switch while accelerating gently it doesn't backfire. If I don't, it does. So what exactly does that thing do? I'm thinking it's some sort of enriching circuit, analogous to the accelerator pump in a carburetor. Am I right? That would at least confirm that I'm running too lean.

Back to the idle screw setting, I'm now suspecting the Auxiliary air valve. I found it (yay). But I only see one hose coming from it. A fat hose, going to the throttle body. Where does it get its air supply? How can I test it?

Thanks,

 

Don't bother looking for an EGR valve. You don't have one.

More later on the throttle switch


Cheers
DD

 

Unless it was an option in Australia only? I hadan EGR setup on my '85 Jag Sovereign.


Comes from the rear exhaust manifold, routes behind the motor and into the rear of the inlet manifold into a plate with three bolts.






As for the air injection, maybe disconnect the main hose and plug it to see if it has any beneficial change, just while you are diagnosing.

 

On USA cars it does exactly as you say....enriches the mixture at full throttle (or nearly full throttle). So, yes, you are running a bit lean. Your manual switch is richening the mixture, alleviating the lean condition.





There's another hose routed underneath the intake manifold....almost impossible to see unless you're under the car looking up.

The only 'test' I know of is to remove the AAV itself and look at the little rotating disc inside. When cold it should rotate open. When hot, it should rotate unless almost all the way closed.

Cheers
DD

 

Somehow the USA-market Ser IIIs never got the EGR valve.

Cheers
DD

 

Thank you very much Doug. Sounds like you know this critter inside and out.

So at least I know I'm running too lean. Any thoughts on what to check next?

 

AFM may have been fettled with or the spring mechanism that operates the VANE door might be the incorrect tension.


With my old E34 BMW 535i that uses the same AFM style, I fine tuned it with the help of a wideband O2 sensor, once I felt everything else was up to scratch though and not faulty.


It involved opening it up, and inside is a spring mechanism that holds tension on the vane door and the more load the engine has the further it opens, which in turn relates a signal to the ecu, on the engine load for air fuel ratios.


The wiper arms that make the contact inside them can become worn over age, and repositioning the contact arms on a fresh area can help. and the final thing is adjusting idle AFR and then adjusting the spring tension/position to alter the AFRs under engine load, took a bit of playing around, but the results on a car that has 250,000kms was well worth the time invested, much smoother idle, much stronger/constant acceleration and better fuel efficiency as I leaned it out a bit, was running 11:1 WOT before adjusting back to about 12.5/13:1 AFR

 

Yeah, I'm starting to think AFM as well. It has definitely been opened before, judging by the sloppy bead of RTV around the cover.

I think my next job is going to be to try and test it, using the info found here:

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

Does anyone know which end is terminal 39 and which is 27? I can figure it out eventually, especially if I take the lid off, but it'd be easier for me if someone could tell me front of car or back or car.

Thanks,

-mB

 

Actually I found another thing to test. I disconnected the Engine Breather Pipe at the throttle body and plugged it. No difference.

There's one more hose coming out the front of the throttle body (for a total of three). It is a short rubber hose running from there up to what looks like a coolant rail by the cylinder head. Is that another vacuum hose, or is that coolant?

Have I found all the hoses connecting to the throttle body now? There's one small vacuum line in back, two large vacuum lines in front (one for idle air and one for the breather pipe) and the third smaller hose in front mentioned above. Am I missing any?

Thanks,

-mB

 

There is another vacuum line underneath facing downwards, roughly underneath the throttle body.


The vacuum line going to the coolant manifold is for a thermo activation switch, I think its already mentioned here, but it can be bypassed.


On my car, just for testing purposes like yourself, I have deleted everything except for fuel pressure, distributor advance and IAC.


If you can get it running on the most basic things and then start incorporating the emission control equipment until it starts running worse, that's one way of finding out the culprit.

 

Don't worry about the air injection pump. it injects air into the exhaust manifold. So, it has no effect on combustion. Caveat, a control valve issue can cause a backfire on deceleration in the exhaust. Kinda neat actually!!


There is a hidden vacuum spigot under the intake, toward the rear as I recall. it has to do with the climate controls. guess how I found t when my car was still 4.2 powered. Flaky AC!!


Carl

 

Thanks again for the help!

I know there is a vacuum port directly under the intake, that (used to) feed into both the AC and the "delay" valve. That has been plugged.

I tested the Air Flow Meter this morning and it seems to come out okay. I tested it with the engine off and the ignition on. Here are the "typical" voltages (on the left) and what I got:

Typical-------------Mine
Pin 6
0-------------------0

Pin 7 (closed)
2.25----------------1.5

Pin 7 (full open)
7.5-----------------5.75

Pin 8
7.6-----------------6

Pin 9
12-----------------10.5

Pin 7 actually read 0 when Fully Closed, but at that point the fuel pump wasn't running either. Pin 7 jumped up to 1.5v BEFORE the fuel pump switch engaged, so to me that means it is definitely in that range anytime the engine was running. There was a smooth transition of voltage on terminal 7 as the flap was moved.

All these came out a bit low, but the car was not running, so the alternator wasn't on. The instructions say that Pin 9 is "12v; nominally 2v below battery". Since the battery itself read 12v, not 14v, I'm thinking that all the voltages should be proportionately low, as they are.

So yippee, I don't need an expensive air flow meter, but I'm no closer to solving the puzzle.

I guess a clogged fuel injector is still a possibility, but from what I've read that surprisingly actually causes a RICH mixture, because the 02 sensor tries to compensate and ends up overdoing it.

It still seems that a vacuum leak is the likely problem. I've found and plugged two, but they didn't really help, except to increase the idle speed.

I have two more lines to check or plug. There's that line coming from the throttle body to the coolant rail, which Stoney says "is for a thermo activation switch". And there's the other "vacuum line underneath facing downwards, roughly underneath the throttle body.", unless that's the same one Carl mentioned.

I mentioned before that I have the idle screw all the way in yet the car still idles at about 950 in park. From what I can figure this may mean that the Auxiliary Air Valve is stuck open. Could that cause the lean backfiring? I was thinking that since it's in series with the idle air screw it would basically have the same effect as having that screw out one or two turns two many; not perfect but it doesn't seem like it would make her run THIS badly.

I'm going out of town for a week now, so it'll be a while before I can do anything more. The temporary tags expire June 23rd. My in-laws come into town on June 9th, so at that point if I haven't found the problem yet I'll be taking it to a Jaguar specialist in Denver, which will no doubt set me back thousands. I really had thought I would be able to fix it myself, but these cars are very humbling.

 

Seems like you have a better mechanical knowledge than half the mechanics out there.


Problem with these older cars is the wear on items, age, heat cycling etc etc


Really seems like you have checked most avenues.


Have you checked fuel pressure and flow?


Do you have access to another running Jag at all? maybe swap a few parts over if possible, like the AFM for example.


Does it run at all better or worse once its warmed up?


Stale fuel? Its really easy to drain these fuel tanks, recommended to as well to remove any foreign material (dirt/rust etc) that has built up at the base of the tanks.

 

OH yeah!!
(';')

 

I redid some of my tests and adjustments. Most check out as they did before. I redid the throttle gap adjustment and now I have no trouble with idling too fast. ----------------------I made a discovery. If I disconnect the O2 sensor the car does not backfire. Reconnect the O2 and it backfires again. So I have a new theory. One of the injectors leaks. So one cylinder runs rich. The O2 sensor reads this and leans out the mixture. And that causes the backfire through the intake. ---------------------So I took out the injectors. Monday they will go in for cleaning and inspection. ------------------In doing so I lost one of the thin white plastic washers that goes under one of the nuts holding a injector retainer plate. How hard is this to get? If the answer is "very hard" then how important is it? ---------------------Thanks, -mB

 

The washer is EAC4289. Google the number and or contact some of 'the usual Jag suppliers'

I doubt that the washer is absolutely essential. I've never really known exactly why they used it to begin with. Others will chime in.

Good that you're having the injectors professionally service. Hope it helps. The other possible explanation is that unplugging the O2 sensor makes the ECU default to a fixed fueling map that's a bit richer....and thus a lean backfire is eliminated

Cheers
DD

 

It was the fuel injectors! #2 and #3 were 25% and 20% clogged. After I put them back IT WORKS! No more backfiring! I am so happy!
:icon_b anana::ic on_mrgreen::icon_steer :


Can you tell?

By the way, I have no idea of what a rabbit with a pizza on its head means.

-mB