Hello!
Purchased my first Jag about 6 weeks ago, a 1986 XJ6, and having a blast. I have sorted through most of the issues (that I know of, lol), but am a little stuck on getting it to start.
Here's the situation:
Once the car starts, it idles smoothly (800-900 RPM), accelerates, and drives great. However, to get it started consistently, I have to manually hold open the Air Flow Meter door (typically stick a hammer handle in it). When I do that, it starts right up, almost immediately dies, then I remove the hammer and it starts right up, I rev the engine, and from there it runs beautifully. The fuel system was completely overhauled by the previous owner, and I have tried the "pre-prime" trick (car in drive with foot on brake and turn key for anywhere from 10-30 seconds). I can hear the pump engage, but she won't start off of it.. Also, it is not just a cold issue - anytime I leave the car for longer than 10 minutes or so I have to do this, and even shorter than that is hit or miss if it will start on it's own.
Diagnostics so far:
1) Lots of vacuum lines/gaskets replaced and hose clamps tightened to check for extra air flow
2) Pulled fuel injectors and nothing seems amiss
3) Ruling out fuel pump and filter because I don't have any misfires/stalling issues when running hard under load (i.e. up a hill, etc.)
4) Check Valve in the air bleed valve is fine (can only blow one way)
5) Pulled the air intake system, replaced seals, poked around to see if anything looks off with it or the throttle (didn't deep dive but everything looked clean and in good working order)
6) Replaced ignition module (more to attempt to address a hot die-ing issue, but might be relevant)
7) Checked fuel pressure regulator for gas in the vacuum line. The regulator itself is rusty, but not leaking
8) Played around with idle screw and AFM Screw

A few questions and theories:
a) Experimented with unplugging the AFM...if running the car dies immediately when it's unplugged, but I'm not sure if that means anything
b) When I unplug the the vacuum line on the Fuel Pressure Regulator, the idle does not change at all...is that normal?

My current theory is that it's either the Fuel Pressure Regulator or the Air Flow Meter itself, but it's weird that I don't have issues while actually driving. Anything else I'm missing or should I just the parts and see if it helps?

Thank you!
Benjamin

 

Not leaking is fine.....but doesn't prove that the regulator is doing it's job !

I'd actually check fuel pressure and go from there. Or, replace the regulator if you have a mind to do so.

Cheers
DD

 

So the car starts if you open the air flow but is hard to start otherwise?

The fuel system provides fuel with that door open, which would be when the car is running. When starting it also provides fuel from the starting signal, so it sounds like you aren't getting that signal. In other words, you have fuel when running but no fuel for startup.

I wish i could remember more details, find yourself an electrical diagram and check out the circuit in question. I can't remember if the starting fuel is on the same relay or its own relay. If you don't have alligator clips and leads for your voltmeter you may need a buddy to turn the car over while you check the circuit

 

The 'start' branch of the fuel pump circuit operates off the starter relay. The 'run' branch of the fuel pump circuit runs off a switch inside the AFM. Appears that both are working in this case. (the engine does run, and the pump is heard operating when the key is turned to 'start')

There is a "cranking signal" input to the ECU (also from the starter relay). AFAIK this provides a small amount of enrichment for starting, hot or cold. AFAIK, again, a fault here wouldn't cause a no-start condition....but I can't swear to that.

Cheers
DD

 

Apart from the cold start injector, that operates for up to 12 seconds with its electric supply via the thermo-time switch, the normal cranking routine will see the ECU giving longer pulses to the injectors based on engine temperature. The Thermotime switch varies the length of time the cold start injector operates. As Roger Bywater says in his article on the XK FI engine; without the cold start injector functioning, the engine will eventually start from the fuel injected via the normal injectors, but obviously needs much longer, and this cranking can test the battery with a lump like the XK engine. All this assumes the fuel pressure is correct. If it's low then despite longer injector pulses the engine will struggle to start.

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

I think the pressure regulator may be suspect here, but I'd also check the cold start injector and thermotime switch too. Unlike modern systems, there are no electronic diagnostics at all, it is a matter of checking through the things that need to operate to start and run the engine. If the fuel pressure is low, it may be OK for low revs, slow speed stuff, so a blast up the road when no law officers are around may show the engine struggling at higher revs and higher loads.

 

Thank you all for your thoughts! Going to start with the Pressure Regulator and go from there...I'm doubtful on the cold start injector because it's an issue for a hot start as well, and I don't believe the injector even operates unless the external temperature is below a certain point (correct me if I'm mistaken, but we're averaging ~90 in the summer so I don't know that it would even flip the switch).

 

A few more vacuum lines and a replaced fuel pressure regulator (tip of the hat to Mr. Boger), and no change...I feel like I'm looking for something incredibly specific. The car idles and runs very smoothly, with no apparent acceleration or "under load" issues. The only issue I have is that when I start, cold or hot, anytime after about 10 minutes of sitting, I have to either crank/run the pump for a long time (only works within 15 minutes or so of shutdown, and only sometimes), or manually open the AFM door (at which point it will start right up)...

Only consistent solve is AFM "door", so assuming fuel
No running issues seems to rule out pump and filter
No running issues+replacements/checks seems to rule out vacuum leaks
Pump engages with starter relay (distinct hum), so seems to rule out obvious electrical
Hot or cold doesn't make a difference so I think that rules out cold start injector

Where else should I look/look again?

Thank you!
Benjamin

 

Hi Benjamin,

If you haven't already, try this.

Prise open the black rectangular box on top of the AFM with a screwdriver.
Inside you will see a long narrow circuit board with two distinct black arcs (the one with a saw tooth edge on one side).
Carefully rotate the pivoting arm with one hand. You can also hold the flap open with one hand if you still have access.
Now with a cotton tip dipped in isopropyl alcohol or rubbing alcohol dab the black strip immediately under where the two copper contacts have just been resting.
Don't use water or solvents as the strips are 30 years old and can't be replaced.
Use the other end of the cotton tip or another one with another helping of alcohol to wipe the entire black strip.
If the cotton tip gets dirty and it should, replace with another one.
A couple of applications should do.

I found that my idle was much better after performing this simple job.
Infact I re did this very task only last week (6 months since last done) as I noticed that my idle in traffic was a little erratic and this sorted my problem.
Even after six months there was a little 'dirt' removed.

Cheers,

Nigel

 

It would help to add the pictures before I submit. Sorry

 

Thanks, Nigel! I'm actually having the opposite issue - it idles great, but struggles to start. Will this procedure help with starting as well?

 

Not skilled in AFM's on this engine. That is a certainty. It sure does look like a complex device!!

So, critical thinking time. What function does the device provide. Added air at start?
My thought as manual opening seems to aid the start. Adding air to an over fueled engine fires it?


Carl

 

HI Benjamin,

In regards to the black strip on the board - 'The board is not a linear resistor. It has multiple outputs not just one. As the wiper moves across the board it reads both the X and Y position of the wiper, and then it relays where the wiper is through different pins. So, if you move or replace the board to correct say a worn spot, you are moving the curve of the X and Y axis, and not just looked at across the board. The ECU is watching a football game as all the fans in the stadium, not just the end zones. Move the ball in any direction and some fan is closer and some farther. That wiper movement changes how it performs. What Bosch did was put that AFM on some kind of special machine and then use a small laser tool to burn away a tiny bit of resistor on each section to get the X and Y axis to read just right as it moved down the board. You can see the small burn marks if you look for them.'

What has happened over many years of operation and particularly at idle and low throttle input is that those two pins on the wiper oscillate over the same small area and can create inconsistent readings for the system.

If you give the strip a clean and I forgot to mention very carefully passing something as thin as a cigarette paper under the pins between them and the black strip when the surfaces are dry, you should notice a little improvement in temperament.

It is a general maintenance item that should be done once in a while and with these cars as we know sometimes it is the little 1% ers which can make a difference.

The only other issue I can think of with yours is whether the flap gap has been set correctly?

Good luck,

Nigel

 

This procedure may solve my car's problem, I will give it a try and see.

Thanks for sharing

 

Hi Nigel,

Please could you ponpo with some arrow to show me the strips that need to be cleaned?

I will appreciate that demonstration.

Thanks

Mike

 

HI Mike,

That big black arcing black strip on the printed board with green rectangles (overprinted with black ones, but you don't need to touch them).

You can't miss it :-)

Cheers,

Nigel

 

Alright, so I replaced the AFM (didn't want to risk screwing something up when I opened it unless I had a backup, so now I can play with the old one if I want), but still no change.

What else should I look at? I have the start process down like clockwork now: remove air filter, insert hammer, crank (it starts immediately then dies...presumably too rich??), remove hammer, crank, and it starts immediately and runs great...then I put the air filter back on and go on my merry way.

Could there be something with the throttle? It's not gunked up, but the engine was rebuilt before I bought so possible not everything was adjusted correctly. I've thought about adjusting the AFM spring, but it looks roughly in the same position as the pictures y'all have posted...help please!

 

1. I still can not get my head around just what the AFM does. I do get the resistance points rather than a band of steady increasing or decreasing. But what does the variable volts do? Add fuel or add air?

Til that is clarified, the effect of inserting s hammer handle will remain a mystery.

2. But, just for a WAG, throttle opening. Butterly firmly seated at rest. No gap, no air til cracked open? Is that what the hammer handle process does ?
adds air, cures an over fuel?

Leaking injector/s

Carl

 

 

Perfect! Thanks. Makes absolute sense, "blows open", fuel added. And, it seems each resistor block tells "something" , how much to add. Hammer handle open, means add a lot!!!

1. If so, it seems to sub for a cold engine enrichment function.

2. Therefore, you can use the S57 to track the wires from the AFM and see where they go. ECU??? That doesn't seem to be an answer to your engines issue.

3. When cold just prime it with a shot of gas If it fires right up, it is a cod fueling issue,

4. Or, manually open the throttle and start it. should nt be needed if all is well. So, if it fires up, it may e an over fuel.

So, we look too much fuel or not enough. ONce determined, track the guilty one.

Carl , , .

 

When you say "prime it with a shot of gas" what is the best way to do that? I've never been sure on exactly what I'm supposed to spray where.

Thank you!