Hi All,

This is my first post on this forum, I am a new owner of a 1986 Jaguar XJ6. I picked this car up as a project a few months ago and discovered it had a bad head gasket. Have since completed a valve job, skimmed the head, installed new head gasket and a bunch of 'while you're in there' items like coolant hoses, vacuum hoses, sensors sending units etc...

The car now runs and drives and seems to perform quite well cold and warm. However, onto the topic of this post I just can't get it to idle smoothly/consistently.

Idle in the cold seems better/smoother and sounds more consistent to my ears, but when warm, it sounds more inconsistent and like a light misfire. When warm started the idle is smoother for ~5 seconds and then returns to the same inconsistent/slight misfire noise.

I have been lurking on this forum a while and read that a truly smooth idle is hard to achieve, I want to know if what I have is as good as it gets and whether the car is actually misfiring or if this is normal.

Here are three videos showing my car at idle.

Engine view: There is a noticeable tremor, not sure how well it comes across.

Exhaust view: This sounds like a light misfire, the exhaust pulses don't seem as consistent as my other older cars. However the pulse/absense of a pulse is not as severe as what i've observed when my other cars misfired.

Tach view: Needle does move around a little. But not too much. I also put the car in drive, that is why the rpm drops around half way.

Known issue:
Massive exhaust leaks, but only beyond the catalytic converter. The Y pipe and mufflers have many rust holes in them.

Here is what I have done:
1. Cleaned injectors, and verified spray pattern.
2. Spark plugs, wires, cap, rotor, DLB198 coil, removed ballast resistor, set pickup clearance at 0.015", timed ~10 BTDC (where it runs/revs best and cool)
3. Tried swapping AFMs, no effect.
4. Coolant Temp Sensor (Bosch OEM 2-wire)
5. Throttle shaft set at 0.002"
6. Smoke tested, extremely small leak found at throttle shaft
7. O2 sensor replaced + added ground wire.
8. Idle screw screwed all the way in. Cleaned internals of air distribution block.
9. Fuel hoses changed, changeover system fixed.
10. New battery + extra body ground added.

Pulled plugs and they looked consistent and normal, maybe a touch lean but not crazy.

I might have missed some things, but I think I have listed everything pertinent to the idle issue.

I am not looking for perfection, but is it safe to continue driving my car in this state? Is it misfiring or is this inconsistency normal? I don't want to be in the situation where I am washing down cylinder walls because of a misfire.

Apologies for the long post, I appreciate the patience. Thanks in advance for any opinions or guidance!

 

Congrats! Mine is an 86 and as many others, never got the smooth idle. There is post or FAQ from a guy that documented his many steps to try to achieve best idle. I will look for it or someone may beat me to it.

Randy

 

It wouldn't hurt to do the capacitor modification, there is lots in the Jag-lovers archives about it. The technical bulletin is here: https://www.jag-lovers.org/xjlovers/xjfaq/efiover.htm

 

Thanks for the reply, the bulletin seems to indicate that it is an overfuelling issue when cold or part warm hence, I did not do this. My car is fine running and driving in all conditions, cold idle is even a little better. But I will try and report back. My main concern is this is a misfire or not. The smooth idle would be a bonus.

 

I would try a few simple tests to see if you can detect any improvement.

If you still have CTS that you removed, with engine warm and the irregular idle occurring:
1. Remove the connector from the CTS and plug it in to the spare room temperature CTS. If the irregularity goes away, then your fuel idle circuit is too lean.
2. Remove vacuum advance hose from distributor and plug with golf tee. Pickup manifold vacuum from manifold and plug it into Distributor. See if you have any difference at idle and acceleration. There is a vacuum advance time delay valve, under the intake manifold that switches throttle body vacuum to manifold vacuum.
3. Timing at 3000 RPM is important to be correct (~33*), not Idle RPM as adjusted by timing. You can check at 3000 RPM with out disconnecting vacuum to distributor) Try book value for idle 14* or 17* for your county & model, and adjust idle mixture on AFM for best idle, reset idle RPM to book (700 or 800) then adjust idle mixture on AFM for highest RPM, then rest Idle back to book.

I had the problem for some time and after numerous actions like you have done, found pickup star rotor on distributor hitting pickup at idle (failing upper distributor bearing).
My final fix was to replace GM 4 pin Module in Ignition amplifier with Aftermarket Pertronix unit (Pertronix D72000 module). My spare amplifier has a new Delphi 4 pin module and I compared them both, and found the Pertronix unit had a smoother at idle. I am using the DLB198 ignition coil and opened my plug gap to .040 (book is .035)
Rgds
David

 

Thank you so much for the detailed explanation.

The car will not restart warm when plugged into the room-temp sensor. Although I could have tried different resistances along the temp sensor curve.

However, I think you were onto something with that vacuum delay unit. https://www.jag-lovers.org/xjlovers/...delayvalve.htm

What I did was connect the distributor vacuum advance to ported vacuum, which is the port above the throttle butterfly (not manifold vacuum). This explains why timing it at spec (17 degrees BDTC) would result in rough running when the vacuum was plugged in. The timing was advancing to 27 ish degrees BTDC, due to bad vacuum delay/regulator applying manifold vacuum to the distributor which I think is strongest at idle/throttle closed. Ported vacuum should be negligible at idle and should not alter timing once the vacuum line is plugged in.

It started raining before I could retime the car, I left off with the idle at 550-600 rpm due to overly retarded timing. It does seem a little smoother/less misfiring, but it was hard to tell at low rpm.

I will retime the car and report back.

I also checked the pickup and distributor shaft and there isn't much play or evidence of contact.

I also have a petronix unit on order and will also report back on these effects.

Thanks again.

 

additionally,

4. The vacuum advance on the distributor of an older car, can be failing with a hardening diaphragm. Replacing the vacuum advance unit is a good rainy-day project, to take one more thing off the table for years.

5. If you have a weak cylinder, you may not be able to get a “perfect” idle… See quote from recent E type forum post.

V12 top end tapping problem solved

Quote ……I set the clearances absolutely spot on and left it for a week while waiting for the gearbox to come back and decided to double check the clearances to find that 2 of them (rear left inlet and rear right exhaust, were now a bit off from where I’d carefully set them! After long discussions with several very helpful expert garages, I was recommended to do a “draw down” test. For anyone (like me) who has never heard of them, they are available for less than £20 and is one of the best investments I have made. Screwing it into the spark plug hole (when each piston is at TDC) and introducing compressed air it not only gives a reading of how leaky each cylinder is, you can actually hear where the air is going. If you can hear it coming out of the exhaust, then it’s an exhaust valve problem. If you hear it out the carb it’s an inlet valve, if from the crank case breather, it’s the piston rings and if it pressurizes the radiator then you have a blown cylinder head gasket. All from a simple tool! In my case the left hand rear exhaust valve and the right hand rear inlet valve were both poor (20% leak on the gauge)….. Unquote

Rgds
David

 

I will start looking for a vacuum canister. I did set all valve clearances at 0.013” when reassembling the head. Given that the cold idle is smooth would indicate to me that the compression is probably ok. Cold, rich engines probably benefit from the extra timing advance.

the weather is supposed to be better tomorrow and I will try to get the car retimed and install the Pertronix d72000. If the idle is still poor, I will invest in a leak down tester and do that test.

Thanks again,
Abhishek

 

One more device that is one way when engine is cold and another when engine is hot…
When the engine is warm and the idle irregular, bypass the “Temperature Valve” with a length of vacuum
hose to Fuel Pressure Regulator; apply and remove manifold vacuum to Fuel Pressure Regulator, to see if there is any
change in idle smoothness, with the higher or lower fuel pressure.

…..The Fuel Pressure Regulator sees manifold vacuum when the fuel rail is cold, and
when it’s hot the “Temperature Valve” closes off manifold vacuum and vents the vacuum line
of the Fuel Pressure Regulator, allowing fuel pressure to rise in the Fuel Rail…

My files are not merged, so I missed this one at first look.
Rgds
David

 

The weather let up a little for me to retime the car tonight. The idle is a lot better. My rather unscientific test for idle smoothness has been to loosen the AFM rubber mounts and see how much it shakes at idle. Before bypassing the valve, the AFM used to shake quite a bit at idle with the mounts loose. Now it is down to a minimal vibration. I still hear a little inconsistency through the exhaust, and can very mildly detect uneven pulses with my hand although again this is much much better than I saw before.

Next week, I will do a fuel pressure test to check the function of that temperature valve. Will also install the D72000. Overall though I am so much happier with the idle now. Thanks again for the help!

 

I did a few different things today:

1. Pulled the plugs and a few cylinders are now showing clearly lean with ashy plugs.
2. Fuel Pressure test (All readings taken at the cold start injector hose with regulator on the rail)

• Pump will get up to 40 psi, when put in D and priming it. However the fuel pressure drops back down to 10 psi within 2 minutes. Is this a bad pump? Is there a check in the Lucas pump? Or is this a bad regulator?
• Fuel pressure while idling is ~29 to 30 psi. Seems like this is on the low side as well.
• Car runs better when removing the regulator vacuum line and plugging it. The fuel pressure with the line plugged is 38psi.
• I get more or less the same readings with the temperature vacuum device on the rail bypassed. (Bypassed it by plumbing manifold vacuum straight to the regulator) around 29 - 30 psi.

I also richened the mixture about 1.5 turns on the AFM just to be on safer side and run a little rich seemed to have a marginal effect. The mixture screw is now about 2 turns from completely bottoming out.

Please let me know how to interpret these issues. Should I get a new pump/regulator or check valve? Anything else I should check?

 

Between Plugs and Fuel pressure readings, I think your slight mixture richening is appropriate. The fuel temperature in the rail is what the Temp Valve is supposed to manage. Once fuel starts flowing, and cool fuel from tank gets to fuel rail, the Temp Valve will lower the FPR pressure.

The dropping pressure after the pump is shut down is regulated by the Air Bleed Valve/and One-way Check Valve, holding pressure in the fuel supply line. It is located in the trunk, next to the fuel filter. Your bleed down numbers are better than most jaguars of your age. The pump can overcome this slight leaking back of fuel to the tanks in normal operation.
Some have added a fuel one way check valve in the fuel line just after the Air Bleed valve, as this valve is only available used. Some have taken it apart for cleaning, and reported improvement. This has no effect on your fuel mix at idle.
I have worse leak down numbers, and will wait till next filter change to install the one-way check valve.

Pump and Regulator seem to be fine from your numbers.
Rgds
David

 

The fuel pressure regulator is an absolute pressure reference. That's why it has the vacuum line, as it references the amount of vacuum to keep the pressure differential between the atmospheric pressure and manifold vacuum the same no matter if idling or full load.

Running rich hides all sorts of sins, but isn't necessarily the proper way to tune the engine, as it loads up the catalytic converter and uses more fuel. I'd want to measure the AFR with a modern wideband gauge and see where you are.

 

I don't see running slightly rich at Idle loading up the Cat ... The O2 sensor & ECU should be giving correct AFR during acceleration and cruising??
Rgds
David

 

Today I put my smoke source down the dipstick tube to figure out if I have any leaks in the crankcase. Found that the crankcase breather cap was leaking. I swapped it out with a new one I had. No real change.

I think given that my plugs clearly show a lean condition, the safer thing to do would be to richen the mixture. I agree that this could make it over rich, I will read the plugs again after I run it a few days. I also filled the tank with some injector cleaner maybe that will help a little. If I still can't sort it out after some use I might invest in having a wideband installed. I have one on my 78 320i and they are certainly very useful to keep the k-jetronic system tuned.

One more thing that I noticed is that the car settles at a slightly different rpm every time I rev it. I wonder if my weights are sticking. Is there an easy way to lubricate the weight pivots? Can I put a few drops of oil under the rotor on these distributors?

 

"One more thing that I noticed is that the car settles at a slightly different rpm every time I rev it. I wonder if my weights are sticking. Is there an easy way to lubricate the weight pivots? Can I put a few drops of oil under the rotor on these distributors?"

Remove Cap, rotor and spark shield. Clean out the junk with some brake cleaner, blow it dry with compressed air, pull the felt plug in the center of the distributor shaft. Fill the hole with some synthetic engine oil, put the felt plug back in and add a few more drops of synthetic engine oil to the felt. Put a few drops of synthetic engine oil into gaps in the advance mechanism and the shaft.

Ideally you would disassemble the distributor down to the shaft for cleaning and lubrication, but this is very dangerous, as it is very easy to stretch the springs and break the small plastic spacers during disassembly; these parts are not available anywhere. I did a complete tear down on several distributors following a detailed article from a Triumph/MGB owner on their forum. I could not find a manual for XJ6 distributor maintenance or overhaul from Jaguar or Lucas.

Others may have a spray lubricant technique to get oil to the weight pivots points with just the spark shield and rotor removed.

Either a hang up of the Vacuum or Centrifugal advance mechanism, can cause the different idle RPM you are experiencing.
Rgds
David

 

"One more thing that I noticed is that the car settles at a slightly different rpm every time I rev it. "

Found another checklist for XJ6 S3 EFI system diagnosis.
Check items 19 & 20.... The over-run valve is located between throttle body and extra air valve.


Rgds
David

 

Today, I did a couple of more things:

1. Tested vacuum advance canister and it seems to hold vacuum and work smoothly. I have a new vacuum canister on order and will install it when it arrives.
2. Lubricated and cleaned all of the distributor internals and set magnetic pickup air gap again, and checked clearance on all trigger points.
3. Installed fuel check valve. Starts are nearly instant now.

The sticking idle problem has reduced, but still there there is 100 rpm ish variation in where it settles. Will check the overrun valve next.

 

I adjusted the overrun valve, there is a lot less rpm float now which is great. I also opened up the Airflow meter plastic cover and cleaned the rheostat contacts. The contacts make a small groove in the track and there were a lot of debris on the track which I removed with rubbing alcohol. This also improved idle quality a little.

The sticking idle is still there. It settles anywhere from 800 - 950 rpms. I also lubricated the AFM flap a little but could not directly reach the bearings. The flap seems to move freely, there are no tight spots.

I think the car is running much better than it did at the start of the thread and I appreciate all the feedback and suggestions very much!

I think I am reaching the point of diminishing returns now and probably should leave the car as it is. I might be expecting too much out of 40 year old car that hasn't lived the easiest life.

 

Time to enjoy now, you are ready for a road trip! Jaguars always get better with spirited driving. They are not driveway queens.
Your results were well documented, and will help others in the future as they inherit a family Jaguar, and begin the adventure.
Rgds
David