XJ6 & XJ12 Series I, II & III 1968-1992

85 XJ6 4.2 Poor Idle/Dies at Idle

 
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Old 06-10-2019, 10:15 AM
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Default 85 XJ6 4.2 Poor Idle/Dies at Idle

I've had my 85 Vanden Plas about five years and it has generally run fine. Lately though it started idling mildly roughly most of the time and now with A/C season in full swing its sometime dying at idle. There are also random drops in idle speed when warmed up that feel like dead misses or nearly dead misses. It runs fine off idle.

I've got some experience with Bosch L-Jetronic systems and with this Jag in particular and I'm running out of ideas for troubleshooting. I'm hoping for some BTDT experience from the grouip. Here's what I've found so far:

Tested Good:

Using a Snap-On Counselor II Engine Analyzer (circa 1990) and a dual channel Tektronix oscilloscope I've tested the AFM and the ignition system. Both seem rock solid. There's a good spark ignition on each cylinder, even when the misses are occurring. I did replace the original ignition system with a Pertronix about six months ago, but again, the scope shows no issue with ignition (at least based on my interpretation). I initially suspected the AFM was dropping out and that was causing fueling to drop, but using the scope the AFM signal looks great both manually pushing it full range and when running while misses are occurring. I see no drops in signal when the problem is occurring and generally it just gives a nice steady voltage that varies appropriately with flow.

I'm getting 17 in Hg idle vacuum (cheap 20 year old Sears gauge) pretty steady all the time. Tried spraying fuel (carb cleaner) at all the likely vacuum leak sources and get no response from the engine. Doesn't seem to be vacuum leak related.

Checked connections at ECU, at grounds on water plenum, white multipin connectors at back of engine, at injector power resistor module. No issues detected.

Adjusted bypass air flow at AFM (idle mixture) using O2 sensor reading to get approximately 0.5V at idle, but have new O2 sensor ordered just to make sure that's right. Its set roughly where it was to start and where most sources (Jag and otherwise) say it should be (number of turns from closed).

Tested fuel pump pressure. Right on target, steady and reacts properly to pulling the vacuum line off the regulator.

O2 sensor seems to be responding properly, but more on that later.

Idle speed is about 900-1000 in park with A/C off. Drops to about 600-625 in Drive with A/C on high.

Valves were adjusted about 4-5 months ago.

Not yet checked:

Charcoal canister connections and wiring.

What's Suspicious:

O2 sensor seems good and switches from rich to lean properly off idle and even at idle mostly. However, it is 4-5 years old, 15,000 mi, so I've ordered a new one just to double check.

All the injectors fire and I've hooked the scope to 2-3 of them and checked the injector voltage trace and they all look good. BUT, I've never had them off the car and therefore they are possibly not in good condition. However, since the problem moves around randomly from hole-to-hole I've been assuming its not injector related. I was hoping I would find the problem was always the same one or two injectors, but this didn't pan out.

The biggest clue comes from the Snap-On analyzer which has a "burn time" test that shows very low burn times (but not zero) on random cylinders when the problem is occurring. By monitoring the spark voltage it can detect when combustion starts and stops. "Good" burn time is around 15 msec, less than 10 msec is not good and I'm seeing them as low as 5 msec. According to the the Snap-On manual this can be several things (all I've checked above) or basically lean mixture/"low fueling". I thought it might be a problem injector, but the low burn time seems to not be limited to a specific injector, although there's four that seem more likely to do it than others. I thought the most likely thing was momentary AFM dropout causing the ECM to reduce fueling, but as I said I see no sign of that on the o-scope. The other clue is it only happens when the engine if fully warm (10-15 min) well past when its moved off of fast idle. This seems to imply something has to get "warm" to create the problem. This leaves me with the ECU, but there seems to be no way to validate that other than swapping it.

Question: Is there any known issue with the ECU that causes this problem? Any sources for known good ECUs to swap in?
Question: Any other thoughts on what I should check?
Question: Any other tests I can run on the AFM? I was monitoring down to 1 msec and didn't see anything.
Question: Thought I might just replace the injectors anyway just to eliminate them. Any recommended source for reconditioning? I saw Jaguar Fuel Injector Service website and was going to use him.

Thanks for any advice!

Ben....
 
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Old 06-10-2019, 10:24 AM
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do the Coolant Temp Sensor test as Doug reccomends.
 
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Old 06-10-2019, 11:29 AM
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If you do decide to get the injectors cleaned - yes SD Faircloth at Jaguar Fuel Injector Service is your man.
 
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Old 06-10-2019, 01:54 PM
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Originally Posted by Jose View Post
do the Coolant Temp Sensor test as Doug reccomends.
OK, I replaced it a few years ago, but its worth checking again.

Thanks, Ben...
 
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Old 06-10-2019, 02:40 PM
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Originally Posted by pjprofili View Post
If you do decide to get the injectors cleaned - yes SD Faircloth at Jaguar Fuel Injector Service is your man.
Thanks, that confirms what I was thinking. Ben....
 
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Old 06-10-2019, 02:46 PM
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I've owned my (1983 build) 1984 XJ-6 since 1989. I have never cleaned its injectors, the engine runs fine. I believe injector cleaning is not neccessary.

are you familiar with AJ6 Engineering? They have an excellent article about the fuel injection system and its symptoms.

I have a link to it in my website in the Contact / Manuals page.
 
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Old 06-10-2019, 08:39 PM
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Originally Posted by bjzwissler View Post
I've had my 85 Vanden Plas about five years and it has generally run fine. Lately though it started idling mildly roughly most of the time and now with A/C season in full swing its sometime dying at idle. There are also random drops in idle speed when warmed up that feel like dead misses or nearly dead misses. It runs fine off idle.

I've got some experience with Bosch L-Jetronic systems and with this Jag in particular and I'm running out of ideas for troubleshooting. I'm hoping for some BTDT experience from the grouip. Here's what I've found so far:

You've covered a lot of ground! Good work.

One thing I didn't see mentioned, though, was setting the throttle blade gap (.002") and making sure the throttle body bore was shiny-clean. Not high-tech, but important ! I consider it a first step in any 'sometimes dies at idle' or other peculiar idle situations

Cheers
DD
 
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Old 06-10-2019, 08:41 PM
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Originally Posted by pjprofili View Post
If you do decide to get the injectors cleaned - yes SD Faircloth at Jaguar Fuel Injector Service is your man.

Yup! Great guy, great service. I've sent many sets of injectors to him, even non-Jaguar injectors

Every Jag I've owned has had some sort of idle quality weirdness at one time or another. Cleaning the injectors has always helped.

Cheers
DD
 
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Old 06-11-2019, 06:23 AM
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Jose,

Thanks for the reminder about the AJ6 site. I had seen it, but went back and reviewed it again.

Doug,

I'll check the throttle plate gap. I had seen that in the manual, but had not put it high on the list as this problem came on rather suddenly. I do think I'll get the injectors cleaned at some point, though I don't think they're the cause of this problem. This car had sat with little use for a some time before I got it and I'd like to know that they're known "good".

Thanks, Ben....
 
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Old 06-11-2019, 01:24 PM
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Hi Ben,
Just two things I thought of from reading other threads here; (1) when checking for vacuum leaks / spraying, did you check around the intake mounting gasket to the head ?, also sealing around the mounting bolts for the igniton amp if still in place or the holes plugged if it's not ? (2) Maybe some of the control wiring going to the coil, I had an 1985 XJ6 that had intermittent running issues caused by the internally corroded coil wiring harness to amp. If all else maybe try my version of the old school "Italian tune-up" = tank of NON-ethanol + heavy sea-foam, empty road, car in 2, drive her hard to high rpm ( don't red line / try to blow it up though ), let off and engine brake back down, repeat several times. That's "cleaned the soot" out of my 1999 Accord V6 "Jaguar support vehicle" when it would not idle / kept stalling when stopped. Or maybe she just got jealous of the Jag getting all the attention ! What ever it was, it "fixed itself" !

Really interested to see what you find is the culprit, good luck !
Cheers,
Brian
 
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Old 06-11-2019, 07:50 PM
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An update. A few issues found, but problem still not gone. I checked the throttle plate. The 0.002" clearance was fine, but there was quite a bit of dirt. Also did a circuit check on the throttle position sensor. It passed, but with some odd results. The was some resistance between open and closed, not a clean 0 ohms to infinite ohms transition. Decided to remove it and check it. Turns out there was no cover on the sensor and the contacts were filled with dirt and oil (see pic)

So, anyone know where I can get a cover or a replacement switch? After cleaning it worked fine and I reinstalled it. Whether it was cleaning the TPS or cleaning the throttle, I had to reduce the bypass air to bring idle speed back down to spec after reassembly. I was hopeful this would solve the problem, but after a thorough warmup the problem returned. I still get hard misses at idle, though the affect of the problem is not as pronounced. It was not as hot here today though so I'll have to wait for mid 80s day before I can say for sure.

Remaining on my list is a new O2 sensor, and I also checked the coolant temp sensor. It was reading about 190 ohms at about 210 coolant manifold temp (after engine shutdown). The spec range is quite wide, but this seem it might be a bit low. I'll order a new sensor as they're pretty cheap anyway.

When I get the new O2 sensor I'll check the calibration of the AFM bypass air screw again. Lots of sites say to adjust to read about 0.5V on the O2 sensor at idle with the O2 sensor disconnected. However, the function of the TPS idle switch is "idle enrichment". Any better suggestions on how to set the bypass screw? I think I'll target 0.5V with the O2 sensor AND the TPS disconnected and then see where the mixture goes when the TPS is reconnected.

Next to check is the vacuum lines for the evap canister and the distributor advance.

This problem is like many I've had --- its not just one cause. It seems with all old cars that by the time the problem has become bad enough to troubleshoot its being caused by multiple problems.

Thanks for your help, Ben....
 
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Old 06-12-2019, 01:20 PM
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I did replace the original ignition system with a Pertronix about six months ago, but again, the scope shows no issue with ignition (at least based on my interpretation)

Which Pertronix unit did you use? full distributor or just replace points? Also, which coil? The 3.0 Ohm coil will not give same spark as OEM system which called for 1.5 Ohm coil. The pertonixs distributor must be matched to coil (although other brands will work fine). Pertronix distributors are set at 8-10 degree advance (with EFI use maximum) and points replacement ignitors are set at OEM static advance setting.

My 84 XJ6 had same symptoms and many attempts to correct, My problem was OEM distributor worn upper bearing, allowing rotor to hit pick-up.

Other trend, just seems to be too lean at idle??

Rgds
David
 
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Old 06-13-2019, 06:09 AM
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David,

Thanks for the response. I replaced the dual pickup coil and amplifier while leaving the original distributor itself. I also used a Pertronix coil just to be sure it was compatible.

I do plan to recheck the distributor, cap and rotor just to make sure they're OK although the oscilloscope readings would say that's not that the problem.

Yes, based on the analyzer saying there are randomly low burn times that indicates (according to the Snap-On manual) that there is low fueling. Given the randomness of which cylinders are having the problem I'm looking for a cause that could affect any cylinder. So not an individual injector or other mechanical fault that would impact a specific cylinder.

Ben.....
 
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Old 06-13-2019, 02:56 PM
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When I get the new O2 sensor I'll check the calibration of the AFM bypass air screw again. Lots of sites say to adjust to read about 0.5V on the O2 sensor at idle with the O2 sensor disconnected. However, the function of the TPS idle switch is "idle enrichment". Any better suggestions on how to set the bypass screw? I think I'll target 0.5V with the O2 sensor AND the TPS disconnected and then see where the mixture goes when the TPS is reconnected.

Ben,
The O2 sensor is not heated, so not sure you are going to get accurate reading at idle? When engine is warm, try turning mixture adjustment one full turn in, wait 30 seconds, then two turns out to see if you get any change in idle quality. If you temporarily installed a heated O2 sensor this may work...
Rgds
David
 
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Old 06-15-2019, 04:42 PM
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Continuing troubleshooting and may have found a clue. The low fueling seems to be occurring almost exclusively on cylinders 1, 3 and 5 only. So that begs the question what do they have in common. Its not the power resistor as it groups cylinders 1, 6 and 5 and 4, 3 and 2 together. ECM? I know the ECU has two injector drivers but I don't know what cylinders are assigned to each one. Cylinders 1,3 and 5 are Pins 31, 14 and 33, while 2, 4 and 6 are 30, 32 and 15. So nothing logical there. Anyone have any knowledge of the internal ECU circuitry?

Thanks, Ben...
 
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Old 06-15-2019, 05:50 PM
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The biggest clue comes from the Snap-On analyzer which has a "burn time" test that shows very low burn times (but not zero) on random cylinders when the problem is occurring. By monitoring the spark voltage it can detect when combustion starts and stops. "Good" burn time is around 15 msec, less than 10 msec is not good and I'm seeing them as low as 5 msec. According to the the Snap-On manual this can be several things (all I've checked above) or basically lean mixture/"low fueling"

Ben

I am thinking that they are all low fuel, 1, 3 & 5 are just lower. Can you adjust idle fuel while monitoring Burn Time, until you get Burn Time on three lowest up to 10 msec??? make precise adjustments (ie. 1 turn, 1/2 turn so you get back to stating point, if this is no help.

Rgds
David
 
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Old 06-18-2019, 08:23 AM
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David,

That's a decent idea, but I've already run with a pretty wide range of bypass screw settings. Very small openings (~ 2 turns) cause some hesitation when warm and transitioning off idle. Anywhere from 5-7 turns seem to run about the same but I do see variation in average O2 sensor readings. I haven't found a great way to decide the optimum position for the screw so I'm open to suggestions. The documentation says that the throttle position switch is used to add a rich bias at idle and WOT, but I don't know how rich and if its feedback based it shouldn't matter a whole lot where the screw is as the feedback will work to correct any error.

There was also a good comment about the O2 sensor being unheated and that the idle values may not be reliable. This is a valid concern, but what I've found is that with the engine fully warmed up, in gear, with A/C on full its warm enough that the O2 sensor is operating and the ECM is functioning in closed loop. In my experience the O2 sensor heater is critical to getting the sensor working fast from a cold start, but not as critical for a running engine, under some load in warm conditions.

I've decided to try swapping ECMs and have an alternate one on the way. Crossing my fingers that's it.

Ben....
 
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Old 06-18-2019, 09:55 AM
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Originally Posted by bjzwissler View Post
David,

That's a decent idea, but I've already run with a pretty wide range of bypass screw settings. Very small openings (~ 2 turns) cause some hesitation when warm and transitioning off idle. Anywhere from 5-7 turns seem to run about the same but I do see variation in average O2 sensor readings. I haven't found a great way to decide the optimum position for the screw so I'm open to suggestions.

Exhaust gas analyzer



The documentation says that the throttle position switch is used to add a rich bias at idle and WOT, but I don't know how rich and if its feedback based it shouldn't matter a whole lot where the screw is as the feedback will work to correct any error.

I don't know for sure but 8-10% enrichment is the number I've seen bandied about. I'm not sure if that's within O2 sensor correction range. The feedback system itself is just a trimming device. The primary mixture control device is still the AFM

Cheers
DD

 
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Old 06-19-2019, 06:45 AM
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Doug,

Thanks for the input. Yes, an exhaust gas analyzer would be the ideal solution, but the old one's seem to be hard to get working and new one's are >$3k so I'm not likely gonna get the chance to try that. I wonder how the shops that offer AFM rebuilds do the calibration?

Agree that the AFM is primary input that determines fueling. Generally as long as the ECU can still achieve a rich/lean switch the system is still in closed loop and able to maintain the desired AFR. Mine is still switching both on idle and off idle so my assumption is both the AFM and bypass screw calibration are not too far off. It does appear that the bypass screw setting has an affect on the average of the O2 sensor output, but its pretty noisy. Some of the VW sites on how to tune their L-jetronic systems recommend disconnecting the O2 sensor from the ECU, forcing open loop fuel control, and then adjusting the bypass to get near stoichiometric (0.5V) at idle and then adjust the internals of the AFM to do the same off-idle. I think I'd have to disconnect the TPS to disable idle enrichment to do that, but then that should get the fueling pretty close to neutral, making it easy for the ECU to compensate with the O2 sensor feedback.

My new, used ECU is supposed to arrive today so I'll see if that has an affect on the problem.

Ben....

Ben...
 
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Old 06-21-2019, 12:15 PM
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SUCCESS! The ECM didn't solve it.... Disappointing but not a huge surprise. That made go back and revisit all my thinking. The low burn times were still the primary sign of a problem. I went back to the Snap On manual and they said ignition could be the cause. I originally discounted that because the other ignition tests showed good and I'd just installed a Pertronix Ignitor and coil six months ago. So I decided to start with replacing the coil and when I did the car wouldn't start at all. The Pertronix coil was 3 ohm so needed no ballast, the new coil required a ballast, but when I put in a lower resistance ballast it worked. I began to suspect supply wiring to the coil so I jumped battery + to the coil positive and the burn times all increased dramatically and no more misses! Root cause found, now where's the wiring problem?

The firewall connector looked great internally. The connector from the right fender well to the engine was a mess. Took it apart and cleaned it and everything else around there and all was well briefly. A longer test ride showed the problem coming back, and once again jumping straight battery power to the coil fixed it. Still searching for the best way to find/fix or jump around the problem. Gotta believe its between the firewall connector and the engine, just sure where.

Also found through this process that the oil pressure switch was leaking badly -- causing the drips I had been wondering about but not doing anything about. The oily mess that was the electrical harness around there was likely the problem, but I can't say for sure. I also retaped a lot of harness and cleaned everything I could find.

Anyway, thanks for all the help and suggestions. It kept me motivated and helped point me in the right direction. Between my 280Z, TR8 and the Jag I think I've learned about all there is to know about L-jetronic, but I'm always surprised... and this time it wasn't the fuel injection at all!!!

Ben....
 
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