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

series 3 1986 4.2 (auto) wont rev.

 
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Old 06-12-2019, 04:00 AM
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Default series 3 1986 4.2 (auto) wont rev.

Hello everyone,

I'm wondering if anyone can help me with a problem i'm having with a series 3 1986 xj6 4.2.
From cold start, It will rev in park fine and idles nicely, However after about 3-5 minutes of running at idle it no longer will rev up, it still idles ok but as soon as i try to rev it, It splutters and coughs and backfires in the intake. I can turn the car off and then back on 10 minutes later and get nice idle, good revving until it gets warm again and then proceeds to cough and splutter again. Thats the issue. (I haven't driven it as it seems silly as ill get stuck out on the road if it dies).

Now, whats been done is fairly extensive so hopefully i can get all the information in here.
Its had:
A new MAF (it says its been calibrated on the sticker....?).
A new fuel pressure regulator.
All new spark plugs.
All new leads and coil .
New distributor cap and rotor.
All the injectors have been removed and professionally cleaned and have good spray patterns.
Im getting 36psi-40psi of fuel pressure at the feed pipe to the rail at idle (both hot and cold and when revving in park).
Its had the tanks drained (they had the water ingress issue) and has had new fuel and filter (pumps ok as good pressure).
Its had the vacuum control valve replaced (the interconnect between the brake booster,inlet manifold and the vacuum assist for the distributor advance).
Earthing strap next to exhaust downpipe good as are all the engine bay relays and fuses have all been checked.
No CAT so no 02 sensor so nothing to check there.
Both dry and wet compression test carried out, All cylinders around 120psi with not much difference between them.
New Auxillary air valve fitted.
Coolant temp sensor tested good and is giving good readings (around 260ohms when hot if memory serves).
Ignition timing set to 6 degrees BTDC at idle without vacuum (I have tried it at 10 degrees BTDC but nothing changes the hot/cold issue)
New cold start injector fitted.

This is everything i can think of, Anyone have any ideas? or know of any other common issues with these models that could point me in the right direction? It happens every time its like clockwork, To me it seems like something thats switching due to thermal differences is causing this because it runs perfectly when cold.

Thanks
 
  #2  
Old 06-12-2019, 07:01 AM
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Originally Posted by abrp2020 View Post
This is everything i can think of, Anyone have any ideas? or know of any other common issues with these models that could point me in the right direction?
Thanks
Yes. Rusty/contaminated fuel tanks

I know you replaced the fuel filter already but remove it and empty the contents into a clean jar. What you see will dictate your next steps. Takes but a few minutes and costs nothing!

Also, were the in-tank fuel screen cleaned or replaced when the fuel was drained?

What's the history on this car?

Cheers
DD

 
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Old 06-12-2019, 03:29 PM
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Originally Posted by Doug View Post
Yes. Rusty/contaminated fuel tanks

I know you replaced the fuel filter already but remove it and empty the contents into a clean jar. What you see will dictate your next steps. Takes but a few minutes and costs nothing!

Also, were the in-tank fuel screen cleaned or replaced when the fuel was drained?

What's the history on this car?

Cheers
DD
Hi, thanks for your response,

No the tanks haven't been cleaned as such, I flushed them through with some fuel but they haven't been replaced, I will check the filter again, I've replaced it a couple of times now, I did drain some fuel yesterday and it was a little dirty, Only thing is, I'm thinking if it was fuel quality related that the fault would occur no matter the temperature of the engine? I can't see the vehicle acting fine when cold and then not when hot if it was the actual fuel. I will double check this though thank you.
 
  #4  
Old 06-12-2019, 08:28 PM
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If the fuel is a little dirty at the pump and filter then it might be very dirty at the pickup screens at the bottom of each tank. What can happen is the crud settles away from the pickups if the car sits for a few hours....but is drawn back into the pickups after running for a bit.

Of course the problem could well have another cause but, personally, I wouldn't go any deeper into the weeds until you're 100% about the fuel situation

Cheers
DD
 
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Old 06-12-2019, 08:47 PM
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Originally Posted by Doug View Post
If the fuel is a little dirty at the pump and filter then it might be very dirty at the pickup screens at the bottom of each tank. What can happen is the crud settles away from the pickups if the car sits for a few hours....but is drawn back into the pickups after running for a bit.

Of course the problem could well have another cause but, personally, I wouldn't go any deeper into the weeds until you're 100% about the fuel situation

Cheers
DD
Hi Doug,

Thanks mate,
Yes it's been a concern on my mind but I must admit I put it to the back of my mind when I saw the consistent fuel pressure at the rail, I would THINK that if there was some restriction that I'd loose pressure as the crud fills the pickups at the tank? I will however have a look at that first.

I've also read that the ignition modules can be a problem? Any thoughts on that?

Thanks.
 
  #6  
Old 06-12-2019, 09:49 PM
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Ignition modules (AC Delco D1906) are known for failing, yes

Most commonly, but not exclusively, they fail after absorbing a lot of engine heat.....like after 45-60 minutes of driving...and return to normal function after cool down. A module failure obviously would hurt ignition but, potentially confound the fuel injection as well. The fuel injection ECU takes a 'trigger' signal from the ignition module

There are some module tests out there but, naturally, they're only meaningful if the tests are performed at the same time the engine is misbehaving. Thus they're most commonly replaced as a guess.


Cheers
DD
 
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Old 06-12-2019, 10:46 PM
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Originally Posted by Doug View Post
Ignition modules (AC Delco D1906) are known for failing, yes

Most commonly, but not exclusively, they fail after absorbing a lot of engine heat.....like after 45-60 minutes of driving...and return to normal function after cool down. A module failure obviously would hurt ignition but, potentially confound the fuel injection as well. The fuel injection ECU takes a 'trigger' signal from the ignition module

There are some module tests out there but, naturally, they're only meaningful if the tests are performed at the same time the engine is misbehaving. Thus they're most commonly replaced as a guess.


Cheers
DD
Hi Doug,

Thanks for your help, I've looked at the coil that's been replaced, it says on the coil (C80R) to use with resistor, I'm sure this means the ballast resistor? It isn't fitted with one, should this been done?

Thanks
 
  #8  
Old 06-12-2019, 11:04 PM
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Most if not all '86 4.2 models (certainly all the USA market '86s) had a ballast resistor but the system seem to work fine without 'em.....as over the years a few have reported tossing 'em over the hedge when they failed and no problems afterward

The system should work Ok with any 12v coil havng about 1.0 ohm primary resistance. I couldn't find the spec on the coil you mentioned. I had best luck on the 4.2s by using a Lucas DLB170...which is actually a V12 coil...without the ballast resistor

Some vendors list coils that are wrong for the Lucas CEI system. Catalog errors. Often a coil for the older OPUS system is listed. As I recall they have much higher primary resistance.

Anyhow.....

Coils often fail in the same way as the modules....after absorbing heat.

If you have an ohm meter check the primary resistance when hot. And the secondary resistance should be about 8k ohms like most any other old style coil.

Cheers
DD
 
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Old 06-13-2019, 12:31 AM
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Originally Posted by Doug View Post
Most if not all '86 4.2 models (certainly all the USA market '86s) had a ballast resistor but the system seem to work fine without 'em.....as over the years a few have reported tossing 'em over the hedge when they failed and no problems afterward

The system should work Ok with any 12v coil havng about 1.0 ohm primary resistance. I couldn't find the spec on the coil you mentioned. I had best luck on the 4.2s by using a Lucas DLB170...which is actually a V12 coil...without the ballast resistor

Some vendors list coils that are wrong for the Lucas CEI system. Catalog errors. Often a coil for the older OPUS system is listed. As I recall they have much higher primary resistance.

Anyhow.....

Coils often fail in the same way as the modules....after absorbing heat.

If you have an ohm meter check the primary resistance when hot. And the secondary resistance should be about 8k ohms like most any other old style coil.

Cheers
DD
Yes I've heard the same thing, I've looked up the specs, it says it's a 12v coil and it has around 1.0ohm of resistance across the primary. The only thing I did notice, I'm not sure what my output from the ignition amplifier should be but with the coil disconnected, under cranking I'm getting no voltage at any of the terminals that go to the coil, with the coil connected up and the engine idling, across the coil primary I'm seeing around 0.6v to 3.5v depending, does this seem low? I was expecting to see up around 8-12v or something like that, it's hard to know without a workshop manual. I'm pretty confident the fuel is good up to the rail as I'm getting clean fuel up there and good pressure, I'm leaning towards an ignition issue but racking my brain to think what might change from cold to warm/hot. It just won't rev when it gets to a certain temperature, with the vehicle revving nicely when cold I think we can assume the intake system is doing its job, and I'm getting fuel. Unless there's a sensor or switch somewhere thats effecting it. I didn't mention but I am running this vehicle off a jumper battery/cables as the main battery has failed, not sure that'll change anything though.

Thanks
 
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Old 06-13-2019, 06:39 AM
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Get a new battery, for sure. And make sure both main grounds are clean, as well as two the "+" posts on the firewall

Cheers
DD
 
  #11  
Old 06-14-2019, 01:58 PM
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No CAT so no 02 sensor so nothing to check there.

I thought all EFI models have O2 sensor for modifying fuel mixture above the idle fuel circuit ??
If you have "Y" exhaust down tube without a fitting for O2 sensor, it may have come from Carb'd model XJ6.

Ignition timing set to 6 degrees BTDC at idle without vacuum (I have tried it at 10 degrees BTDC but nothing changes the hot/cold issue)

My manual is showing 17 degrees BTDC at idle (800 RPM) without vacuum for EFI XJ6s 84-87??

Just to eliminate two more issues that are easily done:
Remove the electrical connector from the MAF and the EFI computer in the trunk and clean the connector pins with spay contact cleaner and reinstall 2-3 times, cleaning each time. Any corrosion will distort signal voltage.
Remove air cleaner assembly and with ignition on run, gear selector in park, you should not hear fuel pump in trunk. Reach in to MAF and move vane with your finger. Pump should come on and vane move freely... no catching.
With potential multiple causes, it is a possess of elimination
Rgds
David
 
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Old 06-14-2019, 02:05 PM
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Originally Posted by David84XJ6 View Post
No CAT so no 02 sensor so nothing to check there.

I thought all EFI models have O2 sensor for modifying fuel mixture above the idle fuel circuit ??
If you have "Y" exhaust down tube without a fitting for O2 sensor, it may have come from Carb'd model XJ6.



Depends on market. Some had cat converters and O2 sensors; some didn't.


Ignition timing set to 6 degrees BTDC at idle without vacuum (I have tried it at 10 degrees BTDC but nothing changes the hot/cold issue)

My manual is showing 17 degrees BTDC at idle (800 RPM) without vacuum for EFI XJ6s 84-87??

Another market variation. The engine should (at least) run smoothly and rev freely at either spec, or anything close to either spec.

Cheers
DD



 
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  #13  
Old 06-19-2019, 12:24 AM
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Originally Posted by Doug View Post


Depends on market. Some had cat converters and O2 sensors; some didn't.


[color=Navy]


Another market variation. The engine should (at least) run smoothly and rev freely at either spec, or anything close to either spec.

Cheers
DD

Hey guys, thanks for your help,

Found something interesting today, I removed the coolant temperature sensor (the one for fuel trim) and connected up my variable resistor which has like a potentiometer knob on it and set it to around 1.6Kohms, which is around what the sensor reads at cold, the engine runs like a dream, idles well, revs up when hot.....any thoughts? I replaced the temp sensor however it hasn't fixed the issue, as soon as I remove the resistor and connect the temp sensor back up the engine runs poorly again.... I'm thinking it's running too lean with the temp sensor connected and with the resistor in place it's riching up the mixture. Possibly MAF not calibrated correctly?

Thanks
 
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Old 06-19-2019, 02:48 PM
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I removed the coolant temperature sensor (the one for fuel trim) and connected up my variable resistor which has like a potentiometer knob on it and set it to around 1.6Kohms, which is around what the sensor reads at cold, the engine runs like a dream, idles well, revs up when hot.....any thoughts? I replaced the temp sensor however it hasn't fixed the issue, as soon as I remove the resistor and connect the temp sensor back up the engine runs poorly again...

Way back when getting a smog check, the shop owner would leave equipment hooked up and running ... and take a smoke break... while I got out my long screw driver with 1/4 universal and a couple of home made Allen attachments and adjusted Idle and mixture. Now I have to sit in customer lounge and watch thru window as he attempts to check timing. I have three white lines 0 - 17 & 30 degrees.. they never ask which one is right, so I know he just pretends to check.

I had a similar experience, I always carried a spare sensor and when I could not get engine to restart after long drive, I just plugged in the cold spare sensor.... engine always fired right up. I just measured the difference between hot and cold sensor, then wired a resistor in series with one of leads to sensor. All good 4 years. I know I am masking a problem, but $3K for exhaust gas analyser???

Looking forward to your solution

Rgds
David
 
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Old 06-19-2019, 03:10 PM
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Originally Posted by David84XJ6 View Post
I removed the coolant temperature sensor (the one for fuel trim) and connected up my variable resistor which has like a potentiometer knob on it and set it to around 1.6Kohms, which is around what the sensor reads at cold, the engine runs like a dream, idles well, revs up when hot.....any thoughts? I replaced the temp sensor however it hasn't fixed the issue, as soon as I remove the resistor and connect the temp sensor back up the engine runs poorly again...

Way back when getting a smog check, the shop owner would leave equipment hooked up and running ... and take a smoke break... while I got out my long screw driver with 1/4 universal and a couple of home made Allen attachments and adjusted Idle and mixture. Now I have to sit in customer lounge and watch thru window as he attempts to check timing. I have three white lines 0 - 17 & 30 degrees.. they never ask which one is right, so I know he just pretends to check.

I had a similar experience, I always carried a spare sensor and when I could not get engine to restart after long drive, I just plugged in the cold spare sensor.... engine always fired right up. I just measured the difference between hot and cold sensor, then wired a resistor in series with one of leads to sensor. All good 4 years. I know I am masking a problem, but $3K for exhaust gas analyser???

Looking forward to your solution

Rgds
David
Thanks David,
Yes I don't have access to a gas analyser so it's making it difficult to determine what's going on and when, all I know is that car runs like a dream with the resistor in place, can you remember which one you used? I think I measured it cold at around 2Kohm.

Thanks
 
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Old 06-19-2019, 11:10 PM
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I look for my notes and could not find details, other than a comment of adjusting 40 degrees C at full warm so may be a .5 K omh resistor??
Rgds David

The CTS can be tested with an ohm meter. Do not start the engine. Pull the connector from the CTS. Touch one probe of the meter to one pin on the CTS. Touch the other probe of the meter to the other pin. The resistance values at various engine temperatures shown below should be within 10%.

0 degrees C (32F) 5.9 k ohms

10 degrees C (50F) 3.7 k ohms

30 degrees C (86F) 1.7 k ohms

50 degrees C (122F) 840 ohms

70 degrees C (158F) 435 ohms

90 degrees C (194F) 250 ohms
 
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Old 06-20-2019, 02:43 PM
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I had a similar experience, I always carried a spare sensor and when I could not get engine to restart after long drive, I just plugged in the cold spare sensor.... engine always fired right up. I just measured the difference between hot and cold sensor, then wired a resistor in series with one of leads to sensor. All good 4 years. I know I am masking a problem.

As I learned later reading "Doug's" explanations of the EFI system and adjustments. Doug, correct me if I still don't have it right!!!

Too lean at idle is vacuum leak or miss-adjusted AFM Idle circuit. The idle mixture adjustment is a regulated vacuum leak by-passing the AFM. To get to correct fuel air ratio at idle you need more air going thru AFM, moving the vane slightly. So if you have found and corrected all vacuum leaks from intake system accessories, then by turning the Idle mixture adjustment screw in, you will reduce the regulated vacuum leak. Now you will have to adjust Idle RPM back up to your target, in my case 800 RPM. When the engine is back to target idle RPM you will have same air with more fuel. At some point the ratio will be correct and the CTS system can go back to adding extra fuel during warm-up only.
The problem surfaced later, as all dealerships all had exhaust gas analyzers, when owners started maintaining cars for themselves and tried to adjust the idle system for minor vacuum leaks without test equipment.
I was frustrated with results of my TS.. all new vacuum hoses, rebuilt cruise bellows, several CTS sensors, rebuilt AFM etc & etc ... still had hot start problem, until found that a cold CTS would make it all work.... so just gave up and made a permanent cold start bias.
Check the other active thread on XJ6 idle problems, he is using the voltage from 02 sensor on a warm engine to check idle circuit Air fuel ratio... I am watching this as it has promise as cheap way to get idle mixture correct.
This is one of those issues that all EFI XJ6 owners have gone through with much frustration. Good luck with your Trouble Shooting!!!
Rgds
David
 

Last edited by David84XJ6; 06-20-2019 at 03:09 PM.
 
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