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

inject.duration XJ12 1979 pre HE

 
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Old 08-26-2016, 03:24 AM
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Default inject.duration XJ12 1979 pre HE

Hello, Has somebody an idea of the 'normal' (hot motor 80)(at idle)(normal atmospheric pressure (sea level)) duration of an injection with this motor and how can one know if the mixture is to lean.
Is there a list of correction factors on this duration in the case MPS, CTS,throttle resistance changes.

Thnks
 
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Old 08-26-2016, 03:57 AM
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No idea, never needed that info with any of mine.

MOST, not ALL, MAP units have an allen key adjustment hidden under the Black end cap, and this is/can be used to CAREFULLY adjust the idle mixture.

The MAP's are known to split the internal bellows, and repairs are mentioned form time to time, but I have never had the pleasure.

I have some specs in my huge stash in the shed, and will go digging over the weekend and post back if someone else does not come up first.
 
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Old 08-26-2016, 08:01 AM
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Default duration data measured

Originally Posted by Grant Francis View Post
No idea, never needed that info with any of mine.

MOST, not ALL, MAP units have an allen key adjustment hidden under the Black end cap, and this is/can be used to CAREFULLY adjust the idle mixture.

The MAP's are known to split the internal bellows, and repairs are mentioned form time to time, but I have never had the pleasure.

I have some specs in my huge stash in the shed, and will go digging over the weekend and post back if someone else does not come up first.
Hallo!
I have been doiing some measurements of the injection duration with my bitscope and have found this:
Motor 40 (coolant) avg inj duration: 2.45 msec (measured on 2 inj.groups)
75C 1.7 (already many misfires)
79C (prestall) 1,63
I guess normal injection duration is higher; I plan to try to influence it by fooling the D-tronic with the CTS when hot in order to have a pulse of about 2msec at idle and further on do some more measurements on MPS and CTS; What else could cause the car to stall when hot, knowing the data above?
Haven I read somewhere there's a potmeter on the ECU to do a adjust of the duration?
 

Last edited by patpin; 08-26-2016 at 08:13 AM.
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Old 08-26-2016, 04:45 PM
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Yes there is a "pot" on the ECU for idle mixture trimming.

That MAP adjustment is also able to trim things a tad.

I will look thru the huge paper files in the next day or 2 and see what I find. Work at the moment, so no access.
 
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Old 08-26-2016, 09:16 PM
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This may help.

Adjusting the Manifold Pressure Sensor

The idle adjustment knob on the ECU has 15 "clicks" or positions on it. On our (1976) you can turn the knob all the way in both directions and, while you can detect degradation in idle quality, it never cuts out.

Judging by your numbers, it looks like your pulse width is way too small..... Adjusting the MPS is "easy" so it may be worthwhile trying that, as you already have a scope.

How is the quality of the vac line from the manifolds to the MPS?

Whats your fuel pressure?
 

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Old 08-27-2016, 03:27 AM
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Originally Posted by Sarc View Post
This may help.

Adjusting the Manifold Pressure Sensor

The idle adjustment knob on the ECU has 15 "clicks" or positions on it. On our (1976) you can turn the knob all the way in both directions and, while you can detect degradation in idle quality, it never cuts out.

Judging by your numbers, it looks like your pulse width is way too small..... Adjusting the MPS is "easy" so it may be worthwhile trying that, as you already have a scope.

How is the quality of the vac line from the manifolds to the MPS?

Whats your fuel pressure?
Hallo! F.Press 1.9 BAR and 1.7 on the other (but I have to calibrate both manometers. Car runs very good until 5 min before it stalls at about 78C. I'll check vac lines today. I have measured the vacuum at the booster it was 20 psi.
Is there a way to know if the problem is in the MPS (other than testing if it holds vacuum), rather than elsewhere.
About holding vacuum. I read that the MPS should hold 0.5psi during 10 seconds in which it could only go to 0.45. Shouldnt that be Bar instead of psi?

Another 'thing', that influences the duration even more I guess, is the CTS. I have measured 37 ohm when hot which is also below specs. But on the other hand I read somewhere that there is a cutoff at 300 ohm, in the sense that any resistance below 300 wouldn't influence the ECU any more. What do you think?
 

Last edited by patpin; 08-27-2016 at 08:04 AM.
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Old 08-27-2016, 09:44 AM
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I'd replace the coolant temp sensor anyway, unless you know its already been replaced recently. Cheap and easy and eliminates one variable.

We were lucky that, after 25 years, "awakening" the MPS with a swift blow from a hammer worked a treat so I don't have any experience in troubleshooting them. I did, however snap up a good used one when it came up on ebay "just in case"

As part of your troubleshooting, if you happen to have a variable resistor, you might want to connect that to the CTS wiring and see if you can keep the car running as it warms up. In general, lowering the resistance decreases the pulse width.

37 ohms is way off, as you said. I'll post the spec table I have just in case.
 
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Old 08-27-2016, 09:48 AM
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Your CTS appears to be an order of magnitude off......

 
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Old 08-27-2016, 03:55 PM
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Originally Posted by Sarc View Post
I'd replace the coolant temp sensor anyway, unless you know its already been replaced recently. Cheap and easy and eliminates one variable.

We were lucky that, after 25 years, "awakening" the MPS with a swift blow from a hammer worked a treat so I don't have any experience in troubleshooting them. I did, however snap up a good used one when it came up on ebay "just in case"

As part of your troubleshooting, if you happen to have a variable resistor, you might want to connect that to the CTS wiring and see if you can keep the car running as it warms up. In general, lowering the resistance decreases the pulse width.

37 ohms is way off, as you said. I'll post the spec table I have just in case.
THanks I'll try a potmeter. today I checked the coolant temp gauge and discoverd that it was 10C to low!! So when the car was stalling each time at 75 to 78C, in fact it was already 88C
Do you happen to know at what temo the fan should turn on?
 
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Old 08-27-2016, 05:06 PM
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Originally Posted by Sarc View Post
This may help.

How is the quality of the vac line from the manifolds to the MPS?
?
Lines seem OK; my MPScannot be adjusted; the showed side in yr link is filled with epoxy...
 
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Old 08-27-2016, 09:46 PM
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If you depress the gas pedal very slightly, just enough to move the throttle pedestal until the "I'm at idle" microswitch disconnects, and hold the gas pedal there, do you still get the stalling?

Im not suggesting you rev it to keep it going, just trying to understand if your issue is only when idling, or also off idle.

Anyway, the CTS needs changed..... Fan should come on around 88
 
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Old 08-27-2016, 10:20 PM
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Originally Posted by patpin View Post
THanks I'll try a potmeter. today I checked the coolant temp gauge and discoverd that it was 10C to low!! So when the car was stalling each time at 75 to 78C, in fact it was already 88C
Do you happen to know at what temo the fan should turn on?
My 3 V12's have an 85c switch in the lower water pump inlet casting for that electric fan.
 
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Old 08-28-2016, 02:48 AM
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Originally Posted by Sarc View Post
If you depress the gas pedal very slightly, just enough to move the throttle pedestal until the "I'm at idle" microswitch disconnects, and hold the gas pedal there, do you still get the stalling?

Im not suggesting you rev it to keep it going, just trying to understand if your issue is only when idling, or also off idle.

Anyway, the CTS needs changed..... Fan should come on around 88
I'll have the CTS changed.
I'll check the idle switch and do the test.
As for the fan: the maual mentions opening start orf thermostats at 88 and full open at 94. It also mentions the fan should be stopping at 92C. Isnt 88 to soon for the fan?
 

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Old 08-28-2016, 03:14 AM
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Originally Posted by Grant Francis View Post
My 3 V12's have an 85c switch in the lower water pump inlet casting for that electric fan.
Do you mean it's contacts for the fan are closing at 85C already? The manual speaks of thermostats opening at 88 (starting) and fully open at 94C?
 
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Old 08-28-2016, 07:45 AM
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Yep.

Bottom hose temps. About 20c below top hose temps, and waaaaaaay more reliable for sensing things like fan switches etc.

Your 88c is the "crack temp" of that stat, so fully open (and lost control of the system at 100c).

Top hose temps fluctuate too much for reliable sensing, always have, and always will.

V12 thermostat fiasco explained.doc

Might help a tad.
 
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Old 08-28-2016, 10:55 AM
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I had it in my head that on my XJS I fitted a lower on temp switch that comes on at 82 instead of the usual 88, but I defer to Grant on this one for sure.

Post back when you get the new CTS installed and tried out.
 
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Old 08-29-2016, 03:40 AM
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Originally Posted by Grant Francis View Post
Yep.

Bottom hose temps. About 20c below top hose temps, and waaaaaaay more reliable for sensing things like fan switches etc.

Your 88c is the "crack temp" of that stat, so fully open (and lost control of the system at 100c).

Top hose temps fluctuate too much for reliable sensing, always have, and always will.

Attachment 135289

Might help a tad.
Very informatif yr .doc. Thanks. I wonder why a electrical fan continues to work when motor is of. Th only thing it can do is cooling the radiator and whats in in, since waterpump is not functioning? Or do I miss something? Thermosiphon action?
 
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Old 08-29-2016, 04:48 AM
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If that electric fan is working, due to bottom hose temp bringing it on), when the engine is shut down, it will continue to to work until that switch seperates the contacts for the relay earth.

It was meant to assist post suttdown heat soak, fact?, dunno.

All mine have twin thermo Efans, and none run after shutdown, and they survive our 45c+ summers just fine.
 
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Old 08-29-2016, 06:26 AM
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Found some of my early day scribblings for the D Jetronic system.

MAP testing:

Term 7 & 15 = 85.5 to 94.5 ohms.
Term 8 & 10 = 346.5 to 353.5 ohms.
Term 7 or 15 and ground = open circuit.
Term 8 or 10 and ground = open circuit.

All my PreHE had adjustable MAP units, and I remember that spec was market specific. That allen screw was "sealed" with a dob of Yellow paint.

I have not had a MAP that was faulty, most were out of range, and I adjusted that screw by ear with the engine running , and eventually got all those engines running fine.

Fuel pressure was a PAIN, and I eventually got 2 pressure gauges, and that made it easier.

One car was sweet at 29psi, the other was sweet at 31psi. The spec is the spec, but the engine knows what it wants.

The 3rd I fitted a HE fuel rail and one FPR, set at 30psi, good as I wanted it.
 
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Old 08-29-2016, 10:57 AM
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Originally Posted by Grant Francis View Post
The spec is the spec, but the engine knows what it wants.
So true.
 

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