LH-jet fuel delivery "stuttering"
07-28-2016, 06:09 AM
#1
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Thread Starter
LH-jet fuel delivery "stuttering"
Hi,
I have a Euro 1984 928S manual with the LH-jetronic system. Problem is that it kind of "stutters" at the point when I just press the gas pedal to maintain speed. If I keep the gas pedal at this point when it starts to stutter, it just escalates the stuttering more and more until i have to press the pedal or release pedal pressure to stop the stuttering, because it gets so harsh. That will only take some seconds.
I have experienced the same thing in other (older) manual gear cars, mostly when in 1st gear following slow traffic. But in my 928 it is worse, and present also in 2nd and 3rd and at higher speed.
When accelerating everything is very smooth and linear, when off the gas everything is fine. it is only present when the pedal is pressed a little just to keep the speed. It is present in all gears, but most noticably in 1st 2nd and 3rd.
To me it seems like a fuel balance problem, not ignition. This is no misfire kind of problem. Could it be the MAF thats out of spec? Any other ideas?
Attachment 1355705
I have a Euro 1984 928S manual with the LH-jetronic system. Problem is that it kind of "stutters" at the point when I just press the gas pedal to maintain speed. If I keep the gas pedal at this point when it starts to stutter, it just escalates the stuttering more and more until i have to press the pedal or release pedal pressure to stop the stuttering, because it gets so harsh. That will only take some seconds.
I have experienced the same thing in other (older) manual gear cars, mostly when in 1st gear following slow traffic. But in my 928 it is worse, and present also in 2nd and 3rd and at higher speed.
When accelerating everything is very smooth and linear, when off the gas everything is fine. it is only present when the pedal is pressed a little just to keep the speed. It is present in all gears, but most noticably in 1st 2nd and 3rd.
To me it seems like a fuel balance problem, not ignition. This is no misfire kind of problem. Could it be the MAF thats out of spec? Any other ideas?
Attachment 1355705
Last edited by paalw; 07-28-2016 at 07:10 AM.
07-28-2016, 07:30 AM
#2
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07-28-2016, 08:05 AM
#3
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Do you know if your injectors have ever been serviced (cleaned and balanced?)
If the answer is no...
If the answer is no...
07-28-2016, 09:21 AM
#5
Pro
Thread Starter
It is a swedish market model and has these options:
M062- Version for Sweden
I154- Control unit for improved emissions
Is I154 the add on box you are talking about?
Other that that i believe it is 100% original. No cats.
M062- Version for Sweden
I154- Control unit for improved emissions
Is I154 the add on box you are talking about?
Other that that i believe it is 100% original. No cats.
Last edited by paalw; 07-28-2016 at 09:42 AM.
07-28-2016, 01:06 PM
#6
Rennlist Member
Sounds like symptoms of weak mixture.
Either the idle mixture has been set too weak (should be around 1% with the air pump running);
if you can't adjust it up to 1% then the MAF needs rebuilding.
But it could also be poorly flowing injectors.
The Swedish version had somewhat lower compression and also revise ignition mapping which blunts the performance. If you run good petrol then you can fit one of my performance chips to perk the car up, once you have solved the weak mixture issue.
Either the idle mixture has been set too weak (should be around 1% with the air pump running);
if you can't adjust it up to 1% then the MAF needs rebuilding.
But it could also be poorly flowing injectors.
The Swedish version had somewhat lower compression and also revise ignition mapping which blunts the performance. If you run good petrol then you can fit one of my performance chips to perk the car up, once you have solved the weak mixture issue.
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07-28-2016, 04:55 PM
#10
Rennlist Member
The default setting is 386 ohms, measured at pins 3 & 5 of the disconnected MAF. If the MAF is badly aged, then you will need a higher resistance. The maximum resistance is 1000 ohms. But if you need that high a resistance the MAF needs replacing.
Idle speed is set using the slotted screw almost immediately below the middle of the cross brace. Turn anti clockwise to increase idle rpm.
Idle speed is set using the slotted screw almost immediately below the middle of the cross brace. Turn anti clockwise to increase idle rpm.
07-29-2016, 01:44 PM
#11
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John must have just come back from the pub. 
382 ohms is 'zero' between pin 6 (CO) and 3 or 4 (grounds) at the MAF. Turning CW richens.
(14 and 25 at the LH plug - https://rennlist.com/forums/928-foru...2-pinouts.html)
382 ohms is 'zero' between pin 6 (CO) and 3 or 4 (grounds) at the MAF. Turning CW richens.
(14 and 25 at the LH plug - https://rennlist.com/forums/928-foru...2-pinouts.html)
08-11-2016, 05:23 PM
#13
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Thread Starter
Ok, I have done some testing.
The first thing I did was to adjust the idle screw, turning it counterclockwise around one turn until idle was at around 675-700 with engine warm, no AC and no lights. Did a test run and as expected it had no impact on the stuttering fuel delivery. But at least now idle is correct.
I then removed the MAF and measured resistance as it sat to 690 ohms. After that I tested min and max settings with my multimeter, which I found was 0 and 970. Then i tuned it in to "zero" aka 382, and went for a test run. It now was noticeably even worse. In addition there was some minor movement on the rpm needle at idle, that I can't remember having seen before.
So i drove back, removed the MAF again and set it to max (970) and did a new test run. It was now better than before, the best one so far and actually acceptable, though not completely as good/smooth as it should be.
Can any conclusions be drawn from this? Is my MAF good or bad? Could it still be cloggy injectors?
The first thing I did was to adjust the idle screw, turning it counterclockwise around one turn until idle was at around 675-700 with engine warm, no AC and no lights. Did a test run and as expected it had no impact on the stuttering fuel delivery. But at least now idle is correct.
I then removed the MAF and measured resistance as it sat to 690 ohms. After that I tested min and max settings with my multimeter, which I found was 0 and 970. Then i tuned it in to "zero" aka 382, and went for a test run. It now was noticeably even worse. In addition there was some minor movement on the rpm needle at idle, that I can't remember having seen before.
So i drove back, removed the MAF again and set it to max (970) and did a new test run. It was now better than before, the best one so far and actually acceptable, though not completely as good/smooth as it should be.
Can any conclusions be drawn from this? Is my MAF good or bad? Could it still be cloggy injectors?
08-12-2016, 07:35 AM
#15
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The Euro's are notorious for being on the lean side. You still may get some of these symptoms after the MAF replacement. After you get the car sorted be sure to buy a set of chips from John Speake. This made a huge difference in our Euro. Don't add the chips until everything else is in good condition.