World's WORST engine management system - 944S

[kdjones2000]

So this POS called Bosch Motronic "adaptive idle speed volumetric efficiency regulation" is possibly the world's worst engine manage system.

No matter what I do to this thing, I cannot prevent the car from randomly stalling when coming off revs while the A/C is engaged. At least one time during any short local road trip, this f*cking Motronic unit will forget that the car actually needs to idle at its preset RPM, and allow the engine to stumble and stall.

Sometimes it's from a low RPM to idle, others from high revs to idle, but stall it does.

Everything in my power has been done to make this thing work like it should: Valve job, good compression, NTCII checked, fuel pressure in spec, new O2 sensor, good injectors, plugs/rotor/cap, all new hoses, new gaskets, evap system checked, AOS checked, throttle body reseal, TPS checked, cleaned ICV (not ISV, that is for engines that actually work properly apparently). There are no adjustments possible on this system for idle, and even minor adjustments to the throttle body stop result in poor running.

Car runs fine otherwise, and idles great with the A/C off. They just totally failed on this system with A/C which is required in this part of the world.

Stoooopid Germans on this system..... BOOOOOOOO!!!!

Alright, end of rant.

[Pauld_94S2]

Out of curiosity, and hopefully to amend your POS Motronic, have you watched the manifold vacuum when driving and going back to idle?

What I am thinking is whether the ICV is not able to compensate since it is at its limit in one direction, and can't react quickly enough.

The other would be if you have another AFM that you could try?
I have one in a driver 944S I could send you, as long as I get it back. The other option woudl be to try one from an S2.

I know some of the 85/1 944s had an issue where the fuel (and this applies to all 944s, including the 944S, yours) is shut off when the throttle is closed, and then went below idle and stumbled before it recovered. The DME is supposed to turn the fuel injectors back on at something like 11-1200 RPMs so the engine can idle correctly.

If it is working correctly, upon closing the throttle, the tach should quickly drop to about 1200 RPM and then slowly settle to idle. I'm assuming yours continues falling below idle and then recovering to idle poorly.

Does act this way, or is the problem only when the AC compressor kicks on?

BTW, it may also be a firmware issue in the DME EPROM. Trying a different DME, or using an aftermarket "racing" chip might change/improve the idle/compressor problem.

Or PM me and we can talk.......

Best of luck, Paul

PS Here's a link to the Bosch L-Jetronic manual:
http://www.cardiagnostics.be/-now/Educational_sites_bestanden/BOSCH%20L-Jetronic%20Injection%20Manual.pdf

[v2rocket_aka944]

did you check the idle contact in the throttle position switch?
same switch as used on the NA.

the later model cars have a higher "injectors back on" RPM on drop-throttle, something like 1600. in a properly working car you can watch the tach as the engine "catches itself from falling", much smoother than the 82-85/1 models.

[kdjones2000]

It works fine without the A/C engaged, going to ~1200, then dropping. It consistently works fine with "normal" workloads.

With A/C engaged, sometimes it will go to ~1200 and then drop to idle just fine. Other times it goes through that, down to ~700, then back up to normal idle and stays there. Other times it will go down to ~500, stumble and recover to normal idle. And other times, it will go to ~500, and stall.

I have been playing with this now for about 2 weeks, trying to figure out if it's when the engine is cold, warming up, or hot. Happens in all situations. Doesn't even seem to be related to ambient temps. Of course my range of temperatures is limited here, with lows in the high 70's and highs in the mid 100's.

I have a friend with an S2; is that the same AFM as on the S? Looks to be based upon PET. I know that the injectors are the same.

This car is supposed to be a starter car for my kid, so random stalling is not an option, as it will lead to loss of power brakes and steering.

Just frustrating as heck.

I am about to go through the entire 944S DME test plan at the DME connector, to see if there's anything that I am missing.

Spencer, yes, TPS was checked out and works properly, both idle and WOT. Idle contact is reached every time the throttle is released, so it's not sticking. AFM is smooth throughout its range, although I haven't voltage tested it yet. Factory plugs are in all adjustment points on this engine still.

[DannoXYZ]

Have you electronically measured the idle-switch in TPS at DME connector?

I've seen many that clicked OK, but electronically the idle-switch is busted/dead and doesn't signal DME that throttle is at idle. DME doesn't control the ICV to manage idle-speed unless it properly detects that throttle is closed.

also try with O2-sensor disconnected.

[Pauld_94S2]

OK, sounds like you have covered the engine and fuel basics, and more.

Since there is never a problem when the AC is off (never, right?), perhaps something related to the AC system occasionally has a different load to the engine, or an or electrical change.

So let's take a different direction:

1. Is the system R12, or converted to 134a? Professionally converted or DIY? Looking to see if the system is overcharged and causes compressor stall when engaging.

2. When the AC has been on for a while what are the vent and ambient temps? High pressures can create heavier load on compressor when the clutch kicks in.

3. Do you have a gauge system to measure the suction/discharge pressures on the AC compressor when the idle speed drops? If so, can you collect some data about the pressures vs. the idle speed drop? Once again looking for high pressures.

4. With the engine idling, does the idle speed drop when the AC compressor kicks in? Does the idle speed change every time, or intermittently?

5. Really reaching here, but if you turn the AC on from the dash to engage AC compressor clutch while closing the throttle, does the compressor clutch come on and the idle drops low at the same time?

6. Same test, but if you have the belt off the AC compressor and turn the AC on from the dash while letting up on the throttle, does the compressor clutch come on and the idle drops low?

For #5 & 6: Looking to see if the clutch current causes the alternator or battery voltage to drop, caused by resistance in a ground or 12V power somewhere. You could also check the battery voltage at the DME, but measure it on the AC volts scale since it will be a short lived transient.)

I'll think about it some more.........

[kdjones2000]

Originally Posted by DannoXYZ

Have you electronically measured the idle-switch in TPS at DME connector?

I've seen many that clicked OK, but electronically the idle-switch is busted/dead and doesn't signal DME that throttle is at idle. DME doesn't control the ICV to manage idle-speed unless it properly detects that throttle is closed.

also try with O2-sensor disconnected.

New O2 sensor as well, forgot about that one. Also checked the NTCII sensor and it passed.

Car passed emissions with flying colors.

Electrically tested TPS at idle and WOT, at the sensor. As I said previously, I am going to go through the DME Test plan at the DME connector this weekend, just to see if there's anything there.

[kdjones2000]

Good suggestions, Paul, but I am quite confident that the A/C system is not causing the issue itself. The idle doesn't change at all when A/C is engaged/disengaged, as the DME appears to be compensating correctly.

I am using a different refrigerant for A/C on this car, which actually uses lower pressures than either R12 or R134a. The load on the compressor and engine is lower than with R134a for sure, based upon direct experience.

The climate control in the car currently is not working 100%, meaning that not all flaps and solenoids seem to be activating correctly. So if I scratch my head and project, maybe the signal from the CC unit to the DME is not always sending the signal for A/C on? I am going to figure out which pin that is and see if it's energized. Based upon the stable idle when A/C is engaged, I doubt that this is a cause however. I have another CC unit that I am planning on swapping in - will see if that makes a difference.

Regarding 5 & 6, I can play around with that and see if there's anything there. I don't have bouncing gauges or other power/ground issues with this car presently, however.

Originally Posted by Pauld_94S2

OK, sounds like you have covered the engine and fuel basics, and more.

Since there is never a problem when the AC is off (never, right?), perhaps something related to the AC system occasionally has a different load to the engine, or an or electrical change.

So let's take a different direction:

1. Is the system R12, or converted to 134a? Professionally converted or DIY? Looking to see if the system is overcharged and causes compressor stall when engaging.

2. When the AC has been on for a while what are the vent and ambient temps? High pressures can create heavier load on compressor when the clutch kicks in.

3. Do you have a gauge system to measure the suction/discharge pressures on the AC compressor when the idle speed drops? If so, can you collect some data about the pressures vs. the idle speed drop? Once again looking for high pressures.

4. With the engine idling, does the idle speed drop when the AC compressor kicks in? Does the idle speed change every time, or intermittently?

5. Really reaching here, but if you turn the AC on from the dash to engage AC compressor clutch while closing the throttle, does the compressor clutch come on and the idle drops low at the same time?

6. Same test, but if you have the belt off the AC compressor and turn the AC on from the dash while letting up on the throttle, does the compressor clutch come on and the idle drops low?

For #5 & 6: Looking to see if the clutch current causes the alternator or battery voltage to drop, caused by resistance in a ground or 12V power somewhere. You could also check the battery voltage at the DME, but measure it on the AC volts scale since it will be a short lived transient.)

I'll think about it some more.........

[9FF]

Originally Posted by Pauld_94S2

...Looking to see if the clutch current causes the alternator or battery voltage to drop.......

...also the extra load when you turn the A/C on will cause a drop in voltage if your voltage regulator is going bad. The regulator is a cheap replaceable component in the alternator, might be worth changing anyway as part of your diagnosis.

[Pauld_94S2]

Slightly off topic, but one point I would like to make.

When you test a sensor, we usually do the test at the sensor.

However, that's only part of the picture, sensors should be also checked at the DME.

There are several feet of wire and a few connectors that the sensor's signal voltage goes through before it gets to the DME. These are all problematic and can be the real issue that often goes undiagnosed even though we believe the DME is getting the output of a "good" sensor because we checked it.

[Pauld_94S2]

Just did a little research and it is possible that the Motronic (at least in version 1.1-1.3 and 2.1 and later) has adaptive learning.

Try disconnecting the battery for 15-30 minutes and see if any changes after reconnecting and restart.

https://en.wikipedia.org/wiki/Motronic

The ML 2.1 system integrates an advanced engine management with 2 knock sensors provision for adaptive fuel & timing adjustment, purge canister control, precision sequential fuel control & diagnostics (pre OBD-1). Fuel enrichment during cold-start is achieved by altering the timing of the main injectors based on engine temperature. The idle speed is also fully controlled by the digital Motronic unit, including fast-idle during warm-up. Updated variants ML 2.10.1 thru 2.5 add MAF Mass Air Flow sensor logic and direct fire ignition coils per cylinder. Motronic 2.1 is used in the Porsche 4 cyl 16V 944S/S2/968 and the 6 cyl Boxer Carrera 964 & 993, Opel/Vauxhall, FIAT & Alfa Romeo engines.

[kdjones2000]

The '88 version of 2.1 has some enhancements over this '87 version, including fault codes and the ability to read those with a scanner, plus some persistent memory while the battery is connected.

The '87 does not. I can't find too much documentation related to the '87 and its differences.





Anyone know how to "adapt" a Motronic system without a Tester? Maybe I just need to take this into a Porsche shop who has the tester and get this done? I am not sure if this is only for '88+ cars, or if this works on '87's as well.

[Pauld_94S2]

Getting a little off topic, but "adaption" means the engine management system can store and make adjustments in key emission control and driveability settings.

I don't think, especially in our older DMEs, that you can externally modify settings in the DME.

That's why owners buy "racing" chips, or change the engine management computer to M-Tune, VEMS, or Mega/MicroSquirt, or similar products.

[kdjones2000]

Paul, all of the above is in the 16V Porsche factory workshop manual, so whatever they are doing with their "reset" of the adaptive idle or whatnot works with our cars. Just not sure if it's only for '88+ or includes '87 cars.

I am not interested in externally modifying it; only to get it back to factory specs, which I assume include NOT STALLING WHILE THE A/C IS ON.

[djnolan]

The AFM can be cleaned with MAF cleaner and the AFM contacts can be cleaned with an eraser. Did you connect a vacuum gauge and check for at least 17Hg?

I have the same car and have never seen this problem, however I occasionally stall it when hot, clutching 1st gear with A/C on.

[v2rocket_aka944]

Test I thought of...
try jumpering the radiator fans to go high speed, leave the AC off...
does it stall coming back to idle?

AC compressor puts a big load on the engine, but AC on means fans go on high speed which puts a big load of its own thru the alternator. Mike mentioned the Voltage Regulator could be sketchy which could make the alternator act like a brake when 40 amps of rad fans turn on

[djnolan]

Here is a similar problem:

https://rennlist.com/forums/928-forum/502693-ac-causes-stall.html

https://rennlist.com/forums/968-forum/151373-help-ac-causes-engine-to-seriously-stumble.html

Also put a volt meter on the battery when checking the alternator. Should not drop below 12.5v or so.

[thom4782]

I don't have deep experience with the 944 yet, but I recently chased an idle problem that was seemingly solved after two fixes.

One fix was to replace the intake manifold gaskets. Likely not relevant here.

The other was to replace the alternator. My electrical symptoms were the output was OK when the AC was off, but dropped too low when the it was engaged.

Perhaps the comments relating to the electrical system issues may be pointing in the right direction.

[mattdavis11]

My bet is on the a/c compressor clutch coil shorting to ground. They tend to do that when shaft seals go, even if slowly.

[kdjones2000]

Originally Posted by mattdavis11

My bet is on the a/c compressor clutch coil shorting to ground. They tend to do that when shaft seals go, even if slowly.

A/C compressor was just resealed and clutch checked out. Don't think that's the issue guys.

Still working on the Test Plan...

You can also add to the original list of things done that I had already checked/tightened grounds (clutch job), and replaced and gapped the speed sensor, plus resealed the injectors.

EDIT: Alright, I have an idea of what's going on here.

Idle vacuum is only coming in at 10-11in Hg. I am thinking that the intake camshaft is a tooth off.

When I did the valve job, I took off the camshafts and wired in the chain so that their timing didn't change. The workshop manual states that you are supposed to have 7 links between marks on the camshafts, and when I put it back together I only counted 6. Rather than change things, I figured leave well enough alone and put it back together as it was. Someone earlier must have messed it up.

So I am guessing that I have late intake valve timing on this. I guess it should look like this:



Seems a bit strange since I get 160-170psig for compression across all 4 cylinders, which is spot on for 10.9:1.

If the intake is off by a full tooth, shouldn't I have bent valves? There was no interference when I turned it over by hand, and the car runs fine other than this stupid idle issue, and I've put 400+ miles on it.

EDIT of EDIT:

Back to the drawing board. It looks like the spacing is correct, only that they aligned things to the inner link, not the outer link. Either way, it's 7 links spacing.



Back to the Test Plan.

Voltage at the battery:

Battery: 12.56V standing
Running, consumers off: 13.54V
Running, A/C on, fans off: 13.43V
Running, A/C on, fans on: 13.34V

Don't think that's the issue...

I am going to send this into the local Por$che shop for smoke testing and CO testing - see if they can figure it out.

Other ideas about low vacuum?