3.2 Engine Stalling - Stumped

[wnsgc]

Excuse the crappy video, and the screen glare, but here is a short video of an oscilloscope connected to my fuel injector harness.

1985 Porsche 911 Oscilloscope Video - YouTube

[ischmitz]

Is that with the engine running? Idling?
Why is the waveform cutting out towards the end of the video? Real or scope issue (poor triggering)?
What is the total duration of the injection pulse? Looks like less than 1millisec?

What is the diagnostic conclusion for this video? Can you put it into context to understand why are you showing it?

[rick-l]

What is that dropout in the hold pulses? That can't be normal.

[wnsgc]

The video was taken at engine idle, before we took it out on the road and got it to stall. After we got back, we hooked the o-scope up to Mark's 87 944, just to see what it would look like, realizing the 944 and 911 are not the same. The 944 didn't have the subsequent pulses, it looked similar to a heart-beat on an EKG, without the thick bands in-between.

[steely]

hard to see a drop out or even graticule (hold = <1.7ms?). - decent shot for in the field, but can you open up the time base a bit on the scope? Maybe vary the rpm so you get a feel for yourself.

[ischmitz]

The "band" is the hold phase produced by the peak&hold driver. The 911 3.2, 924 and early 944 all use the same driver circuit do deal with 4 or 6 low-impedance injectors in parallel. The hold phase is required to reduce the current through the injectors while keeping them open.

Late 944 and 964,993 and onwards have high-impedance injectors driven sequentially. They don't need a peak & hold. They use a simple low-side switch. On the scope you will see a solid pulse for the injection duration. Simple rule: if the DME is a 35 pin version with 2 rows of pins you will see the "band". If it is the 55 pin version with three rows of pins there is no "band".

Ingo

[wnsgc]

Greetings once again,

I should receive my new injectors today from Motorman. I'm reviewing the instructions in the Bentley manual and want to make sure I get this right:

LEFT SIDE:
1) Remove heater tubes, heater blower fan and heat exchanger feed tube
2) Remove fuel rails by:
A) Disconnect left fuel rail supply and return lines
B) Cut wire ties, and disconnect injector harness connectors
C) Remove fuel rail mounting bolts
D) Carefully pull up on fuel rail and remove injectors
3) To remove each injector
A) Remove locking clip holding each injector to fuel rail
B) Pull each injector from the rail
4) Replace each injector
A) Connect to fuel rail
B) Insert locking clip
5) Re-install fuel rail, tighten to 5.5 ft lbs.

RIGHT SIDE:
1) Remove air filter housing, filter, element
2) Loosen bolts that hold A/C compressor in place, remove belt, place compressor on towel near bumper.
3) Follow steps 2 - 5 from LEFT SIDE.

Questions:
1) Does the system have pressure when the system is off? If so, can I do anything to remove the pressure to the fuel lines?
2) How difficult is it to remove the U shaped clips from the injectors, which allow the injector to be removed from the fuel rail?
3) How much fuel should I see when i disconnect the fuel supply/return lines?
4) How much fuel should I see when I remove the injector from the fuel rail?
5) Any other tips?

As usual, thanks in advance.
Scott

[rick-l]

You will see fuel at all 4 places that can easily be collected on a rag.

You can take the clips off with your fingers.

I'm not sure pulling the fuel rail out with the injectors attached is going to work. I ended up removing the clips and pulling the rail off the injectors and then pulling the injectors one by one. Some were glued in there pretty good. I put them back in installed in the rail however.

[Tippy]

1. When you crack a line, pressure will bleed off rapidly until the damper and regulators pump down.
2. I use needle nose. They aren't hard to remove, just sheetmetal.
3. Not much, a couple/few rags will do
4. Very little
5. Patience and wiggle everything gently. It'll work free. Use vasoline on the new o-rings to aid assembly back into rails and intake bosses.

[wnsgc]

6 new-to-me fuel injectors, courtesy of MotorMan in Michigan. Ordered on Friday, received in NJ on Monday. Installation will occur either tonight or tomorrow. Details thereafter regarding success or failure of my stalling issue!

[bigel]

Originally Posted by Tippy

5. Patience and wiggle everything gently. It'll work free. Use vasoline on the new o-rings to aid assembly back into rails and intake bosses.

I would advise against vasoline on the o-rings...petroleum jelly will deteriorate those o-rings! Use a good silicone lubricant.

[wnsgc]

Can someone give me an example of a good silicone lubricant? Originally my plan was to use motor oil (Mobil-1) on the o-rings, but if I should keep away from petroleum jelly, I assume I should also keep away from motor oil. What about WD-40?

[rick-l]

Gas is the recommended lubricant. Dip the top when you put them in the rails and slop some around with a brush on the manifold.

[ischmitz]

Not sure if motor oil is a bad choice - it would have been my first guess. Gas sounds good, too.

I really like Hylomar blue from Permatex as gasket dressing on O-rings of all sorts. It is non-setting and keeps the O-rings lubed for years. Unfortunately, there is rumor it was discontinued? I grabbed several tubes a while back. Check your LAPS (local auto parts store) if they carry it.

waiting in anticipation for whether the new injectors fixed the issues you are having.....

[Tippy]

Ingo, Hylomar (original brand, not Permatex) should be plenty available as our jet engines specifically call out for it and in aviation you cannot deviate.

[wnsgc]

Greetings folks,

Here's an update. Tonight, I installed the new fuel injectors, and went for a test run. I banged thru 1st, 2nd, 3rd and into 4th, and it didn't die (normally it would die at 5k RPM's in 2nd or 3rd). Then I turned around, and started to come home. This time, at 5k RPM's, it didn't die, but it started to hesitate, so I downshifted and came to a stop. At this point, the car was idling very erratically, and eventually died. The engine sounded like it was running on 5 injectors. Each time it stalled, it started right back up (something it didn't do before), but again it wasn't running right, so I limped home. When I got into my driveway, I began to check the injector wires, pulling each one off, then reconnecting it. Eventually it began to run better. I revved the engine and it sounded fine.

Since it's now dark, I'm calling it a night, but I'll check the injector wires tomorrow and make sure I did the installation correctly, but this is much different and in my opinion, much better than it has been, as it didn't die (like turning off a light switch), and each time it did stall, it started right back up. More to come...

[wnsgc]

Greetings folks,

Here is yet another update to my never ending saga of the stalling 911. Last I told you, I replaced the injectors, took it for a spin, and this time, instead of the car dying at 5k RPM's in 3rd, it started to run erratically (i.e. like running on 5 cylinders). When the car got into this state, at idle, it would run like it was missing. So, tonight, Mark and I hooked up the o-scope to the injectors again, just to see what happens when it gets into this new state. When the car began to miss at 5k RPM's, the injector pattern in the scope did not change. This of course, is different from last week, when the injectors flat-lined. So it appears we did fix one problem, but discovered something else. Next we connected a fuel pressure gauge to the fuel rail and did a pressure test. At idle, the fuel pressure was 38lbs, when we took off it stayed at 38lbs, but then when the car went into that "funky state" where it appears to be running on 5 cylinders, the fuel pressure spike the meter at 100lbs. Could this indicate a bad fuel pressure regulator (as Otto stated last week), or is it a bad fuel damper, or both.

As usual, any and all suggestions are welcome. One more question, does the fuel pressure regulator from an 87 944 (part # 944-110-198-03-M14) work in an 85 911 (part # 928-110-198-01-M14)?

Thanks.

[Tippy]

Look into the fuel return line too. If it can't drain fuel back to the tank, fuel pressure can go up.

Scratch that, the issue would be worse at lower RPM's than higher.

[rick-l]

The fuel pressure regulator is also supposed to regulate the pressure relative to the manifold absolute pressure. When you open the throttle the pressure you measure relative to atmosphere should go up.

[techman1]

I am recalling the no-run, unplugging ANY one of the injectors and it runs scenario.

That seems to say less current is required and it runs. Only two paths it could be, positive or negative. Please check all ground wires around the manifold. Maybe even put in a few temporary ones as a test. If that fails it must be the positive side.