3.2 Motronic engine running rough backfiring

[DRACO A5OG]

Oh No Brother, Keep Writing, this is very intriguing!

Need to open her up and take a closer look inside.

[kidrock]

WAAAYYYYY too technical for my pea brain. Methinks I may just send the whole board in to a specialist just to have everything checked out at some point in the future....after all, I'm pretty sure the DME isn't built to last forever. Thanks for the info anyway, Steely.

[shoguneagle]

Dan, thanks for a considerable amount of information regarding the DME. I am still proceeding along the suggestions I have obtained from this thread. I am in complete agreement that I may have problems other than the over servicing and vacuum leaks I have found. I am always suspicious of corrections that seem far too simple; I always am looking for other causes and will proceed through the thread suggestions with this fact in mind. I may have other causes as has been suggested. Just let me know what you think may be causing the flames and backfiring.

I am going down the list and will make sure everything on the engine is operating correctly. This is where the project building process takes on interesting situations. Thus the challenge is to learn, understand, and keep going until the engine problems and related problems are solved.

What I have done is to build an entire 911 Carrera front suspension, engine, clutch, transaxle, brakes, drive axles, rear bearings, etc. into the 914 chassis. This includes the 911 Carrera oil tank. I am running headers with short M&K mufflers until I get the engine straightened out and then I will have special mufflers and cats put on the car. I am building this car to operate in California since I will eventually move it there.

Dan, I find your information very detailed and somewhat informing. I think it is something that we all have to address since our cars are getting older and the DME is definitely getting more scarce. I have seen further developments come out of the information that you have been writing about thus creating more understanding and practical application.

I am going to take the information given in this thread and digest it including all the possibilities that could be causing my problem. I am going to the Parts Heaven Parts Swap this weekend. The traveling from Flagstaff, AZ along with the information provided in this thread should give me time to think about the problem(s).

Please keep the comments and ideas coming since they are definitely helping me to under the DME system and its components.

Steve Hurt

[rusnak]

Somewhere I've got the Dme Test Plan. Might make good airplane reading material. You need an ocilloscope though.

[shoguneagle]

I have been going over items that are not stock and what I have changed. The engine is running two separate headers matched to the engine. The right bank (911 installation left) is operating with the oxygen sensor and this is the one that is not backfiring with flames; the left bank (911 installation right) is operating without a oxygen sensor (I am setup to change the oxygen sensor from one bank to the other) is the one backfiring with flames. In addition, I have built in ports for gas mix measurement on both sides.

The original 911 exhaust had both banks going into one exhaust tube with the oxygen sensor before the CAT.

Wondering if this change in the exhaust system could be contributing to the problem??

Thinking of changing the oxygen sensor from present location to the other side. What do you think??

Thinking of doing a complete gas analysis for both sides. Again, what do you think??

Just trying to get as complete foundation to the problem as possible before concentrating on the problem.

Steve Hurt

[rusnak]

The 914 has a fairly long tailpipe. I'd install the bung in the tailpipe, before the muffler. I doubt changing side will do anything. You should have a complete loop between the fuel rails. Do you have this? Is the fuel pressure regulator and fuel pressure dampener vacuum control working? Idle position switch? Wot switch? These things get checked every engine installation. Etc etc. I would at minimum connect an air fuel ratio gauge and check closed loop operation, and do a lean stop test, and rich stop test.

[shoguneagle]

Thanks, rusnak.

Those are some of the checks i want to hear about. I have a complete loop between the fuel rails. I like to know more about the tests for the fuel pressure regulator, fuel pressure dampener vacuum control, idle position switch, WOT switch, etc. I have done somewhat a minimum tests on the fuel pressure and idle position switch. I am glad you mentioned these and the others. Yes, I agree they all need to be checked in greater detail. This now has become one of my initial objectives. I am sure there may be other items to check. Will be looking for these; included will be the air fuel ratio, close loop operation, lean stop, and rich stop.

Thanks,
Steve Hurt

[rusnak]

I see. You might check out the "Porsche 911 Carrera Service Manual" by Bentley Publishers, and "Bosch Fuel Injection and Engine Management", by Charles Probst. Two excellent, excellent resoueces. Usual suspects - Amazon or Ebay are you best bet. And beg borrow or steal an LM-1 wideband fuel air instrument by Innovate.

[shoguneagle]

An update on this thread since many valuable sources of information and ideas were provided by many.

The engine is up an running without backfiring out one of the banks. It appears I had several vacuum leaks involving the systems. Engine runs strong and with authority. I have been going through the checks involving the Motronic systems and they are checking out. I will continue to do these complete checks including all the ones suggested in this thread.

I have one characteristic develop: the engine starts and runs very well during cold and hot status. It now has taken on a characteristic of wanting to shut down both under load (running down the highway) and when in neutral. The engine seems to go to shutoff position and then comes on strong when it "catches itself" in the shutoff position. It appears to be getting gradually worse.

Need some input on the above characteristic problem.

Thanks again for all the foregoing information and I really liked the way the thread took off and people contributions.

Even got a source of information involving the DME at high technical levels and maybe I can find a source for total repairs of it.

Thanks again.

Steve Hurt

[DRACO A5OG]

Check your dizzy rotor for slop.

You may also want to check the AFM, to insure you are are gettig proper readings through out the travel.

Also, I assume she is trying to shut down when gas pedal is not depressed, correct. I would check the idle position micro switch at the throttle body.

Check also the Fuel Filter strainer is not clogged with debris.

[Lorenfb]

"The reference sensor must send magnetic pulses to the dme or it will misfire."

- rusnak -

Totally incorrect as usual! The 3.2 DME ECM only needs the reference signal
to start. Once the engine starts, the ECM counts the speed signal teeth for
all the needed info. If the engine requires the reference signal after it starts
to run properly, then the speed signal is most likely bad, e.g. bad speed sensor.

[rusnak]

Loren, I suggest that you actually try it with a second reference sensor out of the car, as I have. You will have to retract your statement. The dme needs signal beyond startup from the reference sensor. You can also reference JW's thread on his "no start" related to a missing ref signal. He also confirmed that idle was misfiring without reference signal, so you may step down from your high horse now, your Royal Highness......

[DRACO A5OG]

Oh No, you didn't?

[kidrock]

[rusnak]

Shoot, I'll even make a Youtube video and prove my point if he makes me.

[kidrock]

Originally Posted by rusnak

Shoot, I'll even make a Youtube video and prove my point if he makes me.

I've got your back, stud.

[Lorenfb]

"I suggest that you actually try it with a second reference sensor out of the car, as I have."

- rusnak -

Sounds like someone's located a major problem with their engine management
system!

Been doing that for over 15 yrs with all types of BMWs (528s, 533s, 735s),
Porsche 944s, and 911 3.2s, as they all have the basic same early type
DME ECMs, i.e. running the engine without the ref sensor and having it
run with a perfect idle and rev perfectly also.


Bottom line: READ THE OTHER THREADS WHERE THE FLYWHEEL PIN
PROBLEM OCCURRED AND A SECOND REF SENSOR IMMEDIATELY
STARTED THE ENGINE BY JUST A MOMENTARY WAVE NEXT TO METAL
AND WASN'T NECESSARY ONCE THE ENGINE STARTED.!!!!


"You can also reference JW's thread on his "no start" related to a missing ref signal. He also confirmed that idle was misfiring without reference signal"

- rusnak -

Right, because the timing of the ref signal was NOT at the right TIME.
If he had done it again and luckily found the right timing, it would
run perfectly. The DME ECM needs the correct point (ref point) to
start its count for the next TDC, if not then the timing will remain
either too advanced or too retarded.

Good luck finding your problem, as this should help you!!!!!!!!!!!!!!!!

So much mis-info gets posted here where DIYs just don't fully analyze
and understand what actually happens and GUESS or re-post some
other hyperbole they found.

[rusnak]

Loren,

Now I caught you just making stuff up. Talk about misleading!! You're a hypocrite!!!!!!!

JW had NO SIGNAL FROM THE REF SENSOR!!!!! THAT WAS HIS PROBLEM!!!! (recall that his customer flywheel had a non-ferrous pin. He chose to re-drop the engine and change the pin back to original)

Note, that when he artifically supplied signal by pulsing the ref sensor, the car started.

Then, in his words, he had to keep pulsing the ref sensor AFTER THE CAR WAS RUNNING to make it run smoothly!!!!!

Now, again.....you are caught in either (1) an outright lie, or (2) you are being lazy and shooting casually from the hip. In either 1 or 2, your tech info is misleading, incorrect, and should be avoided at all costs.

(how's ^this for my imitation of Lorenfb?)....haha!


edit: Here's the original thread for entertainment value: 86 won't start after new flywheel? no injector or coil pulse.

[Lorenfb]

"Then, in his words, he had to keep pulsing the ref sensor AFTER THE CAR WAS RUNNING to make it run smoothly!!!!!"

- rusnak -

WRONG!!!!!!

The 1st pulse did not provide the right timing reference.
A few more random pulses finally got the correct reference
point and then no more pulses were needed.

Stop being such a hard-headed DIY and try unplugging the
reference sensor on a good running car and NOT yours which
needs to be properly worked on by a qualified Porsche tech
who understands Porsche electronics!

[rusnak]

Loren,

WRONG!! stop being a hard-boiled egg-head and get out into the real world and look at cars, not books. While you're at it, actually read JW's post.