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Firing on any 5 Cylinders ! DME issue....

graemewinfield

PCGB Member
Member
My 1985 911 3.2 Carrera is virtually ready for the road.
However it has developed a weird problem. I will only run on 5 cylinders!
It wont start.
The injectors have been replaced so its not a single injector. Its like the load of 6 cylinders is just to much.
But if you disconnect one cylinder it will start.
I doesn't seem to matter which injector you disconnect it will start. That seems to indicate its not the wiring or an injector.
I have ordered a dme relay as the easiest first step but I suppose if that doesn't fix it I need to look at the ECU.

Has anyone seen this before and if so what was the issue ?
 
Hi When were the TDC and Speed reference sensors last changed.
these are now 30 years old and starting to give trouble.

Charles.



graemewinfield said:
My 1985 911 3.2 Carrera is virtually ready for the road.
However it has developed a weird problem. I will only run on 5 cylinders!
It wont start.
The injectors have been replaced so its not a single injector. Its like the load of 6 cylinders is just to much.
But if you disconnect one cylinder it will start.
I doesn't seem to matter which injector you disconnect it will start. That seems to indicate its not the wiring or an injector.
I have ordered a dme relay as the easiest first step but I suppose if that doesn't fix it I need to look at the ECU.

Has anyone seen this before and if so what was the issue ?


 
Got them off ebay 83-92 FUEL INJECTOR 0280150158. Cant tell you much more about them.
I cleaned the original injectors and tested them, put them back on and it did the same.

Note I haven't run this car for 10 years. 2 of the original injectors were either blocked or not firing but it started, just a bit rough.
I decided I would change the injectors.
 
The problem seems to be when you connect the 6th injector I think the voltage drops there isnt a pulse and none of the injectors fire ?
Disconnect one and they work. Very confused, maybe tomorrow will be a better day....
 
Hi graeme. I had a similar but not identical issue earlier this year. My 3.2 had been off the road and garaged For 6 years, and when I prepared it for MOT etc, there was misfire which we couldn’t quite get to the bottom of. It also was not obvious what cylinder was misfiring. I did the easy stuff first, plugs, leads, distributor, coil, but the problem persisted. I never tried to run it with one lead disconnected so never replicated the problem you were having. Helpfully, I got some great advice from Charles M, and I note you have him on the case. The next thing i tried were the injectors, they were tested by a specialist and found to be corrosion damaged (water in the fuel maybe) and beyond refurbishment. I bought new injectors from Porsche (very expensive but I wasn’t convinced by the after market option). All this work did improve the running, but it didn’t fix it. It was also failing the MOT on emissions. I then replaced the TDC and speed sensors with after market options. The vehicle did run much better, the misfire was gone, but failed again on emissions. I then had the airflow meter refurbed/re manufactured at a reasonable price. At last, the vehicle is going great, better than its ever been in the 15 years I have owned it. It’s difficult to know what exactly fixed the problem, and given the age of the car, some of these parts were probably overdue for replacement, so money well spent. Hope this helps
 
Hi Charles
I don't believe the TDC and Speed reference sensors have never been changed.
I have owned it since 1987 and they haven't since then.
Its only done 60000 miles and the only work done is an alternator about 20 years ago and the clutch in 2005.


I don't think the TDC and Speed reference sensors are the issue, the more I think about it the more I suspect the high current power feed supplied to the DME relay or the DME Relay. I suspect a bad connection either relay contacts or a soldered joint.

However I think I may source the sensors today and change them anyway.
If they are starting to go it may prevent future problems.


 
graemewinfield said:
I don't think the TDC and Speed reference sensors are the issue, the more I think about it the more I suspect the high current power feed supplied to the DME relay or the DME Relay. I suspect a bad connection either relay contacts or a soldered joint.

That is a possible failure mode; the injectors get a constant 12v supply via the DME relay and are then fired by the ECU providing an earth supply. On the 3.2 the injectors are batch fired left and right; if the current draw of three injectors is too high for the relay to cope with unplugging one might just let the other two fire. I would also check all the earth connections and both battery connections to ensure they're clean and tight. Any signs of water ingress under the left seat where the ECU and DME relay sit?
 
No leaks or water issues, the whole car is superb. It's like it was in 87 when I bought it.
The fault has just happened while it's been in the body shop, although it was running a little rough when I started it up first time in 15 years. I thought new fuel and a bit of injector cleaner might corrected it. Obviously didn't, I suspect it's been sitting too long and either corroded the contactors over the years in the relay or as you say battery or earth connections.



 
But Type 911 you supply Bosch sensors.
Things to check earth lead gear box to body any green corrosion change it.
Dose it have an immobiliser dose it cut the DME circuit.
Another problem is inlet manifold gaskets strange misfiring.

Charles.
graemewinfield said:
Hi Charles
I don't believe the TDC and Speed reference sensors have never been changed.
I have owned it since 1987 and they haven't since then.
Its only done 60000 miles and the only work done is an alternator about 20 years ago and the clutch in 2005.


I don't think the TDC and Speed reference sensors are the issue, the more I think about it the more I suspect the high current power feed supplied to the DME relay or the DME Relay. I suspect a bad connection either relay contacts or a soldered joint.

However I think I may source the sensors today and change them anyway.
If they are starting to go it may prevent future problems.


 
Just to clarify, the TDC sensor, which is NLA does not give the ECU any values, its for setting up the TDC with a Bosch hammer

The 2 reference sensors are the ones to change, both the same, they give all sorts of problems when coming to the end of their life

We list the Bosch versions and an OEM quality Germany option which is a bit cheaper!
 
Thank for your all your input

Well progress has been made, I decided on a plan which narrowed the fault down.
We have found that with all 6 new injectors installed (These were bought off Ebay and are not Bosch) it will not start.
You disconnect one injector, and it will start but runs on 5 cylinders. (I wonder if anyone with a relay failure ever tried that.)
If you just plug one of the original injectors in it starts but again only on 5 cylinders as its not fitted. However you can feel the injector switching.
If you install that injector it doesn't start. This makes a little sense as you have 2.5 bar pushing it closed so the current required is higher.
If you plug in one of the new injectors but don't fit it, you can feel it switching.

So, I checked all the wiring and earths.
Then I got a new DME relay (if not the problem I planned to keep it as a spare so not wasted money).
All the original Bosch injectors were professionally cleaned and tested.
Clive Atthowe Engine tuning in Norwich did a terrific job, £180 covered the lot. Testing, striping, rebuilding/parts then retesting, a hell of a lot cheaper than I paid for 6 injectors ! (I am embarrassed to say how much I spent.)

The plan.
Step one, change the DME relay. Result - Still won't start with 6 new injectors
Step two, swap out the new injectors for the old ones. Result - It now starts and runs

So it was the new injectors. Individually they work but if you change all 6 and the load is too much. The limit I think is one in each bank. I suspect it's the ECU and earlier units can't cope with the load.
Whatever they didn't work on a 1985 Carrera Targa.
Lessons learnt; these aftermarket injectors will work individually but they aren't the same. And cleaning with carb cleaner and a battery to check they click isn't good enough, take them to a professional with the right equipment.

An old 911 can leave her garage now and go to classics at the castle at the end of September.
The cars looking awesome I will stick up a picture (when the rain stops) and she is MOT'd next week.







 
Many thanks for sharing your frustrating issue, I liked the logic you used and the determination to hopefully resolve the problem.. Though it does not seem that you found the actual cause..? I can agree that the issue seemed to cause an electrical overload which killed the ability to fire it up with all six injectors... However where would such a possible overload be found.. I guess the contacts in the DME relay might have had some crud on them that minimised the contact area to the extent it could not handle all six.. I guess using a link to short out across the pins in the relay base that provide power to the circuit might have proven the point..?
Sometimes just disturbing kit can resolve an issue but without proof positive I ever tend to be left with a nagging doubt..
Franny has made a series of vids on restoring a 3.2 Carrera that are very informative and detailed warts and all. The links below are relating to issues with injectors, DME and rough running which you might find informative..?
https://youtu.be/FeU-dYI9-fs
https://youtu.be/M9yIbuXYW0g
 
I believe the problem is the ecu will not handle the current required. With out a circuit diagram of the ecu I can't be certain but my guess is that between pins 14 and 15 and ground there is a FET (field effect transistor) that switches to fire the injectors. I suspect the FET can't handle the current.
 
Many thanks for responding. Overload seems entirely logical given your sequence of tests. I guess without definate data on design parameters you have done the best you can.. The main objective achieved..Well done.
 

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