stax

Greetings;
I am new to the forum, so bare with me please.
I have an early production 1984 Carrera that randomly misfires followed with the engine shutting down.
I have monitored the fuel pressure both at idle and driving.

31 pounds while driving (fluctuating)
36 pounds at Idle

Seems normal and it runs fairly well. However when it misfires the fuel
pressure rises briefly before the engine shuts down.

The Idle stabilizer is rattling as it should.
I have tested the head temp sensor with a meter: 720 ohms warm.
So the question is; What is next step in the process ?
Any help would be appreciated.
Cheers
Stax

rusnak

Is the vacuum line connected to the fuel pressure regulator and pressure dampener?

stax

Yes it is on both damper and regulator.
Just to reiterate; The engine suddenly misfires and stalls.
It is as if the ignition key is turned off, but the fuel pressure still holds.
thanks for you reply.
cheers'
Stax

Amber Gramps

I'd look at the crank position sensor wires. They may be grounding out if they are cracked and brittle. They pass through behind the heater blower and the sensors are on the driver's side of the bell housing.

Could also be the DME relay going bad.

Sellavo66

Hi, I also have an 84 Carrera that does the same thing��. I have checked and changed ground straps,plugs,coil,ignition switch, ignition wires,cylinder head temp sensor,Dme relay,sent Dme out to be resoldered,adjusted valves,checked wiring.... Still does it! If I hit a bump it cuts out for a milisecond or will cut right out and I have to restart. I sent to a reptuable shop and they can not figure it out. I feel your pain.

Amber Gramps

^^^Sounds like the CPS.

rusnak

It would be fairly easy to check the CPS gap too, so definitely do that first. Is there play in the distributor shaft? Maybe take it out and give the mechanism a good cleaning and lubrication while you check the bushing for wear. A bad O2 sensor will also confuse the DME. You can check that with a multimeter, but if it's old, then just replace it.

ECU Doctors

You can also check your CPS using a multimeter like this:

theiceman

Those sensors look like a nightmare to get to ..

911 Rod

Actually it's a easy job and almost enjoyable. Both sensors come out with the bracket. You do need an old sensor to set the gap.

Synchromesh

An update: changed both the speed and position sensors for brand new ones today. Grommet was very unpleasant to get loose but I got it. I didn't take the bracket out so didn't have to set the gap. However, this didn't really change much in the way the engine behaves.

It really became bad now. Basically when cold it'll start and run for a short while and then die at idle. Then it'll just keep dying and dying for 5-10 mins. After that it'll start and run with problem nearly disappearing.

stax

Greetings:
I have just resumed working on the misbehaving ’84 Carrera.
I have completed the list of things to check assigned by
my fellow Rennlisters. Thanks for all of your input.

The engine still randomly misfires on a average of once or twice every 30 to 90 seconds. <sigh>

I noticed the DME relay clicks at the same instant that the engine glitches.



With a Tektronix 465B oscilloscope I began a trace at the DME duel relay.
The fuel pump relay is the one doing the clicking.
Traced to pin 20 of the 35 pin connector port on the Motronic box. Opened the Motronic box and further traced
through several components all the way to a pin on the 8850 cpu chip.

So the glitch originates at the CPU.
At this point I cannot go any further until I find 3 items .

1) A break out box…….I prefer to make my own. I just need to find a source of the 35pin connectors used on the Motronic Box.

2) The data sheet for the Intel 8850 40 DIP uC chip. (I hear laughing….)

2) The schematic for the Motronic ECU box. ( I hear a chorus of laughing…..)

Any suggestions?……………………………………..

I can think of one myself……………Scrap the Motronic box ( scavenge the connector of corse.) and look into

building a modern replacement…………………………………..Mega Squirt ???



Cheers
Stax

proporsche

Hi boys...
i do not see anywhere that you have changed the fuel filter..If it is too old it will stop the fuel delivery..
Ivan

09mageec

Imagine if this was the problem all along.... lol

SARGEPUG

Welcome and I had a similar situation and went through a lot of troubleshooting. Here's my thread and it wound up being the air meter.

https://rennlist.com/forums/911-foru...d-startup.html