86 635CSI engine randomly cuts out

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mattv228

86 635CSI engine randomly cuts out

Post by mattv228 »

im at a loss now. my car will be driving fine then it will decide to cut out randomly. i mean the engine just instantly dies. i thought it was the fuel pump but i rigged that to a switch in the mean time to figure this problem out due to it being sporatic. it has a new fuel pump, pre supply, ECU, both crank sensors, coil, wires, cap and rotor. and new engine main relay. i cant figure this out. it did have a bad wire on the rear crank sensor but i fixed that and for 2 days i had no spark to the motor but today it started fine. it usually only happens when i get up to 4th gear. anyone have any ideas?
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Brucey
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Post by Brucey »

my instinct is that you might have an intermittent connection somewhere, a fuel supply problem, or possibly a bad wiring harness.

I would suggest that when the engine won't run, you probe the harness from the ECU end and see if all the sensors read OK .

The other thing to be sure of is that (despite your bypass switch?) that the OBC immobiliser relay isn't able to inhibit fuel pump or other operation. These are known to fail, although it usually manifests a no-start rather than a no-run.

If necessary, I'd suggest that you fit a fuel pressure gauge, and rig up telltale lamps that show injector signal and ignition signal. This should show you if you are missing one or all when the motor quits.

cheers
~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
mattv228

Post by mattv228 »

Brucey wrote:my instinct is that you might have an intermittent connection somewhere, a fuel supply problem, or possibly a bad wiring harness.

I would suggest that when the engine won't run, you probe the harness from the ECU end and see if all the sensors read OK .

The other thing to be sure of is that (despite your bypass switch?) that the OBC immobiliser relay isn't able to inhibit fuel pump or other operation. These are known to fail, although it usually manifests a no-start rather than a no-run.

If necessary, I'd suggest that you fit a fuel pressure gauge, and rig up telltale lamps that show injector signal and ignition signal. This should show you if you are missing one or all when the motor quits.

cheers
ill try the pressure gauge. and i have fuel i know of because i can hear the pump running. where is the immobilizer relay to know if that needs changing or not. because the car will die and i wont be able to start the car until everything cools down
Chris Wright

Post by Chris Wright »

It could be a bad ignition switch also. The power flows from the ignition switch, through the immobilizer (OBC Relay) and energizes the Main relay. The output of the Main relay in-turn energizes the fuel pump relay. To check the immobilizer, go to the heading: Checking the OBC Relay: below.

_____________________________

This procedure "No Start" procedure is for cars from '83 to '87, the '88/'89 have their main relay wired slightly different and different Ref. and Speed Sensors.

Break down of components needed to get spark to the engine:
Ignition Switch, OBC Relay, Main Relay, Coil, Reference Sensor, ECU

____________
  • The Engine Cranks Over
First check for spark:

Pull a plug wire and get an extra spark plug* and plug it in. Position the plug so that the metal shell is grounded and try to start the car. Watch the plug to see if you are getting a spark. If you are getting spark, skip to the next section to check for fuel.

If you are not getting spark at the plug, test again at the coil tower. if you get spark at the coil but not the spark plug, check the Distributor Cap and Rotor.

(* or use a spark tester, these can be had from an auto parts store. You may also need an extra plug wire to plug in and test at the coil tower.)

If no spark from either the Coil or Spark Plug:

Checking the ignition Switch and Coil:
  • Check, with the key on, for voltage at terminal 15 (+) on the coil (voltage will appear at Term 1 also). If no voltage, the ignition switch or wires to it may be bad. Wiggle the ignition switch to see if it is intermittent**.

    Check, with the key off, the resistance between Terminals 1 (-) and 15 (+), of the coil. It should be around 0.50 Ohms. The resistance between terminal 15 (+) and the high tension center tower should be around 6000 Ohms. Replace the coil if it has high or infinite readings (Open) or little or no resistance (Short).
Checking the OBC Relay:
  • Next, pull the Main relay and with the key on, check for voltage at pin 86 and pin 30 in the relay holder (the numbers are molded into the bottom of the relay).

    If there is no voltage at pin 30, check the Red wires at the Positive Battery terminal (Clean and tighten them regardless).

    If there is no voltage at pin 86, check the OBC relay box under the driver's side kick panel, by jumping:
    • New Style OBC (10/'84):
      the two green wires (pins 3 & 4) in connector C2.

      Old Style OBC:
      pins 14 (GN) and 15 (GN/RD) in the black connector C1 (See page 116/117 in the ETM for pin locations).
    If there is no voltage at pin 3 of C2 (New Style), or pin 14 of C1 (Old Style), the ignition switch may be bad (both terminal 15 on the coil and the OBC pins above are powered from pin 15 of the ignition switch).

    If voltage is present at pins 86 and 30: It is easiest to just jumper the Main Relay as in the Photo below or swap in a new/good main relay and then skip to the "Checking the Speed and Reference Sensors" below. Otherwise check the main relay as in the "Checking the Main Relay" steps after the Photo.
Jumping the Main and Fuel Pump Relays:
  • Image
    _________
Checking the Main Relay:
  • Check the circuit diagram on the side of the Main Relay to see if it has two separate contacts for the output pins 87 and 87b or if it just has one contact going to two pin 87's. Clean the relay prongs and the sockets in the holder base and plug the Main relay back in.

    If there is just the one contact: With the key on and the Main relay plugged back in, check for voltage at the RD/WH wire in an fuel injector plug (or use a "noid" light on the injector plug).

    If there are two contacts: With the key on, check for voltage at BOTH terminals 87 and 87b of the Main relay. (You have to wrap a wire around each pin and plug the relay back in and test for voltage at each wire. Don't short between the pins!! One terminal can be good, suppling power to the fuel pump, and the other bad, not suppling power to the ECU, or vice versa.
    • Alternately, to check both relay contacts and the Start Input: With the key on, check for voltage at Pin 4 (Start Input) and Pin 35 in the ECU harness plug, and at the RD/WH wire in an fuel injector plug (or use a "noid" light on the injector plug).
    If there is no voltage in the tests above: replace the Main Relay. As stated, it might be easier to just replace the Main relay out of hand in the first place, instead of doing the testing. It is also a good idea to keep a spare Main relay in the trunk Tool Kit and just replace it (and the Fuel Pump Relay) out of hand if the car stops out on the road.
Checking the Speed and Reference Sensors:
  • Check the plugs to the sensors first, the plastic can get brittle with age and they can crack around the wire locking bail leaving them loose. Then check their resistance as below (won't hurt to check them both hot and cold if the car's hard starting is temperature sensitive):
<a><img src="http://bigcoupe.com/ims/pic.php?u=21GvpX2&i=349" border="0"></a>
_______________

The resistance test above is simpler, and gives you info on the condition of the crank sensors, but the AC voltage output test below is a bit more conclusive, as it also checks for the rare instance of a missing Ref. pin for the Ref. Sensor in the flywheel (Automatic transmission only):
  • 1. Pull rubber boots back off of the two plugs on the sensor lead side (not the harness side). Pull the main relay & fuel pump relay to disable the FI system.
    2. Set your voltmeter to AC volts and back probe the black and yellow wires while cranking the engine.
    • Speed Sensor (Black Connector - reads flywheel Teeth) -- 4.0 AC volts max while cranking the engine.
      Ref. Sensor (Grey Connector - reads flywheel Ref. Pin) ------ 0.4 AC volts max while cranking the engine.
_______________
Sulimon
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Re: 86 635CSI engine randomly cuts out

Post by Sulimon »

mattv228 wrote:im at a loss now. my car will be driving fine then it will decide to cut out randomly. i mean the engine just instantly dies. i thought it was the fuel pump but i rigged that to a switch in the mean time to figure this problem out due to it being sporatic. it has a new fuel pump, pre supply, ECU, both crank sensors, coil, wires, cap and rotor. and new engine main relay. i cant figure this out. it did have a bad wire on the rear crank sensor but i fixed that and for 2 days i had no spark to the motor but today it started fine. it usually only happens when i get up to 4th gear. anyone have any ideas?
Yes this happened to me also. Same model even. It turned out to be two plugs in the wiring harness that run along the top of the engine. One at the front and one at the back of the engine. Both connections were on the way out with heat and age. I doctored mine for a better contact and never had that problem again. I intend to reterminate with replacement plugs asap...
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SORIANOTRON
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Re: 86 635CSI engine randomly cuts out

Post by SORIANOTRON »

Sulimon,

PM sent I believe I also have narrowed my issue down to these same 2 wires on top of the engine.

Anyone have pictures of said wires?

Ill try to post some of mine shortly.
635911
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Re: 86 635CSI engine randomly cuts out

Post by 635911 »

Photos would help me with my (similar) issue.

Thanks,

Toby
1986 635 CSi Lapis Blau with Anthrazit buffalo
1988 911 3.2 231HP Grand Prix white (Sold - 13 years of Zuffenhausen fun))
1984 BMW R80 G/S Dunkel Blau
1982 Yamaha SR500 scrambler
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SORIANOTRON
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Re: 86 635CSI engine randomly cuts out

Post by SORIANOTRON »

here you go

the 2 connections at the top have previously been jerry rigged with heat sensitive tape, they look like they have seen better days and I was initially thinking this was the root of the problem until I noticed the connection from the FUEL LINE? to the engine is FRAYED/MELTED pretty bad....

Image

Can anyone identify the melted/frayed connection/part?

Thanks guys
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SORIANOTRON
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Re: 86 635CSI engine randomly cuts out

Post by SORIANOTRON »

^ this is actually called a “Earth Strap For Rocker Cover”

Ordered a replacement, will update once it has been swapped.
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SORIANOTRON
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Re: 86 635CSI engine randomly cuts out

Post by SORIANOTRON »

635911 wrote: Wed Oct 31, 2018 10:56 pm Photos would help me with my (similar) issue.

Thanks,

Toby
Toby the 2 wires on the top i was referencing were the SPEED SENSOR & REFERENCE SENSOR.
Image
635911
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Re: 86 635CSI engine randomly cuts out

Post by 635911 »

OK, thanks. I thought there may have been something else I had missed.

Cheers,

Toby
1986 635 CSi Lapis Blau with Anthrazit buffalo
1988 911 3.2 231HP Grand Prix white (Sold - 13 years of Zuffenhausen fun))
1984 BMW R80 G/S Dunkel Blau
1982 Yamaha SR500 scrambler
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TN_M6
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Re: 86 635CSI engine randomly cuts out

Post by TN_M6 »

If not mentioned when the cars instantly dies it must be electrical on the ignition/computer side. You can pull the fuel pump relay or fuse and it will still run briefly off residual pressure. Fuel loss is not instantaneous unless it's injector pulse.

On the speed and reference sensors one can be unplugged (ref) AFTER starting and the car should still run but with speed NO. Mix them up and the car will NOT run. You can also test each for proper resistance at about 1000 ohms between 1&2 and near infinite between anything and pin 3. Another good test is the AC voltage while cranking. Pretty low on the speed and about 1 vAC on reference to see if they have a signal.

So there are a host of good suggestions above and sorry if I duplicate but one that may be missing is the C101 connector that brings the body electrical to the engine harness. The coil power for example comes from the ignition switch independent of the DME and fuel control. A bad connection there can mean no spark even when you appear to have injectors firing and fuel pressure.

If it will restart coasting and kick back in, clutch or automatic will do this if it is a bad connection. You can wiggle the ignition switch and head for bumps in the road to try that.

If the ignition must be cycled for it to go again and that almost always does the trick, it's probably a weak relay like main, fuel, or code that must be cycled.

If it was not mentioned anywhere above the ETM at Wedophones is a must have reference for all the schematics. A long piece of wire and a DMM to let you watch various suspects under the hood is simple and effective way to rule things out.
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SORIANOTRON
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Re: 86 635CSI engine randomly cuts out

Post by SORIANOTRON »

so just a follow up for future reference,

I ended up swapping the ground strap and nada

I ended up discovering there was no spark and traced the issue it to be the speed sensor like most previously mentioned
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SORIANOTRON
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Re: 86 635CSI engine randomly cuts out

Post by SORIANOTRON »

SORIANOTRON wrote: Thu Nov 01, 2018 6:44 pm
635911 wrote: Wed Oct 31, 2018 10:56 pm Photos would help me with my (similar) issue.

Thanks,

Toby
Toby the 2 wires on the top i was referencing were the SPEED SENSOR & REFERENCE SENSOR.
Image

are the speed and reference sensor the same part #?

I recently replaced the "Crankshaft Sensor (Position Transmitter)" part #12521287784

and this fixed my issue, now the same issue is returning Im wondering if it is the other sensor?
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Brucey
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Re: 86 635CSI engine randomly cuts out

Post by Brucey »

they are indeed the same pn and are just labelled differently when installed in the car, so you don't get them mixed up.

It could indeed be a dodgy sensor (the one you didn't replace before, of course....) but as with any intermittent electrical issue the trick is to catch it definitely not working (i.e. when the fault is present) so that you know for sure what the problem really is. All too often the problem appears to have been solved by replacing a bunch of parts and either

a) it isn't really fixed at all or
b) you still don't know which of the potentially useful spare parts you took off the car is actually any good or not.

FWIW I carry spare sensors in the back of my car.

cheers
~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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SORIANOTRON
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Re: 86 635CSI engine randomly cuts out

Post by SORIANOTRON »

Hey guys just wanted to follow up with my issue....

So after swapping out BOTH the speed and reference sensor I was still having my car cut out/die after it would heat up for about 10-15 minutes.

Pulled the ECU and noticed right away some of the connections seemed burnt, took it to my local mechanic (not a dealer) and had him scan the ECU

BOOM

Sure enough the ECU was bad.

Had sent the OG out to be rebuilt by ModuleExperts in Florida but unfortunately they didnt have the parts to rebuild.

So I ended up buying a completely rebuilt Bosch 059 unit from PROGRAMA INC.

Plugged it in and BOOM havent had the engine cut out/stall issue since.

Hope this info is helpful for someone experiencing the same symptoms in the future.
baders
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Re: 86 635CSI engine randomly cuts out

Post by baders »

Great result and thanks for following up.
1986 635csi LOCUTUS / Arktis Blau Metallic / Pearl Beige / S38B36/ Close ratio dogleg 5sp
2002 E46 M3 / Topaz Blau Metallic / Black Nappa / 6 speed SMG software tuned / Bilstein PSS10 Coilovers / CMP RACP reinforcement
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