E30 M20 C191 Plug Replacement

The C191 plug connects the fuel injector harness to the rest of the electrical system. A bad C191 plug can lead to all sorts of problems –

  • hard starting
  • rough idle
  • lack of power
  • fault codes for fuel injectors or temp sensor

The location of the C191 plug makes it susceptible to water. Check out the corrosion on my plugs:

The connectors are not available separately from BMW. You have to buy a new fuel injector harness or engine harness. But why replace one defect with another? So that’s why a lot of E30 fixers have went aftermarket. I chose a motorsport/military spec Deutsch DT connector. I really should have read up on this during my engine build and it would have saved a lot of headache later on.

Do not hardwire the harnesses together. Although it may be tempting as an easy fix, there are major drawbacks to it. They should be separated so the harness can be removed during manifold or cylinder head service.

Replacing the connectors means cutting them off, exposing the wires, crimping pins to the ends, and inserting them into the Deutsch connector. There’s no easy way to do this with the harness on the car but removing it is very difficult too (and is only half of the harness).

The Deutsch connector is not for the weak. First, the connector set that I ordered from Amazon came with no instructions or diagram (no surprise). Videos and online descriptions were no help. The pins crimp differently than the usual crimp connectors. My multipurpose crimp tool did not work well and I settled for ordinary needle-nose pliers. Access and room to work is terrible. Once the wires are crimped they have to be pushed through the rubber seal and then locked down in the connector.

Here’s where a diagram would have come in handy: I didn’t know which pin went where. I wrongly assumed the male pins went in the male connector. After getting all 12 pins in place I realized it was the other way around and the pins weren’t making contact inside. Doh! Live and learn.

The male pins fit in the larger female housing and get locked in with the green wedgelock. The female receptacles fit in the male connector and should be flush with the housing (or just below the surface of the orange wedgelock once installed). I had to carefully pull them all out and run them through the opposite connector. Once that was done, probing with my volt meter confirmed the injector power line had 12.5v with the key on. I also probed the pin on the DME connector and the coolant temp sensor and both had the same resistance. So my little wiring project should be a success!

So why are the injectors not spraying and the engine still not starting?

To read more on this situation, see my M20 starter post by clicking here.

Click here for my C191 connector photos.

E30 M20 C191 Plug Replacement

E30 M20 Starter Problems


My 323 did not start a month or so ago. I turned the key but there was no response from the starter. I had strong lights on the dash and 12v on my battery gauge so I didn’t suspect a weak battery. I was able to roll and bump start the engine.

After that there were no issues for a few weeks until it happened again. Because it was intermittent, and I have other key issues, I first suspected that the ignition switch was failing. The switch is no longer available from BMW. And I think the current ignition switch is from an E30, which is NLA too. I looked into a keyless push-button start system but did not commit to buying one. Then the car acted normally for another few weeks.

In the meantime I did a lot of research. One possibility was the engine electrical grounds. I re-used all of the E21 grounds so I’m not missing any. It’s possible it could use more but given the intermittent behavior I mostly ruled this out. That left the starter and starter solenoid itself.

One helpful feature in older BMWs is the ability to bypass the key and activate the starter from the diagnostic port. By jumping pins 11 and 14 you give power to the starter solenoid. If the key is off the engine spins without starting (such as when adjusting valve clearance). But with the key on, giving power to the fuel pump, cranking the engine can make it start and run.

If the engine doesn’t crank with the key:
Key on / pins 11-14 wired = if the engine cranks, the key is the problem.
Key on / pins 11-14 wired = if the engine doesn’t crank, the key is not the problem.
Follow up these tests with more cranking on the key to confirm.

As useful as that test is, my problem turned out to be the starter. I had no cranking on the key so I jumped the pins and… no cranking. To be honest, most Internet diagnosis pointed to the starter anyway. But I was hoping for an easier fix since changing a BMW starter can be a big pain. I smacked the starter body with a long extension and a rubber mallet (using care not to touch the terminals). Follow a couple of whacks with a turn of the key. After the 9th or 10th whack the engine fired up.

After all that guess I need a starter. And maybe some better grounds just for good measure.

Update 1: a new Bosch starter did not resolve the no crank issue.

The starter swap went smoothly and took just under an hour. But when I turned the key there was just silence, like before. I jumped pins 11 and 14 again and it cranked. Interesting. I let it crank but it didn’t actually run. After checking connections for ten minutes I was able to get it to run but it was very very rough. I plugged in a Peake scan tool and there were tons of codes – coolant temp sensor, fuel injector #3, air flow meter, etc. I cleared them instead of recording more. When I checked again there was only the coolant temp sensor fault. I checked spark plug wires and the idle control valve and some other connections and started it again. This time it ran smoother but idled low. I know I have idle and throttle position sensor issues. The TPS is not perfectly set and I have tinkered with the idle control screw on the throttle body. Perhaps in my reassembly after the starter swap I knocked something loose? I raised the idle via the screw and that helped. After the engine warmed up a bit the misfire, or whatever it was, cleared up and it sounded as good as before. But the no crank issue continues to be an intermittent and annoying problem.

I’m going to go through the grounds next. It sounds like a good idea even if it doesn’t fully fix my issue.

I will also pull the steering column covers off and inspect the ignition wiring.

Update 2: don’t use FedEx Smart Post for shipping – it takes forever. Even though I’m not in a hurry it’s just annoying knowing I could have had the order in half the time. 

Update 3: the starting problem is getting worse. I have plenty of cranking power but it does not want to catch for the first 30 or so cranks. Then when it does it runs really rough.

The fault code for the coolant temp sensor persists. After reading on the excellent site  www.e30zone.com, the blue coolant temp sensor on the thermostat housing tells the ECU to richen the fuel mixture until the O2 sensor is warm enough to control fuel mapping. It’s a replacement for the cold start fuel injector (the “7th injector”) used on earlier systems. Why am I getting the code?

E30zone recommends a procedure for testing the wiring and sensor using a multimeter. I started with the sensor and it tested fine! That could really only mean one thing: wiring. Ugh. 

Update 4: the C191 connector for the fuel injection harness was full of corrosion. The pins were that special shade of blue that promises to shelve any plans of conducting electricity through them. I replaced the C191 connector with a Deutsch connector. And still the engine cranked but did not start.

Update 5: after confirming the Deutsch connector is passing current I pulled the ECU and checked for corrosion and found none. I then checked the file loaded on the Ostrich chip emulator in case it was corrupt or missing (it passed). So the ECU is probably functioning normally. 

On to fuses and relays. On my Euro 1980 323i there doesn’t appear to be a fuel pump relay. And my relays are arrayed differently than the images for a US 320i online. There is a fuse though so I pulled it out, inspected it, rubbed a little Scotch-Brite on the contacts, and reinstalled it.

There is a series of relays for the E30 M20 – DME, O2 heater, and fuel vapor purge valve. From the notes on the M20 swap wiring I found that the green/violet wire on one of the relays mates to the fuel pump power in the E21 harness. So I jumpered pins 30 and 87 and it fired right up!!! Bad relay? I plugged the relay back in and it still started. Maybe loose or faulty relay operation? It was a new relay when I installed the engine but I will carry a spare anyway.

I let the car warm up and had a functioning temp gauge and the engine revved and sounded fantastic so my C191 fix was a succces. Maybe I’ll get a few more driving days in before winter storage. 

E30 M20 Starter Problems

M20 Engine Specs

Here are the specs of my initial M20B28 engine build.

I started with a standard M20 2.5 that was mostly complete but partially disassembled. A 2.7 crankshaft, connecting rods, and flat-top pistons came with it. The 2.7 rods and pistons are commonly used in turbo builds.

I used the 325i rods and custom Wiseco pistons. I could not use the stock i pistons because they are too tall with the long stroke of the 2.7 crank.

Here are the final specs:

Type: M20B28, SOHC, 12-valve, inline-6, 325i iron block (bored) and 325i aluminum head, port fuel injection
Displacement: 2,758cc / 168cu in.
Bore: 85mm
Stroke: 81mm
Camshaft: stock 325i
Compression ratio: 9.5:1
Fuel injectors: 18.25psi @ 3.0 bar
Intake: stock 325i AFM and intake manifold, K&N cone filter
Exhaust: stock 325i exhaust manifolds, stock E21 323i non-cat exhaust
ECU: Bosch 0261200173, custom chip tuning
Power: 190hp (est.)
Torque: 190ft-lbs (est.)


M20 Engine Specs

M20 Conversion Basics

Could there be an easier engine swap than an E30 325i M20 into a E21 323i? Actually, yes: an S52 into an E36 is pretty plug and play. But this almost as easy. There are a few really good guides online, including the ones covering 320i swaps. They didn’t always apply to my project so here’s what I learned along the way.

First, here are the best resources I found:

Todor’s: http://www.todor.info/repair/e21swap/

bf.com thread: http://www.bimmerforums.com/forum/showthread.php?1045649-How-to-swap-M20s-E21-323i-to-Motronic-E30-325i

The swap is easy enough that you don’t need a lot of how-to. This is what you will need:

Complete M20 engine with wiring and ECU (get it as complete as you can)

E30 air flow meter

E30 cone filter intake (the E30 air box might fit but I didn’t try)

E30 throttle cable

New rubber fuel lines from the frame rail to the E30 fuel rail.

E21 fuse box plug and pins

E21 fuse box plug

E30 C101 connector. The top should be on the engine wiring but get the body-side for a factory look.

If you have a 1988+ M20 you will need the 1987 cooling system parts (water pump and hoses). Some of the hoses can come from the E21 system.

An extra E30 front exhaust manifold. The E30 exhaust layout has the manifold outlets next to each other. The E21 has them spread apart. This might vary from car to car, however. I could have used the E21 manifolds but they looked too rusted on the studs. On my car I could not use the E30 rear manifold and had to source a front one. Everything bolted up from that point.

E30 oxygen sensor and bung. And realize that the bung cannot be welded to a rusty exhaust pipe.

You will probably want to relocate the battery to the trunk. That required a battery tray, battery cable, junction box for the engine bay, and wiring from the junction to the starter and alternator. And you will need to cut several holes in the interior sheet metal.

You also need to cut a hole in the firewall for the engine ECU so it can mount under the dash, a la E30.

The E21 flywheel and clutch will bolt on to the E30 M20. Some of the parts are actually the same anyway.

Make sure the engine is complete and running. If you buy a non-running engine upfront you will be chasing those issues later instead of driving it. I chased a no-start issue for weeks because the engine I bought needed basic maintenance. Here is what tripped me up once the engine was in the car:

  • fuel pump relay
  • short in the fuel rail
  • alternator voltage regulator
  • alternator ground
  • TPS adjustment
  • throttle stop screw

They’re not expensive so make sure this stuff is taken care of.

You will need the motor mount arms from the E21 (which you won’t get until the engine is out of the car).

As for the actual swap itself, I did it in my driveway with a Harbor Freight engine hoist. You have to pull the hood off and the latch mechanism and rods need two people to safely remove.

I pulled the engine with the transmission attached but it was SCARY. I won’t be doing that again. It came out pretty well but the two pieces together made it a struggle to reinstall.

Here is the wiring that needs to be done:

oil pressure light in instrument cluster
fuel pump power
alternator charge light in cluster
ignition coil power
ECU power
power to starter
coolant housing temp to cluster
RPM signal to tachometer. This was the only connection that did not work. I ended up running a dedicated wire from the coil to the C101 plug (bypassing the E30 ECU).

Here is my wiring inventory:


I was very pleased with the results and how straight-forward this was. It was a great learning experience and I’m enjoying the fruits of my labor.

in it’s new home


Here are the links to my engine build and pics from the swap: http://douging.smugmug.com/Cars/BMW-M20-Engine-Restoration

M20 Conversion Basics