No Start - Broken MPS Wire Options

[MDTerp]

I spent the weekend trying to diagnose a no start issue with no luck. Here’s a short description of the condition and what I’ve already checked. Any help is appreciated.

Drove the car to Carlisle two weeks ago, 180+ miles round trip with no issues. Tried to start a week later and the engine cranks but won’t catch and doesn’t sound like it tries to catch.

1975 2.0 D-Jet with 74 Case/Heads/Exhaust and 75 ECU/MPS
4k miles on engine rebuild and car has always started up easily and idled well
Stock Cam
8.0 EU Pistons
123 Ignition Distributor - Installed last year
Bosch Blue Coil - Installed last year
FJ67 Fuel Injectors - Installed with rebuild

I always try to look at the last parts I touched which were:
Replaced hose between expansion fuel tank and filler neck
Added heater hoses between electric fan and J pipes
(both done after trip and before no start)
These don’t seem related but I assumed I bumped a connection loose somewhere when installing the hose to the J pipes but I couldn’t find anything.

Checked fuel pressure at front side of the rail of cylinders 1&2 which was 28 psi when key in on position and while cranking. It slowly drops over 20mins or so. Pulled injectors from 1&2 and cranked with injectors in bottles. I confirmed fuel spray, although it didn't seem consistent.

Connected a timing light to coil, battery and #1 spark plug wire and the light would not flash when cranked. Pulled cylinder 4 spark plug wire and saw weak yellow spark with spare plug attached to wire and threads held against ground.

Checked #7 pin on 12 pin connection on relay board and measured 12V to ground with key in on position. Checked positive side of coil to ground with key in on position and measured 12V to ground.

Pulled distributor cap and everything looks new and normal.

Replaced Bosch Blue coil with older coil (which worked when removed) and no change in starting.

This also gave me an excuse to replace my 10+ year old battery so I have a new charged battery and no change to the no start situation.

This week I will pull the plugs and see if they are fouled. I might try replacing the 123 distributor with the old stock one. I also have a spare 052 ECU I can try. I'm sure this is something stupid I missed.

Edited: fixed typo for clarity

[iankarr]

You know the drill…air…fuel….spark. Seems like you have the first two. The no flash on the timing light and weak spark leads me to suspect the coil and, secondarily the dizzy. I’d swap the coil first.

[Lockwodo]

You know the drill…air…fuel….spark. Seems like you have the first two. The no flash on the timing light and weak spark leads me to suspect the coil and, secondarily the dizzy. I’d swap the coil first.

(IMG:style_emoticons/default/agree.gif)

[bkrantz]

Remember that shielded plug wires can make some timing light pickups send no or weak signal.

[g911]

I am assuming you have bypassed or eliminated seat belt interlock module under the passenger seat? My 74 had one and it wouldn’t start until I bypassed it.

[Lockwodo]

I am assuming you have bypassed or eliminated seat belt interlock module under the passenger seat? My 74 had one and it wouldn’t start until I bypassed it.

My '74 2.0 starts regardless of whether the seat belts are buckled. Does that mean I either don't have the module or it's has already been bypassed?

[r_towle]

Wiring to distributor got knocked off putting on heater tubes….because you touched it last

[Van B]

Wiring to distributor got knocked off putting on heater tubes….because you touched it last

(IMG:style_emoticons/default/agree.gif) Always.

In college I was replacing the head gasket on my 944. I knocked a vacuum line loose from a T fitting on the firewall as I pulled the head and never noticed. Car wouldn’t start. After a week and redoing everything I could think of, including pulling the head again, I finally noticed the line I knocked loose on day one…

[iankarr]

Wiring to distributor got knocked off putting on heater tubes….because you touched it last

Excellent point! Check the simple stuff first...

[MDTerp]

All great suggestions, thanks everyone. It does feel like something must have been knocked off. I’ll spend some more time on it tonight and give an update.

[MDTerp]

Seat belt interlock is bypassed with Yellow & Yellow/Red wires under seat jumped.

Started checking all connections and found this on the MPS. Will confirm it's the root cause. Looks like it's time for Jeff's FI Wiring Harness.

[Van B]

I could be wrong, but that doesn’t look fresh.

[Lockwodo]

A burned connection like that could mean that the MPS has shorted internally. You should verify the resistances for the primary and secondary coils are in spec, and that the MPS holds vacuum.

[MDTerp]

I could be wrong, but that doesn’t look fresh.

I agree, I think it was hanging on by just a couple strands and I must have knocked it off completely when installing the hose. When I pulled the terminal it was obvious someone had done a repair previously which I think is why there is a burn mark on the connector.

The car started up immediately after a quick repair. I'm happy to find the issue but more work to do for permanent solution. I'll also check the MPS is still working correctly as suggested but no difference in the idle when I started it.

Thanks for the help.

[Van B]

I hadn't considered progressive fraying. It certainly is strange to have just that one wire broken. I'd be interested to know about any causal factors you discover.

Congratulations on the fix though!

[arbitrary]

That external mark looks like from a soldering iron.

Although not visible on the side of the wire that’s exposed soldering electrical connectors on engines isn’t always a good idea. The solder creeps up the wire and makes it very stiff; fatigue over time or mishandling can cause a break.

[StarBear]

Hooray! Hope you can make the NE Gathering again.

[MDTerp]

That external mark looks like from a soldering iron.

Although not visible on the side of the wire that’s exposed soldering electrical connectors on engines isn’t always a good idea. The solder creeps up the wire and makes it very stiff; fatigue over time or mishandling can cause a break.

Yes, there was a lot of solder on the end of the terminal which makes sense that it stiffened the wire and then fatigued. Looking at the mark on the plastic the terminal must have been laying on it when it was soldered.

Still might replace the whole harness, but will consider repairing this connection. I don't have a lot of experience repairing connectors like this.

[MDTerp]

Hooray! Hope you can make the NE Gathering again.

I definitely plan to be there. We had a great time last year. Hopefully this is the only issue I have to resolve to get ready.

[r_towle]

I think if your wires break that easily, it might be time for a new harness.
What do they cost now?
More than a long flatbed ride?