I have done a resistance check of all the coils I have, between pins 3 and 4. I have 16 coils, most of them either fairly new or brand new. These are the readings I get
7 coils read around .385
5 coils read around .950
4 coils read around 1.100
I'm not sure what a correct or good reading would be.

I did and experiment and installed all coils with readings around the .950 mark as I have 8 in that range. Only 7 in the .385 range. Installed and started the car. Same result, car starts for about 2 seconds then stalls. Fuse # 18 blows.

The engine wiring loom in the car was changed when I started having problems with the car in 2019. When I got the new to me engine wiring harness (both sections, from engine to firewall and from firewall to computer) I went through every wire and did a continuity check. Every wire checked out. Just mentioning this as possible pertinent information.

 

I'm thinking about installing one coil and trying to start the car. If the fuse doesn't blow, connect another until I find the cylinder that blows the fuse. What do you think?

 

That's quite a spread. The .385 seems a tad low, but if you're still getting the same result with the .950 then you can ignore that for now. I don't know the correct primary resistance for the coils, but the 'general' is around 0.5-0.6 ohms, apparently.

I'd try measuring across connections 3 and 4 of each coil plug connector next to check for any issues in the loom.

 

Yes, I will measure the coil connectors next. There is a short somewhere.

 

Fingers crossed!

I'd expect a P035x code somewhere, but possibly not.

 

Ok, I did a resistance check on the coil wires pins 3/4. I'm not sure what the numbers mean

Cylinder
1. 3.3
2. 3.2
3. 3.3
4. 3.3
5. 2.2
6. 2.3
7. 2.4
8. 2.0

With key on, all coil plugs have power on pin 2.

 

That, taken with the coil measurements you have, means there's no obvious short across the coil primary paths.

Pin 2 is the feedback pulse to the ECM. They are joined together in groups of 4, one group for cylinders each bank 1A,2B,3B,3A, and the other for 1B,2A,3A,4B. They confirm spark back to the ECM.

Can you do a resistance check between each pin 3 loom connection and ground? This carries the 'coil fire' signal from the ECM.

 

I get .2 on all cylinders except on one which is .3.

 

Drat, this is elusive.
Those resistance values are considerably lower than I'd expect. The ECM will output a ground to each pin 3 connection when it wants to fire the coil in the relevant cylinder. At rest, I'd expect those outputs to be 'floating', so there would be ~12V on both sides of the coil with the ignition on. With the pin 3 resistance to ground at ~0.3 ohms that wouldn't be the case.

I can't reconcile this at the moment with the fuse not blowing with the ignition on, engine not running. Have just checked and re-checked that I've given you the correct pin numbers - 2000 & 2001 electrical guides both agree.

One more check:- can you get to the connections with a meter with the coil plugged in - possibly from the back of the coil connectors - and check for 12V at each pin 3 with the ignition on?

 

First off Michaelh, thank you for persevering with me. I know the solution is elusive. I'll try this tomorrow. Everything that has happened to this car in the last 2 1/2 years has been unexplainable.

 

Yes, I've been following your threads. I spent a similar amount of time tracking down an intermittent misfire/no start on the MGB. Perhaps we were bad in a previous life...

Just so we're definitely on the same page, the 2000 electrical guide shows the coil connector pins numbered out of sequence. (The 2003 guide is similar, except the sequence is different again.) I'd expect the pins to be 1,2,3,4 across the connector.

Comparing wire colours to pin numbers, the EG shows:

1 - black
2 - yellow/green
3 - green with a different colour tracer for each cylinder in the bank - e.g; cyl 1A is green/blue, 2B is green/white etc.
4 -- red/white

Does this tally with what you have?

 

Yes, we must have been very bad in a past life

OK, I was thinking this might be a problem. None of the electrical diagrams match the color coding of my harness. I'm attaching a picture of one of the coil plugs. My 2000 electrical diagram shows from left to right pins 3-2-4-1 but the colors don't match. I also just rechecked resistance on pin 3 which looks like it's the left most pin and did get .1. Of course I may me using the wrong pins.

 

OK - If your replacement harness was from a '99, then we're nearly there:



If the secondary colour on the slate wire can be loosely described as 'light green', then the colours in your pic match coil 1B.

It also confirms the connector pins are numbered sequentially.

 

Yes, that is coil wire 1B. I'm going to redo the resistance test as I was not numbering the wires sequentially. I get back with the new readings.

 

So we're not as mad as they think we are

 

I guess not so mad I redid the resistance test on the coils and found 1 that was way off. Installed the best of 8 and the car started. It runs and I must thank you for that. What a journey. Now I am stuck where I was before the no start condition. Engine runs rough and throws codes for coils on cylinders 3, 4, and 7. Same as it did before, and I know those coils are good. So are the plugs, injectors, etc. I will do a coil test tomorrow to see if those cylinders are actually firing. Michaelh, thank you for sticking with me through this. I can only imagine what you went through with the MGB

 

More progress Well done!
So you're back with the P035x codes, the motor will run sweet for ~5 seconds and then start running roughly? If so, did you have exactly the same symptoms before you replaced the harness?


PS - The MGB was a walk in the park compared to what you're facing. It was ultimately a process of elimination, and the solution was something simple, but I'd never encountered it before.

 

I'll recap what the issue was.
I took the car out of winter storage April 2019. I decided to change the coils as it had all originals installed. After changing the coils, the engine ran rough and I got the codes for bad coils on cylinders 3, 5, and 7. Nothing I have done since then has changed that.
New coils
Spark plugs
new wiring harness
new fuel injectors
re-manufactured ECU
Checked every ground point on the chassis
I always use Techron in the fuel tank every now and then

For about 1 month in the summer of 2020 the car all of a sudden started running well. No rhyme or reason. I drove it for about 1 month. One morning, going to breakfast with my wife, we got about 1 mile away from home and it started running rough. I got it home and it's been in the garage every since. At this point, I'm not sure what to do. I'll check to see if there is or is not a signal on those 3 coil packs. I'm questioning if there is a timing problem. Before changing out the coils, the car ran smoothly with an occasional restricted performance for a minute or so. That was why I decided to put new coils in.

 

Do you still get the 5 seconds of grace from startup before the rough running kicks in?

 

Probably more like 2 seconds. Just enough for me to think, maybe, no.