Engine fires, runs, then quits immediately

[82DMC12]

Yes, the most likely reason is that something in your VOD rebuild left your engine with a decent-size vacuum leak. That would lean out the mixture at any given CO screw setting.

Recommending you run a smoke test to verify/locate any vac leak(s). Ideally you can repair those and then adjust the mixture/CO screw closer to where it used to be. Smart of you to count the adjustment turns to get it running. And smart of you to notice and report that you had to richen the mixture.

So many potential leak points are in play in a VOD job...the Pipe of Agony and its seal, any of the o-rings, vac hoses and components they attach to, the idle speed motor and more.

But what would a vacuum leak have to do with the opening pressure of the injectors? The car was not running as I was setting the CO screw to see if I can get the injectors to start dripping or spraying. I had to turn it clockwise quite a bit in order to get the injectors to start singing. I can see a vacuum leak being a problem while adjusting the screw while the engine is running on a dwell meter, but not with just the RPM relay jumped. Know what I mean?

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[82DMC12]

Update for tonight -

The car is now running! The CO is set roughly at this point as I have not put my dwell meter on it or checked any other engine settings besides timing. I just have it set so the injectors barely stopped dripping, and double-checked by tapping the plate a few times and letting the pressure return and check for drips again. That worked well enough so the car started on the first try. It sounded a bit rough so I put the timing light on and found it was probably around 20 deg instead of 13. Got that adjusted easily so I'm right on the mark and it sounds better now. I didn't run it long because I still need to bleed the cooling system after refilling it and I don't want to do that until I replace the coolant hoses under the fuel tank plate which I found started to leak a bit after I put a pressure tester on the expansion tank.

So, I'll get those in this weekend and then bleed the coolant, and then see what I find with the dwell meter. I'll probably put a new O2 sensor in too since I haven't done that in forever.

I also turned the Accel ignition coil around and swapped the terminals so I now have the wires on the correct posts, but amazingly that didn't seem to do much with the ignition. Weird!

I am out of town for work Thurs and Fri so I won't have much more to add until the weekend, but I'm still trying to understand how the CO got so far off. The initial injector crack setting can only be governed by primary and control fuel pressure. It must be related to the PPR o-ring replacement I did. Perhaps my CO was set LAST TIME with the old o-rings which were maybe not so good and tight?

[DMC-Ron]

...except you shouldn't be showing 12V on the white/yellow wire, ever. You can put 12V on the coil (+) as a test, but don't leave it that way...

Ron,
I think there should be + voltage on the W/Y wire. When cranking the power (+12) flows thru a single resistor, in run, the power flows thru both resistors. After going thru the resistors power goes thru the W/Y wire to the coil. If you pull the W/Y wire off the coil and measure, you should see +12 volts. If you measure connected to the coil the voltage will be lower because of the resistor drop and the coil load. I'm guessing 5 or 6 volts is about right.

Ron

Coil Resistor Circuit.JPG

[Helirich]

Update for tonight -

The car is now running! The CO is set roughly at this point as I have not put my dwell meter on it or checked any other engine settings besides timing. I just have it set so the injectors barely stopped dripping, and double-checked by tapping the plate a few times and letting the pressure return and check for drips again. That worked well enough so the car started on the first try. It sounded a bit rough so I put the timing light on and found it was probably around 20 deg instead of 13. Got that adjusted easily so I'm right on the mark and it sounds better now. I didn't run it long because I still need to bleed the cooling system after refilling it and I don't want to do that until I replace the coolant hoses under the fuel tank plate which I found started to leak a bit after I put a pressure tester on the expansion tank.

So, I'll get those in this weekend and then bleed the coolant, and then see what I find with the dwell meter. I'll probably put a new O2 sensor in too since I haven't done that in forever.

I also turned the Accel ignition coil around and swapped the terminals so I now have the wires on the correct posts, but amazingly that didn't seem to do much with the ignition. Weird!

I am out of town for work Thurs and Fri so I won't have much more to add until the weekend, but I'm still trying to understand how the CO got so far off. The initial injector crack setting can only be governed by primary and control fuel pressure. It must be related to the PPR o-ring replacement I did. Perhaps my CO was set LAST TIME with the old o-rings which were maybe not so good and tight?

That’s good news. One thing that could have happened is the warm up regulator was not reducing control pressure enough. There are several reasons this can occur. That makes it harder for idle vacuum to pull the plate down. I know you said you checked this, but maybe it’s fine at rest and only occurs when running. If this is the problem, you might find it will be too rich after warm up.

[82DMC12]

That’s good news. One thing that could have happened is the warm up regulator was not reducing control pressure enough. There are several reasons this can occur. That makes it harder for idle vacuum to pull the plate down. I know you said you checked this, but maybe it’s fine at rest and only occurs when running. If this is the problem, you might find it will be too rich after warm up.

I'm trying to think about this logically. Anyone who thinks I am not describing this properly, feel free to chime in and have the discussion!

We know the metering pin in the FD is always riding on the roller connected to the "see-saw". The CO screw is able to lift the roller or drop the roller so that the metering pin is "higher" or "lower" in the FD when the air plate is at a neutral position. So, basically the CO screw sets the initial position of the pin within the FD. If we have the RPM relay jumped, we have a constant fuel pressure supplied to the FD and it's just waiting to be routed to the injectors. If the pressure is sufficient, the fuel will spray through the injectors because the opening pressure of the injectors is fixed. There should be zero fuel going to the injectors when the metering plate is in a rest/neutral position, or else the engine would flood while jumping the RPM relay.

If the initial setting of the CO screw is too lean (CCW), the pin will require more travel (air plate pushed down further) to get the fuel to move to the upper half of the FD and then to the injectors. If the initial setting of the CO screw is too rich (CW) the injectors will be leaking into the cylinders.

The only thing that can change the initial CO setting is a change in primary pressure, a change in control pressure, or a change in the physical location of the FD (such as using too thick of an o-ring underneath it).

I did change the O-ring with a new one from DMCMW and I did replace the o-rings on the PPR which could have affected the primary pressure. Maybe between both of those changes, it affected the relative spacial relationship of the metering pin to the slits in the FD.

Since all this is done without the engine running, vacuum cannot be an influence in the initial setting (RPM relay jumped).

Now that I think about it, I don't believe control pressure can influence this setting either, since we do not have any downward force on the meter plate. The control pressure simply provides resistance to the plate so that it requires more air flow to move the pin during a warm condition.

What do you all think? This was written on my first mug of coffee.

[Helirich]

I agree with what youÂ’re saying if the fuel is supposed to spray the minute the plate goes down. IÂ’m not sure if this is true. I very well could be wrong, but IÂ’ve not read at what point the fuel should spray.

For example, let’s say when the motor is idling normally, the plate is down 1/2”. Then I suppose if the fuel started spraying when the plate is down 3/8”, it could run fine. You turn the key, the engine vacuum sucks it down 3/8”, it starts and sucks down further to the 1/2” mark. If you give it throttle, it sucks down further yet increasing fuel for power.

So if your WUR was not reducing control pressure enough, it would be harder for the vacuum to suck the plate down. If idle vacuum could only suck it down 1/4”, it would stall right after the cold start fuel ended. In this scenario, if you adjusted the mixture screw so that a 1/4” of plate travel gives it some fuel, it will probably run. But of course, when you give it more throttle, it maybe too rich.

I completely made up these numbers. I donÂ’t have any idea how far the plate moves at idle. IÂ’m just giving you my logic here. And a possible reason the mixture screw would have to be adjusted that far because of a bad WUR.

Is there a reference that says the fuel must spray with in the first 1/8” of travel?

Edit, please dismiss the stupid “A’s” in my post. My stupid IPad does that all the time on this site. I don’t know why.

[Rich]

But what would a vacuum leak have to do with the opening pressure of the injectors? The car was not running as I was setting the CO screw to see if I can get the injectors to start dripping or spraying. I had to turn it clockwise quite a bit in order to get the injectors to start singing. I can see a vacuum leak being a problem while adjusting the screw while the engine is running on a dwell meter, but not with just the RPM relay jumped. Know what I mean?

Got it. You're right.

Thought you were by now chasing a mixture problem while engine ran, not the original non-start and injector spray issue. Just a misread on my part.

Good to hear you got it running today and are back where you expect the CO setting to be.

[82DMC12]

I agree with what youÂ’re saying if the fuel is supposed to spray the minute the plate goes down. IÂ’m not sure if this is true. I very well could be wrong, but IÂ’ve not read at what point the fuel should spray.

For example, let’s say when the motor is idling normally, the plate is down 1/2”. Then I suppose if the fuel started spraying when the plate is down 3/8”, it could run fine. You turn the key, the engine vacuum sucks it down 3/8”, it starts and sucks down further to the 1/2” mark. If you give it throttle, it sucks down further yet increasing fuel for power.

So if your WUR was not reducing control pressure enough, it would be harder for the vacuum to suck the plate down. If idle vacuum could only suck it down 1/4”, it would stall right after the cold start fuel ended. In this scenario, if you adjusted the mixture screw so that a 1/4” of plate travel gives it some fuel, it will probably run. But of course, when you give it more throttle, it maybe too rich.

I completely made up these numbers. I donÂ’t have any idea how far the plate moves at idle. IÂ’m just giving you my logic here. And a possible reason the mixture screw would have to be adjusted that far because of a bad WUR.

Is there a reference that says the fuel must spray with in the first 1/8” of travel?

Edit, please dismiss the stupid “A’s” in my post. My stupid IPad does that all the time on this site. I don’t know why.

All good conversation here. I'm not sure there's a specification for how far down the plate should go before fuel sprays but my guess is basically immediately because once the CSV stops firing, you need fuel to run, and the vacuum, as you said, is pulling the plate downward. So any downward pull has to result in fuel spray.

I have some Bosch books at home so I'll see if it says anything about this measurement.

My understanding is that from a practical standpoint, after doing a dwell fine-tune, that the CO screw will end up being slightly lean compared to the rough "crack setting" I have done so far.

[Ron]

Ron,
I think there should be + voltage on the W/Y wire.
...

Ron

Uggg...I don't know what I was thinking about, with it removed. ...It would obviously read the battery voltage with the meter itself being in series.

Thanks!
(It's Ron's fault ;-)

[Bitsyncmaster]

Here is what you should expect for coil voltage on our DeLorean.

Assuming you have the stock blue coil, 12.5 volt battery voltage, engine not running, stock wiring and stock ignition ECU, stock resistor setup.

Primary coil resistance is 0.40 ohms at room temp. Ignition ECU drives negative coil to 0.7 volts. So not counting harness resistance the coil voltage will be 4.07 volts (measured from the + and - coil terminals). This is probably the highest voltage you should expect since the harness wire is dropping voltage not accounted for with that calculation.

FYI:
The copper (in the coil and in the car harness) resistance increases 0.393 % per deg. C. My testing was done at room temp (70 deg.F or 20 deg.C).

Normal designed coil current is 5 to 7 amps on most all older cars.