Engine Miss Under Load and With Light Throttle - Diagnosis Help Request

[DeloreanJoshQ]

Hello Everyone,

I've run 10,500 miles/16 months since rebuilding the engine (with all new or rebuilt fuel and ignition system parts) without any issues, save for a 30-year-old short heater hose that blew in 95 degree weather. So I have had alot of fun in a fairly short amount of time with my Delorean!

However, starting yesterday morning, I have come across the following problem symptoms:

-Under load(when taking off from a stop and after shifting and applying gas through each gear) I notice 1-2 seconds of engine miss and it then clears up
-When holding light throttle (1/4 travel- no accelerating or decelerating) I get intermittent missing
-I detect a slight miss at idle
-Hot or cold engine does not affect the symptoms, engine starts normal/with ease, wide-open-throttle after 1-2 seconds is normal/excellent

Here is what I have checked so far:
-Checked all of the vacuum lines per the vacuum diagram, except for the ported vacuum switch that is under the intake manifold.
-Checked all plug wire connections for tightness at the spark plugs and distributor.
-Checked the coil wire at distributor and coil for tightness.
-Checked wiring for tightness at coil and ballast resistor on firewall.

Before I buy possibly unnecessary parts (new wires, plugs, cap, rotor, fuel pump, etc.), I thought I would reach out to the community for help.

If you have any more suggestions, I would greatly appreciate them.

Thanks!

Josh Q

[deloumis]

To me it really sounds like one of your wires are are going out. Test them with a timing light.

[DeloreanJoshQ]

To me it really sounds like one of your wires are are going out. Test them with a test light.

How do you do that with a timing light? Would testing the resistance of each wire do just as well? I have Taylor brand 8mm wires. Anyone know what the resistance should be per foot?

[deloumis]

How do you do that with a timing light? Would testing the resistance of each wire do just as well? I have Taylor brand 8mm wires. Anyone know what the resistance should be per foot?

Hook up power and ground normally, hook up to cylinder 1 wire. Look at the light, it should be a steady strobe. If it is, that wire is working fine, go to next wire, cylinder 2. Do the same for each and you will know if one is not working because you will see the light going off and on most likely. If you do find one is not working continue testing each wire. Do not assume it is your wires that are bad it could be your plugs or cap also. Before checking each cylinder start by checking the coil wire. Also should be a steady strobe almost as if it is just on. Should be perfectly steady or something is wrong, that goes for each cylinder wire as well.

Not sure about testing resistance.

[Bitsyncmaster]

My guess would also be an ignition problem. Clean the pins on the resistor block, pull and reinstall the bulkhead connectors, pull and reinstall the ignition ECU connector. If that does not cure the problem, I would borrow another ignition ECU and try that.

[DMC1983]

My guess would also be an ignition problem. Clean the pins on the resistor block, pull and reinstall the bulkhead connectors, pull and reinstall the ignition ECU connector. If that does not cure the problem, I would borrow another ignition ECU and try that.

can't rule out the possibly of the distributor hold down nut working loose

[David T]

Sounds like a dying ignition coil. I would suspect an ignition system problem over a fuel system problem but either is possible. Intermittent problems can be hard to track down. The easiest way is to substitute known good parts and see if the trouble goes away.
David Teitelbaum

[DeLorean03]

These symptoms are similar to what I had going on - my problem was my fuel injectors were dirty and did not have the correct spray pattern.

The unfortunate problem with your symptoms, as others have already suggested, is it can go fuel or ignition. You could measure the resistance of each spark plug wire to see if it measures any resistance (good) or infinite value (bad). The bummer with that is the work it takes to do such a simple test.

This brings up a really good point: we need a reference how-to for ignition systems. How do we determine if the coil is bad, how the ballast resistor is bad - those kinds of things would be VERY helpful to know.

[DeloreanJoshQ]

-Alright, I am going to test the primary and secondary resistance of my Pertronix coil. I'll need to get the specs from them that it should read on my voltmeter.

-Regarding R/R of the spark plug wires, when I put them on it was before the intake was installed. What is the easiest way to remove them? Can it be done without removing the fuel distributor? If I'm going that far, I may as well replace the spark plugs in case one has a hairline crack in the porcelain?

-I did check all of the bulk head connections and they seemed to be ok.

Thanks for all of the help!!

[DeLorean03]

To test the resistance, you have to pull each plug wire off the distributor ._. .....

Once you do that, you just take your multimeter, set it on the appropriate resistance value (try each one until you get a value once you do the next step), and then take each multimeter leads and put them on opposite metal ends of the plug wire.


I'd have a set ready just in case you find one with infinite resistance. Then I'd measure all of my new plug wires resistances to make sure I wasn't putting in a "new" broken part.

That's just one of many things it could be, but at least that gets you started with the suggestion I made. Do any others that don't lead to taking the fuel mist/mixture unit loose to get to the ignition dist first though =/ ...