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 =/ ...

[deloumis]

Taking all the wires just to realize is a lot of work especially if you remove them and find out it was the coil. That's why I gave you a simple test to do. I always use that method just to confirm there is fire in that cylinder and working correctly.

[DeLorean03]

Taking all the wires just to realize is a lot of work especially if you remove them and find out it was the coil. That's why I gave you a simple test to do. I always use that method just to confirm there is fire in that cylinder and working correctly.

That is kinda why I said...

"Do any others that don't lead to taking the fuel mist/mixture unit loose to get to the ignition dist first though =/ ..."

No need for him to do all that work when it could be something simple first.

[DeloreanJoshQ]

OK, so here are the results of the Pertronix Flamethrower 1.5 ohm 40,000 volt coil test (including subtracting initial resistance by touching both test leads together). Again, the problem is not intermittent and cold or hot it is always there. I tested the coil cold.

Primary Resistance (setting multimeter to 200 ohm-touching black to (-) and red to (+)) - 1.4-1.5 ohm fluctuating
Secondary Resistance (setting multimeter to 200K ohm -touching black to (-) and red to the coil wire terminal) - 9.9 steady

What do you all think?

[DeloreanJoshQ]

OK, so here are the results of the Pertronix Flamethrower 1.5 ohm 40,000 volt coil test (including subtracting initial resistance by touching both test leads together). Again, the problem is not intermittent and cold or hot it is always there. I tested the coil cold.

Primary Resistance (setting multimeter to 200 ohm-touching black to (-) and red to (+)) - 1.4-1.5 ohm fluctuating
Secondary Resistance (setting multimeter to 200K ohm -touching black to (-) and red to the coil wire terminal) - 9.9 steady

What do you all think?

I had a new coil tested and it read 1.6 for primary and 9.5 for secondary.
Any thoughts?

[deloumis]

If you have a new coil slap it on and go for a test drive. I still believe it is the wires, have you ran any test in them?

[David T]

Now that you mentioned you have a Pertonics coil I would definitely try another coil and you may have burnt up your ignition wires, distributor cap, and rotor. I would also pull and examine the spark plugs.
David Teitelbaum

[DeloreanJoshQ]

-The wires are Taylor 8mm Spiro Pro that are advertised at 350 ohms per foot resistance. They are custom sold by John Hervey.
I'm not sure I understand how a Pertronix 40K volt coil would burn these up as these wires have been used with hotter coils in V8 applications.

-I ordered Bosch silver tip HR6DS from Autozone S/O..I'm going to wait until I get these to install them before I start pulling wires to resistance check them.
-I'll try a stock-style ignition coil just for kicks...
-I cleaned all of the pins on the resistor block and also researched the wiring. The block was previously wired with the lower left side having a wire connector going to it (on the "bent" tab). According to research on here and the electrical wiring diagram, it is supposed to go to the upper right side of the block. So I corrected that. However, it had no affect on performance....
-I R/Rd the yellow, blue, and white bulkhead connectors and also ignition ECU connector under the opposite side of the fuse block area.
-I checked the distributor and it is tight.

None of these checks or changes had any effect....so now I am going to wait and decide what to do with the plugs, wires, cap, rotor, and coil......

[DeloreanJoshQ]

I checked and the MSD 8.5mm Superconductor wires Hervey sells are only 40-50 ohms of resistance per foot. If I need to replace the wires, I will purchase these over another set of Taylors....but first I'm going to check and replace the spark plugs..

[Bill Robertson's Sock Puppet #1]

Pertronix coils are fine. Many owners are running them or their MSD counterparts no problem. A common mistake some newbies make is to orient the coil and wire it by the label on the side of the canister rather than the + and - signs on the top. White with yellow goes to positive and white with gray goes to negative.

Unless you have upgraded your ignition to HEI your Pertronix coil is no hotter than a Bosch blue coil anyway.

Spiral wound wires rarely go bad. That is the biggest reason to use them over Bosch or Bougicord wires, which do fail with time.

Ignition modules can certainly fail with time but they usually fail altogether.

I suspect your problem is cap and rotor wear. Pull them off and look inside. While your rotor is out measure resistance across it. Many owners do not realize that Bosch rotors have a resistor potted inside. Anything more than 5K ohms is too much.

[DMCMW Dave]

[QUOTE=DeloreanJoshQ;73952
None of these checks or changes had any effect....so now I am going to wait and decide what to do with the plugs, wires, cap, rotor, and coil......[/QUOTE]

What plug gap are you running? That has much more impact on system voltage and therefore stress on the cap and rotor than plug wire resistance or coil type. .

[DeloreanJoshQ]

What plug gap are you running? That has much more impact on system voltage and therefore stress on the cap and rotor than plug wire resistance or coil type. .

Stock...I'll double check it (after 10K miles) when I pull them.

Also, the coil is hooked up correctly....

[Bill Robertson's Sock Puppet #1]

Caps and rotors do not last forever. Every time a spark jumps their gap it takes a tiny bit of metal with it. Eventually the gap gets too big for spark to jump consistently. It's no different than erosion of spark plug electrodes.

Because they are so difficult to replace DeLorean caps and rotors sometimes get left in too long.

[DeloreanJoshQ]

I removed the spark plugs tonight.

-The passenger side bank all look pretty even with a tan-ish/white color
-The driver side bank- the middle plug and plug closest to the rear bumper were black fuel fouled(bumper one more than the middle one). The one closest to the front of the car was pretty close in comparison to the passenger side bank. The gaps were all consistent and were narrow (i didn't check them with a tool though)
-The middle plug wire was stubborn to get off and when I tried to persuade it, the metal connector stayed on the plug and separated from the wire, ugh.
-I was originally told to pull all of the fuel injectors, removed the linkage from the front of the engine and remove the allen bolts that hold the center section to the "W" pipe to lift the center section out and get to the distributor. I started doing this until I tried to figure out an easy way to get to the bottom two allen bolts that hold the center section to the "W" pipe.

I then made a call and was also told that I can just remove the back hanger brackets and carefully wedge the center section up, which gives just enough room to reach the clips on the distributor cap and then slide the cap out with the wires attached and pull the rotor off.
Does anyone have any more tricks?

So on the way are new plugs, cap, rotor, and now I need wires. I am assuming the trouble cylinders were the two fuel fouled ones on the driver side which would narrow down to plugs, wires, or the cap/rotor.

Any new thoughts on these latest discoveries?

Thanks again for all of the help!!

[DeloreanJoshQ]

I replaced all of the ignition components, including the coil (pertronix) and the problem was still present; it was not ignition related as I originally thought.
So I pulled the fuel injectors from cylinders 5 and 6(which had-fuel fouled spark plugs) and replaced them with clean injectors. They didn't have visible dirt as much as they had black "fuel fouled soot" on them similar to the spark plugs.

Anyway, the problem went away after replacing those injectors.

Problem Solved.

Thanks for all of the help everyone!!

[DeLorean03]

Thank YOU for telling us what it turned out to be. This is the kind if stuff I like to see - helps those of us trying to diagnose issues something to go on. I can't stand it when someone asks for help, gets suggestions, and then fixes the problem and never tells everyone else what it was. Quite possibly my #1 pet peeve on ANY hobby forum I am a part of.

[DeloreanJoshQ]

Problem has started to redevelop...get a slight hesitation upon acceleration and not consistent; sometimes not noticable.
The problem is not as bad as before, but definitely annoying.

Could it be the fuel filter? I haven't changed it in 12K miles/2 years. I was going to wait until I upgraded to the new DMCH fuel pump/sender kit.

Any other ideas what would cause this? Fuel accumulator? The problem happens when warmed up. Can't recall if it happens when cold. Still testing...

Thanks for all of the help!

[DeLorean03]

Did you replace all 6 injectors - or just #5 and 6?

I'd be wiling to bet if 1-4 stayed the same, another one started to act up.

FWIW, I haven't changed the fuel filter in 9 years.

[Doogie]

Like they were the first time around, I would first suspect the fuel lines going to those injectors bring dirty/collapsed/old preventing proper pressure (and therefore proper spray pattern leading to incomplete burning and "sooting" of the injectors), or (hopefully not) the fuel distributor being clogged somehow that 5 and 6 don't get their proper pressure leading to above..

...or..(and I'm spit-balling here) question why it's 5 and 6 and none of the others that feed from the same Fuel System. On my Model T, Cylinder number 1 always fouls way before 2,3, and 4 because the intake manifold is right behind the fan, and believe it or not cools the air going to #1, (actually forming ice on the intake manifold in that area). To get a proper burn, you have to adjust the carb a little rich, and it fowls the plug early.

So, why are 5 and 6 "sooting"? Intake leak in that area? Temperature difference on that part of the head? Can you see inside the intake where the injectors drop in? Is it "sooty" compared to the others?

..again...just spit balling....

Doogie

[vwdmc16]

Ive had this same issue for the last 18 months, simply replaced my 18k mile plugs and the car hasnt run this smooth in years! Yeah easy fixes!

[David T]

I would do a compression test. You might need a valve adjustment.
David Teitelbaum

[DeloreanJoshQ]

I have the stainless steel fuel lines (less than 2 years old). I originally cleaned out the first two trouble injectors. Perhaps I have to do the same with the other 4. I also did a valve adjustment a year and a half ago..but I suppose it is possible they need re-adjusted (don't really make a lot of noise though).
I'll report back once I solve the problem. thanks for the help!

[DeloreanJoshQ]

Just a follow up. The problem started developing again. So I bought 6 new Bosch injectors and installed them. No hesitation or stumbling under load or acceleration at all. When I pulled out the old ones the driver side bank was more "sooty" than the passenger side and again, suspect. If I have a problem again in the near future, then I will suspect something is fouling out the injectors.

But for now, she runs better than she ever has with the new injectors!

[David T]

Don't throw away the old injectors. They can probably be cleaned.
David Teitelbaum

[DeloreanJoshQ]

I still have them. I believe I can sell them as "cores" to interested people in the community, correct?

[DMCMW Dave]

Don't throw away the old injectors. They can probably be cleaned.
David Teitelbaum

Depends on how they got dirty. I hear of more problems trying to get them cleaned than I do people happy with the process. I won't remove rust inside. Which is usully the problem.

[Ron]

I hear of more problems trying to get them cleaned than I do people happy with the process
:hihi2: Yeah, like 90%