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Ignition Issues: Replacing the Spark Plugs and Coil

DeLorean Repairs, Maintenance and Upgrades

The DeLorean needs routine maintenance and the occasional, more significant refurbishing.  Beyond that there are also a number of customizations and upgrades to improve performance, reliability and functionality. 

Ignition Issues: Replacing the Spark Plugs and Coil

Joe Angell

The three inch lip to get the DeLorean into the garage.

I managed to limp the DeLorean to the garage yesterday, rather than towing it there.  The trick was to leave early enough in the morning that there weren't enough cars on the road to cause too much traffic during the twenty minute drive.

During the trip, I found that acceleration up hill was pretty bad, with my maximum RPMs being around 2000 when accelerating.  If I put the gas all the way down, I'd hear a series of "pops" from the exhaust.  Googling revealed that this is likely unburned gas combusting in the exhaust itself.  When I took my foot off the gas a little, the RPMs would go up 100 or 200 and the popping would stop.

Other than the slow acceleration, I made it to the garage without incident.  I was even able to roll the car onto two boards and clear the three inch lip into the stall without any real problems, beyond the usual slight scraping of the bottom of the car.

Ignition System Diagnosis

I came back in the afternoon to work on the car, after the engine had cooled down.  Since the popping noises indicated unburned gas, I started to look into ignition system issues.  It is possible that the car is just running really rich, but I don't think that's what is happening here. 

My first test was to start the car, and then start pulling spark plugs.  The idea is that I can find which cylinder is missing by listening for the RPMs to drop; if they don't, that cylinder isn't firing.  I only tried this for cylinders 1, 2, 3 and 6, since those were most accessible.  When i pulled 6, the RPM didn't seem to change.

Replacing the Coil

At this point I decided to try replacing my rather old Bosch coil, thinking that might be the culprit.  I tested the coil and found that it was indeed reading low:

  1. Disconnect the wires from the coil.
  2. Set your multimeter to the 1 ohm range.
  3. Touch the negative lead of a multimeter to the negative terminal of the coil.
  4. Touch the positive lead of the multimeter to the positive terminal of the coil.  The resistance should be around 0.8 to 1.3 ohms.  Mine was reading 0.5 ohms.
  5. Set your multimeter to the 10k ohm range. 
  6. With the negative lead still touching the negative terminal of the coil, touch the positive lead of the multimeter to the distributor connection at the top of the coil.  The resistance should be between 5.5k ohms and 8.5k ohms.  I had trouble getting any reading at all on mine, implying that it was very dirty or corroded.

There are three auto parts stores in walking distance, and after failing to find the coil at NAPA I got an MDS 8200 series from Advance Auto.   Replacement is very simple.

  1. Remove the old coil bracket by extracting the two bolts holding it to the firewall with a 13mm socket.
  2. Disconnect the three wires from the top of the coil.
  3. Loosen the screw-and-nut that holds the coil in the bracket.  Mine was very well rusted, and once I got it open I couldn't get it closed again (I broke the tip if my flathead screwdriver attempting to do so). 
  4. Remove the male blade connector adaptor from the positive side of the old coil with an 8mm or 5/16 inch socket.
  5. Mount the male adaptor onto the positive side of the new coil.
  6. Remove the female blade connector adaptor from the negative side of the old coil with an 8mm socket.
  7. Mount the female adaptor onto the positive side of the new coil.
  8. Slide the new coil into the bracket and tighten the nut-and-screw with an 8mm socket.  I wound up cutting my screw in half with a Dremel and a cut-off disc and replacing it with a new 45mm M5 bolt, nut and washers from Home Depot.  Be careful not to get a bolt that's too long -- I got 50mm bolt the first time, and then wasn't able to mount the bracket on the firewall because the bolt blocked access to the mounting holes.  The MSD coil is mounted "backwards" (label facing the firewall) so that the positive and negative terminals are on the same sides as they were with the Bosch, although I'm not sure this really matters, as the wires seem long enough either way.
  9. Mount the bracket back on the firewall with the two bolts, washers, and a 13mm socket
  10. Reconnect the distributor wire to the center of the coil
  11. Connect the white/slate wire to the negative side of the coil.  This wire should have a male blade connector on it. 
  12. Connect the white/yellow wire to the positive side of the coil.  This wires should have a female blade connector on it. 

Note that the yellow/white wire goes to the positive side of the coil, and should have a female blade connector on it.  If the adaptors on your coil are backwards, you may have to swap them.

Unfortunately, this had no effect on my misfire issues.  When in idle, I could rev up to 5000 RPMs with no problem, but if put the car in drive, held my foot on the brake and reved, I wouldn't be able to get over 1500 RPM and could clearly hear the engine missing.

Changing the Spark Plugs

I figured I might as well replace the spark plugs next.  There were some suggestions on how to test them for cracks or bench test them out of the engine, but since a full set of TR5 plugs was $18 at NAPA I thought I'd just replace them all.  I later learned that you should use only the original Bosch HR6DS whenever possible, as apparently other owners have had problems with seemingly identical plugs from other vendors.  Note that the Bosch Platinum plugs are not recommended, as other owners have reported them failing catastrophically (the ceramic breaking apart). 

Changing the spark plugs is pretty easy, at least on cylinders 1, 2, 3 and 6.  Cylinders 4 and 5 require moving the idle air motor out of the way, and possible popping out some injectors and removing the cold start valve to get better access.

  1. Remove the air cleaner box.
  2. Remove the spark plug wire from its socket.   The cap simply pops off the head.
  3. Using a spark plug socket and an extension, remove the old spark plug from the engine.
  4. Gap the new spark plugs to 0.024-0.028.  This is done by inserting the gap tool between the prongs of the plug, and rotating it until it doesn't turn anymore.  This is the current gap.  If it is too small, you can pry the prongs out a bit with the tool by tilting it.  If it's too wide, you can close the gap using a pipe wrench or by pushing it against the ground other hard surface.  If you push it down too far, you'll need to pry it back open.  I was able to use a pair of pliers on either side to bend it open again.
  5. Insert the new plug into the engine and tighten it to 13-15 ft lbs of torque using your torque wrench.  Be careful not to over-tighten it, or else you may damage the threads in the aluminum head and either not be able to get it out again, or not be able to secure it properly. 
  6. Re-secure the spark plug wire to the plug.  Make sure that the long neck inside the cap slides over the end of the plug.  The cap will lock into place on the head by pressure.

Unfortunately, this also did nothing to improve my issues. 

Next Steps

On Tuesday morning I'm going to check the engine timing.  I'm also going to check to make sure the distributor wires are actually connected correctly, and that the distributor cap and rotor are in good condition.  Dealing with the distributor is a pain, because it is located on the back of the engine and partially under the fuel metering unit and the intake manifold, and is most easily dealt with by at least shifting the metering unit and removing the engine cover.

Parts

  • Six (6) Bosch HR6DS spark plugs.  Cross-overs are not recommended; for whatever reason, the PRV6 is a bit finicky about what kind of plugs are used.  Do not use Bosch Platinum plugs, as they have been reported to catastrophically fail in some cases.
  • Ignition coil.  I used an MDS 8200 series coil, purchased from Advance Auto.

Tools

  • Socket set
  • Socket extensions, in order to reach the spark plugs.
  • Flathead screwdriver
  • Spark plug gap tool.  The simple disk shaped ones are about a dollar at many auto parts stores.   You can use a feeler gauge if you prefer.
  • Inch-pound range torque wrench for tightening the spark plugs
  • Spark plug socket.  Mine was part of my socket set.
  • 1/4 to 3/8 inch socket adaptor.  I needed this because my torque wrench took 1/4" sockets, but the spark plug socket was 3/8".

 

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tags Fuel, Spark