In the life of a 258-powered Jeep there is a time when it's drivability becomes unacceptable. By now, many of these Jeeps have at least 100k miles on the odometer and performance is starting to suffer. The major player in this poor performance is the Carter BBD carberators. Luckily for us, there is a number of well-documented fixes for the BBD (see Jeepwire). Another area which produces numerous problems is the stock Prestolite and Duraspark ignitions used on the 258-equipped Jeeps.  Hard starts, hesitation and poor fuel economy can be a thing of the past with an upgrade to GM HEI.
 

Since 1975, General Motors has addressed the needs for efficient combustion with its High-Energy Ignition system (HEI). These HEI systems fire a considerably "hotter" (increased voltage) and "fatter" (higher amperage) spark across the spark plug electrodes. The GM HEI ignition is vastly superior to stock AMC Jeep ignitions in every possible way. The HEI distributor combines all ignition components in one unit. The most notable results of upgrading your stock Jeep to GM HEI include:

 

Increased horsepower & torque

Improved acceleration & idle

Decreased fuel consumption & emissions

Computer controlled 258 owners with the electronic Carter BBD carberator read this.
 
 

GM HEI Distributor = $30-$60
Purchase your HEI from a salvage yard. A remanufactured unit from a local auto parts chain will cost well over $100.  Start the search for an HEI from vehicles of the years 76-82 with the GM 250. These distributors were most common in the Chevy Nova, vans and trucks. You will be looking for the HEI  with the coil built into the cap. The proper HEI will already have 3 wires coming from the distributor body that plug into the coil on top of the cap. Note: It is likely that an HEI from 1976-1977 will have remote mounted coils. These units will work just as well as the others.

New distributor drive gear = $25
The stock HEI gear will not work with AMC engines. In most cases the stock Jeep gear will not work with the HEI so I suggest you dont try it. The proper part number is 3208615. Use this part number at a Chrysler dealer (and pay a lot of money) or better yet, order it from Four Wheel Drive Hardware. This is the gear found on 83 and later AMC V8 engines and will fit perfectly on the HEI's distributor shaft.

Replacement Spark Plug wires = $25
The GM HEI requires different style spark plug wires than those used on a stock Jeep ignition. I used plug wires from a 77-83 Ford Fairmont with a 200 inline 6. They were a little long but are the closest match found so far. You have the option of buying the types of plug wires you cut to size. This will ensure you don't have wires that are too short or so long that they lean up against a heat source. Buy quality wires. Another option would be to buy wires for an 8cyl. This way you will get two extra (spare) wires at the same cost.

Misc. wire and connectors, vacuum line = $5-$10
A few feet of 10 - 14 gauge ignition wire and some 10 - 14 gauge female spade connectors will be used. A longer length of the proper vacuum hose may be needed for the HEI’s vacuum advance cannister. The stock hose on mine was an inch or two short after installing the HEI.
 

Centech Wiring sells the above HEI style connectors ($4.25) if you want 'factory' style wiring. The quality of their products is top of the line. They also offer complete wiring harnesses and power steering components as well as other Jeep related products. Check em out some time.
 

Replacement Rotor & Cap = $25
It is a good idea to replace these items on a salvaged HEI unit. Buy a quality set! Blue Streak is generally regarded as the best and is highly recommended.

Optional: Adjustable Vacuum Advance =  $20
An adjustable vacuum advance unit can be used if you experience 'pinging' or poor fuel economy or if you feel the HEI's advance is not working well with your setup. The fact of the matter is you can pretty much drop in the HEI and forget about it in most cases with the stock advance working very well. Crane Cams makes an excellent adjustable vacuum advance kit and is available through Summit Racing or most speed shops.
 
 
 

Note: There are mini rebuild/recurve kits available for the HEI distributor. These can be found at most local auto parts retailers or Summit Racing. Again, these are not nessecsary but some people like to tinker.
 

Thoroughly examine and clean the HEI distributor. Remove the original rotor and check the spring posts. They should not be dished by wear. Check the mechanical advance weights and make sure they move freely. Carb cleaner can be used to loosen any lightly rusted areas and will work great to clean up the components. Apply a few drops of a light oil to the moving parts. Make sure to wipe up as much excess oil as possible to keep from attracting dirt. A wire brush can be used to remove any grease or buildup found on the distributor's stem & body. Replace the old rotor with a new one. Replace the drive gear on the HEI. I suggest putting the old cap on the distributor to protect the inner components of the HEI during the gear removal process. Use a hammer & punch to drive the pin out of the drive gear and stem of the distributor. The old gear can now be removed and the new gear & pin installed. Remove the old cap.

Note: Those with pre-79 258's may need to grind or file the part of the HEI distributor shaft that drives the oil pump. This may be needed to get the HEI distributor to seat properly in the engine block. Check the fit and if needed file or grind the HEI stem until it closely matches the stock stem. It shouldn't require much if anything at all.
 
 

Disconnect the battery. Unplug the wiring from the stock distributor, coil and the stock ignition module (found under the windshield washer fluid reservoir on the driver's side fender well). Remove the old coil from the jeep. The large size of the HEI distributor blocks access to the stock coil's mounting bracket. You will not be able to remove it once the HEI is installed.

Finding 0° TDC on the compression stroke:
Remove your spark plugs.  Place a wine cork (or the finger of a second person) in the #1 spark plug hole. Don't force the cork into the hole, we want it to be able to come out when the piston compresses. Place a (3/4"?) wrench or socket on the vibration damper's bolt, rotate clockwise until the wine cork pops out so you now know this is the compression stroke. Note: Always rotate the engine in a "clockwise" direction as you look at the vibration damper. Continue turning until the timing mark shows 0° TDC. Make sure you have not made a complete 360° turn. If you did, this will be the exhaust stroke and you will need to start over.
 

Installing the HEI:
Lift the stock distributor straight out. If you have a tough time getting it out, lightly tap the base of the distributor with a hammer to loosen it.

Take the newly assembled HEI and estimate the position you would like it installed in. You determine where you want the #1 plug terminal to be. By having the engine at 0° TDC whatever terminal the rotor points to will be #1.
The stem on the HEI is what drives the oil pump. Take a long flathead screwdriver and inside of the engine block you can turn the oil pump slot so that it is going to line up with the stem on the distributor. If we put the HEI in the Jeep as is, the rotor will rotate clockwise due to the directional teeth on the drive gear and in turn will be one tooth off. To compensate for this, turn the rotor one tooth counter-clockwise before installing in the jeep.  Now you can install the distributor body in any position you want, as long as the rotor points to a spark plug wire terminal on the cap. You may need to play with the position of the distributor if you have any clearance problems or if you want to position the vacuum advance unit away from the front of the vehicle. I had a minor clearance problem between the HEI cap and the engine block that was remedied by slightly rotating the distributor body. I also had to bend my dipstick (be careful if you do this) to allow the new distributor to fit. Standing in front of the jeep and looking towards the rear, my advance points to 8 o'clock and my # 1 plug terminal points to 6 o'clock.  With the distributor  properly seated the rotor should now be perfectly lined up with a spark plug wire terminal on the distributor cap. This terminal is now #1. Do not force the HEI into position. It should seat rather smoothly. If you experience problems then you do not have the oil pump shaft lined up or you  may not be seating the drive gear properly. Note: Most HEI distributors have a notch on the body that will align with an exact position on the distributor cap... in short, there is only one position the cap can properly seat on the distributor body. Tighten down the distributor hold down clamp once you have the new HEI installed.

Note: Remember to mark the plug wire positioning on the new cap...  often they are marked from the factory... by rotating the distributor and using a position not factory,  it could get very confusing, especially if you take the Jeep to another mechanic who spends some time thinking position 5 is position 1... and then tears apart your timing case to “cure” the problem!
 
 

Spark plugs & spark plug wires:
Re-gap your spark plugs to .045".

Install the new spark plug wires in the proper order.
 

Wiring the HEI:
This is essentially a one wire hookup. The HEI distributor has two marked terminals. One is a power source and the other is for a tach signal. A simple wiring diagram

HEI distributors require 12 volts during the ignition ‘crank’ and ‘run’ positions.  I used the 10-gauge red wire that fed power to my old Durapsark ignition module. If you choose this method you must make sure you use the wire that enters the ignition module and not the wire that exits. The exiting wire has resistance somewhere along the line and supplies the required 12 volts during the 'crank' position but only 9.6(?) volts during the 'run' position.  Because everyone has different vehicles you may not have the same wire I used (YJ Wrangler owners see links below). A few other options for supplying the proper voltage:

1. Use the positive wire from the stock Jeep coil to trigger a 12 volt 30 amp relay.  Relays are inexpensive and available at most automotive parts retailers. Wiring diagrams are included with the relays and installation is simple.

2. Run a new wire from a position in the fuse block that gets the proper voltage during the proper ignition positions.

Make sure you don't supply the new distributor with a constant 12 volts. You will eventually drain your battery and you may also have an interesting time shutting off your engine once you start it .

Those of you with factory tachometers are probably done with the wiring. My factory tach works fine even though it is not hooked up to the HEI. If you have an aftermarket tach or if you are no longer getting a signal from your existing unit then simply use the 'tach' terminal on the HEI.

Note: It is important to me that everyone understands this swap is as permanent as you would like it be. You can easily turn back at any time and return to the stock ignition setup if necessary. It is also a good idea to carry around the stock jeep ignition components as a backup.

Replace or reinstall the vacuum hose to the HEI’s vacuum advance. Use the stock vacuum port from the carb. (Ported vacuum will work best in most cases)

Timing:
Start the engine and set your timing. Mine is now set at 9 BTDC at 1600 rpm’s.  Follow the specifications of your model and  year. Then make fine adjustments if necessary.

Vacuum Curve for a 1985 CJ7

Troubleshooting Page