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The Ignition Bypass: A solution for problematic 4.2L BBD Jeep engines

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Editor's Note

This modification is not legal for EPA regulated U.S. street vehicles. Regardless of the outcome, in the United States, it should be used only on off-road vehicles and not on emission controlled vehicles. This material is the account of one person's experience and was generated on a vehicle that such modifications were legally permitted to be made upon. Performing this modification on emissions controlled vehicles could subject the user to a mandatory reversal of the modifcation and/ or a fine. Neither the author nor ORN assume any liability from your use of this material.

All CJs and YJs with the BBD equipped 258 (4.2L) engine are starting to get old. One of the seemingly inevitable consequences of aging in these vehicles is the ECM (Electronic Control Module) computer or other portions of the stepper motor controlled carburetor or the complex pulse air emissions system failing. The performance of the vehicle can be dramatically altered by any sort of failure or even a mild degradation in the ECM or emissions system. These systems control both the ignition timing and the air to fuel ratio on these vehicles, and if either is off then a significant loss in power or overall driveability may result.

Typical symptoms of a malfunctioning ECM or emissions system may include: surging or missing at idle, knocking or pinging, loss of overall power and/or an increasing inability to maintain freeway speeds. In extreme cases the ignition may actually cut out completely while running or the engine may not start. There are other reasons that this modification may need to be done as well. Any change to a different type of carburetor, or to a TBI fuel injection system such as those offered by Holley or Howell may confuse the ECM resulting in the the spark timing being off. The addition of high performance parts or engine modification such as head modifications, headers, or intake modifications may also require that this sort of modification be done because the modified engine may operate outside of the parameters for which the ECM was designed.

This modification will do two things. It will rewire the ignition wiring to be basically the same as that found in an '78 to '82 CJ (with the 258 engine, but without the computer controlled version of the Carter BBD carburetor) and it will make the stock carburetor with its computer controlled stepper motor work with the rewired ignition. These instruction are based on my own personal experience and hundreds of Jeeps have been modified in this way since the first version of this modification was published.

The following steps all apply to anyone keeping the BBD carburetor. Steps one and two may be skipped by those changing to another type of carburetor or to fuel injection.

Warm up the Jeep long enough for it to be operating in closed loop mode if the ECM is still functional.

Screw in the idle jets so that the needles that are controlled by the stepper motor move. The purpose of this is to fool the ECM into putting the stepper motor driven needles into the full rich position. When this happens, you will see that the needles will be all pushed the way to the inside of the carb and the tone of the air going through the carb will get much lower and the engine will slow down and start to miss. At this point you should shut off the engine and disconnect the plug for the stepper motors on the back of the carb. The plug stays disconnected from now on.

Before the modification

An alternative to steps 1 and 2 is to remove the stepper motor from the back of the carburetor and fully extend the needles. Replace the stepper motor while being careful to not push the needles into the body of the stepper motor.

Changing the wiring: On '83 and later CJ's and YJs with the 4.2 the purple and orange wires from the distributor go to the computer under the dash instead of directly to the ignition module like earlier CJ's which were equipped with the Dura Spark ignition module. In summary: Orange wire at the distributor connects will connect to the orange wire at the ignition module and the purple wire from distributor connects to purple wire at ignition module.

To do this you will have to pull the wires out of the split loom covering and follow them carefully. The orange and purple wires from the distributor both go into the firewall between the battery and valve cover, and there there is also an orange wire from the ignition module that goes into the firewall at the same place. Cut the two orange wires and splice them together. Cut the purple wire here as well .

After the modification

There is no purple wire coming out from the plug near the ignition module, so you will have to add a section of wire. Find where the purple wire is that goes into the plug from the ignition module is, splice your new wire into that purple wire and then run the new wire up from the splice to where you cut the other purple wire and connect the new wire to the cut purple wire (near the firewall). I used 16 gauge wire for the section I added and I have always used insulated spade terminals at all the connections. A perfectionist would have soldered and used heat shrink tubing to insulate the connections.

Disconnect the vacuum line going to the distributor and plug the port that the it originally went to. Now attach the disconnected vacuum line from the distributor to a tee that you will have to install in the the ported vacuum line. The ported vacuum line is attached to a fitting on the BBD carburetor located on the side towards the valve cover, about 1/2 way up the carb.

Set the timing. Disconnect the vacuum line at the distributor and plug it. Set the timing to 8 degrees BTDC. When I first started the test vehicle the timing was retarded (about 6 degrees ATDC). Don't let the motor run for long like this or it may be damaged. Know how to adjust your timing, have your timing marks clean and the distributor clamp loosened ahead of time so you can advance the timing to 8 degrees BTDC (Before Top Dead Center) quickly.

My results: My 258 now revs smoothly to 4000 RPM, that's right 4000 RPM! Before this it would only get to 3000 RPM and then start to bog and miss. It also has a bit more power below 3000 RPM and the low end torque is definitely stronger at 600 RPM than it used to be. I idled up a fairly steep driveway in 1st gear high range and it went down to 400 RPM and kept pulling, before the conversion the engine always died when I did this in this driveway.

Since the original article appeared, I have received hundreds of e-mail messages about this modification (literally - at least 2 messages a week, sometimes 2 a day for up to a month after a new newsgroup or mailing list discovered the article which was originally published in 1996). Out of these hundreds of messages only two people have told me that they did not see much of an improvement, while most of the rest seem to have been as amazed as I was by the improvement. I'm very pleased that all those CJs and YJs are running better and I hope that this reworked version of the modification will help even more people.

Frequently Asked Questions on the ignition modification:

These are the most common question that I've been asked during the last few years.

As always, your results may vary depending on the condition of your engine, carburetor and ignition system.

Q. Is there a kit available for this modification?

A. No. The materials are readily available at any auto parts store and should cost under $5. All you will really need about 6 feet of 16 gauge wire and some wire connectors.

Q. Is the modification emissions legal for emission controlled vehicles?

A. In the United States, this modification is not legal on it's own. It is possible that steps 3 through 5 may be 49 state legal with the use of certain types of fuel injection that are already certified for 49 state use. Please contact the fuel injection system manufacturer or your local state regulatory agency for this information. It may also be legal in some states when replacing an engine in an older vehicle (without the BBD system) with an engine from a year with a BBD system. I have no specific information on what is or is not legal in any particular state or with any particular system(s) and I will not be able to answer any questions or give any advice relating to legality. Remember that you are altering the emissions control equipment and in many US states this is illegal.

If you live or operate the vehicle in a country other than the United States, the legality of this modification will depend entirely upon the laws of your country. You will need to contant the relevant government agency in your country for this information.

Q. My state has no visual inspection. Will my Jeep pass the smog test (sniffer test ) with the modification?

A. First, I can't really tell you that, among other things it will depend on the condition of the rest of your engine and how well it is tuned. I can tell you that in my Jeep with this modification that I have passed a sniffer test with results as good as a new 4.0L fuel injected Jeep engine and I have also failed the test miserably, depending on the the condition of the engine and its state of tune. In general, some people who have a BBD system that is malfunctioning or who have idle difficulties may find the modification to be of some benefit because it may help reduce misfires at idle. Each misfire sends a puff of unburned hydrocarbons out the exhaust and lessens your chances of passing the test. Your results will absolutely vary and we make no claims of any kind regarding the impact of this modification on any emmisions testing in any country.

Q. Do you ever get pinging or spark knock?

A. Not since I did the ignition modification. I've found that I can advance the timing as far as 12 degrees BTDC and still have no spark knock on some engines. Of course , your results may vary.

Q. Does it really help power?

A. That depends on the condition of your current emissions system and ECM. There will be a more dramatic difference for those with problems and a smaller difference for those who's systems are in perfect condition. Your results will vary

Q. Does it help off-road?

A. It has for me. My engine now has more low end torque and be much more controllable a low speeds. Top end power in the 3000 to 4000 RPM range is greatly enhanced and sand hills are much easier. An engine with a perfect emissions control system and ECM may not see much of an increase in top end power. By now it should almost go without saying, but your results may vary.

Another situation where this simple modification may be of use is if the ECM malfunctions or fails on the trail. This simple modification would provide a way to bypass the ECM so you can drive your vehicle off the trail and to a repair facility.

Q. Will this hurt my fuel economy?

A. My fuel economy improved. Your results may vary.

Q. Will "putting the stepper needles to full rich position" cause extra fuel to be added to my engine?

A. No. The computer controlled BBD system is designed to bleed in extra air to make the engine run leaner, not add extra fuel. More air is added to the mixture as the needles are pulled out of the carb (into the stepper motor body). From my experience, the air/fuel mixture will be nearly the same as a pre-computer controlled BBD when the needles are in full rich position.

Q. The needles on my stepper motor won't move. What should I do?

A. See step 2A.

Q. Will this help my 4.0L engine?

A. NO! This has been asked a few times. Don't even think about trying this on a 4.0L engine. The 4.0 fuel injection system is totally different and far superior to the 4.2L engines carbureted system. The 4.0 needs no such ignition modification to perform well, but some aftermarket ignition modules may be used to increase performance.

Q. Will this work on my early XJ with the carbureted 2.5L engine?

A. The wiring portion is the same, but the XJ carburetor is very different. I have never attempted this on an XJ 2.5 L, so I don't know how well it works. If you decide to try a modification like this, you do so at your own risk, but drop me a note and let me know what happened.

Q. Will this work on my later Grand Wagoneer or J-10 with a 360?

A. No. These do not use the same system and are optimally wired to start with. Their carburetors are not computer controlled.

Q. There are a lot of wires, which are the correct wires to modify?

A. You will have to identify the wires yourself, remove the split loom from the wiring under the hood and follow the wires carefully. Replace the split loom covering when you are done.

Q. Do I need to do this with the Mopar Fuel Injection kit?

A. Absolutely not. The Mopar kit includes a new ignition system that completely replaces the system described here. It functions perfectly as supplied. Replacing a faulty BBD system with a newer, legal, fuel injected system is the best way to solve the problems inherent with the BBD system, unfortunately is it also quite expensive.

Q. Is this compatible with aftermarket ignition upgrades?

A. This modification should work fine with most aftermarket coil upgrades as well as many systems like those available from Jacobs, MSD, Crane and others. This upgrade should also work fine with the home-brewed GM HEI distributor swap.

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