Autosport Labs

1987 924s

MAP Megajolt ignition system

Fuel controlled by the DME

Started as a slight hesitation and grew in to a major misfire. It would rev to about 4K and then cut out completely. If I keep Th throttle open it would backfire. rev to 4 k drop to 2k backfire Rev to 4K... If I release the gas when it hits the 4K wall it will drop to idle and idle just fine. Eventually it backfired so bad that the J tube on the AFM came off.

I disconnected the megajolt ignition (put it in limp mode) no change other than it was running at a static 10 deg of timing) changed out the EDIS module no change.

Replace the AFM and the backfiring and hunting mode is gone but it still will not rev past 4K and the car now the car is running like it is restricted. There is also a new rattle in the exhaust and it has that really hot cat smell.

My thought is that my AFM has been failing and it caused my cat to go south. I am now considering installing a "test pipe" Anyone else got any idea's?

Other things that are working. Fuel pump testes out ok. Fuel filter was changed a while back (regular maintenance) I dont suspect bad gas.

Ok went outside and forgot to hit send so I will continue the saga.

Cat was shattered with a big chunk covering the outlet. I suspect that the backfiring shattered it. In either case I took the cat back off and removed the piece stuck in the outlet a with a quick rev of the motor and it was empty. Button that back up and take it for a drive. Ok for a little while but then the symptoms are back with the backfiring and not revving. Go home and look at the wires. They are arking like a SOB. It was a lightning storm!!! I have been meaning to replace those as they were an old set from my wife's mustang I used to get the EDIS thing up and running. A quick trip to the parts store and I get a set of Bosch lifetime warranty set (for a 94 mustang with a 3.8 V6) and all for $36!!! I swap them out in the lot at auto-zone and no luck. In fact the problem is whose. It will only rev to 2K and the backfires with the clay cleaned out sound like my old rug-er. People were seriously looking for cover until they figured out what it was. It was amusing. So now what?. .. . . I try a couple of things while working the total and as I wiggle the wiring harness between the EDIS and the Sensor and the Megajolt it all of a sudden kicks in. I jump in and start driving home. I made it about 3/4 of the way home and the problem comes back. I limp it home as I can not rev it above 1.500 or it backfires horribly.

So tomorrow I am going to rip the harness apart and separate the wires for the PIP and SAW room the wires that got to the VR sensor on the cam. I was even thinking I would try shielding them from on another.

If this works I will remove the EDIS system and put the stock setup back temporary and I will re do the harness with everything in a much heavier shielded wires.

I am assuming that the PIP and SAW are getting interfered with by the signal from the VR sensor. OR.. . . Wait. I put the test lead that comes off the EDIS in the harness. It was just cut and caped. It is really a tachometer out lead from the EDIS unit. I bet that this is some how shorting the PIP and saw. It is caped in about the exact area in the harness where I wiggled it and it stared to work. Ok I will have to inspect that before I rip it apart!!!

The good side what when it was running with the cleaned out cat and the new wires it was hauling but!!

Any other idea's?

Hi Dean,

It does sound like a bad connection somewhere from how you described the events. Also noteworthy is how the problem was still there with the megajolt disconnected. In addition to re-wiring, double check all of your ground connections. Finally, is your battery/alternator in good shape?

Let us know what you find out.

Is the VR sensor mounted solidly? If the bracket can spring or the sensor can wobble, it's cause sparks all over the place - watching the runtime on the laptop and looking for rpm spikes will help with that.

Photo of my mount. I dont think it is the problem.



Since we are getting 6-12 inches of snow tonight and tomorrow I will have to put this project no hold till Thursday. I am going to undo the wiring loom I made and pull out the two dead leads from the EDIS Unit. Since the EDIS unit is mounted with in a foot of the VR sensor I am also going to take the shielded wire for it out as well and separate it in its own sheath. Lastly I am going to remove the Tach wire I have in the loom (from the old coil back to the cab) and wire the touch directly to the MJ. It is not the problem as I cut that wire at the MJ in the cab so the wire s dead in the harness at the moment and I still have the problem but I want to get the wire out of the harness to eliminate a possible problem in the future.

The more I think about this I am betting that the tack out form the EDIS unit is some how shorting / arcing to either the VR or more than Likely the PIP and SAW wires.

brentp wrote:Hi Dean,

It does sound like a bad connection somewhere from how you described the events. Also noteworthy is how the problem was still there with the megajolt disconnected. In addition to re-wiring, double check all of your ground connections. Finally, is your battery/alternator in good shape?

Let us know what you find out.

Hi Brent

This is absolutly NOT a MJLJ problem. This is a wiring issue for sure. If it is the test / tach lead from teh EDIS should I ground it?

Hi Dean,

if you're talking about the "IDM" line output from the EDIS, you should just clip it short and cap it. Since it's an output, you shouldn't ground it.

Figured out the problem. The cold weather up here has caused the space between the sensor and the trigger wheel to grow. I push on the sensor moving it closer to the wheel and the car runs perfect. Remove pressure and it runs as described.

Now how to fix it

Ohya I pulled the three unused wires out of the wiring harness and caped them to the side. It did not change anything

With the motor off I took a small screwdriver and and hammer and tapped on the metal bracket of the sensor bending it towards the gear / trigger wheel and the car is running ok now. It still has a miss every now and again and seems to reset so I am going to see about re designing the trigger wheel to a larger wheel and hopefully use a stock ford sensor.

In the interim I am going to replace the sensor with a new one. I am also going to take a file with the car running and "trim" the death of the gear in order to get it exactly round. At the moment I am only off by a very small amount but if you look at how these sensors are mounted and the tolerance to the trigger wheel that they use on the RX7 motor this may be a critical step in making this setup work properly.

So it is back to the researching of gears, sprockets and cogs. = >>> new thread time once I get some info together.


EDIT: I may actually re install the stock ignition to see if the miss goes away. I have left it all in place making this a 10 minute swap over. I actually have kept all the parts and tools to do it in the cae just in case. For the record I have put close to 3000 mile on my car with the MSLJ. I am hoping to drive it for a year as my DD and see how it holds up.

We have the same sort of issue on two minis. one running Forced induction one N/A. the revs drop to zero and the program freezes then it all returns. The only way we can stop it is to change the plugs or close the gap, but the gap closing doesn't last long. We have just swapped from NGK plugs to Bosh super 4's to see if there is any difference

I think that you will find that the cooling of the motor is really that is causing it to run better. as it heat up the space between the VR sensor and the trigger wheel grows and the signal degrades causing the problem. (At least this is what i have found to be the cause) What trigger wheel and sensor are you using?