TFI Upgrade Question

[Allen Garneau]

Okay I'm a newbie so please don't slam me if I say something wrong. My 90 YJ 4.2 started running like crap (rough idle, stalling) so after reading a bunch of threads on this forum I decided on the TFI upgrade and a new MC2100 carb.

I did the TFI upgrade first. I'm not all that mechanical but it was easy enough. Noticed an instant improvement in idle and high-end rpms, but after about 200 miles I started having a problem. Seems now after the engine heats up (usually 10 to 15 minutes or so) the jeep starts bucking and lurching. It's intermittent, but at highway speeds (most of my driving), you'd swear the body was about to come off the frame. Figuring I did something wrong, I took it to my local shop to have it looked at and to let them install the carb for me (since my confidence was shot).

Got it back Saturday. Carb is awesome, very smooth, but still have the lurching and bucking after 10-15 minutes of driving. The mechanic said he looked at the TFI upgrade and everything looked good and was hooked up properly. The TFI package came with Splitfire 8mm wires, a new cap and rotor, and a Pro-Comp 50,000 volt ignition coil. I Installed Splitfire spark plugs gapped to .045.

The problem didn't exist before the ignition upgrade and installing the carb did nothing to improve it (didn't figure it would). Anyone have any suggestions on where I should start?

Thanks,
Al

[TSM_88YJ]

Did you do the nutter bypass? If not the ECM is still controlling your ignition and can't talk to the carter(since it's gone) and is freaking out going into a limp home mode.

The nutter is simple enough to do.

The ignition module is on the drivers side under the coolant jug.

On the ignition module you will see a purple and an orange wire.

Cut them.

On the distributor you will see an orange and purple wire. Cut them near the fire wall.

Get same size wire and splice the orange to the orange and purple to the purple.

You have bypassed the ECM, taking out of the mix, it is no longer doing anything.

You will probably need to readjust the timing.

This should be it.

I had the same problems. Did the swap and everything ran great for awhile, then turned to crap. FIANLLY did the nutter bypass and fixed the problems.

[ProjectNick]

Another vote for the nutter bypass. Even though you had a mechanic adjust the carb. I'd still check your choke if I were you. I did the nutter bypass before I even started driving my jeep(which is now, I still have it in the garage waiting for final details)

[Allen Garneau]

OK. So based on your advice and the fact that I needed the choke adjusted on the new carb, I swung by my local shop after work tonight. I showed the mechanic your description of the nutter bypass. Seemed easy enough so we did it. We hooked the orange and purple from the ecm unit to the wires from the firewall, bypassing the distributor. Tried starting it, nothing. Was that correct? Since we couldn't get spark, we also tried from the firewall to the to other end that was cut off from the ecm. Still nothing. Since it was now dark and cold, we figured no harm, no foul, let's just splice everything back to the way it was and I can drive home. Spliced it back up, still nothing. Can't get any spark. Anybody know what might have happened? We checked the fuses and everything is fine. Could something else have shorted?

The problem I originally described with the lurching started after the TFI upgrade but before the carb install so I really didn't think the nutter bypass would help but I figured I'd give it a shot. Once I get it running again I'm going to switch back to my original coil and give it a try. Maybe the new coil is bad?

Any help now would be great since I no longer have an operating jeep and the 60 mile ride to work in DC on my motorcycle isn't much fun when the low tonight will be 25F. Thanks again.

Al

[kloppk]

Since your problem started BEFORE replacing the carb my bet would be a defective coil. It may crap out once it gets hot. Is it clamped to the block like the old coil? If so you might also try to relocate it on the fender well away from the hot block.

[TSM_88YJ]

OK. So based on your advice and the fact that I needed the choke adjusted on the new carb, I swung by my local shop after work tonight. I showed the mechanic your description of the nutter bypass. Seemed easy enough so we did it. We hooked the orange and purple from the ecm unit to the wires from the firewall, bypassing the distributor. Tried starting it, nothing. Was that correct? Since we couldn't get spark, we also tried from the firewall to the to other end that was cut off from the ecm. Still nothing. Since it was now dark and cold, we figured no harm, no foul, let's just splice everything back to the way it was and I can drive home. Spliced it back up, still nothing. Can't get any spark. Anybody know what might have happened? We checked the fuses and everything is fine. Could something else have shorted?

The problem I originally described with the lurching started after the TFI upgrade but before the carb install so I really didn't think the nutter bypass would help but I figured I'd give it a shot. Once I get it running again I'm going to switch back to my original coil and give it a try. Maybe the new coil is bad?

Any help now would be great since I no longer have an operating jeep and the 60 mile ride to work in DC on my motorcycle isn't much fun when the low tonight will be 25F. Thanks again.

Al


Okay I may have thrown you off by saying cut near the fire wall.

You found the orange and purple comming out of the firewall (probably twisted together). You cut those. Cutting near the fire wall should leave you with a pretty decent length of purple and orange comming FROM the distributor.

You connect the orange and purple from the ignition module TO the orange and purple of the distributor. You are bypassing the ECM which is behind your glove box.

You are wiring the igniton module directly to the distributor bypassing the ECM.

[Allen Garneau]

Cool, that was the only way I didn't try it. I also received an email from the guy on the internet that sold me the TFI upgrade. He is sending me a new coil out today since it sounds to him like the coil may have been bad as well. I'll try rewiring it tonight and see if it starts up. Oh, and I located the new coil on the fender well so that shouldn't be an issue. Thanks.