Tach jumping around right after misfiring

[roadbratt]

While my car is misfiring(i still cant find the solution to the misfiring), the tach jumps around(mostly jumps up from the incorrect rpm to the correct rpm, usually 2k more) right before it starts correcting itself. Anybody know what would cause the tach to jump up like that? I thought it would be the crankshaft sensor, but I was told that could cause the tach to drop to 0, but it never has... closest it has come to 0 is when it idles, at 500 rpm. If anybody has any ideas, let me know. Thanks.

btw, car is a 1992 SHO MTX, with about 147k miles on it.

-Andrew

 

[MOSHO92]

Possibly the CID (cam sensor) or the DIS module.
My DIS went out a year ago and threw funny codes and had misfires and a jumpy tach.

 

[sdpatt]

Check the codes, but I suspect you will find either a faulty CID sensor or poorly grounded or connected DIS module.

 

[projectSHO89]

Don't know the history of your car's problem, but if it misfires badly and the TACH looses about a third of the RPMs it should be showing, my guess would be that you are dropping two cylinders.

The TACH signal is a summation of signals including the individual coil drive signals. If one coil pair is not being fired due to a fault in or around the DIS or coil pack, the TACH signal will drop proportionately.

A timing light on the plug wires while misfiring will quickly identify if a coil pair is not firing.

Steve

<small>[ June 17, 2003, 09:04 AM: Message edited by: projectSHO89 ]</small>

 

[MO-KAN SHO]

I actually just had this exact problem last week. It would misfire for a while while the tach jumps around, then unexplicably, would straighten up.

*MY* problem seems to have been a loose molex plug on the coil pack. Found it by accident, unplugged that molex to try and test. Found a little corrosion on the plug, and the rubber seal on the female end of the plug was loose. I straightened out the rubber seal, and reinserted the plug, then zip tied it together temporarily to help diagnose. I figured if it did it again with that zip tied, I knew I'd need to do more troubleshooting. Two weeks now, and no more problems, I'm planning on permanently repairing it this weekend.

You might want to try the same thing... I figure 99 cents for a pack of zip ties ain't a bad expense for troubleshooting. And that's assuming you don't have a couple zip ties laying around the house.

Let me know what you find out....

Ben

 

[drivinhard]

cam sensor

 

[BeatDaSHO]

Your problem is most definately the DIS module. I just had a friend stop by my house because his car was doing the exact same thing. Number 2 and 6 cylinders were dropping out and a code for the coil pack was thrown. I replaced the coil pack and it was still doing it. I tried my DIS module from my car on his and it worked perfectly. So i tried his DIS module on my car and number 2 and 6 cylinders weren't firing. It is the DIS.

Greg

 

[projectSHO89]

MO-Kan's experience points out exactly what I said about the IDM/TACH signal being a SUMMATION of signals.

With an open circuit at one of the coil wires, the DIS cannot generate the signals necessary for a proper IDM/TACH signal.

More troubleshooting is definitly in order. Just keep in mind that all the inputs including the CID signal, the CPS signal, and the three coil signals are ALL required for a proper TACH.

Steve

 

[roadbratt]

Thanks for all the help guys! I forgot to mention something, it might aid me a bit better, but while the car isnt missing, if you gun the engine while you're driving, it would start missing again, and it would correct itself as soon as you let up on the pedal. I just wanted to add that incase you guys have anything to add.

I'm taking the car tomorrow to my dad's friend who owns a garage, and after he puts on new tires, we're going to have him look at the car. I'll keep in touch. Thanks

-Andrew

 

[roadbratt]

Hey guys,
Just to let you know, I fixed the misfire with a 20 dollar replacement DIS module I found on ebay that came from another 92 SHO. I know that the part numbers did differ on the old one and the newer one, and they looked exactly the same (the text was the same), so maybe you could use different dis part #'s. Thanks.