Tach bounces occasionally.

Recently my tach has started jumping occasionally and I am wondering what might cause this. is this an indication that my crankshaft position sensor might be going bad? 98 xj 4.0

Doubt it.

While the crank position sensor IS the engine speed sensor, it just feeds info to the PCM, same for the temp/CTS sensor, etc.

Might want to check your dash grounds and clean the connections at the PCM.

Part of the reason I was asking is because I've been having some ertic engine behavior it is died twice while driving and it's been running rough and jerking intermittently as well

It misses when the tach jumps right ?

dobe

not always.... but i think I may have found it, was poking around through the wires and found a bad connection on the engine to firewall ground. Hopefully that was the source of ky woes

I have this same problem and it just started about a month ago. I noticed it only happens during WOT and above 3,000 RPM. It'll bounce down about 200 or 300 RPM on the tach, but the engine doesn't miss. I did develop a rough idle though.

With that said, I have felt that it is down on top end power. Not drastically, but it is noticeable to me because I drive a decent grade on the pass during the week. My gut is telling me this is a ground issue like Joe stated, but I'm thinking more of an engine ground. I'm also going to replace my distributor because it's under warranty and I'm thinking the CPS inside it is suspect as well.

Alll little background -
Mine is a 95 4.0L 5spd. New Crank Position Sensor, distributor, coil, wires, plugs, air temp sensor, TPS, cat, new headers, new 4-port injectors, plus a few other odds and ends over the past few months to prepare for the dreaded smog test. Now that this problem developed, I'm a bit worried to try to smog it. (problem happened after the parts install, but weeks after everything was done, so I know it isn't a part swap issue)

If I figure something out, I'll let you know.

I did get mine all sorted out so if it helps I can give you the run down on what I did. Initially my xj was idling low and rough and would stalled out on me a couple times. Also I have an automatic and it would not downshift when I stepped on it and acceleration was also poor on its own. This initially led me to believe the tps was shot so I replaced with one from Napa. This improved the idle quality by far and gave me bsck ky downshifting when I went to step on it. I hadnt changed plugs in 20-30k so I went ahead and did new filters and plugs as routine but this made very little difference. I followed up by disconnecting and cleaningthe MAP and Cks (my cks is less than two years old). Which also made little difference, So i started testing ground continuity and found my engine to firewall ground was bad so I replaced that. At that point I started telling myself that that that's what was responsible for my erratic behavior, but I spoke to someone who corrected my thoughts by telling me that the firewall ground is merely a secondary path to ground so should not cause issues on its own. And sure enough later I noticed my tach jump on the freeway again. Up to this point I had been trying to avoid doing the Cks but I was running out of options. I started paying much closer attention to the engine when driving and in addition to feeling that lack of power I also noticed that the vibration from the engine were more than it used to be (was almost un noticeable in a lifted truck with off road tires.) and I also noticed I very slight misfire kn occasion. I had been thinkjng the cks for a while but not wantkng to replace it but I was fiarly certain that all this was pointing me in that direction. I picked up kne from chryslef (my old one was an auto parts brand) and installed. Since then I have had no issues and my heep is back to running flawlessly. Also just as a side note I found a tiny piece of metal on the end of the sensor( my cover is missing) and I am inclinex to think this may have affected the field enough to mess with the mag pick up. I cleaned up the old one and wi keep it for an emergency spare in case I ever need it.