85f-bird

Alrighty, so
Today i get in my car go for my morning drive to the airport and when i get to the airport and park i notice that my tach has a little jump in it that i've never noticed before. It's almost a "hestitation" where the RPM's may be at 850 or so but then it'll bounce to 1000 and then to 1050 and then drop off at 850. So i turned off the radio, turned off the Heat and listened to the engine while the tach was doing this lil' bouncy thingie and the engine sounds fine, no revs or erratic sounds when the rpm's jump. Also, when i was leaving and getting back on the highway i noticed that it did the same thing, but it did it while accelerating. When i had the lousy ignition components and bad MAF it would do the go up, slow down, go up slow down thing, and thats the basic idea that the tach is putting out, yet i could hear/ feal engine hesitation w/ it...so is it a tach issue, an electrical issue, or a "who knows" issues. LOL. Thanks in advance, gonna go drive it now, so hopefully i'll tell ya if it's sitll there, and i'm srue it will be.

85f-bird

Well, took the car out and yes it's still there....little jump 50-75 rpms , no engine noise that seems to corrospond w/ it, does anyone know the symptons of a failing tach?

momokings

I have the same thing going on with mine. I was curious about that, lets see what the wise ones say about it. I think mine only does it when idle.

2_point8_boy

Check where the wires run to the coil. Find the wire coming out of it that has a link that you can take apart, I think it's a white wire w/black stripe. Take that wire off the coil and spread the little connections apart just a little. See if that helps, you might no be making a good connection. Also you could check to see where the plug wires run in relationship to that wire. They aren't supposed to giv off any interference, but they might and could be inducing an extra voltage onto the tach wire that it's picking up.

momokings

I think Lee and I figured it out....it has to do with timing and distribuitor cap. Needs to be tuned.

polagrilr

Yeah I just noticed mine started doing that a few days ago, it seemed to coincide with the cold weather and me using the heater all the time...it'll jump around between 2000 and 2200 but after about ten minutes on the freeway it settles down and acts normal...?

85f-bird

hey pola,
can you feel your engine doing anything, it'd be nearly impossible to not notice a 200rpm jump in an engine, even with a incredibly quiet exhaust system you could definately hear it...heck in our planes we can notice a full 25 rpm increase or drop, ya just feel it. I looked at the wires around my coil and all seemed in good standing, thinking that my tach may be on the way out, but we'll see in the near to distant future i guess.

polagrilr

well luckily the engine seems as normal as can be expected and everyone else ive talked to is putting their money on the tach being the problem...but if i end up on the side of the freeway getting it towed home i will let you all know

85f-bird

pola, all my automotive folks are saying the same thing (tach issues) but i'll be sure to inform u just the same if it's not. still ticks me off to lose the tach after 18 freakin years. LOL. Although from the factory they're guaranteed to fail....eventually.

Joe_L

My 87 Firebird had the jump you were talking about for a while, then eventually it started jumping up 500 RPm without any engine revs. So I decided to hunt it down. It turned out that the wire that runs to the coil as 2_point8_boy already explained above had started to break off. So I simply replaced the wire and worked good till I sold it.