2000 Bucking?

OK folks, lurked for a while and have enjoyed the site immensely so here's my first post and my first problem.

Bone stock 2000 4.0L, auto, 76K miles.

Bucks intermittently and I MEAN intermittently.

Bought it used about 2 months ago. Thought it was doing it when going up hills but wasn't sure. Then it started doing it more often while cruising. Finally I got a check engine light. Eureka! It said misfire on #3 plug. OK, new plugs and air filter. Problem went away for almost a month. So much for eureka, it started again this past Thursday. Not a very hard buck and at times it feels like it's loading up too. Also noticed yesterday on the way home that while cruising uphill around 50-55 it would buck a bit, so I watched the tach and it would hover about 2000 and then it almost sounded/felt like it downshifted and the revs came up about 300 RPM. It also shifts weird when it's acting up. Almost like it's searching for a gear. If you try to accellerate lightly it wants to immediately drop all the way into passing gear. If you floor it it seems to run fine.
Guys, help me out here. I'm lost. I hate to take it to a mechanic because I know it won't do it for him and honestly I'd rather fix it myself but I have no clue where to begin on this one.

Thanks in advance!

Longhunter

PS - I should probably also add that it seems to idle just fine. A little rugh when cold started but only for maybe 10-15 seconds.

Is it giving you the same code when you put the scan tool on it? It may be the plug has fouled again.
Check the cap for carbon tracking which will cause the symptoms you describe and will go away with a new plug. The new plug has less resistance so the spark looks for the easiest route to ground . After the plug starts to wear in then the carbon track again becomes the easiest path to ground again.

It may be worth it to have a computer diagnostic run on it. Should cost around $100 and it will help pinpoint the problem.

And there is no Distributer on a 2000.

Unfortunately I'm not getting a CEL this time. It only happened that one time.
I've considered the diagnostics but was hoping for an easier route. Guess it's not going to be that simple.

Longhunter

Just brainstorming here but a bad throttle position sensor could give you similar symptoms....I believe there is a testing procedure for them which would always be recommended over throwing parts at it. Good Luck.

sounds like a bad TPS to me

Thanks gentlemen!

Now can someone tell me how to check the TPS to know if it's bad?

Thanks again!

Longhunter

There are three wires that go into the connector which attaches to the TPS.
Ground, reference voltage of 5VDC and voltage that goes back to the ECM so the throttle plate position is known by the computer...
Take a multi-meter (Volt/Ohm meter) and read each wire to ground until you find one that reads 5VDC; that is the reference voltage. You need to measure the DC voltage across the other two with the ignition in the ON position, but not running. With the throttle plate closed; you should read approximately .5VDC. Open the throttle wide and read what the voltage is...; it should be about 4.8VDC.
IF when you slowly open the throttle plate the voltage is irratic; then the contacts inside the TPS are oxidized or broken which is causing the problem.

CharlesS,
Many thanks!!!!!


Longhunter