RPM drops when shifting 3-4th gear

[mrrdeac]

On one of my 2002 GT's (138K miles) I have been experiencing a hesitation (two or three times) as the transmission is shifting from 3 to 4th gear. When this happens, the RPM's drop about 500 RPM. The car has recently had the MAF replaced and new AC Delco 41-110 plugs and new plug wires (AC Delco 746BB). Also, I replaced the fuel filter and PCV. When it hesitates, if you let up on the gas pedal slightly (or if you give it more gas) the hesitation will stop. By the way, there is no SES light. It only seems to do this as the transmission is shifting from 3 to 4.

The transmission was shifting hard but I went through Bob Naylor's regiment of "sea foam" and two fluid changes (new filter, of course). Transmission is now shifting smooooooth. I don't think it is transmission related (a slip would cause RPM's to go up not down). Any suggestions where to look?

[jd-autotech]

if you access to a back pressure gauge you should pull pre converter o2 sensor and put gauge in and see if you got more than 11/2 psi your converter could be beginning to plug up it wouldnt be the first grand prix with that problem they even extended the converter warranty to 100,000 miles if this your problem have the dealer check it for warranty coverage before u pay someone

[mrrdeac]

Catalytic converter was replaced at 75K under warranty. Symptom was very different. In that case, reakky could not accelerate or get RPM above 2000.

[grandprixgtx00]

hmm...if you drive the the shifter in the "3" position instead of the "D" position does it stop? I'm wondering if there is a problem with the torque converter not wanting to locking up. i would have the car scanned with a transmission capable scanner and see if any codes pop up

[mrrdeac]

Good idea. I will give it a try. Do you expect the RPM's to drop when the torque converter does not lock?

[grandprixgtx00]

when this happens, are you doing highway speeds? how fast are you going? and i guess give us a little more info on where the RPM's are. are they around 2000 and drop down to 1500, or...

[mrrdeac]

Approximate speed is about 45 MPH. I am not sure about RPM's but I think they are about 2200 and fall to about 1700. Talking to others and researching this it seems to fit what others call "shudder" that occurs when the torque converter fails to lock. Does that sound right?


Oh, I almost forgot, i did not see the problem when I manually shifted. But that was only a couple of times and the "shudder" is very intermittent. Thanks for sticking with me on this one.

[tblake]

When was the last time a full tune up was done on this motor. Many "shudder" at lockup (45mph) conditions are caused by secondar ignition missfires.

[mrrdeac]

Good to hear from you again Tim. In the past week I replaced all plugs (AC Delco Iridium's), new wires, new fuel filter, new PCV valve (I did it because I thought this was a possible cause of the shudder).

Mitch

[tblake]

Originally Posted by mrrdeac

Good to hear from you again Tim. In the past week I replaced all plugs (AC Delco Iridium's), new wires, new fuel filter, new PCV valve (I did it because I thought this was a possible cause of the shudder).

Mitch

Uh oh...

When did this "shudder" start to happen? Right after the new plugs and wires were installed?

Couple years back, I had an 02 GP that I just finished replacing the LIM gaskets. Afterwards, it started, ran, and idled good. On the road, it ran good up untill the point when the torque converter locked up at 45mph in shuddered. It did not do this before the LIM gaskets, so I spent a couple days checking for vacuum leaks and things like that. Then I replaced the plugs and wires. Turns out the plug boot on #4 cylinder had a small cut in it. At highway speeds in lockup, the spark jumped through the side of the boot rather across the spark plug gap causing a missfire or "shudder".

[BNaylor]

It doesn't sound like shudder to me. You'll know when you get either launch shudder or shudder when the torque converter locks or during shifting. For example launch shudder is pretty violent. The front end will bounce up and down or feel shaky.

Shudder = convulsion effect.

I'd get the tranny scanned or checked out with a scan tool capable of turning the torque converter on or off.

BTW - Could be "chuggle" or "fishbite".

[mrrdeac]

Actually the "shudder" began before the tune up. So I am not thinking that the tune up is causing the problem. I originally believed that was the issue when the problem began (and the plugs were really bad so I thought I had the problem fixed with the tune up but the problem was still there) By the way, you mention a slight cut in the boot on the wire. A GM mechanic showed me a quick way to check for that. He put about a pinch of salt in a spray bottle of water and sprayed the wires while the engine was running. The openings in the wire literally lit up with electrical arcs.

I guess I am not sure if it is truly shudder. There is only a slight couple of hesitations at about 45MPH as I am going from 3rd to 4th gear. The RPM's drop during that time. I will try to get a trans capable scan

Bob, I am not sure about "chuggle" and "fishbite". First time I ever heard these terms. Could you elaborate on these?

[tblake]

Originally Posted by mrrdeac

....By the way, you mention a slight cut in the boot on the wire. A GM mechanic showed me a quick way to check for that. He put about a pinch of salt in a spray bottle of water and sprayed the wires while the engine was running. The openings in the wire literally lit up with electrical arcs.....

Funny you mention that. Thats exactly how I found what was causing my missfire issue on the 02.

This one is going back a ways! 2006.... whoh!

http://www.automotiveforums.com/vbul...d.php?t=656443

[BNaylor]

Originally Posted by mrrdeac

Bob, I am not sure about "chuggle" and "fishbite". First time I ever heard these terms. Could you elaborate on these?

Common terms and used by GM in their tech bulletins.

Based on your symptom it doesn't sound like these either. This is when the torque converter lock and unlocks abnormally causing what feels like a misfire condition or a bucking feel. Normally under a load like going up a hill and in 4th gear (OD) only.

A drop of 500 rpms going from 3rd to 4th gear could be normal and especially if the torque converter locks up during the shift or right after depending on your speed....45 mph or more.

[mrrdeac]

I watched it a little closer today. RPM's dropped about 100 to 200 RPM's. And it follows more your description as to when it occurs. But it alsmost always happens 2 or 3 times. It is very intermittent and it does seem to happen more with slight hills. But it is more like a hesistation (but very similar to a miss fire in feel). I agree, I need to get a scan for codes from teh transmission (most people want to charge for the service, so I am still looking for some place to get the scan).