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Acting strange on 1-2 shift on full throttle pull... sometimes

J

jmiska

New member
Very occasionally, on a full throttle standing start, I've had something odd happen when it should shift from 1-2. It first happened on the first full throttle pull after we put the rebuilt motor back in, and I chalked it up to the computer learning the new behavior of the engine. However, it just did it a couple times today (3000 miles later), and it was repeatable for one time.

Here's what it did today. It pulls very hard through first right up to 6000 RPM, and at the exact moment it would normally do a shift, it suddenly loses power, drops to about 5000, and pulses quickly between 5000 and 5500 like a rev limiter is limiting fuel pressure or ignition. If I back off from the floor just the slightest bit it immediately drops into second and begins pulling hard as normal.

It's only done this a few times, and only since the motor rebuild. Tonight, after the first time it did it, I immediately made another full throttle pull and it did the exact same thing. At 90% throttle it made a totally normal shift into second every time (at about 5500 RPM). After I shut the car off and then started it back up to try again, it didn't repeat, even on a full throttle pull.

One thing I noticed after putting the engine back together, is that the 1-2 shift seems to happen at 6000 RPM on a full throttle run, where I remember it happening about 5500 before. I can't be totally sure, though, where it used to shift. It's 6000 now in any case, which is the hard redline on the tach, of course.

Here's some background: 2005 GTP. Transmission was fully gone through 45,000 miles ago with all new soft parts, pump and converter, and has been perfect since. The engine was recently gone through for a freshening up, which included a new cam. The cam was a Sealed Power "direct replacement" (though, according to Federal-Mogul, isn't an exact factory grind, but is designed to not require new fuel and timing curves or operate incorrectly with factory programming of the PCM while increasing performance). I'm not sure what that means in terms of how far off factory grind it is, but it does seem to rev a bit freer with the new cam. It also got a 3" ZZP catted downpipe at the time, so I'm not sure what contributed to the slightly quicker revving. For all practical purposes, it's basically stock, but like new.
 


Thanks for the quick reply. So am I hitting a normal rev limiter because it's running too high before shifting? That's what it felt like. I would assume a rev limiter would kick in at the 6000 RPM redline, but what would cause a delayed shift? I feel like what's happening is computer controlled, and not the transmission just failing to shift.
 
That would cause it to over-rev. Do you know at what RPM the factory rev limiter hits? Is it right at 6000?
 


Okay. So for whatever reason it's delaying the shift slightly. Is the rev-limiter's behavior such that it continues limiting power until the throttle is backed off? That's what I saw twice tonight, where it held low power until the moment I lifted slightly, at which point I didn't even feel the shift, but was in second and back accelerating at full strength.
 
if its hitting the limiter it will cut out and back on, like a ricer in a parking lot with his dumb foot to the floor.

if you let off from wot it will always up shift asap no matter the rpm's.
 
:-) then yeah, it hit the rev limiter. I need to find why it's now shifting at 6000 under full throttle where it used to shift at 5500. Thanks for the help. Really appreciate it.
 
make sure you have no vac leaks and the maf is clean and working. all plugs are back on ect ect. aka the once over real good.

you should really get a tune for it, it will feel so much better.
 
Thanks again. I've felt like I should get a tune. I'm pretty old school (carb and distributor tuning) and have never dove into a PCM before in my life (though I am 100% comfortable with diagnosing modern cars and with all things electronic). Not sure where to start with a tune, though, that won't put my engine at risk. It's my daily driver and I use it a ton for long-distance highway runs. Still, with the engine all back at good factory specs it's the right time to think about maximizing its potential.

I'm going to crawl over the whole engine this weekend though looking for vacuum leaks like you suggest. I do expect there could be one after putting it all back together. I had found several dry-rotted joints when taking it apart, but may have missed one.
 


with the stock tune im willing to bet your even running a bit lean. cams want more fuel, where you'd change carb jets back in the day, now we just up the fuel in the maf file. instead of turning the distributor to time it, or messing with springs to advance it faster, we simply tell it to make more timing.

im from the old school myself, when i first got a tuner i was just looking at it scared to death to make a change in fear of killing it lol not so much any more.
 
I've thought about that fact that I might now be lean. Between the higher flowing converter/pipe (assuming it is) and the cam, I've been figuring that the engine would need more fuel, and hoping that the computer would recognize that and give it the fuel it needs. Now that you've got me thinking about it, I'm going to pull a plug tomorrow and take a look. I haven't read the plugs since it went back together.

It's funny. The last car I built was an old Chevy Monza with a nice 1964 225" Buick six. That was totally un-stock and tuned the old-school way to match fuel and timing with the intake, heads and exhaust I had built for it. Then I got into boats and ran a couple with small block Chevys that I built carbs and ignition for (still have one with an old SB Chev that runs like an old champ). This is the first car I've started thinking about performance on in years. I got it as a highway cruiser and commuter because I drive a ton, but the more I drive it and work on it, the more the old bug keeps getting stronger and stronger for more horsepower...
 
mine was supposed to just be a car, and well, little by little my mod list has grown. as long as you dont drive it like you stole it daily, they last as a daily driver modded.
 
Scotty, you're the man. Problem found and solved. I discovered that the big vacuum elbow for the PCV valve was split and leaking. For now I taped it and secured that with a zip tie. I took a good drive just now and shifts are right back exactly at 5600 and no hitting the rev limiter. I also checked a couple plugs. Damn those get hot on this car. They're new plugs and wires only 3000 miles ago, but already the wires were pretty well stuck to the plugs and needed a lot of careful twisting to break free. Anyway, the plugs looked just right, so I'm not too concerned about being lean.

Still, I do see a tuning job in the not too distant future...

Thanks again for the advice. It helped me find the issue. Thankfully it was something very simple.
 


Don't just tune the motor. These Trannys have an abysmal tunes from the factory. A Trans Cooler, Transgo shift kit and a good trans tune and you'll think you have a brand new car.

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