First off, thanks to any and all that take the time to read and give their input. Now on to the meat and potatoes of the situation. I recently purchased a 1997 SSE. The person I bought the car from replaced the original engine/trans with units from a 1997 Grand Prix GT that had been rolled. The car drove out decent, had just a couple of minor things I thought needed to be addressed.

First, the trans shifts hard. Not firm, but hard. I had thought it may be from the fluid draining when the donor was rolled. Now I am wondering if there is a compatibility problem. Edit: There is a code, 1670 I think, TCC, input and output speeds differ. I cannot feel any slipping.

Second, the mileage isn't near where it should be.

Since the purchase, I have replaced both O2 sensors and the cat as I chase the MPG and "catalyst inefficient" code. Only to late did I find this site that says not to use the Bosch units. Mileage hasn't changed much. I track every tank, 18.6 on the last one.

My questions are: Is anyone familiar with the GP trans and are there differences that I need to address to use it successfully in the bonny?

And secondly, I bought the car strictly for fuel economy over my diesel truck. MPG is a big concern. My research had shown this car should get 28MPG on the highway. I'm not much over half of that. Any suggestions?

Plugs are new AC'*; I don't drive hard, almost all of my driving is at a steady 60MPH.

its a gear raito problem the grand prix gt had a different gear ratio than the bonneville se or sse
your bonneville is probly a 3.06 fdr
I believe the grand prix is a 3.29 fdr
you will need to get a custom pcm or put a trans from a bonneville with your gear ratio in
your probly not getting and TCC lock up and that would explain your mpg issues

Speaking of transmissions, how did they get a 4T65-E to work with a 4T60-E programmed PCM? That'* my question.

Can you take a picture of what you have going on under the hood? I am curious to see whether that car has a 60e or 65e installed.

A 97 GP and a 97 SSE would both have the 4T65E. There'd be no compatability issue other than what'* already been described with the gear ratios.

I would ditch the Bosch O2 sensor. The one behind the catalytic converter isn't as big of a deal, but the one on the exhaust manifold at the firewall is your weak link. Change that to a Delco O2 and you'll see a big improvement in your MPGs. 3800s and Bosch equipment don't mix. At all.

Actually, the 4T65E did not show up for the H-Body L36 until 1998, the '96 and '97 L36'* had the 4T60E. Only the '97 L67 had the 65E.

But the '97 GP did have the 4T65E.

Correct.

My bad.

Anyway, back on topic (sorry about that)... Ditch the Bosch O2.

OK - Bosch sensor = bad. Shouldn't be a problem getting that changed.

Trans is still a issue. I had thought reading the forums here that the car had the 60, I am also thinking 3.06 ratio.

If the GP had a 65, with the 3.2? ratio, wouldn't things like the speedometer be incorrect? It seems very close based on the electronic "signs" placed around here to warn you if you are going to fast. Is there a visual difference between the two trans?

I can feel three distinct shifts (4 gears) They are hard enough to rattle the dash. Is the TCC lockup beyond that, or is it the fourth gear?

Looking at car-parts.com, the only other donors for a trans are the regal and the 88. I don't see any mention of the GP. Kinda figured this wasn't going to be good.

For the gentlemen that requested a picture - what do you want to see? I would have to find a host site, or maybe could PM it?

Thanks for all the input so far.

From what I've heard the only 97 L36 powered cars to recieve the 4T65E were the Park Avenue and the Riviera.
Also IIRC I've read on the GP boards that the 97 GP SE and GT recieved the 4T60E.

I continued to read up on this transmission issue. From what I understand, only the 4T60-e had a vacuum modulator, the 4T65-e did not.

The GP trans does indeed have the vacuum modulator on it, ruling out the 4T65-e being swapped in.

So now, what are my options for using the GP trans. The gear ratio is different, is this the only item? If so, can the PCM be modified?

The other concern is the transmission wiring harness. Does anyone have any information on this? I do not know if the GP had the transmission shift mode selector, that the bonne'* had. Maybe the wiring pin-out is different?

I am still looking for a actual bonne transmission to put back in it, thus eliminating all the problems, but have not been successful so far and I am a little limited in cash flow since I just purchased the car a month ago. The savings in MPG was supposed to replenish the bank account, but with the mileage issue that is not happening.

Thanks again for the help