Hey guys, I have a 98 C5 that had a 5.7 with 230,000+ miles (yikes) wasn't taken care of so good with the previous owner. Fast forwards some months into owning it, the block cracks on me, bummer, but oh well. Mind you, the person before me put a new transmission into it, he confirmed it by lifting it and still seeing the tags on it. Never had a problem with the trans on the 5.7, and I go and find a 6.0 LQ4 and install it, and once it's all said and done, I drive it home fine. I wait say 2 hours til I drive again, and I keep missing 3rd gear? I call my guy back and he's sh*t out of luck says he's not a professional on trans and that I should call to get it rebuilt. I know it isn't the issue cause I can reverse and go forward, just very slowly. Was talking to 2 mechanic friends, and they both said it can be the shift solenoid in the transmission. On top of that, when I inspected the engine, I noticed on the top of the engine driver side closest to the fuel injectors, it's a hose I believe to be my radiator hose, wasnt clamped on correctly and can come on and off with ease. Spent too much money to go the wrong direction, maybe somebody can help out. Also got a bunch of codes on my 02 sensor. Could that play a part as well?
the engine runs great, mechanic mentioned I neeed a tune and that's obvious. But why am I missing 3rd gear? I can't go past 30-40 mph but other than that it runs good. Send help plz

 

Welcome to the forum! Since you have not had the new engine tuned, I'm guessing that you don't know your current HP/TQ levels. You haven't really given us enough detail about your setup to be able to guess as that engine can put out anywhere from 300-1000+ HP. The stock automatic transmission (I assume you have an A4 as you mentioned shift solenoids) can only hold about 450 HP reliably. You could get lucky and hold closer to 500, but that is a crap shoot and on the bleeding edge of what it can take. I had just had my stock transmission rebuilt before installing my supercharger, and 3rd gear shredded on the dyno. If I had to guess, I'm thinking you blew the transmission with too much power. It's also possible that whatever you were doing that cracked the LS1 block, also damaged the transmission. Keep in mind that (with an upgraded clutch) the MN6 can handle a lot more power and abuse than the A4 can. Rather than trying to repair the existing transmission, my advice would be to send it out to RPM transmission to have it upgraded. I don't know what your ultimate goals are for the car. However, if you call them, they can advise you on what level of transmission you need. Let us know if you have any other questions, and we'll be glad to help in any way we can.

 

So to be clear, you still have the stock 5.7 mapping in the car with the new motor? I would first get an appropriate tune in the car to ensure your shift points and various tables are correct for the motor you now have. It could be programming so why spend money on the trans (that was working fine before) until you know for sure it is mechanical and not software related-