My son and I just replaced a manual transmission in his 1997 A4 1.8T. We were so happy to get it done... tried to start it... nothing but crap.

Some background:

1) I replaced the original transmission with one from a 1999 A4. I did not change any of the sensors that cam with the 'newer' transmission.
2) Four sensors were disconnected: wastegate control, Power Stage (why is called this is beyond me), and the two on the tranny. The upper O2 sensor was unscrewed and set aside.
3) While re-assembleing, the Power Stage sensor came out of the air box. I did not remember which way it went on, so Im not sure if it is on the way (or spun 180 from where it should be).
4) Eveything else went together with a problem.
5) Car was up on jacks for 2 weeks.
6) Battery was disconnected for 2 weeks.
7) When we restarted it, we found a gas leak near the passanger rear time where the gas lines come down from the tank. The leak went away after a few minutes.

Whats is happening:

1) When cold, car backfires. sound like its into the intake, not exhaust. Stepping on the gas hard makes the car die.
2) Will not idle without help.
3) Evenutally, we can get the warmed up by babying it. At this point, I can drive it and shift through all gears (well, at least the tranny works now ) If the rpm is below 3k, stepping hard on the gas will just bog the engine down, and above 3k, it will wind up to about 5.5k, then just sit there.
4) Check engine lite is not on.
5) OBD2 scan does not show error messages.
6) My son and I spent 5 hours so far looking at every hose and connection on the car.


We did spend alot of time around the rear top of the engine, since the bolt there (and almost everywhere else) was impossible to reach. I noticed that there is a connector/sensor there. If we broke it, could it be the problem. It appears OK.

Can anyone suggest where to start looking!!!
PLEASE HELP!!!

 

Well i can tell you it has nothing to do with the transmission thats causing this thats for sure...
The only thing you could have messed up taking out the trans was the RPM sensor or harness, if you did that the car would just crank and wouldn't start at all.
You need to use the VAG COM to fully diagnose this issue, a standard OBD2 code reader won't do the job. www.ross-tech.com download it to your computer and buy a VAG cable pronto.

Most likely its a blown vaccum check valve, or throttle body adaption, somthing like that.

 

Well after hours of looking, the only thing we found was that the hose clamp for the input of the intercooler was missing. Now, the can be driven, it will accelerate at a decent speed, but the rpms will not go over 5100.

Also, I mis-typed.... the tranmission was out of a 1999 passat. Again, I did not change any of the sensors on the transmission.

Any ideas?

 

When I had my 01 a4 up on jackstands for a couple of months, when putting it back on the road, I had two bad coils, which gave me the symptoms you are describing. After changing the two coils, the other two went bad within a couple of weeks. Just a thought. You should check OBDII to see if you are having any misfires.

 

is the CEL flashing when its bogging down? you said 4 sensors were disconnected, are they now reconnected? by 'power stage' sensor in the airbox, do you mean the Mass air flow sensor? the side of the tube with the plastic mesh points upstream of the airflow. if you got that backwards it could cause these problems.

 

The Maf housing should have an arrow on the side of the housing, it needs to point in the direction that the air flow needs to go.