3VZE TOTAL Timing Advance
 

My wife has a '94 T4R with a stock 3VZE/5spd with 200k miles. Recently started to "stumble" a lot when driving, whether under load or cruising. Usually under 3k rpm and sometimes at idle though it usually idles pretty smooth. I replaced and calibrated the TPS with no noticeable change. Now I've moved onto checking timing, I get 10* BTDC at idle which is spot on, but the TOTAL advance @ 2k rpm is about 44* BTDC. This seems way high, I'm only familiar with old carburetor-points ignition air cooled VWs and we generally set total advance to 32* BTDC and it isn't all in until 3.2k rpm. This seems way off, is there something that could cause excessive timing advance that I'm missing?

I don't know how you guessed the 44* advance, but, an advance at about 2k rpm seems normal I believe once the TPS reports it is not in idle condition the computer advances the timing. Also, if at idle it is at 10* then it is exactly where it needs to be.

Stumbling can be a fuel problem, especially under load. It could be a bad spark problem, so your distributor rotor and cap would need to be checked.

Less likely, but it could be an Air Flow Meter problem.

The FSM has procedures on how to check ALL of these things. Definitely don't start buying aftermarket parts in efforts to fix it. From my experience aftermarket electronic parts are BARELY within spec. I went through 2 brand new and 1 refurbished distributors before I took one off my donor '95 and re-did the harness and it worked perfectly, had 150k miles, OEM.

Test your fuel pressure and check the rotor and cap on dist, very easy to do.

I'm assuming you measured the 44* while parked, thus no load on the engine. In that case that much advance is completely reasonable. The computer has a multidimensional lookup table based on rpm, airflow, fuel flow, and throttle setting, as well as input from the knock sensor, to advance timing. Low fuel flow and mostly closed throttle allows for a lot of advance before knocking is a problem. When load and airflow increase, knocking is more likely, so timing advance will be less.

I have data on ignition timing I've taken from the diag port while driving. Generally at 2000 rpm, timing advance varies from about 30 degrees to 65 degrees, depending on load and throttle setting.