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Anastasia,
I've come across a few circumstances that might lead to the symptoms that you're experiencing, and I'll list them in the order that I think they're most likely to occur:
1. Bad AMM. Pull your 007 AMM and check the platinum wire. If it's broken, the car is running in a full-rich mixture. Even if it's not visibly broken, it would be a good idea to swap the AMM with a known good unit (I keep a backup 007 AMM in my vehicle at all times, right next to the spare Fuel Pressure Regulator, Main Relay, and Radio Supression Relay). Check your airbox thermostat while you're doing this (if you still have the preheater hose in place).
2. Mixed up spark plug wires. I saw this on an '89 740 Turbo and it was hard to diagnose. If you switch a pair of spark plugs on a pair of even-firing cylinders (like 1&4 or 2&3), the car will seem to run ok, but it will only be firing on two cylinders. (This was not easily noticable since it's an even pair and does not cause a rough idle. The 2 mixed up plugs were firing on the exhaust stroke rather than the compression stroke and thus caused no significant engine vibration.) For a picture of how the spark plug wires are supposed to go, read the following post. It would be worth your time to double check this:
http://www.brickboard.com/RWD/index.htm?id=752158
3. Bad 02 sensor. If it's dying (loosing calibration) or has broken off, the car will be running slightly rich and your gas mileage will suffer. Read the following post for cheap replacement options. (always buy Bosch, and change them every 5 years or so for the best fuel economy)
http://www.brickboard.com/RWD/index.htm?id=731387
4. Plugged/melted/obstructed cat. There's a CO2 sensor test port just before the cat that you can remove and vent some of the exhaust. You'll need an 11, 12, or 13mm socket to remove the plug (I can't remember which one), and this will provide an exhaust vent about the size of your pinky finger. While not much of an improvement it will help a bit.
-However, you should be able to unbolt the exhaust clamp just before the cat and look into the cat using a flashlight. Look for clogged passegeways in the Honeycomb element (due to black carbon buildup), parts from the O2 sensor, and fins from the Turbo exhaust turbine. If it looks like the ceramic honeycomb core has paritally melted, you'll need to replace the cat (and also check/fix a really-rich fuel mixture that caused the problem).
-Lastly, while you have the cat unbolted, here's a quick test you can perform (assuming you still have the factory exhaust system on the car). Put the exhaust clamp back together, but with the tubes offset and only use two of the bolts. Have the cat inlet tube hanging below the exhaust mainfold outlet tube (so that it looks like the cat is scooping clean air from the road surface), and arrange the triangular clamp so that you can hold it in this position using two of the bolt holes in each half of the clamp. Drive the car and see if your 'bog' symptoms are gone. Note: The exhaust will be significantly louder, but not as loud as a naturally aspirated car under similar circumstances. The turbo does a LOT to absorb the sound as it leaves the cylinders and you should be a few steps quieter than a Honda with a 2.5" exhaust and Fart-Can muffler. If you do decide to hit Wide-Open-Throttle, try to do it in an area with dense woods on both sides of the road (just to be kind to the others who don't like noisy exhausts). While driving, listen to the sound of the Turbo. The variable pitch sound of the air rushing through the rear turbine makes a very distinctive noise that is not drowned by the loud exhaust. The turbine sound changes in pitch with a slight time delay in comparison to the exhaust sound that is matched to the engine RPMs.
5. Disconnected or stuck Wastegate Arm. (unlikely to cause the symptoms you mentioned, but remotely possible) If your turbo is just barely getting into the yellow and doesn't want to boost any farther than that, check the wastegate actuator. If the little C-clip is missing, the wastegate arm and the actuating rod will seperate from each other, thus causing the wastegate to be open all the time. If everything looks normal, try removing the c-clip, smearing anti-seize on the rod/arm connection, and re-installing the c-clip. This will eliminate any binding or resistance to proper operation. The arm and rod attachment is the upper right Brass colored "S" shaped piece of metal in the following picture Click Here, and on the far right side of this picture. Pictures borrowed from Anthony Hyde's Turbo Page.
God bless,
Fitz Fitzgerald.
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'87 Blue 245, NA 236K
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