Clouds of black smoke upon acceleration with black velvet sparkplugs on '86 760T. Car was running very well after I put in new injectors, ECT and AMM, set base idle and did many other things a few months ago. Sparkplugs ran very clean. A few days ago I put a Dawes device set at 10 psi and car ran very nicely. I briefly used the power once or twice but did not abuse it. Last night I took a fist long drive and felt misfire like from running too rich. When I came home I took out the Bosch platinum sparkplugs and they looked like black velvet. This morning I put in a set of new NGK BPR6ES. Took a drive and accelerating in hilly country the car was without power and put out clouds of thick black smoke. All I can think of oxygen sensor. It is one thing I have not changed. Moreover, I put some silicone on a couple of turbo hoses recently to replace them after I did some work on the car. Silicone, I hear, can damage the sensor. I checked the sensor this morning with a voltmeter and I thought voltage change was a bit slower than I remembered, but I am not sure. Any and all help with be greatly appreciated.

Rafael, I agree with John. Definitly a boost leak. I think you went a little overboard with 10 psi. It would have been better to slowly increase the boost from say 6 or 7. I'm running about stock, 7-8, and it's just fine for my old ultra low friction engine. If I increase it too much, I might throw a rod. I'm thinking of a replacement engine.
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Warren Bain - '86 744Ti M46 >285Kmi, '96 965 >99Kmi Wifemobile near Manassas Va.. I get e-mail replies to my postings.

Hello Warren:

Thank you for your reply. I increased boost first to 8 psi, then to 10 psi. Still, you can imagine, I have been accelerating the car more or less hesitantly, corcerned, as you say, with throwing a rod, even thinking of old bearings worn down to the copper. Hoses are new and vacuum is tight, but this morning I checked O2 voltage and I though it fluctuated less widely and more slowly than normal. I checked O2 voltage again now and it is stuck at .87 volts. Looks like I need a new O2 sensor, does it not?

Would you replace or rebuild the engine? Where I am replacement is not an option unless I have the engine shipped from stateside. A rebuild seems easier because I put a remanufactured head and turbo 50K ago. Hopefully, these are holding up well and would require little work done to them. Putting in rings, or pistons and rings, bearings and such does not seem too daunting. What do you think?

Best regards.

I cast my ballot for an induction leak. Your new found boost to 10 psi has probably split a hose. Remove the hoses to inspect them insid and out. The hose at the throttle body ruptures on the outside of the bend. The intercooler discharge hose goes on the bottom where you can't see it without removal.

Hello John:

Thank you for your reply, it is always nice to hear from you. Hoses are new, but I will take a look. Vacuum is high, 8 O'clock at idle in Park and 9 O'clock at idle in Drive, and the vacuum/boost needle hit bottom like a rock when I lift the accelerator. Would it do this if there were an induction leak? I checked O2 voltage this morning and I thought it fluctuated less widely than normal and a bit slowly. I checked it again now and it is stuck at .87 volts. Best regards.

Some induction leaks only show up under boost. Under normal driving they don't. I have experienced this, but not with new hoses. The O2 sensor takes a few minutes to get warm, but if it gives steady voltage when warm, it is shot. I bought the universal Bosch O2 sensor (it was $40 US) from IPS and spliced the wires.

Silicone will poison the sensor. Sounds like a classic rich-running problem and if the O2 sensor is aged and poisoned, then that's surely the explanation. Are you sure your fuel pressure regulator is fine?
Also: many have reported less-than-optimal running with Bosch plats in turbos.

O2 sensor is certainly aged (original!) and probably poisoned from silicone I sprayed on turbo hoses to make it easier put them back on. FPR is also original, but its vacuum line is dry without any gas. Plugs are now brand new NGK BPR6ES, enerally favored at Brickboard, Turbobricks and IPD, but it ran very well Bosch platinum until yesterday. What do you suggest, put in new O2 sensor and FPR? Thank you very much for your help.

See John's note about induction leak: I think he's closer to the truth. But I still think you need an O2 sensor.

On my 88 760, The O2 sensor is out of the loop under boost (the ECU ignores it and adds fuel based on the AMM reading) so I didn't think a bad O2 could affect the car under moderate to heavy acceleration.
Steve
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88 765 w/2stage boost & G-valve

This is interesting! Please, tell me more.

Well I was born in a ............... Seriously do you have a specific question? I'll try to help.
Steve
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88 765 w/2stage boost & G-valve

O2 sensor out of the loop under boost, so ECU reads AMM signal alone? On the '86 760, I seem to remember rising voltage at O2 wire approaching .9 volts in direct relation to rpm and boost.

That sounds correct. During non boost conditions the ECU samples (from O2 sens) the mix and adjusts, that's why you see the readings bouncing around from about .2 to .7 When the readings go steady the fuel is being added according to the air entering the engine. Yes, you need a richer mix under boost.
Steve
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88 765 w/2stage boost & G-valve

Hoses are new. Vacuum is high. I checked O2 sensor voltage this morning and I thought it fluctuated less widely and more slowly than normal. I checked it now again, and it is stuck at .87 volts. Looks like I need an O2 sensor, does it not?