Black Smoke at WOT

[llll1l1ll]

Hey all, '88 325is here.

A quick search of the forum yielded little results to my problem. When I go WOT, I get black smoke out of the exhaust after 4K. In fact, I get black smoke anytime I go over 4K. For a while, I thought it was carbon; however, I'm starting to think it might be a fuel issue.

For the record, I have a new TPS and CPS. The DME is a 173 - the original was a 153. The stomp test gave the result of one blink. The cap, rotor, and plugs are a little less than one year old. The wires are maybe, maybe six months old. I listened for vacuum leaks and I only found one but I plugged it. There might be some very, extremely slight leaks, but I don't think it would make the car spit smoke like that.

The only thing I think it could be is my O2 sensor. Although it, too, is also hardly half a year old, I did have a problem with fuel in my car where an injector wouldn't shut off and would just start dumping fuel to the point where it was seeping into the exhaust manifold. This could possibly have fouled the sensor, but to my knowledge the fuel didn't make it that far down.

If it's any help, my car also idles kind of shaky. It'll stay steady, but it'll make the car shake. The motor mounts aren't even a month old.

Is the smoke normal? Anybody have any ideas?

Thanks in advance,

Pete

[Dissembler]

black smoke means your dumping way to much fuel. I'd start by checking your fuel pressure. After that pull the fuel injectors and test them to make sure they arent leaking. If they are, have them rebuilt. I'd also replace the air filter while your at it.

Its also very possible that the your O2 sensor is dead. if your car was running really rich, it could have fouled the o2 sensor in no time.

[jasonsnoddy]

rollin coal

[jlevie]

Based on what I've gleaned from various sources and from a good bit of dyno time tuning Spec E30 engines I have a working understanding of the behavior of a stock engine management system.

The behavior of the engiine management system at WOT is as follows:

Below 4500rpm the DME uses engine temperature, rpm and AFM data.
Above 4500rpm the DME only uses engine temperature and rpm.
O2 sensor data isn't used at all at WOT, but learned adaptation from the O2 sensor is applied to the WOT maps.

With stock injectors and normal fuel pressure the A/F ratio from about 5000rpm and up will be darn close to stoich. It may be slightly rich if the O2 sensor is aged (useful life is not more than 100k). but the adaptation limit unlikely to be allow a rich enough condition to throw black smoke.

Therefore I'd suspect excessive fuel pressure (bad FPR or an obstructed return line), bad injectors, or a bad ECT. Hoever, since the stomp test only results in one blink (it should return 1444 if no stored faults) the DME could also be a suspect.

[llll1l1ll]

ECT would be the coolant temp sensor for the DME, right? I'm going to go ahead and order that part and see if I can't find some obstructions on any of the FPR lines. I'll also see if I can't find a cheapo FPR to replace mine if it is faulty. I do have a very slight fuel leak, but it's on a feed line leading up to the FPR.

Then if that doesn't work, I'll try another O2 sensor. I remember when I replaced it, the engine ran super smooth at idle. It's just that it doesn't run rich when just hanging out and driving around.

I forgot to mention my car rarely sees anything above 4,500 rpm. It will see it maybe once or twice per month. IIRC, I did one time goose it a whole bunch to see if it would get rid of some black smoke and it did after the fourth or fifth pull.

Like I said, it's not a whole lotta black smoke, but it's enough for me to notice in the mirror (particularly at night).

[jlevie]

That is a bit different from the impression you gave in the original post. There it sounded like each and every time the engine exceeded 4500rpm it produced black smoke.

If it throws black smoke only a time or two after being regularly driven at more modest rpms you are most likely seeing accumulated carbon being burned out of the heads. That may still be an indication of an engine that is running slightly rich (what do the plugs look like), or the carbon could be from oil burning.

The nicest thing you can do for the engine is to pretty regularly run it up to at about 5500rpm. That can be done in the lower gears and not require high road speed.

[llll1l1ll]

I pulled the plugs the other day and the front ones (1-3) look fairly normal (a bit white), while the rears (4-6) look pretty charred and fouled (lots of black gook). I actually had to pull them since one of the spark plugs had actually closed its gap due to buildup, thus not firing. They smelled like a mix of fuel and oil.