Smooth cold engine, then rough warm engine

Won't that just make the car run in the "open-loop"? Not sure how this will tell me anything. Besides, I think my mechanic already tried swapping the Ox sensor and still the problem persistsed.

I will look into the throttle switches. Can you please elaborate as to how these may be causing my problem?

Thanks.

To me 'black smoke' says that oil is getting into the cylinder somehow.

My first guess would be a valve seals.

Charles.

Well, as soon as I disconnect the temp sensor (while the engine is running) it goes into an open-loop and the exhaust clears right up. It also idles a little higher but that is because of the default settings (of the open-loop).

If oil was getting in, then I would see black smoke all the time. Furthermore, I don't think that oil creeping into the cylinder would result in rough idle and very poor acceleration. Also, the smoke smells very gasoline-ish, so I assume I'm getting a gas-rich mix.

This is just perplexing me (and my mechanic, who has also consulted some Bosch techs) to no end.

Have you ever seen a car running with a bad O2 sensor? If not, it runs how you describe your situation. When you say swapping the sensor do you simply mean that he disconnected it? If you're not sure, disconnect it right now and run the car.

A bad full throttle switch will cause an erratic idle minus the black smoke though. Disconnect and bridge the connector with the car running, see what happens.. Both switches should have 0 ohm resistance.

This sounds promising!

Let me ask you this: Won't disconnecting the Ox sensor also throw the engine into the open-loop mode? Remember, my car runs fine when I disconnect temp sensor because its in this loop (but my temp sensor is fine). If disconnecting the Ox sensor also puts me in this loop, how does this tell me anything?

You almost certainly have an electrical problem. Check the grounds. Specifically, examine the wire that connects to the driver's side rear of the head. It is not unusual for it to break. Other things wrong might be a vacuum leak or a different sensor with a bad connection (i.e. corroded terminals) like the MAF sensor at the air filter housing.

You were right, disconnecting the o2 sensor is the same as disconnecting the coolant temp sensor. They're both in the same system so you have it narrowed down to closed loop running mode now. It's probably not fuel related, this tells you, for instance..., and you already figured out it's not burning oil, too...

Bob

Just to interject...

Black smoke is too rich Blue is certainly oil White is antifreeze (not to be confused with an engine warming up on a cold day)

My guess is the o2 sensor. The o2 tells the computer that the fuel ratio is either rich or lean. If the o2 says "im lean" then the computer richens the mixture. Most people don't change their o2 sensors until they fail, and it looks like yours might have. BUT check the wires on the back of the head like previosly mentioned.

just my opinion will

What is the dfference between open and closed loop?

I know my mechanic checked a bunch of wiring and grounds, but I will have him look into what you suggest below.

Thanks.

If you disconnect the O2 sensor and the problem resolves itself, the problem is the O2 sensor.

As for it be>This sounds promising!

Open loop mode means the amount of fuel used is determined without regard to the reading from the coolant temp sensor or the oxygen sensor, and maybe other sensors.

Closed loop mode means it uses a readings from these sensors to feed back into the formula telling it how much fuel to squirt, given how much air it measures.

I was actually figur>What is the dfference between open and closed loop?

This is not true. See other postings. Disconnecting the oxygen sensor does more than just throw away that sensor's reading, it puts the engine's computer into a different running mode.

Sorry, should have qualified the statement with: In my experience, on

4 different Digicrap cars, I disconnected the O2 sensor and what *sounds* like the same problem resolved itself. So I replaced the sensor and setup the engine each time to see it run mint.

Sure, my anecdotal reports are worth crap but O2 sensors >>>If you disconnect the O2 sensor and the problem resolves itself, the

I had a similar problem in a different make, ran fine when cold, but ran rich (black smoke) after warmup. A little underhood diagnosis led me to believe it was a bad fuel injector (different noise than others).

However, as it was still under emissions warranty, I took it to the dealer. Three visits later, and after changing the ECU, MAF meter, O2 sensor, and other parts, they replaced the fuel injectors. Problem solved.

Sorry for the delay in getting back. I asked my mechanic if he had checked the O2 sensor. Because it had been replaced in February 2002, he didn't think it would fail so quickly. Is he correct? Can an O2 go that fast (little over a year)?

He did though, check the voltage on it and it seemed ok. He now suspects that the O2 sensor can be faulty even if its votage reading is "normal".

He said he also checked all the wiring and grounds and they checked out OK.

Next step is to get the car back in a swap in a new O2 sensor.

I will keep you all posted.

I've read all the different response. Yet no one mentioned the cold start injector remaining on after the engine was started. You also have an idle stablizer which the element may burn out or stuck. Or a simple vacuum leak this engines will run rough with the slighest leak when warm. I've had numerous VW's and the O2 sensor did little. We had several vehicles we maintained in my motorpool in Germany. JUst some food for thought. Enjoy.

Thanks for your reply. The idle stabilizer was replaced early in the year, so I don't think it is this. As for the vacuum leak, I may have this, but if I did, would it not run rough even after I disconnect the temp sensor putting the engine in an open-loop?

This is why I think I have an electrical issue. The engine runs fine as soon as I disconnect the temp sensor. If it was a mechanical problem, it would still manifest itself in the open-loop. At least this is how I understand it.

I will forward your suggestions to my mechanic and have him double check the idle stabilizer. I am leaving the cart with him this weekend.

I hope to provide an update here soon.

Cheers,

The Dude

injector remaining on after the engine was started. You also have an idle stablizer which the element may burn out or stuck. Or a simple vacuum leak this engines will run rough with the slighest leak when warm. I've had numerous VW's and the O2 sensor did little. We had several vehicles we maintained in my motorpool in Germany. JUst some food for thought. Enjoy.

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