[JSVSouth]

Hi all.

I'm currently restoring my c900 N/A, I am pretty sure it is running rich. The symptoms are that it blows smoke from the tailpipe, and the spark plugs are all black, despite being almost new.

The smoke is worse at startup, but continues when at operating temperature. It smells like fumes/petrol. As for the plugs, I replaced them very recently, and they're already completely black and sooty on all 4 cylinders.

Where would you guys start with this? I was thinking either O2 sensor or coolant temp sensor. Any input would be apprecitated.

Thanks.

 

[16saabs]

does your CEL light come on?, could be O2 AMM, as possibly sounds like one or other or both are not 100% disconnect. cold start sensor(between 2-3 cyl on inlet manifold, and see if that alters things, as if it's its 'set' in cold start you will getting rich fueling

 

[Sipes216]

follow the bently procedure for testing the maf sensor, then check the o2 sensor. could be either of these causing an over-rich scenario.

 

[peva]

Could also be a ruptured FPR. Pull FPR hose off the inlet manifold and look for signs of fuel in the hose. Could also run it with the FPR hose off and check for signs of fuel dripping from the hose.

 

[JSVSouth]

No CEL on at the moment. Thanks all for the suggestions, I'll give them all a try tomorrow and see if I get anywhere.

 

[Sipes216]

depending how the sensors fail, you won't get a check engine light. this goes for even new modern cars too. car thinks it's running a-ok!

 

[JSVSouth]

Okay, did a bit more investigating today.

Carried out the Bentley test of the MAF, (turned engine off, checked for wire to briefly glow) and that was fine. I also unplugged the MAF whilst the engine was running, and all that resulted in was a rough idle. So I'm ruling that out.

I disconnected the coolant temp sensor whilst the engine was running, and that caused a rough idle also. I also tested the sensor's resistance as advised in the Bentley manual, and got a reading of 305 Ω when warm, which is what it should be. I'm ruling this out.

As for the fuel pressure regulator, I disconnected the vacuum line from the top of the FPR leading to the intake manifold, and blew it out with compressed air. It wasn't blocked. Then I ran the engine very briefly without the vacuum line connected, and there was no leaking fuel from the top of the FPR, meaning it's not ruptured. I also pulled the fuel return line off, and it was flowing freely. So I'm ruling the FPR out.

So unless I'm missing something obvious, I think it's the O2 sensor. Maybe it's stuck in open loop, or maybe it's just giving a bad reading. Unless anyone has any more suggestions, I might go ahead and change the O2 sensor.

Cheers.

 

[Sipes216]

An oxygen sensor would be a not too bad idea. Unless you have an oscilisxope, you can't really test them too accuratelynin this kind of case. Your checking everything else leads this to high plausibility.
Of that sensor does not come loose easy, or with a bit of force, stop, remove the pipe segment, and take it to a shop. Don't **** those threads up.

 

[peva]

The oxygen sensor is easy to check. All you need is a voltmeter. The voltage on the signal wire should cycle around 0.5 volts whilst the engine is running. The NGK site is as good as any; http://www.ngkntk.co.uk/index.php/technical-centre/lambda-sensors/how-to-test-lambda-sensors/

 

[Sipes216]

You can have good voltage but bad cross counts.