Puff of black smoke at startup

Sting · Post by **Sting** » 09 Oct 2004, 19:22

1996 850
When I first start my 850 a puff of black smoke will be emitted from exhaust. After the car starts it vanishes. I recently had my annual emissions check and have noe issues. it passed with flying colors. I have no CEL light, all maintenace completed including new battery, plugs, C&R, MAF and fuel filter. I have cleaned the TB and IAC as well. Car runs great with the exception of intermittant start problems where the engine takes 6-7 cranks to turn over, other times it starts no problem. But I am curious bout the black smoke at start up for a couple of seconds. Any ideas?

TAFinley · Post by **TAFinley** » 12 Oct 2004, 23:23

Black smoke is an indicator of very rich mixture. You say the engine cranks a bit before starting, and that's a significant clue.

It's possible that one of the fuel injectors is leaking. Normally, pressure is maintained in the fuel system after engine shutdown. Because of this pressure, a leaking injector would discharge fuel to its cylinder until the pressure was near zero. Burning off that excess fuel would lead to the black smoke on startup. Also, because the pressure in the fuel system is down to zero or near zero, a number of cranks on startup would be required to rebuild the pressure sufficiently for the engine to start.

Guest · Post by **Guest** » 14 Oct 2004, 11:44

Terrific diagnosis! How do I determine which injector is bad?

TAFinley · Post by **TAFinley** » 17 Oct 2004, 20:29

Thanks for the kind words about my diagnosis. Actually, it's just a hypothesis.

As I said, a leaking injector will lower the fuel pressure to near zero. I don't know what tools/equipment or expertise you have, so I'll tell you the easiest "tool-less" way to determine if the fuel pressure is dropping when the car is turned off.

Shut the car down and wait at least a half-hour or so. While you're waiting, locate the fuel filter. It's on the passenger side of the car just forward of the rear axle. In the fuel line between the fuel filter and the rear axle is a valve just like a tire valve. Remove the cap from the valve. Now, here's where you have to be careful. Get a fairly large rag (half a T-shirt or so), put on eye protection, place the rag just under the valve and depress it the same way you would a tire valve to release the air; ie, depress the pin in the center with a small screwdriver or something similar. Fuel should spray out with a good bit of pressure. If fuel doesn't spray out, you are losing pressure and that lends credence to the leaking injector hypothesis. If the fuel does spray out, then you're not losing pressure and the fuel injector hypothesis has been ruled out. Back to square one.

Assuming that you are losing pressure, you need to find the leaking injector. I would suggest that you start by looking at the spark plugs. The plug on the leaking cylinder will most likely be fuel or carbon fouled. Pull the plugs, keeping them in order so you know which cylinder each one came from, and compare them. The one that has more black deposits on it or is wet came from the cylinder with the leaky injector.

Sting · Post by **Sting** » 17 Oct 2004, 21:35

Followed your suggestion but turns out none of my injectors were leaking. It was a worthwhile activity however, as I was able to clean in and around the ports to ensure my seals were good. After all this time I discovered (as a result of pulling the fuel rail), that the small elbow connected to the FPR (absorber) had a pin hole and was twisted-- cutting off airflow. Replacing and reinserting the elbow seems to have cured my annoying start problem. If I had not looked for a leaky injector, I would likely not have see the kinked hose and subsequently the pinhole- Hats off to you!

Thanks again-sometimes what you're not looking for, leads to find what you are looking for.......

Post by **matthew1** » 18 Oct 2004, 12:49

Yea, TA that was a heck of a good hypothesis.

I put a link to this on my main page because it will certainly help others: https://www.matthewsvolvosite.com