Sniffer Tube Questions '81 GTSi

I have been working some time on tracking down a misfire on the right (front) bank. I finally got the bright idea of checking the four little sniffer tubes at the front firewall (I think that is what they are called).

I found that the third one from the left, which I assume goes to the number 3 cylinder, produced nothing at all. The other three had a regular pulse of air, and got warm as the engine warmed up. But number 3 had no flow at all.

Even if the cylinder is not firing properly, should there not be some flow at this tube? Or does that indicate that the cylinder is just not firing? Is it possible that the tube got blocked somehow, or have I located my problem cylinder?

Would a blocked fuel injector cause this problem, or does it have to be a stuck valve (big problem)?

I am thinking my next step is to try a leak down test of this cylinder, but I don't know if that is even possible with the engine in, and I would have to find TDC for the front bank.

Any help would be appreciated before I start pulling the head off. Thanks.

 

I should have mentioned in the previous post that the spark is good to all the cylinders. I spent the last two weekends making sure of that. Also, it occurs to me that if one of the valves is stuck closed, a leak down test would not really indicate that there is anything wrong. Of course for a valve to be stuck closed the stem would probably have to be bent so I am back to pulling the head. Help!

 

There is nothing at all at the #3 sniffer tube. I used a piece of vacuum hose from the tube to my ear and there was no air movement at all. All the other tubes have a steady pulse that I can feel with my finger and hear very clearly with the hose.

That is why I thought it may be blocked. It was totally silent. The capped end of the tube is actually a long way from the manifold, so it would take a very long piece of wire to reach all the way down. I thought about putting some oil in the tube to see if it drains down, proving that there is not an obstruction, but I am not sure if that is a good idea.

The tip about using the distributor is good. I will do that if it comes to it.

 

I'll try that tomorrow with some compressed air. I might try a little 3 in 1 oil overnite. If it drains then it should be open and I am back to dead cylinder.

 

I put some motor oil in the #3 sniffer tube and it was indeed clogged. I ran a piece of wire clothes hanger down it and it opened right up. There seemed to be a sooty black build up, but it was soft. I sprayed a little wd40 down to tube for good measure. The next day I fired it up, and the #3 tube ran exactly the same as the others, so I am back to square 1, but at least there is one more question answered.

I am back to thinking it is a bad crank position sensor, so I will switch them out this weekend to test for that. The lower sensor is easy to get to with the car on stands, but the upper one is looking like it will be a *****.