hello everybody,

I have an 1989 Z51 6 spd. It backfires when I slow down. Car idles steady 700 rpm 16 inches of vacuum and the timming according to my mechanic is spot on. I pulled the recently installed plugs a couple of days ago and they look fine. No oil, carbon build up, etc.
When I slow down the car backfires out the mufflers. It was tolerable but now it is loud and stalling the car. The SES is on. Last time my mechanic checked the code it was a lean code. He thought it was something with the vacuum.
We assembled the vacuum lines to the carbon canister and installed the O2 sensor. There was a small backfire but the car ran fine.

I was looking at an old post at the SunCr suggested
"Block/clamp off the air hose to the right header and see if it goes away. If so, the Check Valve is broken which means it's sucking air which is why the O2 is staying Lean. Surprised there's no Code."
Problem is I do not know what is the check valve.

I have attached pictures of what I think he is talking about.
I went to the index and the exhaust section of the SM and could not find Check valve. Sorry

Thank you

 

I removed the check valve and plugged the vacuum line. Did not work. And now probably unrelated I lost my spreedometer and MPG indicator. The avg MPG works but the instant is not indicating.

Later and thank you.

 

16 inHg at idle is too low. It should be more around 20. I would run a leak down test to start off with.

 

Mal adjusted valves or burnt valves or timing off (doubtful of last one of it runs good other than backfires).

 

Another forum suggested that the injectors are NFG. Evidently the quenched injectors used in 1989 had a problem in that the gasoline disolved part of the injector casing causing shorts and no function. I ordered and replaced the injectors.

Really screwed up the job. ion that I did not realize that even though the runners will both up to either side of the engine there is a lefty and a righty. So I am correcting that over sight.
But, one note. I was under the impression that the runners and plenum had to be removed to take out the injectors.
Actually the plenum must be removed. The runners can stay in place. If I had realized that the job would have taken three or four hours.
Just FYI for any one else.