This was posted also in a continuation of the “Unrepeatable Miracle … again” thread, however I wanted to draw from the experience and knowledge of those specifically familiar with the Chevy/GM TBI systems.
Update. I accessed the van’s self-diagnostics, and received a code which simply states the fuel mixture is too rich. I knew that already because of the black smoke, so that diagnostic code was not particularly useful, except that I did not get any other codes, which makes me think the problem is NOT the MAP sensor or the engine coolant sensor because if there is a problem with those units, there should be an associated code which is flashed.
I had always suspected that the O2 sensor might be going because before I moth-balled the vehicle, very occasionally the engine would start to emit black smoke and run terribly, and then that would stop and it would be OK again. So a few days ago I replaced the O2 sensor with a new Bosch unit. The engine started right up by turning the key and ran pretty well, only emitting white smoke (water vapor) because it was cold outside. I was hoping that as it warmed up the van would run great, but instead what happened was that the engine idle very gradually got slower and slower, and then it started emitting black smoke and the “check engine” light came on, and again the diagnostic was showing the same code as before … too rich. Now my brand new O2 sensor is fouled as well.
Next I visually inspected all the vacuum hoses that I could see well ( the 3 vacuum connections on the front of the TBI are very hard to see and access for they are right under the dash. I did find two hoses that were kind of cracked around the nipple, so I cut the bad part off and reattached. Then I pulled all my spark plugs AGAIN and cleaned them with carb cleaner and a toothbrush and a file and replaced them into the van. When I started up again, the engine seemed to run even better for yet a longer duration, but then again, gradually the idle got slower and slower, and then it started to smoke and the check engine light came on with the same code as before.
So now, my main suspect is the fuel pressure regulator diaphragm, which I didn’t even look at when I previously replaced the 2 tbi injectors. I took the regulator unit apart, but am not quite sure what to make of what I have found. There was some dried and hardened residual fuel residue which I cleaned out with carb cleaner. The rubber diaphragm itself seemed intact and certainly better than a picture I saw online where the rubber was stretched and sort of ballooned. However what I did find, was that the circular metal disc which sort of rocks back and forth in the diaphragm unit had uneven wear where that disc contacts the end of a metal tube for the purpose of blocking the fuel flow. So I am wondering if fuel may be slipping by when that should be completely shut, and perhaps that is causing the problem.
I’ve already dropped almost $300 on parts and fuel tank cleaning, and am not eager to spend yet $30 more on a regulator diaphragm which may or may not solve the “running too rich” problem. Any thoughts from anyone as to what I should do from here ? The behavior of the engine is very consistent. If it hasn’t been started for a day or so, the engine starts right up and seems to run quite nicely for a while, but then the idle speed gradually gets slower and slower, and then it starts to emit black smoke and the diagnostic emits the “running rich” code.
Thanks for any help you can give.