K87ZZ4

A couple of years ago, I posted about a problem I was having with my 87 running rich and occasionally setting a Code 45 (Rich Exhaust), plus the MPG was way off (30-40%) between what the dash showed and what I calculated:

https://www.corvetteforum.com/forums...ning-rich.html

It failed an emissions test, many years ago, so I took it to a shop for diagnosis and they concluded it was the diverter valve in the A.I.R. system (no replacement parts available).

I tried a bunch of things, on my own, to try to figure out what was causing the problem.

• I disconnected (and plugged) the tubes in the A.I.R. system (no difference).
• I replaced the IAC.
• I checked the FPR vacuum hose for fuel (it's dry).
• I put in a new O2 sensor.
• I checked the resistance on the injectors (a pretty consistent 17.6-17.7 ohms each).
• I checked the fuel pressure (42 PSI on prime, running at 40. Holds pressure fine).
• I pulled spark plugs (all pretty fouled with black soot).
• I checked for vacuum leaks (no surges in engine speed).
• I disconnected (and plugged) the lines from the charcoal canister (EECS)(no difference).

I collected data and found the Block Learn (BLM) at 108 nearly all the time, the O2 super high 800+ mv most of the time, and very few cross-counts (Rich ALL the time).

I finally gave up and took it in to a different shop. They ran it through a full battery of tests and decided it must be the ECM because they saw some erratic data and everything else checked out. They replaced the ECM and all is well! It was a case of scrambled brains in the computer! I had wondered if my custom chip could be at fault, but it's running fine with the custom chip now.

Comparing data now, to then, I see the Base Pulse Correction would erratically bounce between 60's to 100's and now it's 120's and 130's most all the time. If I had known more about what I was looking at, and what was 'normal', I *might* have been able to focus on the erratic behavior of the Base Pulse Correction, but I'm not sure I would have concluded that the ECM had lost its mind.

I've read about others with similar symptoms, so I'm posting this just in case my experience helps someone else...

don hall

Unfortunately, your experience confirms some shops are more competent than others......

Fiberbundle

What was the name of the shop that performed the correct diagnostics? Those of us in the Northwest are always looking for quality work on our C4s.

K87ZZ4

Brad's Custom Auto in north Seattle (Lake City area).