I have a 1989 TPI on a 383 using the stock MAF setup and 24 lb injectors. The car was running well with the exception of a high RPM vibration which I determined to be coming from the crank (the long block was purchased pre-assembled). I pulled the engine balanced the crank and put it back together. Now on starting the engine the LC-1 WB O2 sensor indicates an AFR in excess of 20:1 while TunerproRT shows an O2 reading of ~80 mv (the LC-1 feeds a simulated NB signal to the ECM). Meanwhile the BLMs rise to 160 and max out. Assuming I had a lean condition I increased the fuel pressure via the AFPR to 55 psi, no change in the O2 reading. I then readjusted the fuel pressure to 45 psi (with the vacuum hose removed) and using the real time function of Tunerpro I changed the fuel injector value from 24 to 19 then to 30 with no change in the BLM or O2 voltage. I then disconnected the LC-1 and reinstalled the 3 wire O2 sensor that I had previously used. On initial startup (after allowing time for the sensor to heat up ) the voltage was ~ 0.585 then it rapidly decreased to 0.00 mv (as read by TunerproRT). It then pegged at 0.00 mv with no variation whatsoever despite revving the engine. It made zero cross counts. I checked for vacuum leaks and did not locate any. I am using thick header gaskets and do not hear or feel an exhaust leak. I used ultra black rtv sealant on the intake manifold which I understand to be sensor safe. I removed the MAF sensor wire in an attempt to force limp home mode and the LC-1 read ~10.7:1 AFR, this made the car run slightly better then at the indicated 20:1 AFR. I have tried two different chips and two different bins with no change in the indicated O2 readings. The only code indicated in Tunerpro is the MAF sensor code (after I disconnected it). I am unsure what to check/change next to troubleshoot this issue. Any suggestions would be appreciated.

Any thoughts on this issue?

Do you have a smog pump on this setup?

I think your lc-1 sensor has taken a dirt nap

If it's going to zero, then something is grounding the wire going to the O2 sensor. What does it read with the wire disconnected? The ECM supplies approx .5V to the wire that you should be able to measure with a meter with the wire unhooked from the O2 sensor.

afremont, he is using a wideband with a simulated output. The wire from the ECM no longer goes to the oxygen sensor, it goes to the analog output on the wideband controller.

Ok, but the ECM should still be supplying power to the wire. With the wire unplugged, if it's at ground potential then it's either shorted somewhere or the ECM is broke.

Learn something new every day. I was never aware that the ECM supplied any sort of voltage to the O2 sensor.... of course I don't have a stock ecm

According to what I've read in the service manual, the ECM does it so that it will read something resembling a valid signal when the engine is cold. It also makes it easy to diagnose shorted/open wiring.

I will try checking for the .5 v tomorrow. Today I checked the spark plugs on the driver side (side with the O2 sensor) and they were all sooty black. I rechecked all vacuum hoses and did not find a leak. I changed the fuel injectors to some 28 lbs ford units I had previously used with no change. I removed all of the engine grounds, used a wire brush on them then reattached them, no change. Today I was using the 3 wire O2 sensor, not the LC-1. It would start at ~585 mv with the engine off then, after starting rapidly decrease down to 0.00 mv after the engine is started, however, if I gunned the engine the O2 sensor would go to about 800mv for a second then fall back to 0, so it is not stuck on zero, it just seems to have excess oxygen for some reason. I believe I hear a sort of sucking noise coming from the driver side of the engine but have not found a leak. Is it possible to have bumped the intake manifold gasket during reassembly such that a leak is occurring beneath the intake manifold? The smog pump has been removed, the smog pump connections are attached to each other so as not to leak and the EGR has been blocked and turned off in the bin. Thanks for the suggestions so far.

If you have access to an exhaust analyzer, you should test the exhaust stream with an complete stand alone analyzer. Or use your LC1 (unhooked on your ECM) and your Laptop. You must verify, is the exhaust really lean or is your 0.00V shown by your ECM false.

Black spark plugs indicated not a lean condition.......

Hard to find vacuum leaks can cause by a leaking brake booster or a blown, damaged intake gasket.

BTW: I fight with a similar problem, but my O² stay at high voltage and the BLM and INT goes in the deep 80's......... 1992 Z28 problems (1992 Z28 rough idle and many more problems!)

Same exact issues with my 383 as well. Here's where I'm at:

Engine will only start with starting fluid, once started it will continue to run although it has a good miss. My O2 readings go to zero when the ECM goes into closed loop and the BLM maxes at 160. Fuel pressure is set at 38 psi at idle. I have aftermarket fuel pump and Accel 30lb injectors running a Stealth Ram.

Plugs were black indicating a rich condition although the ECM thinks it's lean.

I thought maybe it was weak spark... Installed a Crane Hi-6, new coil, wires and MSD distributor, no change.

The cam isn't that big .224/.230 but I wonder if the overlap is tricking the O2 into a false condition.

If I shoot each header tube with a heat gun no two cylinders are the same. The temp ranges from a low of 190 to a hight of 425. I'm a a loss.

Food for thought, take it for what it is worth

rich to the point of misfiring will show LEAN as the exhaust still has oxygen in it.

So today I pulled the intake manifold and reinstalled the intake manifold gaskets very carefully to ensure they were not the issue. I then put the engine back together, set the timing and the exact same issue arose. The 3 wire O2 sensor moved gradually from 558 mv 0.00 mv over about 5 seconds. However, when I revved the engine up to ~2500 rpm the MV went to 800. So I removed the 3 wire O2 sensor and put the LC-1 wide band back in. At idle the LC-1 was indicating about 18:1 AFR, but when I revved up the motor the AFR came to the the 14.5 range. So I drove the car and during driving with the RPM in the normal range the AFR indicated ~14.5 +/- a few tenths. Then when I stopped and the car idled the AFR rose back up to 18:1. I am going to try a different MAF sensor tomorrow to see if that makes a difference. Any suggestions as to why I am seeing different AFRs at different RPM values? I don't have a miss and I am running at a constant RPM so neither acceleration or power enrichment would explain the difference.

I've never tuned MAF, but on my VE based system I have different AFR targets depending on engine load (map and rpm). Maybe you are seeing something similar?

18.1 idle is pretty lean. Our cars seem to like to idle nice and rich (14.0 :1 in my case) leaner then that and I get a lean miss at idle.

Replace the MAF with a known working unit. Once I replaced the MAF the engine ran much better however, the starting issue is still going on. The O2 readings improved but the BLM continues to max out at 160.

Mine sat for so long I'm thinking the injectors may have old fuel plugging them up, I'm going to replace the injectors tonight and see if it makes a change.

So today I started the car and experienced the same lean condition at idle, however when I rev the engine up to about 2500 rpm the AFR come down to ~14.7 (which is the commanded AFR in my bin). So I took the car for a drive. Under normal driving conditions the engine acted as expected. The AFR maintained about 14.7 +/- a few tenths, under acceleration power enrichment kicked in and the AFR fell to ~12:1 but when I put the car in neutral and let it idle the AFR begins to climb and hovers around the 18 to 19:1 range. I switched out my MAF sensor with a Micro-tech unit and experienced the exact same lean condition at idle. So what I am left with is an engine that indicates lean at low rpm/idle and operates and indicates normal AFR at operating speed. What could cause this?

IAC Valve stucked?????

So, check this out. As I had stated in a prior post my 383 did a lot of the same issues as yours.

After speaking with a guy who works with Accel DFI, he found out I was running the yellow top 30lb Accel injectors. He told me those were junk as the company Accel used to manufacture them had a problem, he had several cars with tuning issues running those injectors.

I ordered a new set of 30lb injectors from Summit, they showed up last night and I stuffed them in. Problem solved. The engine fired right up, O2 transitions are normal, BLM at 128 so... Looks like the issue was bad injectors.

I have solved the issue the interest of bringing this thread to a resolution I will post it. Since I was in search of a vacuum leak I created a smoke machine which consisted of a foot of 3.5 inch pvc pipe capped on both ends, but with a screw off cap on one end. I then drilled and tapped each end and screwed a 3/8 inch barb fitting into one end and a air compressor nipple connected to a regulator into the other end. I lit a smoke bomb, dropped it into the pipe, screwed the apparatus together and with ~1-2 psi from the air compressor hooked the barb fitting to the vacuum line which normally would attach to the power brake booster. After about 10 seconds smoke started pouring out from the driver side intake manifold where the front most runner bolts on. I disassembled the intake and pulled the driver side runner to assess the gasket. At this time I realized I had left some critical information out of my earlier post. I have installed an Edelbrock Hi Flo base plate and runners on the engine, then I used a stock Felpro gasket setup to reassemble the engine. But, the Edelbrock Hi Flo baseplate uses a different gasket on the driver side frontmost runner to baseplate connection then stock. It in fact uses the same gasket as the stock passenger side. So my vacuum leak was due to using the wrong gasket. I installed the properly fitting gasket, reassembled the intake and now the car works properly again. The moral to this story is to always eyeball the gaskets on installation to ensure they fit properly. Had I done that I never would have had an issue.