asilomar244

I have a '94 LT1 in a conversion that is CA legal and runs great. A few months ago it starting missing under load. Tested fuel pressure, steady at 40psi. Replaced fuel filter and pump as they were original, vehicle is 1982. Replaced coil pack and ICM as they were also original on the LT1, no change. Replaced the opti with a high quality AC Delco reman unit, not cheap. Cap on old rotor showed oxidation on one point and scorching. So was hopeful that was the source of the problem.


Buttoned everything back up. Started on 3rd try, sounded great. Went for 5 mile test drive, no error code. No more missing under load except for a couple slight one on downshift. Compared to before, 95% improvement. Today, puttering down the hill from my house it stalled. Hard to start, doesn't want to rev. Still no error codes. Will check fuel pressure tonight and connect it to Scan94/95 to monitor O2 and fuel trim. A bit out of ideas. I have a replacement knock sensor, but don't think these symptoms are consistent with retardation of timing.

Any ideas?

Will update with more info later tonight. I have family in town and they need my regular daily driver, so this ride needs to get on the road asap. Any guidance is most appreciated. Thanks!

blown94

Is your water pump leaking by any chance? They like to contaminate the opti, and before you tell me it's a new opti, idc. Also, if the new opti made it much better and then it died and wouldn't start back, and that wasn't a symptom before replacing it, I would say defect. Usually they throw a pickup sensor code, but not always. One thing to check is your opti extension harness for corrosion/contamination or loose pins and make sure it didn't work out@the opti itself

asilomar244

Thanks blown94, checked the water pump closely tonight looked dry. New opti came with sealed connector, chose not to cut away seal and check pins there. Checked pins at harness connection looked good.

Fuel pressure stable at 32-34psi. A bit low, problem? New version of the same fuel pump I have used for 6 yrs. Engine ran rough right away, would barely rev, multiple DTCs (including low res pulse on opti):


Cleared DTCs, restarted engine, got only 1 (opti again, high/low res pulse):


Cleared DTCs, restarted engine, no more popped up. Ran for 5min without another engine code. Here is a capture of the feed. Alternator is low at 11V, I am going to replace that tomorrow. No knock sensor retard.


Still running rough. Checked PCV valve, seated in intake looked ok. Reseated air plenum at MAF inlet and checked post MAF on plenum for any leaks. The stock baffle was removed years ago and I blocked the junction and sealed with RTV. It was a bit loose. I hose clamped and the engine ran better. Took it for a test drive. Drove pretty good, a huge improvement over yesterday. Did ok under high load (3rd gear uphill @ 20mph). Got onto open road and it stumbled at 75-90% throttle. So not completely out of the woods. Cleaned the block on the plenum junction, fresh RTV setting now.

Any thoughts?

Injuneer

Your Cell 16 (idle) long term fuel trims are maxed out at 160 (adding 25% extra fuel).

Do you still have the AIR pump functional? In the photo, the system is in open loop (O2's ready, CLT above threshold, but did not reach the timeout on the 205-second timer), and it shows a "1" for the AIR pump relay, indicating the pump should be running if its installed. Since you indicate the car is CA legal, the pump must still be there. Normally, in open loop, with the AIR pump running, and the O2 sensors both "ready", you should be seeing very lean readings (0.00X V) on the O2 sensors. Yours are reading extremely rich at 0.900+ V. Pump may not be running, check valves may be frozen closed, rubber hoses may be melted.

The very high LTFT's jack up the injector pulse width, which at 4.00+ millisec, are above what you would normally see with an MAF reading of 7.89 G/Sec.

asilomar244

The AIR pump is setup and does turn on (dealt with wiring and DTC on that a month ago after setting up new fuse/relay block). The pump itself is original, I will check output flow on it tonight. Check valves are in the AIR pump? No melted hoses.

Injuneer

The check valves are the black "cones" at the exhaust manifolds:

Courtesy of Shoebox

http://shbox.com/1/diverters.jpg

asilomar244

Pulled both AIR check valves. Both in good order, unidirectional flow. Cleaned and reinstalled. Replaced alternator- only slight change to 12.0V. Finished air plenum repair and reinstalled. Started engine, ran rough right away. Worse than last night during test drive. Intermittent? Was bad for a full 10min test tonight (not driving). No DTCs. The one shown below is from pulling the coolant temp sensor connector. No change, reconnected. The LTFT dropped as did the O2 sensor readings, but still high.

Considering the car ran pretty well the first drive and ok last night, intermittent issue due to defective opti? I am going to call the distributor tomorrow, please let me know if you have any ideas.


Note: The engine RPM reading is incorrect, this pic was taken with the engine at or near idle...

Injuneer

In this photo, the PCM is in closed loop. The O2 sensors will be cycling rapidly back and forth between 0.1xx - 0.9xx V. You just happened to picture a frame where they were on the low side. There is limited value in a single frame like this. A data log over a longer period of time is much more useful.

The RPM error may be pointing to an erratic cam position signal from the Opti.

What is the MAF reading.... too fuzzy to read?

asilomar244

The MAF reading is 16.44 in this screenshot. I just ordered a second opti. Ugh.

Injuneer

That's a very high reading, if the engine is really at ~800 RPM (not the value shown), and the throttle is closed, as indicated.

asilomar244

During the initial troubleshoot, I replaced the MAF with an OEM Delphi unit. Do you think it could be defective? I'm not aware of any calibration for those sensors.

I'll disconnect it tonight to see if that helps.

Injuneer

I review these logs all the time, and my guideline is 6 - 8 G/Sec at 800 RPM idle. That is usually accompanied by a base injector pulse width in the range of 1.5 - 2.0 milliSec. Again, hard to see, but your pulse widths appear to be high (~4 milliSec).

The MAF sensor output is a frequency signal. There is a calibration table programmed into the PCM that converts frequency (Hz) to grams/second air flow. The PCM takes that mass air flow value, divides by the target A/F ratio (14.7:1 in closed loop, non-PE mode) to get the required mass fuel flow. It multiplies the calculated "normal" mass fuel flow X (LTFT/128) X (STFT/128) to get the corrected flow, based on your O2 sensor feedback. Then it looks at the injector constants (dependant on the size of the injectors) to determine the pulse width required to deliver that amount of fuel.

What appears to be causing the high air flow is the fact that the idle air control (IAC) shows it is open 100 "counts" (steps), while that value would normally be in the range of 20 - 40 counts.

The odd thing is, if your MAF sensor was indicating higher than actual air flow, the PCM would be adding higher than required fuel, the O2 sensors would pick up the "rich" condition, and start to correct by pulling the LTFT's down below 128, not pushing them above 128.

You don't mention anything about your engine.... is it stock, or modified? If modified, does it have stock injectors or larger than stock injectors?

asilomar244

Stock motor (including injectors, PCM) except for the following mods:

De-shrouded slightly ported intakes (rebuilt)
TPI exhaust manifolds (modified w/ 1/2" OD EGR tube)
K&N cone air filter
Modified wiring harness (VATS delete)

asilomar244

Disconnected MAF at startup, engine ran into high rev range great three times. Then began to stumble and died. Couldn't restart for a minute, when I did, it was running rough with the MAF disconnected. Reconnected, no change. Replacement opti arrives Friday. Unless anyone has guidance in a different direction, I am going to hope the current reman unit (AC Delco) is defective and replace it.

Injuneer

What "intakes"? What does that mean?


Do you possibly mean TPIS headers? No one make replacement stock manifolds, and "TPI" would be the manifolds from a 3rd Gen (possibly an L98) and would not have the D-ports like an LT1.


A complete CAI, or just a filter? If just a cone filter, where is it connected?


Do you mean a resistor added to the key read circuit, and/or a module to simulate the 30/50Hz "fuel permit" signal to the ECM?