Hey guys I've been struggling trying to figure this out. Starting a month back driving down the road the car would loose power for a split second and come back it did that a few days at random. Then a couple of weeks ago I died at a light struggled to get it back going. No check engine light. Now not confident in driving I've been trying to troubleshoot.

When it dies you can immediately hear the fuel pump cycle. But it would cycle multiple times at different intervals seemed strange. Threw a gauge on it and it holds 42 never drops below when priming it gets close to 45. I've had issues in the past so I also changed the icm. Ohmed out the coil shows good. Checked ground on the pcm to the passenger side of the block. The ignition switch wiring looks good. I have an remote start on the car which bypasses the switch and the car would still not start. I bypassed the remote start to be sure that was not immobilizing the car.

Yesterday while troubleshooting I could not get the car to start so I checked for spark at the coil, i had it. Checked for spark at the plugs and once it arced to the frame the car started (strange). Connected the laptop back up to run scan9495 and I have code 36 which I never had before last night. From what I read code 36 should not cause the car to stall but recent posts seem to saying there's a chance it can. I didn't have this code until today. When I ran scan9495 earlier in the week it would randomly flash random dtcs but never activate the light.

Poking around on scan9495 I noticed my ignition volts seem to be dropping. I read that's the battery voltage to the pcm? My guess is when that voltage drops longer the car dies? Or when it's not there the car does not start?

Any ideas?
Thanks


The car
95 lt1.
Lt4 hotcam
1.6 rockers
Hooker long tubes catless y borla muffler
Mail order pcm tune with lt4 knock module
Engine was rebuilt about 10 years ago with some head porting
Ram air conversion
Air pump delete
Opti is about 12 years old with around 12k on it.
Walbro 255 fuel pump
160* thermostat
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Have you checked the 12 volt wiring to the PCM? Loose? Damaged connector pin? The PCM can't operate below 5 volts. But it should set a code for low voltage (DTC 50). But if it's a total loss of voltage at that short period, maybe it can’t set the code.

The fact the fuel pump is starting intermittently would also seem to indicate the PCM is briefly losing 12 volt power or ground.

The PCM has two grounds. As I recall the second ground is on the stud on the driver side head that holds the ICM/coil bracket. Check the wiring diagram in the factory service manual. See near the top of this PCM diagram for the grounds to the PCM connector. The factory service manual will show the ground number and location. It will also show the splices and any intermediate connectors and where they are located.

http://shbox.com/1/1995_pcm1.jpg

There are also four PCM pins that receive 12 volt power from 2 different sources. See near top of this diagram:

http://shbox.com/1/1995_pcm3.jpg

You should also be concerned about the 20 volt spikes. That may be related to a problem with the voltage regulator in the alternator.

The PCM is not going into closed loop. It should be, because the coolant temp is high enough and the run timer (206 seconds) to time out. But it can’t go into closed loop because the driver side (Bank 1) O2 sensor is “not ready”. Seems it may be stuck at close to 0.450 volts, which would indicate it is not active.

The Bank 1 long term fuel trim (LTFT/BLM) is bottomed out at 108 (pulling out about 15% fuel), the lower limit for that correction of a rich condition.

MAP looks a little low, although part of that could be due to the LT4 HOT cam.

 

Hey Injuneer, I have not checked the specific 12v pins for the pcm getting voltage, I did reseat all connectors earlier in the week. Both grounds I have taken off and cleaned this week as well the one down on the passenger side of the block and the drivers head at the icm.

I didn't even take notice of the voltage spikes. I had my battery tender hooked up during this I'll have to test without that and if it's still there replace the alternator then.

The pcm does attempt closed loop but rpms start dropping and it bounces out of it. Both o2s move up to 6xx mV before it goes into closed loop.

The long term fuel trim I do not know anything about.

Tomorrow I will check all 12v connections. I guess my main concern should be getting the voltage stable and that should solve my shutting down and not starting issue. After that then focus on getting the car to run properly.

Thanks for the assistance I appreciate it.