goser

Hi gents, looking for some help with my 1994 Z28 M6


Last two days the car stalled waiting in the drive-through for my morning coffee. First time thought I just stalled it from poor clutch control, but it wouldn't restart until some employees pushed me down the hill and I bump-started it (Big thanks to them!). Ran fine rest of the day. Next day, same drive-through, rpm's dropped off a couple times before it cut out idling in neutral. Took a few tries to restart but gave it lots of juice when it did... I was pretty low on fuel so gassed up and it ran fine rest of the day. Was planning to clean my IAC and do a re-learn to see if that helped, but thought maybe it was coincidence from bad driving and bad gas.


Today, driving along in 5th, engine just died completely. Got to the side of the road and got it to start after a few tries but died almost immediately. Eventually got it going again and made it a couple hundred feet before it conked out again. Turned off the a/c, got it started again, and kept the revs above 2000 the rest of the way home, although it seemed fine other than running rough for a short time.


So... where do I start? Plugs, wires, and opto were replaced last year... the shop did a shoddy job and I replaced the plug wires again after one melted, but otherwise that system should be fine unless the opto died already or they messed up something else. I replaced coil and ICM last year as well... thinking I should space out the ICM now, as maybe the problem is heat related from the warmer weather we've had this week. Figure I'll replace fuel filter and check pressure once I get my gauge back from a buddy I loaned it to. Suspect there may be vacuum leaks, but would that cause stalling when cruising at highway speed?


Totally lost as to where to start looking, so any help is appreciated!

Injuneer

At any point have you scanned it for codes?

What brand is the Opti? Lot of poor quality stuff on the market. Have you had the ICM tested? Definitely follow up with the fuel pressure.

Is the engine stock or modified? Does it still have a functional EGR system?

goser

Thanks for the reply!

Engine is stock, with egr. The opto I put in was a cardone (not reman.)

Haven't scanned for codes, I just ordered the parts tonight to build the usb to obd cable.

How would I test icm? (I might still have the original icm and coil around somewhere, maybe I'll swap the original back on and space it out.)

Cheers

Injuneer

Auto parts stores can test the ICM. Make sure they get it good and hot, to replicate the under hood environment.

Pulling the codes is important because the Opti and ICM codes that can shut down the engine and prevent it from starting do not turn on the SES light.

Asked about EGR, because an EGR valve sticking open can cause stumbling and stalling.

goser

Still waiting for my OBD cables to arrive, but was able to check fuel pressure finally.

Key on engine off: 41PSI while priming, 37 after pump shut off. 34 at idle, 41 with regulator vacuum pulled. As far as I've read, this is within spec, although right on the low end.

I thought I saw, just for a second while running, the pressure drop to around 20. It was so quick I'm not even sure it really happened, but if it did that would probably be the pump dying, ya?

goser

Haven't had much time lately, so haven't changed fuel filter or done any other diagnostics, but I did get a chance to build my OBD1 cable (the guy who created scan 9495 is a genius!)

Scan showed code 16 (low res pulse), which apparently shuts off pump and injectors, so I guess that's the culprit.

Going to look for wiring harness issues to the opti tomorrow, but anything else that could cause this except my new opti or the wiring to it?

Battery was a tad low when I metered it the first time it stalled (12.3 or so) and since it's been sitting for a while today it's at 10.1 and won't crank... I read somewhere low battery can throw code 16... could that be the issue? Or does the battery need to be really dead for that to happen?

Thanks all!

Injuneer

If the battery gets low enough to affect the system operation, you would get DTC 50 for “system voltage low”. “Normal” voltage is considered 9.6 to 16.0 volts. DTC 50 sets when the voltage drops below 8.0 volts.

DTC 16 is totally dependent on how many high resolution pulses appear before a low resolution pulse is detected by the PCM. The cam position module requires 12 volt power, and the PCM provides 5 volt reference signals that are used to generate the 0 to 5 volt square wave high and low resolution pulses. I haven’t seen a case where DTC 16 was caused by low battery voltage.

If it’s running and you have DTC 16 it's a “stored” code. I've seen that set on initial cranking, when it takes an excessive amount of time for the PCM to “find” the low res pulse signal. So it isn’t 100% conclusive that's what is causing the stall/no start.

As you noted, start by checking the short Opti harness from the gray connector on the bracket on the passenger side of the intake manifold to the Opti (some newer replacement Opti's include that harness piece permanently attached to the Opti). Look for damaged pins, missing insulation, and corrosion on the connectors, particularly on the Opti end where the ozone generated by high voltage discharge in the non-vented (93/94) Opti produces a greenish-blue crud. Make sure the connector clip that holds two pieces together is not broken.

If it stalls again, while cranking the engine to restart it, watch the tach to see if the needle jumps up a couple hundred RPM, which would indicate the PCM is receiving the low resolution pulse from the Opti. The optical cam position sensor module in the Opti can be sensitive to heat soak causing loss of signal. But yours seems to recover fairly quickly, allowing a restart.

The absence of DTC 41 and 42 would seem to rule out problems w/ the ICM.

goser

Thanks so much for the advice injuneer! Things are not looking good for our heroes...

I was pretty sure the shop that did the opti didn't replace the short jumper (and after looking at it I'm 100% sure), but unfortunately all the connections look good and clean. Put a meter on Pin A and got 5V at the opti connector.

On the upside, while I was poking around I noticed the evac vacuum elbow at the throttle body is totally cracked and rotten. Probably nothing to do with the stalling, but can only help matters to fix that leak!

Injuneer

Are you aware of Shoebox's LT1 help site? He has a procedure for verifying the low res pulse is reaching the PCM/ICM.

Note that verifying 5 volts on pin A and B just tells you that the PCM is supplying the reference voltage. With the engine running, you have to detect a 0 to 5 volt square wave on A and B to verify the Opti optical module is actually using the reference to deliver the pulses. Can be done with a cheap oscilloscope, or by checking the voltage on pins A and B with the AC volts setting. Or, if the engine isn't running, key on, using the DC volts scale, check the voltage on pins A and B while slowly rotating the crank. the low res signal will only switch 4 times for one crank rev, so it's easy to detect with the meter. The high res switches 180 times per crank rev, so the crank needs to be turned slowly to read it.

4th Gen LT1 F-Body Tech Aids

4th Gen LT1 F-body Tech Articles

And yes, GaryDoug, author of Scan9495 is a genius (and a graduate electrical engineer), and he doesn't even own an LT1.

goser

Hi gents, since my last post I ended up needing to drive my truck around for work for a while, but a few weeks ago I got the car out again and it ran like a champ for weeks! Just when I thought maybe clearing the code had resolved things... it died in the same coffee shop parking lot (the ladies there are never going to let me live this down!)

Just like the first time, where I thought I may have stalled it by accident, this time I definitely stalled it out rushing to do a three point turn. Tried for hours to restart it, a couple times it caught and ran for a few seconds before dying, and eventually had it towed home. Of course, when I hooked up the code reader again it had a code 16. Luckily it started now and I was able to park it in the garage for the winter.

The two constants seem to be that the problem appears after I stall it, and both times I had recently been through an automatic car wash. I'm guessing both are coincidental, but maybe someone knows if there's a correlation here?

I'm thinking I'll replace the opti over the winter... I'd like to get the vented MSD one, but the price is pretty steep. I'll need to look over the instructions again, but I'm not sure I'm comfortable with the DIY venting option. I'm guessing this opti is already shot, but would just replacing the cap/rotor with the vented MSD cap help matters? My backup plan at the moment is to buy a new Spectra or Cardone opti and install the MSD cap on it for the venting. Anything else I should be considering here?

Thanks!

Injuneer

When it was cranking and wouldn’t start, was the needle on the tach moving upward a few hundred RPM?

This points to an intermittent problem with the optical module in the Opti, OR an intermittent problem with the short harness/connector. I know you checked the harness and connector and said they were “good and clean”. But maybe the fact that both times the stalling/no start/DTC 16 occurred right after a car wash, it may not be a coincidence. Water may be entering at the harness connector.

My personal recommendation for a replacement is to avoid the MSD. Since they introduced it, it has been plagued by one problem after another. A couple years ago I was personally involved in providing MSD with data logs that showed an extraneous signal interference screwing up the way the engine runs - a massive idle surge, and rough running. This data was developed from analyzing PCM data logs from three different MSD Opti owners with similar problems.

Eventually MSD indicated they found the problem, claimed they rebuilt the Opti returned by one of the owners, and returned it to him. He eventually reinstalled it, replacing a cheap Chinese rip-off that was actually working fairly well. The engine would not start with the rebuilt MSD Opti. He returned it again, they sent him a new one and third time was a charm, the engine actually stared. Another one of the three people with the problem had his MSD replaced before I got involved, and his engine would not start with the replacement.

The only Opti that is currently getting positive feedback is the one assembled by Petris. A few people here have tried it, but no negative feedback. The spline drive (93/94) unit has a vented cap. Cost is $400.

https://petrisenterprises.com/