69_427_SBC

Having starting issues in my vette. Posted previously about it here.

Have since taken it a few steps further, with no conclusion:

Car:

- 1987 L98 Z51, stock, 4+3
- VATS key bypassed
- Starter Enable Relay bypassed

Symptom: Car cranks, fires a few times, but fails to idle and dies. No codes or security light.

Diagnosis: No injectors firing, ever (checked with noid light, and verified that light works on another vehicle)

Troubleshooting:

- Fuel pressure good, pre-crank, during cranking, and while briefly running
- TPS voltages good (0.60-4.38V range)
- Disconnected MAF, no change to symptom
- Injector resistances all checked good (16.9 ohm)
- Spark is good during cranking, during running, and even for a few cycles after quitting (checked with timing light)
- Rear distributor plug connected (with signal to ECM)
- Pulled cap; no signs of issue in distributor
- Noticed "tach" wire to distro was disconnected; located and connected, no change to symptom
- Oil pressure shows 11psi or greater during cranking and running

Given all this, a question: what things could cause the ECM to refuse to fire the injectors? From what I've read:
-- No distro signal to ECM, so it doesn't think the engine is cranking (distro plug was plugged in... bad module?)
-- VATS system failure/alert (should light up a code 46, though)
-- ECM failure (seems unlikely)
-- Severe injector wiring issue (again unlikely)
-- TPS malfunction causing the ECM to think throttle is open (TPS checked good)

Seems like the most likely option is the module, but could a bad module be driving good spark yet be failing to signal the ECM? Seems like the ECM would use the same signal for both.

Ideas welcome!

antfarmer2

Injector fuses? Ignition module?

69_427_SBC

Injector fuses is an excellent point! Top of the list for tomorrow.

Also ordered a new AC Delco module, figured it's good to have a spare around even if it doesn't fix the problem.

antfarmer2

Don't forget the thermal grease.

Cliff Harris

If the ignition module is putting out DRPs (Distributor Reference Pulses) the ECM will turn on the fuel pump through the fuel pump relay. The ignition module generates DRPs any time the engine is rotating, whether cranking or running. You said fuel pressure is good, so this appears to not be a problem (unless the fuel pressure is up because of the 2 second fuel rail prime when the ignition is turned on. If the injectors are not firing then the fuel pressure will remain high). Does the fuel pump run when cranking the engine?

The ECM uses the DRPs to synchronize the firing of the injectors to the rotation of the engine. If the DRPs are not present then the injectors will not fire.

Another thing that will prevent the injectors from firing is a bad TPS or connection to the TPS. If the ECM thinks the TPS is in "clear flood" mode it will not fire the injectors.

The ECM looks for a 30 Hz signal from the VATS module and will not fire the injectors if that signal is missing. The ECM will set an error code 46 if the VATS signal is not present.

69_427_SBC

Excellent question, I will verify. Not sure how this would tie to the injectors not firing, unless it all ties back to the reference pulses not making it to the ECM.

As I understand, the ECM simply provides spark advance input to the distributor, so it could still fire (as I observed) without the ECM getting pulses. Is this true? If so, I may have my culprit (module or wiring between module and ECM).

Also looks like I should verify TPS wiring beyond the function of the sensor itself. Added to the list.

antfarmer2

You did clean and check connections on the distributor had another member had a bad connection on his tach filter there.

DanielRicany

Are you getting injector pulse? Maybe bad gas? Maybe cam timing is off? (rare)

I would try running it on starting fluid.

Cliff Harris

No. The ICM has a basic advance curve built into it and will run the engine without the ECM. You can test this by disconnecting the EST connector, which disables the input from the ECM.

69_427_SBC

All the connections within the distributor were clean and assembled, and the ECM signal wires make it at least into the weather pack connector on the harness.

Signs continue to point to the ECM not getting reference pulses from the module. I currently have the module out and am bringing it to Autozone to be tested soon. I'll update with results!

69_427_SBC

Module tested good, but their test equipment seemed to be checking only spark info and had no indication of ECM comm performance. So I guess I'll try the new module when it comes, and check wiring until then.

antfarmer2

[QUOTE=69_427_SBC;1592453170]Module tested good, but their test equipment seemed to be checking only spark info and had no indication of ECM comm performance. So I guess I'll try the new module when it comes, and check wiring until

Try other stores.

Cliff Harris

I had my bad ICM tested 4 times at the local Autozone. It passed all 4 times. It only failed when it got hot. Apparently their test didn't get it hot enough, although it was hot to the touch when they finished.

69_427_SBC

Module arrives this week; I figure I'll just try that.

Makes sense. My car is failing to work at ambient temp, so I would expect a tester to show the issue IF it was looking for ECM comms - and this one wasn't. So the new module in the car will be my test.

I'm travelling this week so will drop off the radar for a bit, but I'll be back!

69_427_SBC

Ok, guys. I have to swallow my pride a bit for this update. After much fooling around with the distributor and ECM wiring, I finally realized that the reason I didn't have a Code 46 was... the diagnostic display panel plug was partially out of its socket and thus the "Service Engine Soon" light was never illuminated. Yes, this means I didn't get the Code 12, and that should have been my sign. Lesson learned.

So that means I'm back to a VATS issue. My first idea was to just get the ECM reprogrammed to remove VATS (and other stuff), but two quotes of $500 and $350 later I decided to hold off on that. So I did some digging and bought one of these:

http://www.ebay.com/itm/281813159375

Just installed it (with the 30Hz setting) and... it still doesn't work. No run, and a Code 46.

At this point, I'm at a loss, as the wire that now runs from this device is the ECM's only point of reference to the VATS system, and it should be looking for and receiving a 5V, 30Hz signal. All I can think of is:

- The eBay device is not working as designed, or
- The eBay device is not the correct design

Before I go buy an oscilloscope to test the first point, does anyone have any other ideas?

RichardH

According to Howell Engine developments service manual for both Corvette TPI Fuel injection and for the LT1 engines a code 46 is a VATS code stored in the ECM that has a production Mem-cal. To test a code 46, test for non-pulsing injectors by connecting a 6 or 12 volt test light across the terminals on any injector connector (removed from injector) and crank the engine. If the light does not flash while cranking, the ECM most likely has a VATS program or Mem-cal in it. (NOTE: this test will also show if you are net getting a distributor reference signal.)

The book also mentions that the ignition key might have to be reprogrammed, though I'm skeptical on the key as an issue.

I found this book extremely helpful in troubleshooting my 1987 L-98 circuitry.

69_427_SBC

Ok, I have a conclusion here. The ECM is NOT, repeat NOT, looking for a 5V, 30Hz signal. What it is looking for is for the VATS pin to be grounded at a 30Hz rate. I asked and eventually answered this question in another thread here.

I was able to use the eBay bypass in conjunction with a cheap MOSFET transistor to accomplish this (description at link), and the car is now running.