Art-97-Z28

Been getting some computer diagnostics done on my stock 97 Z28 LT1 in order to get it running again. The car had been running OK but with a bit of a miss. If I was driving it every day, it would fire right up, however if I let it sit for more than 24hours without use it would behave like the engine was flooded and be very difficult to start up. The only way I could get it going was by removing the fuel injector fuse and it generally took 4 or 5 attempts at this. We changed the plugs, wires and opti distributor and it still wouldn't run. Hooked up to the shops diagnostic computer the rpms were showing 138,420 even though the engine was not running. We tried another PCM and it still showed the same value 138,420. Could this be related to the crankshaft position sensor or wiring? Why does it always show exactly that number? Any other ideas?

Dave89IROC

Try a different scan tool

Injuneer

The PCM reads the RPM from the cam position sensor in the Opti. It has to be rotating to produce a signal. There would have to be some sort of high frequency RFI on the circuit. Without the engine running it's hard to see where any RFI would originate.

bobdec

Quick question .. long answer.. RPM to a scanner is just a byte or two bytes of data buffer (depending on software) from the PCM. The buffer is loaded every so many microseconds from calculations derived from The OPTI pulses in OBD1 and '96-97 OBD2's, and from the crankshaft sensor on later non distributor (opti) OBD2's. The scanner then reads a much larger block of all sensor data buffers (128 bytes or larger) 8-20 times a second depending on it's speed and pCM availability. It then converts the binary data in the RPM field byte(s) to a viewable RPM number. Apparently your scanner is reading the PCM RPM buffer and displaying it as 138420. As an example .. Datamaster displays 50 RPM and desired idle of 3188 with key on eng off on OBD1. Not sure what JET DST or other scan software display in that case.
Your hard start sounds like flooding. If you hold throttle to the floor when cranking it should kill the injectors. Try that and let us know if it works..

Art-97-Z28

`Thanks for the info. I did of course try the gas pedal to the floor trick, but no luck with starting. Pulling the fuel injector fuse worked in the past, however I can no longer get it started this way either. Got the car running for a few seconds after fiddling with the crank sensor connector and while it was running the RPM reading changed, however it quickly died and the reading went back to 138420. Changed out the crank sensor and fixed up the connector but still same issue. Is the high 138420 RPM number causing the pcm to shut the engine off? So far we have a new opti, opti harness, new pcm and crank sensor. The plugs and wires were changed out about 6 months ago.

Injuneer

The crank sensor does not provide the PCM with RPM info. It's there to provide a rotational velocity from the crank so the PCM can compare it to the known crank rotational irregularities caused by the engine firing, and hence diagnose misfires. The PCM get's RPM info from the CAM position sensor in the Opti, as previously described.

bobdec it trying to explain to you that the scanner is just going to a default conversion of the last data bit it received, AFTER the engine shuts down. There are both an 8-bit and a 16-bit RPM fields. Which one is the scanner reading? DataMaster and FreeScan read both data fields.

As noted way above, try a different scanner and see if you get different results on shutdown.

bobdec

Sometimes I get carried away, "ask me the time and I tell you how to build a watch".. IMO that scanner likes to display 13840 with engine off. Does that shop computer have log capability. When the engine is cranking and 'not starting' it would be nice to see RPM usually starts around 75 RPM, then you should see injector pulse width around 40-60 Ms, if it kicks it will jump to 300 RPM, AFGs will start showing and then RPM will rise fast. As Fred said '97 calculates RPM from OPTI pulses, so that 75-300 reading during cranking will tell if OPTI is sending signal to PCM. Also as stated CPS is only for misfire detection on a '97 not used for spark generation. An OPTI or ICM DCT will kill engine and NOT light SES indicator.

Art-97-Z28

OK, so turns out the first replacement crank sensor was bad and now we are getting more normal numbers when "trying" to start the car. Given that we have also installed a new opti, plugs and wires and tried a new PCM where would be the next logical step? Ignition control module and ignition coils? Fuel injectors?

ACE1252

See if these help....from a 1996 factory service manual.

Crank, but no start.
http://www.ace1252.com/1996_Z28_Fact...ut_no_run1.JPG
http://www.ace1252.com/1996_Z28_Fact...ut_no_run2.JPG
http://www.ace1252.com/1996_Z28_Fact...ut_no_run3.JPG

Hard start....
http://www.ace1252.com/1996_Z28_Fact..._start_001.JPG
http://www.ace1252.com/1996_Z28_Fact..._start_002.JPG
http://www.ace1252.com/1996_Z28_Fact..._start_003.JPG

Help with Opti diagnosis(video in thread) can be found in a sticky at the top of the LT1 forum. Are you sure the signals are getting back to the PCM from the Opti(have you back probed the wires at the PCM with a scope?)? Scanned for codes after trying to start(a couple can be set with no SES light)?