McLovin

Have a 95z28 thats been sitting for a few years, it ran when I parked it. Swapped the transmission, put a stall in, cam headers and a few other things. Recently the car was sold to my brother and we are trying to get it back onto the road.


Now for the problem, car will crank, but will not start. Hooked up my scanmaster, no dtc's. But i noticed something odd, the map senor reads the same thing as the barometer as soon as you turn the key over with the throttle body closed. We have swapped in different map's still get the same reading. Also the car is getting no spark, no injector pulse, and the maf isnt reading anything. The fuel pump does prime when you turn the key over and crank it, but nothing else is happening. I switched the ecm from my built 95 in and get the same readings. We cleaned up all the ground but the one on the motor mount, hes hasnt got a chance to get to it yet. I know my way around LT1's pretty good and am totally stumped by whats going on.

antoine

How is the opti with a scanner hooked up while cranking do you see a rpm signal from the opti

ACE1252

This is from a '96 factory manual, but should help out....the DTC numbers will be different from a 95 to 96. Also, you may need to have the car scanned for trouble codes. I think there is at least one code that will not flag the SES light, but will prevent the car from starting.

Check all your fuses(dash and under hood) and inspect the wiring harness for chewed or damaged wires.
Also, check the high resolution, low resolution, and 5V ref signals for the opti(check out the ABITS video in the opti sticky or start at step 11 in the chart below).

Crank, but no run.....
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

McLovin

No rpm's, no dtc's the thing thats really throws me off is the map reading the same as the barometer. Is that likey to be the opti as well?

ACE1252

Step 6 in the chart I posted states that if there is no scanner RPM reading to go to step 11....which is pointing to what I posted earlier.....no high res/low res opti signals or 5V ref signals for it.....

This gives you a direction to move in....stop scratching your head and start testing what the chart tells you to or do it via the ABITS video. I'm not trying to be mean, but you are not reading what has been posted to help you out.

The video tells you at the 14:40 mark that with both high res and low res signals missing, the engine will not start and not set a code. This brings in to question the 5V ref signal...or the signal harness connected to the opti. There also could be ignition module issues as a possibility too.... as those codes(41 and 42) don't set the SES(video mark 15:50) and will shut down the injectors to prevent flooding.

Start at the 13 min mark....and this does apply to 95's codes and all....
https://www.camaroz28.com/forums/lt1...-video-795663/

McLovin

Like I said I was wondering why the map reading is so far off. Thats what has me scratching my head. I was hoping someone has ran into this before, and could tell me what it is. I have also broke more opti's then you can imagine my fast lt1 spins 6,800rpm's and eats the **** out of them. Any opti I've ever had that didnt put out injector pulse/rpm's/spark ALL tossed a DTC at me rather it be low res or high res. This one does not toss anything and there was nothing wrong with the opti or so I thought when I pulled it off the last car. That is not the part thats got me scartching my head, I added that info because I thought maybe its tied into what ever is causing my map to readings to be off. I'm thinking maybe a mouse ate through some of the wires and was wondering if anyone ran aross all of the readings being so far off together. Also excuse me that I havent ran out to fix my brothers car while hes out of town working. I was trying to toss ideas out there so the next time he comes into town I can have everything covered.


So lets start with this, what causes a map to read the same thing as the barometer when the throttle blades are closed.

ACE1252

But how about missing BOTH? You will not get a code if missing the feed voltage that powers both the High res and Low res signal. If you are missing that voltage(5 volts), then back tracking to find out why it's missing may lead you as to what is jacked up with the MAP......If I'm not mistaken it's got a 5V volt feed as well.....are you starting to see the possible correlation? If that voltage is missing then the MAP reading, I suspect, will be stuck at either the high end or low end of the scale.

I could be wrong, but I don't think there will be a significant difference in the MAP reading vs Atmosphere at cranking speed. I simply don't think enough vacuum will be pulled at 100-120 rpm to make a major dent in the readings. The MAP reading, with the engine static, should be 99-100kpa @ 29.5-29.9in Hg. I will test it tonight on my Z and get back with you.

However, if what I went over in my first paragraph is correct(missing the 5 ref at both places...opti and MAP), then that may explain why your map is not moving. I'll also test what happens to my MAP reading with the sensor unplugged(see if the reading is high or low).

bw_hunter

I'm not sure about this. If the engine is capable of generating 165psi in a compression test at cranking speeds, it has to be pulling a significant vacuum. I'm not sure how to calculate it and I do suspect it's be a lower value than when it's running but it should be measurable.

I also doubt this is the problem with the crank/no start situation. The OP needs to follow the diagnostic and let us know what he finds....

Injuneer

With high vacuum, the mass of air that actually enters the cylinders is reduced, and the compression pressure would be very low. That's why you open the throttle blades to measure the compression.

ACE1252

I just tested my MAP sensor.

• Engine not running the reading is 98kPa(28.93inHg).
• Engine cranking, throttle closed, the reading is 95-98kPa(28.05-28.93inHg).
• With the MAP unplugged, the reading drops to 10kPa(bottom of scale).

Given the above, I think your MAP sensor is fine as is the voltage to it. Based on the info you have provided, the opti high res and low res signals are what you need to target to find your problem. If that checks out, the ignition module/ignition coil is the next place to check.

With the connectors all hooked up, an excellent way to examine these voltages, with a voltmeter, is backprobing the weatherpack connector with a t-pin. Still, you have to be careful not to damage the connector. They are at Wal-Mart in the sewing section.

T-Pins....
http://www.walmart.com/ip/T-Pins-Size-28/19757784

Backprobing a connector....3:20 mark
http://www.youtube.com/watch?v=DylOq484Tws