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C4 Fires won't keep running. Fuel pressures good. Unplugging…

C4 Fires won't keep...

C4 Fires won't keep running. Fuel pressures good. Unplugging maf made no difference. Unable to check fault codes. Also fan comes on with ignition. Tried with vats removed

Mechanic's Assistant: How many miles are on the vehicle? And when was it last serviced?

About a month 97175 miles

Mechanic's Assistant: What is the model/year of your Chevy?

1986 Corvette c4

Mechanic's Assistant: Are you fixing your Corvette yourself? What have you tried so far?

Automatic

Mechanic's Assistant: Anything else you want the mechanic to know before I connect you?

All fuses ok

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Answered in 1 minute by:
7/11/2018
kalamykid
kalamykid, Shop Owner/ Tech
Category: Chevy
Satisfied Customers: 5,514
Experience: I HAVE OWNED MY OWN SHOP FOR OVER THREE YEARS. I HAVE THE TOOLS AND SOFTWARE TO ANSWERE ALMOST ANY Q
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When you say the vats has been removed, how was it removed?

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Customer reply replied 10 days ago
Reprogrammed memcal. Start enable relay bypassed. Was doing this prior to trying this

How long will it stay running?

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Customer reply replied 10 days ago
2 to 3 seconds
Customer reply replied 10 days ago
Injector pulse is present till it stops completely. As it dies down

Do you have a noid light or a test light we can use to check for injector pulse?

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Customer reply replied 10 days ago
Its on a scope trigger does stop until engine does

How old is the ignition module, and pick up coil in the bottom of the distributor?

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Customer reply replied 10 days ago
No idea sorry had car for about 5 years

Do you have a test light so we can do some testing? Also do you still have the chip in your key?

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Customer reply replied 10 days ago
Chips in key have test light
Customer reply replied 10 days ago
Plus resistor bypass box too

Lets make sure the ecm is getting the ground it needs to enable the injectors to stay working. What I need you to do is run a ground wire to the ecm plug for the B6 pin that is a purple wire. This will trick the ecm into thinking the passkey system is active and working correctly.

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Customer reply replied 10 days ago
Ok

Hold on

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Check that pin and see if the wire is a purple wire, or a white wire please.

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B6 is in the 24 pin plug, not the 32 pin

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Customer reply replied 10 days ago
White wire next pink/white in b5 I belive

With the ignition switch in the run position, what is the voltage to the white wire in pin B6?

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Customer reply replied 10 days ago
5 volts

Then the system is not seeing the ground it needs to enable the fuel. Go ahead and run a ground wire to this pin and lets see if it will start and run.

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Customer reply replied 10 days ago
Fires still stops after few seconds

Does this car have a tach in the dash?

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Customer reply replied 10 days ago
electronic dash

Will the tach go to 0 just before the engine dies, or does it slowly drop to 0 as the engine dies?

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Customer reply replied 10 days ago
Slowly with engine

Can you unplug the 4 pin connector on the passenger side of the distributor and see if it will stay running?

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Customer reply replied 10 days ago
How's it unclip ?

Do you have the flat 4 wire plug or the square 4 wire plug?

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That is not the plug. Run the wire out away from the distributor till you come to a black plug, ad unplug the wires there.

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Nope, that is the main power and tach plug. The 4 wire plug is for the est, electronic controlled timing. If you lightly pull on the wires on the passenger side you will see the plug. It is probably tucked behind the distributor where you can not see it.

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Customer reply replied 10 days ago
Found it trying no difference

I hate to say it but everything is pointing to a bad mass air flow. Unplugging it will not tell you anything as it will not run with it unplugged.

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With the ignition switch in the run position can you check the voltage to the dark blue wire, purple wire, and the dark green wire with the mass air flow plugged up please?

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Customer reply replied 10 days ago
OK will have to do another day what should I see.

12 volts on the first two, and anywhere from, .4 volts to 5.0 volts on the last one. The past one is the one I need to know the most, but the other two are feed wires and I need to know if they are powering the mass air flow.

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The dark blue wire should only have voltage for about 5-10 seconds after the ignition switch is turn to run.

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And dumb me just remembered that the oil pressure switch controls the fuel pump after it starts. Check the voltage at the red wire coming out of the oil pressure switch after the engine starts and see if it goes to 12 volts. If it does not replace the oil pressure switch if you have 12 volts to the orange wire. If the orange wire does not have 12 volts to it, let me know and we can go deeper into the wire diagnostics.

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The oil pressure switch is under the distributor on the drives side of the back of the engine.

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Just wanted to check in on you and see if you have found your problem yet?

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Customer reply replied 2 day ago
Black/ white earth when checked black earth. Green 5volts . blue .82 volts purple same
Customer reply replied 2 day ago
Oil pressure red had power
Customer reply replied 2 day ago
Also thermo fan comes on with ignition

What was the voltages for the orange and red wire at the oil pressure switch?

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Customer reply replied 2 day ago
12.2 fuel pressures stays around 45 to 50 psi
Customer reply replied 2 day ago
Injector coil resistances all around 15.8 to 16 ohms

Something is wring in the data, you have to be loosing fire or fuel. So either the fuel pump is not pumping and keeping the right pressure, the injectors are loosing there pulse ground form the computer, or you are loosing spark to the plugs for the engine to shut off.

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Customer reply replied 2 day ago
Everything just slowly drops down its not like a sudden stop? . I'll pick up a scope an recheck injector pulse an spark . tacho slowly dies with engine .

Let me know when you get the scope and we can go over a few things and see what is going on.

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Customer reply replied 2 day ago
OK thanks

You are very welcome.

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Customer reply replied 2 day ago
OK got scope on injectors and kV reader on spark lead. Also got another ecu board which I can use

Watch the rpms, and see if you still have injector pulse when the rpms start to drop.

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Customer reply replied 2 day ago
Yep still does further apart as Slows down
Customer reply replied 2 day ago
Spark is still present too on the kV reader

Does the scope watch the positive voltage to the injectors?

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Customer reply replied 2 day ago
still present

Do you have another mass air flow you can use?

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Customer reply replied 2 day ago
No sorry . have a spare ecu with no memcal I can swap over if that will help eliminate anything ?

With all you have tested, it has to be a bad mass air flow. The computer is working like it should, but what I believe is going on is the computer is not seeing the correct air flow, so it is not letting the fuel mixture set correctly for it to stay running.

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Customer reply replied 2 day ago
OK I will chase one up and retest thanks

You are welcome.

kalamykid
kalamykid, Shop Owner/ Tech
Category: Chevy
Satisfied Customers: 5,514
Experience: I HAVE OWNED MY OWN SHOP FOR OVER THREE YEARS. I HAVE THE TOOLS AND SOFTWARE TO ANSWERE ALMOST ANY Q
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