Hi all,

I have a 1991 c4 with an l98 and I was hoping to get some help with an issue of injectors not firing. A little background, I just got done replacing the intake manifold gasket, injectors and water pump so alot stuff was taken apart, but it was running prior. Car was sitting with a dead battery during this time for a while.

thanks to the info on this site I've been able to run alot of diagnostics below, but all passed. I am not sure what else to check.

I did confirm with a noid light, the injectors are not firing. the car cranks, fuel pump primes, and I have fuel pressure at the rail.

- no error codes
- Injectors do not fire with none plugged in
- injectors do not fire disconnecting one by one
- injectors all show 16 ohms resistance
- injectors are getting 12v power
- both injector fuses are good
- "tach" output on distributor is pulsing
- distributor timed (cyl 1 at tdc)
- TPS shows changing resistance, and is getting 5.4v (did not have a way to check while its plugged in)
- cleaned main ecm ground
- tried with disconnecting oil pressure sender, disconnecting tps, disconnecting MAF
- Vats is currently tricked with resistors, car cranks
- computer 1.3 shows "01 1" which I take to mean that it is sending the 30hz signal for VATS fuel enable
- grounds on ecm plugs are strong

My only theory is that the VATS signal is not getting to the ecm, but to want to confirm this before throwing parts at it. And the computer says it is sending the signal so... I don't see why this would fail just from the car sitting a little while anyway.
B6 (BLUE WIRE TO ECM) DOES NOT SHOW VOLTAGE AT ALL
But, given its a 30hz signal i dont know how that would show up on a voltameter.

 

TPS shows changing resistance, and is getting 5.4v (did not have a way to check while its plugged in)

Is 5.4V what you meant to say?
The TPS 'at rest' should show .54 volts.
My understanding is that if the engine is turning over, VATS is not the issue.

 

I can't check the outgoing voltage from the TPS because of the insulators, but the input power to the tps is 5 volts. I measure the resistance of the tps it went from 7ohms to 3 ohms as you move it (I dont know the math but I assume this steps down the 5 volts to .54 volts...

from what ive learned VATS in a 91 works like this:
- proper resistance is checked by CCM (no individual vats module)
- CCM grounds starter relay allowing it to crank
- CCM sends 30hz signal to ECM allowing injectors fire

It is possible for the CCM to fail halfway and stop sending this 30hz signal but still allow car to crank, but besides checking the computer 1.3 for confirmation of fuel enable, I do not know how to check for this signal...alot of ppl dont even know about this function of VATS so its making it incredibly hard to find info.

 

Take a small paperclip, straighten it, and insert it into the insulator alongside the wire. You must have <2.0 V on the blue wire, but nominal is .54-ish V. If the blue wire is "high", this of course puts the system in Clear Flood, and no injection pulses. If the connection between the TPS and the black ground wire is open, the blue wire goes high, even if the sensor itself is 'good'. Back-probe and verify the voltage on the blue wire.

A primary indication of a VATS inhibit is that the SECURITY light will be solid-ON with the key in RUN. The SECURITY light should never be on, flashing, nothing ever, with the key in RUN. If it is, it can be VATS or something associated with the CCM. You are correct that 1.3 01=1 is fuel enabled. << The SECURITY light is OFF if fuel is enabled.

If the ECM is not receiving the fuel enable signal, it will have Code C46 (and H46) in Module 4. Or it can be blink-coded on the SES by jumpering A&B.

Check the TPS voltage by back-probing the blue wire with the TPS still connected.

 

I checked the tps as recommended, and I am getting .544 volts off the blue wire.

no security light with key turned (it does work), still no codes (minus ccm c/h25 for the courtesy lights since I unplugged them)

 

Just to update this thread (because so many are left incomplete) I see that testing the tach output from the dist is not sufficient and that I need to check a purple/white wire for a signal... I will try and find this on the distributor side, I think it may be a wire under the cap.

additionally, unanswered question from another member in 2021... which port on the ecu side receives the signal to fire injectors? so I can test its making it to the ecu.

 

A few basic tests:

With the key in RUN, is the Service Engine Soon light solid ON? It must be. If = no, the ECM is not powered up.

Does it fire on starting fluid sprayed into the throttle body. If = yes, there is spark and the ignition module is powered and functioning.

Turn key to RUN. Fuel pump should run for 2 seconds and stop. Leave key in RUN. Briefly (about 1/2 second) crank again, stop, fuel pump should be running then stop. If = YES, the ECM received reference pulses. << This is a very simple yet definitive test for reference pulses. If it passes this test, you have reference pulses.

If it fails the 'fuel pump runs again after cranking":
There is a 4-wire weather pack connector on a pigtail that comes out of the base of the distributor and plugs into the engine harness. One of these wires is the pur/wht reference pulses wire. You wouldn't be the first guy to forget to plug this connector back in after installing the distributor. It didn't take me too long to figure it out because I didn't hear the fuel pump running after cranking. This test is easy, and a good one.

 

Yes the light is on.

Yes, runs with starting fluid, i did pull plugs and saw spark.

I do hear the initial prime of the fuel pump but I will check again after cranking. I did check that plug and am sure its connected, but thanks, because I want to confirm I see pulses out of that wire. Will update after lunch hopefully.

 

Update and close:

I was not getting a signal off the purple/white wire coming from the distributor so I replaced the ignition module. The car works now

for the record (and to correct some other incorrect information on this forum) you CAN get spark while NOT getting the signal sent to the ecm to fire the injectors.

 

Deleted.

 

Oh hey no no no u got it all wrong. I meant from the other threads I’ve been reading from like 2002 and 2012 and ect haha

conflicting info on those that confused me so I’m just hoping to get all the results in one thread for future reader.

I AM EXTREMELY GRATEFUL FOR YOU
espeically being the only person who helped me out. Your tip on the purple white wire got my problem fixed. So yes thank you so much!!

sorry that came across as it did. Seriously!!