1990 stalls after starting
Thread Starter
Heel & Toe
Joined: Nov 2024
Posts: 20
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1990 stalls after starting
The neglected '90 C4 now starts easily but only runs about 2 or 3 seconds be before stalling. It has a new Racetronix fuel pump (with a clean tank), regulator diaphragm, filter, and Bosch injectors. Ignition has a new HEI module, cap, rotor, wires and plugs. The pump seems to be making good pressure, but to make sure it's constant I tested with battery 12v right to the fuse to ensue it's not shutting down. After it stalls (with normal wiring) there is almost no residual pressure in the fuel rail. Once I get a SAE connection to the fuel rail I'll check the pressure with a test set I have. The diagnostics don't show any ALDL ECU codes. Learning about ECU module testing now, but looking for other ideas. The car has been off the road since 2017, apparently it had the same problem back then.
Thread Starter
Heel & Toe
Joined: Nov 2024
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Bosch 22 lb. injectors. Using the in dash ALDL for codes, only shows the '12' placeholder with no stored codes. Going by the service manual on this. Engine is all stock (for now) minus the air pump and EGR.
Melting Slicks
Joined: Jul 2005
Posts: 2,207
Likes: 302
Well, you're on the right track with your intent to get a fuel pressure gauge. Re-establishing the basics is where I would start as well...
Fuel pressure
TPS voltage and IAC Reset https://www.thirdgen.org/tpimod2/ (1990 TPS sensors aren't adjustable, but you can check the voltage)
Base timing (if you can get the car to run long enough).... though you might try disconnecting the EST connector and seeing if the car runs... might be a clue on what's wrong.
Could be a host of possibilities, but eliminating the basics is always the best place to start.
Does the car just shut off like a light switch? Or does it kinda stumble before it shuts off?
Fuel pressure
TPS voltage and IAC Reset https://www.thirdgen.org/tpimod2/ (1990 TPS sensors aren't adjustable, but you can check the voltage)
Base timing (if you can get the car to run long enough).... though you might try disconnecting the EST connector and seeing if the car runs... might be a clue on what's wrong.
Could be a host of possibilities, but eliminating the basics is always the best place to start.
Does the car just shut off like a light switch? Or does it kinda stumble before it shuts off?
Thread Starter
Heel & Toe
Joined: Nov 2024
Posts: 20
Likes: 0
Well, you're on the right track with your intent to get a fuel pressure gauge. Re-establishing the basics is where I would start as well...
Fuel pressure
TPS voltage and IAC Reset https://www.thirdgen.org/tpimod2/ (1990 TPS sensors aren't adjustable, but you can check the voltage)
Base timing (if you can get the car to run long enough).... though you might try disconnecting the EST connector and seeing if the car runs... might be a clue on what's wrong.
Could be a host of possibilities, but eliminating the basics is always the best place to start.
Does the car just shut off like a light switch? Or does it kinda stumble before it shuts off?
Fuel pressure
TPS voltage and IAC Reset https://www.thirdgen.org/tpimod2/ (1990 TPS sensors aren't adjustable, but you can check the voltage)
Base timing (if you can get the car to run long enough).... though you might try disconnecting the EST connector and seeing if the car runs... might be a clue on what's wrong.
Could be a host of possibilities, but eliminating the basics is always the best place to start.
Does the car just shut off like a light switch? Or does it kinda stumble before it shuts off?
The engine doesn't suddenly shut off, more like a quick ramp-down to off. On some starts it stumbles, regains power for a second then shuts down.
Key-on the TPS reference voltage at the connector is 5.4 volts.
Thread Starter
Heel & Toe
Joined: Nov 2024
Posts: 20
Likes: 0
Well, you're on the right track with your intent to get a fuel pressure gauge. Re-establishing the basics is where I would start as well...
Base timing (if you can get the car to run long enough).... though you might try disconnecting the EST connector and seeing if the car runs... might be a clue on what's wrong....
Base timing (if you can get the car to run long enough).... though you might try disconnecting the EST connector and seeing if the car runs... might be a clue on what's wrong....
Melting Slicks
Joined: Jul 2005
Posts: 2,207
Likes: 302
OK, fuel pressure key-on is 45 psi. During cranking and running its a steady 40 psi. Never drops lower until after its shut off.
The engine doesn't suddenly shut off, more like a quick ramp-down to off. On some starts it stumbles, regains power for a second then shuts down.
Key-on the TPS reference voltage at the connector is 5.4 volts.
The engine doesn't suddenly shut off, more like a quick ramp-down to off. On some starts it stumbles, regains power for a second then shuts down.
Key-on the TPS reference voltage at the connector is 5.4 volts.
Melting Slicks
Joined: Jul 2005
Posts: 2,207
Likes: 302
From what you're describing it sounds more like fuel related than spark related.
Hmmm.... this is where it helps to have the ability to look at running data. Do you have a bad coolant sensor? MAP sensor? etc... you can get faulty sensors w/o throwing a code unfortunately.
Did you do the IAC reset procedure I linked to?
Hmmm.... this is where it helps to have the ability to look at running data. Do you have a bad coolant sensor? MAP sensor? etc... you can get faulty sensors w/o throwing a code unfortunately.
Did you do the IAC reset procedure I linked to?
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Thread Starter
Heel & Toe
Joined: Nov 2024
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Thread Starter
Heel & Toe
Joined: Nov 2024
Posts: 20
Likes: 0
A bad sensor is more likely to put it in a reduced power limp-home mode rather than shutting the engine down immediately. Oil pressure on the dash reads 40 psi during the brief start, well above the 4 psi shut off threshold.
Thread Starter
Heel & Toe
Joined: Nov 2024
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No change. Any amount of throttle is the same effect. A noid light on a injector connection shows it getting a signal even while the RPM's are dropping off / loosing power.
Melting Slicks
Joined: Jul 2005
Posts: 2,207
Likes: 302
It's been sitting for ~7 years now... have you checked all your vacuum lines and stuff like that?
Another possibility is to ohm out the injectors. Are there any that are out of family on the resistance measurement from the rest?
Thread Starter
Heel & Toe
Joined: Nov 2024
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Not necessarily... For example, a bad MAP sensor that's reading high, but still within the normal voltage output range can fool the ECM into delivering A LOT more fuel than it normally would.
It's been sitting for ~7 years now... have you checked all your vacuum lines and stuff like that?
Another possibility is to ohm out the injectors. Are there any that are out of family on the resistance measurement from the rest?
It's been sitting for ~7 years now... have you checked all your vacuum lines and stuff like that?
Another possibility is to ohm out the injectors. Are there any that are out of family on the resistance measurement from the rest?
Melting Slicks
Joined: Jul 2005
Posts: 2,207
Likes: 302
Hmmm...If you have a windows laptop, you might want to invest in an ALDL to USB cable and download TunerproRT.
Tunerpro is free, so you'd only have to buy the cable.
https://boostednw.com/emulation-cabl...-gm-datalogger
It's an investment, but with these old cars now, being able to see what's actually happening can possibly save money in the long run because it makes it a lot easier and more likely that folks on this message board can diagnose the problem w/o having to randomly throw parts at the car.
You can post datalogs on this site for other people to download and replay, looking at sensor outputs, trouble codes, and other information.... takes a lot of the mystery out of trying to diagnose EFI problems.
Tunerpro is free, so you'd only have to buy the cable.
https://boostednw.com/emulation-cabl...-gm-datalogger
It's an investment, but with these old cars now, being able to see what's actually happening can possibly save money in the long run because it makes it a lot easier and more likely that folks on this message board can diagnose the problem w/o having to randomly throw parts at the car.
You can post datalogs on this site for other people to download and replay, looking at sensor outputs, trouble codes, and other information.... takes a lot of the mystery out of trying to diagnose EFI problems.
Thread Starter
Heel & Toe
Joined: Nov 2024
Posts: 20
Likes: 0
Hmmm...If you have a windows laptop, you might want to invest in an ALDL to USB cable and download TunerproRT.
Tunerpro is free, so you'd only have to buy the cable.
https://boostednw.com/emulation-cabl...-gm-datalogger
It's an investment, but with these old cars now, being able to see what's actually happening can possibly save money in the long run because it makes it a lot easier and more likely that folks on this message board can diagnose the problem w/o having to randomly throw parts at the car.
You can post datalogs on this site for other people to download and replay, looking at sensor outputs, trouble codes, and other information.... takes a lot of the mystery out of trying to diagnose EFI problems.
Tunerpro is free, so you'd only have to buy the cable.
https://boostednw.com/emulation-cabl...-gm-datalogger
It's an investment, but with these old cars now, being able to see what's actually happening can possibly save money in the long run because it makes it a lot easier and more likely that folks on this message board can diagnose the problem w/o having to randomly throw parts at the car.
You can post datalogs on this site for other people to download and replay, looking at sensor outputs, trouble codes, and other information.... takes a lot of the mystery out of trying to diagnose EFI problems.
Pro
Joined: Feb 2009
Posts: 655
Likes: 164
From: Vancouver Washington
You said that you have oil pressure above the required 4 that should keep the fuel pump running. Have you checked voltage at the pump after the initial 2 seconds?
From what you describe it sounds to me like the oil pump pressure signal is not making it to or perhaps, from the ecm. There is also a fuel pump relay in the mix. On my 86 the relay is on the fire wall near the brake booster. Good thing is, you can swap the fuel pump relay with maybe the fan relay or one of the relays near the battery. Mine has an overdrive relay which is exactly the same as the fuel pump relay but you don't have that on the 89.
I hope this helps.
From what you describe it sounds to me like the oil pump pressure signal is not making it to or perhaps, from the ecm. There is also a fuel pump relay in the mix. On my 86 the relay is on the fire wall near the brake booster. Good thing is, you can swap the fuel pump relay with maybe the fan relay or one of the relays near the battery. Mine has an overdrive relay which is exactly the same as the fuel pump relay but you don't have that on the 89.
I hope this helps.
Thread Starter
Heel & Toe
Joined: Nov 2024
Posts: 20
Likes: 0
You said that you have oil pressure above the required 4 that should keep the fuel pump running. Have you checked voltage at the pump after the initial 2 seconds?
From what you describe it sounds to me like the oil pump pressure signal is not making it to or perhaps, from the ecm. There is also a fuel pump relay in the mix. On my 86 the relay is on the fire wall near the brake booster. Good thing is, you can swap the fuel pump relay with maybe the fan relay or one of the relays near the battery. Mine has an overdrive relay which is exactly the same as the fuel pump relay but you don't have that on the 89.
I hope this helps.
From what you describe it sounds to me like the oil pump pressure signal is not making it to or perhaps, from the ecm. There is also a fuel pump relay in the mix. On my 86 the relay is on the fire wall near the brake booster. Good thing is, you can swap the fuel pump relay with maybe the fan relay or one of the relays near the battery. Mine has an overdrive relay which is exactly the same as the fuel pump relay but you don't have that on the 89.
I hope this helps.