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Clayso
Clayso, Master Automobile Technician
Category: Chevy
Satisfied Customers: 880
Experience:  ASE Master Certified Automobile Technician. 30 Years Experience.
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84 Corvette running very rough. Replaced fuel filters,

Customer Question

84 Corvette running very rough. Replaced fuel filters, didn't help. (injectors spraying pretty good) Replaced distributor electronics (pickup magnet broken) didn't help. Replaced plug wires, now both injectors not working at all. Fuel pump runs about 20 seconds when ignition is switched on but have not checked fuel pressure. Injector fuses good. What could have I done to kill the injectors? How can I check for power to the injectors?
Submitted: 9 months ago.
Category: Chevy
Expert:  Clayso replied 9 months ago.

Hi There, I'm Clayso a certified master automobile technician with 30 years of automotive repair and diagnostic experience.

Power to the injectors comes from the #1 & #2 3 amp injector fuses. Verify with a test light there is power at the fuses and on the red and white wires at the injectors. This circuit should have battery voltage during cranking and when the engine is running.

Customer: replied 9 months ago.
I had already checked the two 3 amp fuses. A bit redundant but I removed the injector connectors and used a 9 volt battery to bump across the injector terminals. I got a "click" from the injectors. How many volts at the injector terminals should there be during cranking?
Expert:  Clayso replied 9 months ago.

At least 10 volts. I'm putting together a diagram to find where the 3 amp fuses get their power from right now. I'm sure there is no power at the fuses, but check to make sure.

Let me know

Thanks

Expert:  Clayso replied 9 months ago.

The INJ 3 amp fuses power from the ignition switch when the key is in the on position. There should be battery voltage on both sides of the fuses with the key on and at the injectors on the red wire for #1 injector and on the white wire for #2 injector. Let me know if there is power at the fuses ant at the injector connectors.

Thanks

Customer: replied 9 months ago.
Ok, checked voltage at fuse panel for both injectors. Slightly under 12 volts with the fuses removed, 1 meter lead to hot side, 1 lead to chassis ground. Do I check injector connectors red/white to chassis ground?
Expert:  Clayso replied 9 months ago.

Put the fuses back in and check for battery voltage at the injector connectors. With the key on there should be battery voltage on the red wire for #1 injector and on the white wire for #2 injector.

Customer: replied 9 months ago.
I had already checked for power at the fuse panel and checked fuses with an ohm meter. With ignition switch on; checked power at injector connectors; between red & ground and between white & ground, 12 volts on both. I guess that only 3 or 4 possibilities; no injector signal from the ecm by not receiving a signal from the distributor or a bad ecm; or (and probably most likely) low fuel pump pressure; or a bad throttle body fuel regulator.
Expert:  Clayso replied 9 months ago.

Very true. Does your distributor have the coil on top of the cap or is the coil external from the distributor?

Expert:  Clayso replied 9 months ago.

I'm pretty sure the rough running problem is the EGR valve is leaking. Once I know what strib is in this Vette I can send you the correct diagnostic procedure for the module and pick up coil.

Customer: replied 9 months ago.
On top of the cap.
Expert:  Clayso replied 9 months ago.

I was pretty sure you had the integral coil. Verify the thick pink wire going to the cap has battery voltage during cranking. You will have to remove the cap to check the pick up coil and module. Disconnect the pickup coil from the ignition module. Set your voltmeter to the AC scale and disconnect the pick up coil from the module. On the 2 wire connector from the pick up coil check for 500 millivolts AC or higher. Depending on your meter you may see 0.5 volts AC. This is with the engine spinning over. The link below is text book for checking the module and pick up coil.

Distributor Testing and Inspection

If the pick up coil checks good most likely the module is defective. If there is no AC voltage from the pick up coil the pick up coil is defective.

Customer: replied 9 months ago.
The pickup coil and module are both new as well as the distributor cap and rotor. Only the coil (on top of the cap) is used. In fact I replace the entire distributor. The cost of a pickup coil and module was nearly the same as a new distributor.
Customer: replied 9 months ago.
I know I have spark. It starts right up while spraying starting fluid into the throttle bodies.
Expert:  Clayso replied 9 months ago.

I understand, I just never assume new is good. Injector pulse is also controlled by the ignition module. Do you have a test light? With a test light you can tap ground at the PCM to verify the circuit to the injectors.

Expert:  Clayso replied 9 months ago.

Instead of going through the strib checks, try this first. Disconnect the strib 4 wire connector. On ECM side of the connector. Attach a jumper wire to the purple/white wire. Disconnect the TPS connector. Turn the key on and with the loose end of the jumper tap the grey wire on the TPS connector and listen for the fuel pump to energize and the injector to click.

Let me know if the pump comes on and if the injectors click.

Thanks

Customer: replied 9 months ago.
With the new distributor the engine fired up but ran rough like it did from the beginning. For whatever reason, I stopped getting fuel from the injectors after I replaced the plug wires. I pulled the distributor again today to check for disconnected wires. Didn't see any problem.
Expert:  Clayso replied 9 months ago.

The distributor and the ECM control the injector pulse. Power is supplied to the injectors and the ECM pulses ground to the injectors based on the input from the module in the distributor. Either the ECM failed or there is a problem in the wiring or the strib. When you torn the key on does the check engine light come on?

Customer: replied 9 months ago.
I put a screwdriver against the filler and to my ear and had my wife turn on the ignition. I could hear the pump start. The check engine light does come on. I'll check for a pulse from the distributor tomorrow.
Expert:  Clayso replied 9 months ago.

Cool. Try this quick check to see if the distributor and the ECM can comminucate. Disconnect the strib 4 wire connector. On ECM side of the connector. Attach a jumper wire to the purple/white wire. Disconnect the TPS connector. Turn the key on and with the loose end of the jumper tap the grey wire on the TPS connector and listen for the fuel pump to energize and the injector to click.

I'll get you an Assembly Line Data Link (ALDL) connector view to see if the Check Engine Light (CEL) will flash a #12 code.

I'll be here all day tomorrow. Let me know what all you find.

Thanks

Customer: replied 9 months ago.
I may not have any codes since the battery has been disconnected. I realize a 12 code means there aren't any codes present.
I sure hope its a fuel pressure problem and not the ecm or a broken wire in some obscure location.
Expert:  Clayso replied 9 months ago.

Code 12 is actually a check code for determining the integrity of the ECM's circuit to power and ground.

At the ALDL jump the A terminal to the B terminal and turn the key on. The CEL should flash 12 times 3 times in a row. 12 will flash (*pause**).

ALDL Connector

You are correct, since the battery has been disconnected there will be no codes stored. To check fuel pressure you would need to adapt a gauge in between the fuel line and the throttle bodies. The specs are 9-13 psi.

Customer: replied 9 months ago.
Checking fuel pressure will be a treat since there's no fitting to hook up a gauge. Been thinking of maybe cutting the fuel line and installing a "T".
Expert:  Clayso replied 9 months ago.

At 9-13 psi simply cracking open a line would be enough to verify fuel pressure.

Was the engine running rough at idle? Also how well did it run going down the road?

Customer: replied 9 months ago.
Idle was terrible. Improved somewhat when warmed up. Wouldn't run well enough to even drive it. Would just stall whenever I tried to drive it. It doesn't want to rev up from idle. It seemed like a fuel delivery problem at the beginning. I replaced the inline filter and the fuel pump filter. Didn't help at all. I noticed the distributor cap was (literally) split down one side. Replaced the cap and (burned) rotor. No improvement. Found the pickup coil magnet was broken. That led me to replace the distributor along with the pickup coil and module. No improvement. Installed new plugs. Didn't help. Plug wires looked good but I replaced them anyway. That's when I stopped getting fuel from injectors.
Expert:  Clayso replied 9 months ago.

The easiest place to tie into the fuel system to check pressure is at a filter versus cutting a steel line or trying to adapt to the fittings. Usually on low pressure systems like this, if you have fuel spray any any of the connections when loose and the key on there should be enough pressure for the engine to run.

I use a test light clipped to the battery positive and check the injectors green or light blue wires for a pulse to ground with the engine spinning over. The test light will flash each time the module and the ECM ground the injector.

Once the no start condition is solved you can disconnect the EGR's vacuum and see if the idle improves. If the EGR valve is leaking it basically opens the exhaust straight into the intake, kind of like a vacuum leak. Leaking EGR's are notorious for driveability problems both at idle and under a load. If the EGR valve is flat at the gasket surface you can make a solid gasket out of tin and close the passage from the exhaust to the intake and see if the idle improves.

Let's get it running again and go from there.

Customer: replied 9 months ago.
I'll get back on it tomorrow after a doctor's appointment and mowing the grass.
Expert:  Clayso replied 9 months ago.

That's cool. I'll be here.

Let me know

Thanks

Customer: replied 9 months ago.
I didn't get to work on it until late tonight. Hooked up a noid light and behold, I have signal to injectors (thank God) I don't think there's any need to check fuel pressure. I may have bad hose or fitting on the pump. Since I'm have to remove the pump to check its fittings, I'm just going to replace it with a new one for an 85 model. I'll replace the injector filters as well. I believe the pump decided to shoot its wad at the exact time I changed the plug wires just to confuse me.
Expert:  Clayso replied 9 months ago.

Yeah I kinda wondered where you where, I fell asleep on you. I'd never expected a noid light to flash either, very surprising.

The '84 and '85 fuel specifications are very different. 1984 with fuel pressure at 9-13 psi. 1985 with pressure at 24-39 psi. The fuel pressure regulator is what maintains the pressure at the specifications. I've had fuel pumps crap out at the darnedest times, one minute it's peaches and cream and the next nothing.

Let me know how this works out after changing the pump.

Thanks

Customer: replied 9 months ago.
I read somewhere the 85 model pump works really well on the 84 crossfire setup. I am a bit concerned the extra pressure may damage the regulator diaphragm. Hope the parts are in stock.
Expert:  Clayso replied 9 months ago.

Dead heading the pumps is the only way to determine the pressure differences. Dead heading is done by blocking off the return with the pump in operation. This can also be done on the bench with a gauge hooked directly to the pumps discharge port. I don't have any specs for this procedure, but a 20% increase is about normal. It's just one of those try and see things. Worst case scenario is like you say at the regulators diaphragm.

Customer: replied 9 months ago.
The way things have gone so far, I'll probably have to rebuild the regulator anyway.
Expert:  Clayso replied 9 months ago.

If this vehicle sat for any length of time rebuilding the TBI's and replacing the injectors wouldn't be a bad idea. The links below are for the TBI units.

Fuel Injector Balance Testing

Throttle Body Overhaul

Let me know if you can view and print out the overhaul procedure.

Thanks

Customer: replied 9 months ago.
Just installed a new fuel pump. I went with the stock one. I found the rubber coupler had split on the old one. The old pump was probably ok but why chance it. Anyway, with the new pump, she fired right up and idled perfectly, revved up really good
too. Now its time to sell the s.o.b.
Expert:  Clayso replied 9 months ago.

Right on! I've been hoping for some positive results. So basically the pump was simply pumping fuel back into the tank.

I can understand selling it, 1984 was no glory year for the Corvette that's for sure. The last one I owned was a '78 and the only thing it had going for it was the Silver Anniversary and the ride was more comfortable than my '76. I hope you get a good price and a quick sale.

I hope the information sent was helpful. If you have any questions let me know. You can reply back to me through this thread at any time.

Kindly rate my assistance in a positive manner so I may have credit for helping you.
Positive ratings are greatly appreciated.
Thanks.