Ok, guys. I’m about to start the process of cutting an access door and replacing the fuel pump on my 1990 RS 305 TBI Camaro, since fuel suddenly stopped flowing to the Throttle Body and I’ve tried everything I can think of before this next big ugly task.

But it before I do, I wanted to make sure I sought some final advice.

Symptoms:
Car was running great (less than 90k original miles), and after it was parked for a couple hours, it just cranked and wouldn’t start. Here's what I know and have tried so far.

1. Fuel filter has been replaced (and I noticed fuel continually leaks out the fuel filter when the output side of the line is disconnected).
2. When fuel supply Line is disconnected at the rear of the throttle body, and car is attempted to start, zero fuel leaks out of the line. The throttle body is not getting any fuel.
3. All lines have been blown out to ensure nothing is clogging them up.
4. Inline fuse by the coolant overflow tank is good. Power at that point is 12v (tested with a multimeter in place of the fuse).
5. Pouring fuel into the TB allows the car to start & run.
6. TBI injectors are getting power (verified with noid lights).
7. There is no fuel pressure to the throttle body (verified with pressure gauge on the supply line)
8. I looked for a place to test the power to the fuel pump under the rear of the car, but couldn’t find any wiring to test back there.

You guys have any other ideas for me to look into before I get my cutting wheel out?

Thanks in advance for the help!

 

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MAKE SURE YOU CHECK ALL THE FUSEs FIRST IN BOTH power boxes with a working test light source

 

If you remove the back seat, can you probe the FP wiring there?

 

Possibly. I looked under the car and saw this connector. Is it the fuel pump power source? If so, it gets 8.7 volts.

 

Someone else will have to verify - my car has a carb and block-mounted mechanical FP.

You get 8.7v probing while still connected? Does the voltage jump up when disconnected?

 

So...another entire thread for the same old problem?

https://www.thirdgen.org/forums/tbi/...s-305-tbi.html

1. How do you plan to seal that hole you're about to cut? You get into a collision, a crappy, weak sealing job can allow gasoline from a ruptured tank into the passenger compartment. You won't enjoy burning to death.

2. If you insist on buying and installing a fuel pump without doing the testing/diagnosis ahead of time, you might get caught with a new pump that still doesn't work because there's a broken wire or popped fuse or corroded connection or whatever.

 

I just finished the same repair on my 90 RS yesterday. I replaced the fuel pump and sending unit since I was in the tank. The PO had cut an access door that I found when removing the carpet. I plan to seal it with sheet metal.and black gasket maker. The PO did neither of these things.

 

No, I troubleshot and ruled out the possibilities in my last post, so I created a new thread for a new job - a possible fuel pump replacement. Just making sure first.

So I take it your opinion is that it's not the best idea to cut an access hole to replace the fuel pump? Ok, thanks. What alternative do you suggest, if any?

I do not insist on replacing the pump "without doing the testing/diagnosis ahead of time". Did you read my post and all the steps I took to troubleshoot? I consider the fuel pump replacement option a last resort, based on the results of my troubleshooting. But If you have something else I can do, or at least some constructive advice, I am all ears.

 

Thanks. I would hate to go this route... just don't have the lift or expertise to drop the rear end. Still researching...

 

Thanks. I know of the main fuse box under the steering wheel, on the left. Is there another box somewhere that has a fuse related to the fuel system?

 

No, that plug in the back gets 8.7v while unplugged and the key on.

 

Without being there to test myself, I think you have just found your problem (assuming that that connector IS for the FP. If you pull the FP fuse or relay, is there now no voltage at that connector?

 

On that connector, the BLK/WHT wire is ground. The PPL wire is for the gauge sending unit. With the GRY wire being the +12 volt pump feed from the relay & oil pressure switch.

With the engine off the ECM will only run the fuel pump for a 2-second prime. Then turns off the relay until it gets distributor reference pulses.

So connect the DVM to the BLK/WHT & GRY wire locations in the body connector. And place the DVM where you can see it when you do a key-on, engine-off. Can do a brief crank to get the ECM to re-enable the fuel pump relay for another 2 seconds.

 

Ok, thanks.

1. If that shows that it is getting 12V, but the Fuel Pump still does not pump fuel, is it safe to say the Fuel Pump needs to be replaced?

2. If that plug does not get 12V, where is the fuse or box that feeds it?

 

Prime 12v from the relay bye your master cylinder. Cranking power comes from the ops. Ops/pump/ecm fuse is next to the battery.

 

You DID NOT rule out possibilities in the previous post. You talked about maybe ruling out possibilities, without ever reporting that you actually did. You also had mistakes in logic, leading to false conclusions.

Drop the tank. It's a pain in the azz. Beats gasoline in the passenger compartment if you're in a collision, though.

I already gave you advice, and you never reported back that you accomplished the testing, and what the results were. In fact, you're still looking for the harness in this thread!

What is the fuel pressure? Measuring at the fuel filter location is OK, measuring near the throttle body is better.
What is the voltage to the pump (tested as close to the pump as you have harness access) when the ECM goes through it's two-second prime when the key is turned to "Run"?
Is there excess voltage on the fuel pump ground wire? A broken/corroded ground can stop a pump even if the power to it is good. GM has a nasty habit of using under-size wire; so 1 volt on the ground side (measured as close to the fuel pump as you can get) is probably acceptable. 2 volts would be too high.
What is the current draw of the fuel pump? You can measure amperage anywhere in the circuit. Voltage has to be measured near the pump.

Until you rule out the electrical side of the electric fuel pump, there's no point in cutting the body or dropping the tank. THE PUMP MAY BE FAULTY, but you don't know that until you do the diagnosis.

 

If I understand this correctly, it's not quite accurate.

Yes, there should be a way to run the fuel pump by turning the relay on. On my Luminas, and I think the K1500 TBI is the same, there's a fuel pump jumper wire that can be touched to battery +. This engages the relay, the relay powers the pump.

The oil pressure switch (ops?) does not power the fuel pump during cranking UNLESS the fuel pump relay has failed. It's a secondary system that can run the fuel pump if the fuel pump relay or wire harness has problems, or if the ECM fails to send the "pump ON" signal to the relay. The fuel pump runs off of the oil pressure switch and some wiring shared with the fuel pump relay circuit, once the engine cranks long enough to build oil pressure. If the relay fails, the engine will start, but it typically has to crank longer than if the relay is working.

The primary system involves the ECM driving the fuel pump relay for two seconds when the key is turned to "Run", and also when the ECM detects that the distributor is turning. When this system is working, the two-second "prime" programming is instantly replaced by the "distributor is turning" programming, so that the fuel pump relay is energized the whole time the engine is running.

 

The ecm only does the 2 sec prime then the ops takes over. Ecm pinout not specific to op tbi.

 

88-92 f body tbi

 

I can't read the left side of the first scan. It's like part of the page is cut-off.

I think the important part is where it says: "There should be about 12 volts on CKT...
...tion is turned "ON", or any time reference pulses are received by the ECM."

I suspect the missing bits are "...CKT 465 when the ignition is turned..."

IF (big IF) I'm correct, the fuel pump relay is driven by the ECM via CKT 465 for the two-second prime, and thereafter whenever the distributor is turning--which generates the reference signal the ECM is looking for.

The second scan shows that Circuit 465 is still the fuel pump relay drive, but there's no description of the circuit operation.

If the fuel pump relay isn't driven by the ECM during cranking, long crank time results because there's no retained fuel pressure in the TBI system, the two-second prime is lost, so the engine would have to build oil pressure to get a steady supply of fuel. Or it starts and builds oil pressure in the first two seconds...but that's not realistic in cold weather.

 

Which of these is the Fuel Pump relay?

 

Why are you checking the fp relay if you have 12v to the pump ?
The fp relay has a green/white wire, orange wire, tan with white or grey wire and blk/white.

 

I don’t have 12v to the pump, or at least I’m not sure yet. See post #5 above. I have 8.7v to the plug that I think goes to the pump (but have not yet gotten confirmation on). So I’m hearing I should check the relay next.

 

The wire might change color from the relay through the body connector c100. If you have 12v at the relay the high resistance would be in c100 next to the master cylinder.

 

As stated above, fuel pressure is Zero at the throttle body.

As indicated above, when tested at the closest point I can see near the pump, at the body connector in the picture, it gets 8.7v when the key is in the ON position. Is that the best place to test? If so, I will continue to test while switching the key from off, to on, and then cranking for a couple seconds to see if the meter reads 12v.

Where/how would I test this?

I'm sorry, but I do not know how to test this.

 

Thank you all for your help. From your suggestions above, I will do the following before replacing the pump:

1. Find and check ops/pump/ecm fuse (was told this is possibly next to the battery?)
2. Check the rear body connector with a DVM while key is on and cranking to see if it's supplying 12v to pump.
3. Check for excess voltage on the fuel pump ground wire. (Which wire should I check?)
4. Check current draw of the fuel pump. (How?)
5. Find and check second fuse panel. (Anyone know where this is, and if there is a fuse there relevant to the FP?)

 

OK fellas, calling off the dogs. The fuel pump relay was the problem. Swapped it out and she fired right up. Thanks for all the help. I now understand my car a lot better.

 

Good. Now enjoy the car and have a beer.

3. Check for excess voltage on the fuel pump ground wire. (Which wire should I check?)
4. Check current draw of the fuel pump. (How?)

3. Meter on DC volts. 1 lead to a known good ground. The other to the FP "ground" wire. This will compare the voltage you are getting on that wire (supposed to be ZERO, or very close to it) in reference to an actual ground (zero).
Get it? If you were to measure, say 4 volts, then your fuel pump would only actually be seeing 8 volts (12-4).

4. To use the current function on your meter, you have to move the red lead to the other jack (amps). Set meter for DC amps. Then you have to open the circuit you wish to test (in 1 place) and insert the meter in between. Energize the circuit and read the current draw.

 

Spoke too soon... Car ran for several miles, parked it and then would not start. Same thing... fuel pump relay clicks but the pump does not come on (no whine). I figured the relay was a dud, so I replaced it, but that didn’t do it this time unfortunately.

Checked the inline fuse (by the coolant reservoir), And it’s good.

Are there any other fuses that could affect power to the pump, or should I continue with my points to check, above?

Would the the fact that the relay fixed it temporarily lead you to any likely reasons it’s not getting fuel?

 

Poor or dirty connections at that relay. Wiggle the wires while it is plugged in and key on.
Also check the ground connections.

 

Checked those wires and all seems well. Wiggled them while key was on and the fuel pump just doesn’t turn on.

 

Do you know how to "jump" that relay?

 

I do not.

 

Jumping the ops would give power to the pump or to jump the relay orange to tan/white or it might be grey .

 

Sneak up behind it with a gun and say “reach for the sky”.

 

Do you have 2 male spade connectors and a 6" piece of 14 or 16 gauge wire?
Make a jumper wire out of that. Then according to this picture, figure out which terminal of the relay is "A" and "E". Those would be the contacts of the relay that actually do the switching. Use your wire to jump A to E. KEY ON. If the FP runs, then most likely the problem is the relay. If the FP does not run, then go back to that FP connector under the car and measure for 12 volts. If you now have 12 volts there, then you have bad connections from that point to the FP, or the FP is bad.

 

Doesn't the vehicle have a built-in relay jump wire? Insert a jumper wire into the plastic connector body, touch it to Battery +, listen for the pump.

Once the pump is running, re-test your voltage at the rear connector with the new relay--assuming you can get the relay to engage. Hopefully more voltage than when previously tested back there. In a perfect world, you'd have battery voltage. Since GM uses under-sized wire and the wire connections each have a little resistance, you can expect battery voltage minus about two volts. With the ignition on, engine NOT running, and a fully-charged battery, I'd want to see 10.5 volts minimum.

Amperage test as described previously. You could cram the meter probes into the fuel pump fuse socket after removing the fuse. About 5--6 amps expected with the pump RUNNING.

Ground wire voltage testing as described previously. No more than 2 volts, LESS PREFERRED. Zero volts would be optimum, but you're not going to see that unless your car is wired better than the ones I've tested. About one volt seems typical.

 

Just an update.... Relays were tested good. Voltage all the way through was verified good. Fuel pump just wouldn't power up, so we dropped the rear end and replaced it (as opposed to cutting an access door due to Schurkey's safety warning). After that, all is well.... thanks guys.

 

Excellent.

 

Great stuff. Good choice not hacking up your car!! Just feels better doing it right.