'98 C1500 5.0L RWD No start

[James2406]

Afternoon gents,

My buddies '98 will barely start after a new fuel pump, fuel filter, and TPS sensor.

I have the following codes,
P0123 Throttle Position Sensor (TPS) circuit high voltage
P0453 Fuel Tank Pressure (FTP) sensor circuit high voltage
P0463 Fuel Level sensor circuit high voltage
P0560 System Voltage
P1121 Throttle Position Sensor (TPS) circuit intermittent high voltage

So he replaced the fuel pump, TPS, Fuel filter and still has a very struggling start, that eventually lead to no start, sometimes it will fire for a few seconds then struggle and die.
He then ended up having no fire at all, and I used my habor freight master fuel test kit, and found zero pressure.

Now I have a Autel Maxicheck MX900 and I repaired another buddies '05 tahoe, using it. It's very useful. I commanded his fuel pump on, and found the pressure to slowly decrease. I also use alldata, and the procedure called out that it should hold pressure. We replaced the fuel pump, and it ran alot better and held pressure at the fuel feed line, but still dropped pressure at the rail. Alldata called out for a fuel injector test next for that. It still had a slight miss, and sometimes hard start. Following Alldata, I removed the intake manifold, and tested each injector to find #5 sputtering. We replaced that and not a single issue since. My reason for explaining this is that, my scan tool cannot command the fuel pump on, on this '98. It gives me an error "OUT OF LIMITS RANGE" so I don't know if thats related but there it is.

We dropped the fuel tank, and removed the pump. I reconnected the two connectors, and had him turn the key, no wine, nothing. So we returned it for another, thinking it was DOA or faulty. Then tested the new unit again outside the tank (for a split second) and it works, i can audibly here it running. We then re-installed the tank.

Well now it gets weird. I hooked up my fuel gauge, and I get 10psi KOEO, and it jumps to 55 - 65 when the key is off, and then instantly starts to drop pressure. THEN, sometimes when we cycle the key, I get nothing. To check and see if the pump is working, I removed the fuel filter, and the same process occurs, sometimes I get fuel out of the line, and sometimes I don't when he cycles the key.

I'm going to follow Alldata's procedure for fuel test to a T tomorrow. I ususally just hook one line to a gauge with no relief valve and check that way, but it calls out to remove the relay, remove the fuel pressure, cycle the key, then install the gauge set, cycle the key, and remove the air, the cycle again to test. (Attached image)

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I'm starting to think he has a wiring issue, since I cannot command the fuel pump on with my MX900.

Any thoughts or insight would be very very much appreciated. Thanks guys

 

[Schurkey]

My buddies '98 will barely start

What does that mean? Cranks slowly? Cranks OK but won't run?

after a new fuel pump, fuel filter, and TPS sensor.

What diagnosis was done before replacing the pump and TPS?

I have the following codes,
P0123 Throttle Position Sensor (TPS) circuit high voltage
P0453 Fuel Tank Pressure (FTP) sensor circuit high voltage
P0463 Fuel Level sensor circuit high voltage
P0560 System Voltage
P1121 Throttle Position Sensor (TPS) circuit intermittent high voltage

Before or after you replaced the pump and TPS? Did you clear the TPS codes after replacing the sensor? Have you verified TPS voltage? Have you checked the system voltage?

Clear all the codes, see what comes back.

So he replaced the fuel pump, TPS, Fuel filter and still has a very struggling start, that eventually lead to no start, sometimes it will fire for a few seconds then struggle and die.
He then ended up having no fire at all, and I used my habor freight master fuel test kit, and found zero pressure.

Is "zero fuel pressure" the only problem? Connect a scan tool, verify EVERY sensor and computer output. Make sure the plugs aren't fouled, and the distributor cap isn't pooched especially between the center terminal and the #3 conductor.

I commanded his fuel pump on, and found the pressure to slowly decrease.

The pressure decreased with the pump commanded "on"?

I also use alldata, and the procedure called out that it should hold pressure. We replaced the fuel pump,

Again? How many fuel pumps have you put on this truck?

I removed the intake manifold, and tested each injector to find #5 sputtering. We replaced that and not a single issue since.

You replaced the #5 injector, or the entire spider unit? I didn't think the injectors were individually replaceable.

We dropped the fuel tank, and removed the pump. I reconnected the two connectors, and had him turn the key, no wine, nothing.

So the fuel pump fails to run, intermittently?

So we returned it for another, thinking it was DOA or faulty. Then tested the new unit again outside the tank (for a split second) and it works, i can audibly here it running. We then re-installed the tank.

Well now it gets weird. I hooked up my fuel gauge, and I get 10psi KOEO, and it jumps to 55 - 65 when the key is off, and then instantly starts to drop pressure. THEN, sometimes when we cycle the key, I get nothing.

So the fuel pump is still randomly intermittent, and the system is so screwed-up that the pressure increases when the key is turned off?

To check and see if the pump is working, I removed the fuel filter, and the same process occurs, sometimes I get fuel out of the line, and sometimes I don't when he cycles the key.

Connect a voltmeter to the fuel pump power supply wire as close to the pump as practical--often the most-rearward connector in that harness before it goes up and over the fuel tank. Find out if the pump is getting power when it fails to run. Then do the same thing on the ground side, as close to the pump as practical--find out how much voltage the ground wire has WHEN THE PUMP RUNS.

I'm starting to think he has a wiring issue

Me, too. But it may not be ONLY a wiring issue.

 

[James2406]

What does that mean? Cranks slowly? Cranks OK but won't run?


What diagnosis was done before replacing the pump and TPS?


Before or after you replaced the pump and TPS? Did you clear the TPS codes after replacing the sensor? Have you verified TPS voltage? Have you checked the system voltage?

Clear all the codes, see what comes back.


Is "zero fuel pressure" the only problem? Connect a scan tool, verify EVERY sensor and computer output. Make sure the plugs aren't fouled, and the distributor cap isn't pooched especially between the center terminal and the #3 conductor.


The pressure decreased with the pump commanded "on"?


Again? How many fuel pumps have you put on this truck?


You replaced the #5 injector, or the entire spider unit? I didn't think the injectors were individually replaceable.


So the fuel pump fails to run, intermittently?


So the fuel pump is still randomly intermittent, and the system is so screwed-up that the pressure increases when the key is turned off?


Connect a voltmeter to the fuel pump power supply wire as close to the pump as practical--often the most-rearward connector in that harness before it goes up and over the fuel tank. Find out if the pump is getting power when it fails to run. Then do the same thing on the ground side, as close to the pump as practical--find out how much voltage the ground wire has WHEN THE PUMP RUNS.


Me, too. But it may not be ONLY a wiring issue.

I'll reply to each one of these later when I get a chance,

He did zero diagnostics when he replaced his fuel pump.

the #5 injector was on a different vehicle, please re-read the post.

 

[James2406]

What does that mean? Cranks slowly? Cranks OK but won't run?


What diagnosis was done before replacing the pump and TPS?


Before or after you replaced the pump and TPS? Did you clear the TPS codes after replacing the sensor? Have you verified TPS voltage? Have you checked the system voltage?

Clear all the codes, see what comes back.


Is "zero fuel pressure" the only problem? Connect a scan tool, verify EVERY sensor and computer output. Make sure the plugs aren't fouled, and the distributor cap isn't pooched especially between the center terminal and the #3 conductor.


The pressure decreased with the pump commanded "on"?


Again? How many fuel pumps have you put on this truck?


You replaced the #5 injector, or the entire spider unit? I didn't think the injectors were individually replaceable.


So the fuel pump fails to run, intermittently?


So the fuel pump is still randomly intermittent, and the system is so screwed-up that the pressure increases when the key is turned off?


Connect a voltmeter to the fuel pump power supply wire as close to the pump as practical--often the most-rearward connector in that harness before it goes up and over the fuel tank. Find out if the pump is getting power when it fails to run. Then do the same thing on the ground side, as close to the pump as practical--find out how much voltage the ground wire has WHEN THE PUMP RUNS.


Me, too. But it may not be ONLY a wiring issue.

Cranks fine, till the battery dies. But if it starts, it's struggling really hard, then stalls.


Found that the shredder valve showed no pressure, removed the pump from tank, no noise. New pump outside the tank, noise, now gets pressure, but intermittent, gets 10psi Key on engine off, turning the key off, hits 55+ psi then slowly but steadily drops.

Cleared all codes, came right back. did not verify TPS voltage, swapped in a new one, to test if the new one before it was doa.

other codes are listed above.


I understand part of the post was confusing, that was an example of how I learned how to use the Autel Scan tool to command the pump on, however I get an error on this truck (different vehicle) and the scan tool states "out of limits range" when I attempted to command it on. The '98 C1500 has had two pumps this far, it's my theory with a low fuel level, and constant cranking, he burned up the first replacement pump.


#5 was on the difference vehicle described above.

It seems like it does yes, however I have been told the PCM or ECU whichever, will not allow you to constantly key on engine off, then key off engine off, then key on engine off (cycle it) and continue to build pressure, and that it will stop building pressure for a specific amount of time. I will test this again tomorrow, and verify the first time what my start pressure is, if there is any. I disconnected the fuel filter to check for flow, and got some. However I don't know if it's the correct amount, or not.

Yes, it seems that way which is the weirdest thing ever. Again I will verify this is repeatable tomorrow, but it was 10psi key on engine off, then key off engine off, it jumped to 55+ and slowly dropped off.

I will give this a try, I have a multimeter however I'm still learning electrical, can I test through the rubber shielding around the wire? or do I need a special voltmeter than can do this.

I believe it is a wiring issue, and I don't want to get him to spend the money on a new set of injectors, I can perform the replacement, as Alldata has more than enough information to do so, but I don't want to be a parts swapper, and thats the main reason I'm taking this job on.

 

[James2406]

Got video showing exactly what I'm dealing with.

'98 C1500 No start

 

[Road Trip]

Reply #1

I have the following codes,
P0123 Throttle Position Sensor (TPS) circuit high voltage
P0453 Fuel Tank Pressure (FTP) sensor circuit high voltage
P0463 Fuel Level sensor circuit high voltage
P0560 System Voltage
P1121 Throttle Position Sensor (TPS) circuit intermittent high voltage

Clear all the codes, see what comes back.

Cleared all codes, came right back. did not verify TPS voltage, swapped in a new one, to test if the new one before it was doa.

I have a possible theory about what is causing all these DTCs -and- the flaky fuel pump behavior, but first I need to
explain a little about what all these DTCs share in common. (Hint: If you want to prep yourself a bit, look up DTCs
P0122, P0452, P0462, and P1122, and see what they have in common.)

A little background info on these sensor circuits. In general, the engineers needed a way to differentiate between a
valid yet semi-unpredictable analog signal vs an invalid analog signal due to a missing supporting Reference power
or sensor ground. (I've attached a rough hand-drawn diagram showing the basic layout from a voltage perspective.)

In English, they set up the voltages from the sensor to vary within precisely defined limits. This way, IF the computer
sensed either a 'too low' or 'too high' voltage, then it knows to ignore this out of range input, and if possible substitute
a predetermined middle value to enable 'limp home' functionality, and also set the SES light.

With this in mind, look at your list of DTCs. If only one of those were set, then I would chase that individual circuit.
But when there's a 'global' fail where all these circuits go out of range in the same direction, then this makes me
believe that we've either had either a Power source -or- Grounding failure. Given that losing Power will make the
circuits fail with a too low voltage, while a loss of ground can make these sensors fail with a too high voltage, this
makes me want to inspect/verify the primary grounds between your PCM and your powertrain, and also from the
powertrain to the battery. (!)

****

Q: So how does this tie into your flaky fuel pump operation?

A: In order for the VCM to set up the fuel injection for success, here is what it is programmed to do:

1) With the key turned to the On position (but no further), the VCM will operate the fuel pump for 2 seconds,
bringing up the pressure to the injectors so that the injectors will have fuel ready for metering in order to
ensure quick starting.

If the engine crankshaft remains stationary, the VCM will then stop the fuel pump for safety reasons.

2) Once the human turns the key to the Start position and the cranking commences, the VCM will
immediately return power to the fuel pump, and continue to keep it running as long as it
detects that the engine is still running. (This is done by monitoring for Reference Pulses from the
CKP sensor.)

3) At the end of this drive cycle, once the engine (crank) stops turning, the VCM is programmed to once
again run the fuel pump (via the Fuel Pump Relay) for an additional 2 seconds. Watching your video,
I think I saw this occur once?

Obviously if the VCM is suffering from a marginal ground, then this could explain the DTCs you are experiencing.
Also, if the grounding (and/or Power) to the VCM is flaky, then this can cause the CPU inside the VCM to start/restart
erratically. Speaking of power, the VCM wakes up when power is applied to it from the ignition key, and of course
flaky power is much like a flaky ground -- I always emphasize that any electrical device in your vehicle runs off of the
*net difference* between the positive & ground that it sees, not what's observed at the battery terminals via your
voltmeter's test probes. (!)

In English, if the VCM is being fed flaky power from the ignition key switch then you can end up erratic fuel pump
operation. (But this would not explain those sensors failing with a too high voltage, so I included the flaky power
from the ignition switch simply for completeness.)

****

Alright. After dragging you through that theory, let's try to come up with a common sense troubleshooting
approach. The most elegant approach is to fix the grounding issue generating all those DTCs, and once
they no longer show, see if the fuel pump starts operating reliably.

Then again, if it was me I'd want to know if the Fuel Pump is working properly if fed with good power, so I'd
first pop the Fuel Pump Relay, and using a fused jumper power directly (orange to gray) and see if I don't get
steady fuel pump pressure for as long as the jumper is connected? If fuel pump pressure is now steady and
running reliably, and then go searching for the root cause:

* Bad fuel pump relay or fuel pump relay socket connections?
* Good relay, connections, & wiring, but flaky VCM not controlling the relay properly?
(Tie in, VCM is victim of bad grounding showing up both in failing DTCs -and- fuel pump behavior?)
* VCM is being fed good power & grounds, but is itself failing and is the root cause? Outside chance.)

****

I could go on & on, but this should give you some troubleshooting ideas to try. Note: If you don't already have
the General's '98 Factory Service Manuals for your rig, then I'd seriously consider downloading them at your
earliest convenience over here: ('88+ GMT400 FSMs) Good stuff like the physical location of G103, G104, & G105,
troubleshooting trees, etc.

By the way, thank you for posting the entire list of DTCs as well as the short video showing the fuel pressures.
By providing this data right off the bat I had enough failure info that I could brew up one possible theory.

Gain control over the fuel pump operation. And clear those DTCs that all seem to share a single point of failure.

And be sure to get back to us with whatever you discover.

Happy hunting --

 

[James2406]

Quick update, the truck will run with the MAF unplugged. It still gives me a little trouble to start, but when I plug it back in it might run for 5 minutes or die instantly, but if I raise the engine rpms via the throttle cable, without it plugged in it won't stall, but with it plugged in, it will stall.
Fuel pressure while idle is 55+ and raises up with throttle fine, however fuel pressure also drops with MAF plugged in when I rev the engine.

Going to read through your comment now.

 

[James2406]

Jumped orange to grey and I get a nice steady pressure

 

[James2406]

Gotta be a ground issue, after I jumped the fuel pump, and went to restart, with MAF unplugged, I it's the original problem all over again

 

[Road Trip]

Jumped orange to grey and I get a nice steady pressure

Gotta be a ground issue, after I jumped the fuel pump, and went to restart, with MAF unplugged, I it's the original problem all over again

I would be interested to find out 2 things:

1) With the jumper in & steady pressure, does the truck now start as it should, every time you try it?

2) With the jumper in, does the MAF still negatively affect the engine's operation?

Regarding question #2, I would not necessarily indict the MAF sensor until *after* the grounding
issue(s) being flagged are first resolved. (We could be in the MAF as victim of flaky ground scenario.)

EDIT: If you look at the G103 & G104 ground distribution wiring diagram in reply #6, you can see that
the MAF is shown as depending upon this all-important engine ground. And I also happened to notice
that the actual fuel pump relay control side *also* uses this same ground. (!) Food for thought.

Good progress, though. At least we can quit wondering about the fuel pump and concentrate on
those grounds for now.