To replace fuel pump or not to replace fuel pump?

Kevin from Winnipeg Technician Posted   Latest  
Discussion
Driveability
2005 Chevrolet Silverado 1500 4.8L (V) 4-spd (4L60-E)
Intermittent Stalling

2005 Chevy Silverado customer complains about intermittent stalling. Scan of vehicle revealed no codes. Fuel system tests find a 55 psI pressure reading (spec 55-62... so as you can see it’s on the low side of the scale. Fuel amperage waveform reading is as shown below. Various parts replaced as Indicated by the customer. My shop could not duplicate the concern. Do you use a scope waveform capture of the fuel pump to make a Diagnosis? :)

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Jeffrey from Yuma

 

Manager
 

I think it needs a fuel pump, but I think it also needs some more analysis. Is it a TAC engine? If so, what is the TPS% at a hot no-load idle? Also when it stalls does it restart right away? Does it remain a crank no start condition for more than a few minutes? 

This is the most I’ve ever enjoyed a beta test 😁

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Bob from West Chicago

 

Mobile Technician
 

I want more information. Stall and no start? Stall and starts right up? Not enough info to have a path. Would I use a current capture to call a fuel pump, no. It could lead me to more testing.

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Kevin from Winnipeg

 

Technician
 

The customer describes it as ... “slowly the truck starts chugging out then it stalls”. Then he says he has to wait about 5 minutes and it restarts. He has had the truck at another shop and the “other” shop says there is nothing wrong with the fuel pump.

We had the truck all day and I had my techs test drive it on and off during the morning. With not being able to duplicate the problem.

Bob in your reply you said you would do more testing? can you elaborate? 

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Bob from West Chicago

 

Mobile Technician
 

Volume testing for the pump. I would also check ignition with the symptoms. I've seen as i think most of us have, coils and modules fail, cool down and work again.

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Lane from Louisville

 

Mobile Technician
 

Of course I use current testing for diagnosis of a fuel delivery. I would assume you know your capture doesn’t look good. Being at the bottom end of pressure and seeing your capture, I wouldn’t like that pump. 

Would I call a new pump a fix to your issue, kinda hard to, not knowing any info on health of engine or seeing any type of data. Without seeing the issue happen, all you can do is inform your customer. I would not sweat the ”other” shop

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Kevin from Winnipeg

 

Technician
 

Hi Lane 

Yes I know the scope capture of this pump looks terrible ... matter at fact it probably one of the worst fuel pump pattern I’ve seen in awhile. I generally would never condem a fuel pump based on an amperage waveform capture... but with almost a 3 amp swing ?

The customer has a brand new “carter” fuel pump in the back seat waiting for me if required.

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Lane from Louisville

 

Mobile Technician
 

Might not be doing the truck any better with “that” new pump he has 😂. 

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Kevin from Winnipeg

 

Technician
 

🤓lol

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Scott from Claremont

 

Manager
 

Hi Kevin,

In addition to the test shown I like to do a few single-shot captures showing the stall current of the pump. This technique might be able to uncover resistance in the circuit since the stall current is usually far greater than nominal current. And like Bob mentioned, performing a pressure/volume test will uncover marginally operating system issues. I'm sure that you're aware, gathering test data (before and after) usually helps when trying to nail down intermittent issues, especially when the customer becomes part of the decision making process. If you make a repair and the symptoms persist, at least you can show the customer that you performed a valid service that he or she would (hopefully) feel better about paying for. I see too many situations like this where the shop donates rather than gets compensated for their valiant efforts.

One other thing I would be looking at on this vehicle is TAC_DC plotted over time while in gear and slighting moving the throttle off-idle, I've seen intermittent TAC issues show elevated/erratic DC% due to worn/sticking TB's.

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Dustin from Cleveland

 

Technician
 

What is stall current?

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Scott from Claremont

 

Manager
 

Hi Dustin,

Stall current is the maximum current drawn through the motor at zero RPM. Setting up the scope for a single shot trigger will allow one to see this as shown in the examples below.

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Bob from East Longmeadow

 

Diagnostician
 

How about driving it while current ramping the pump? If you can duplicate the symptom you can analyze the waveform for any changes while the problem is present.

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Juan from Montgomery

 

Mobile Technician
 

I have had luck watching data pids when a fuel pump intermittently fails. Watch for fuel trims and oxygen sensor activity. The times I have caught it the fuel trims go positive and o2's go lean equally on both banks. Other than that, need more info. I also suggest doing some spirited driving to duplicate the concern.

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Glen from Arthur

 

Technician
 

What about the battery cables/ends for poor connection/voltage drop. Also the ground @ the rear cylinder heads. If I am amp testing a FP I also look @ the relay on/off.

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Bill from Rosetown

 

Technician
 

I had one yesterday,but with a 6.0,that was about 1000 RPM slower than yours at key on and 500RPM running amperage near the same measured at PCMB fuse. It looked very similar with similar symptoms. I did a capture of key on in rush and then start/run. I smacked the tank during capture to attempt to induce an issue. My volume fell a little flat at 50 PSI maintained pressure at full load during test drive (60 PSI idle). No lean codes and from the hip 02 readings at WOT near what I expect from a 6.0 which is dancing around a bit, but not diving lean (Yours should Maintain above 800mv I believe, I do not see many 4.8L). I was reluctant to call a pump. I had informed the customer that the pump is showing indications of getting weak and that further voltage and current measurements could be done at the pump, but without duplicating his concern (that only happened a few times within the year) I could not say with certainty that is causing his stall, or that he does not have multiple issues. He also had baffles loose in the exhaust that could have been causing a block off ,although likely never enough to cause a stall from my experience. There is a good possibility there is mouse damage on mine and may have issues caused by that as well. I do not oversell any repair results unless I have a smoking gun. I left it up to my customer to decide whether it was worth it to him for further pump testing and replacement. As much as I can assume my poor readings are what caused his stall I certainly was not able to prove it. Maybe your customer will let you drive it to failure while you run errands and that sort of thing. Doesn't look like I can send the pico file, but if you like I can bring in a couple pics of my 05' from yesterday.

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Robert from Ballston

 

Diagnostician
 

I agree with other replies. Testing the pump at idle only yields inconclusive testing. Yes that waveform raises some serious concern about the pumps health. If it’s not your problem it will be soon. No doubt it’s failing. Is it the cause of your stall? Unknown due to lack of testing during a fail condition. 

I would absolutely recommend testing the pump under several operating conditions. Watching the startup current is a good place to start. Generally I become very concerned with a fuel pump if the start up current raises above the fuse amperage rating. The pump may momentarily draw more than the fuse rating without actually causing the fuse to fail. Those are the ones that usually cause melted wires and poor terminal contact. Most pumps will not exceed twice the operating current on startup if it is still in good shape

Does the pump speed up or slow down under load does the current increase or decrease under load? Current increase and speed decrease indicates a restriction after pump. A speed increase and a current decrease under load is a pump starving for fuel. A speed that decreases and current that decreases is a poor power or ground for pump. 

Hope this helps. 

+1

Martin from Burnaby

 

Instructor
 

Hi Kevin. Yes, I've used pump wave forms and current ramping to back up suspicions of intermittently failing fuel pumps and the one you posted isn't at all healthy appearing. Working in a dealership for many years, these would arrive almost daily at the door and 99% of the time it was the pump and rarely was a waveform part of the diagnosis. 

We'd more often perform routine physical inspections and system testing, but given the life span and failure rate of pumps, there was little time to devote to capturing and discussing wave forms, or at least to getting paid for additional testing, unless the vehicle did have a truly intermittent condition. 

Follow the symptoms and verify cranking fuel pressure after a cold soak, fuel filter for restrictions (frequent) and the debris that it has trapped, plus electrical connections for signs of excessive current damage and/or poor grounds. How long does the engine crank before starting and has battery condition been verified? All electrical testing should begin with a good known battery and system voltage. What is the fuel volume?

Is there an obvious shift in short and long term fuel trims associated with the symptoms? Is there any associated noise from the pump? How well does the system hold pressure after engine shut down and restart after a hot soak?

For whatever benefits there are in analyzing waveforms, a failing fuel pump should also exhibit some physical maladies to support replacement. Quite often, the fuel filter hasn't been changed, resulting in a pump that worked hard drawing more current and as a result overheating the connections. FWIW, I've also captured and viewed new pump wave forms, that didn't appear healthy because the components were not bedded in and that would have been called bad by scopaholics.

Personally, I'd want to experience the failure symptoms first hand, given that the truck has already had multiple parts hung on it. 

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Kevin from Winnipeg

 

Technician
 

Well just a follow up on this pump discussion. With the fact we had this vehicle for a day and we could not replicate the symptoms of the customer, but with the fact it was meeting the bottom end of the pressure spec and almost a 3 amp swing in the captured waveform and the customer describing the symptoms of a failing pump, plus with the fact no codes were setting to point in a different direction I had make the decision to replace his fuel pump. It’s been 2 weeks now and the customer has not called back to indicate my diagnosis was incorrect. I generally would not make a diagnostic decision based off a fuel pump waveform however I feel when you get enough diagnostic direction to point towards a failure I prefer not to waste anymore time testing for this complaint when the testing time could cost more then the actual replacement of the part. There was certainly some added testing procedures that were mentioned by various members and I agree these added tests could provide more direction pointing towards a pump problem , however the waveform to me was the key reason for my diagnosis. 

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Glen from Arthur

 

Technician
 

Did the replacement pump waveform also validate the repair?

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