1998 K2500 5.7L - Hard Start/Low Power

[Pollo Dustino]

Vehicle
1998 GMC K2500 Suburban 4x4, 5.7L V8

Work Done
New fuel pump, Tru-Grade brand, P/N D5331028 (With California emission. With Module Code GFN. Without Low Lubricity Fuel System Sender. Wiring Harness Included. Internal Strainer Included)
New fuel filter
Locking gas cap

Concern

I bought the vehicle surplus from my job in August of 2022. Never had an issue starting, fired up virtually instantaneously. Had the factory fuel pump which could be heard outside the vehicle when running. One day I was driving and the vehicle stalled, would not restart. Could not hear the pump and it would not respond to kicking the fuel tank. Had it towed to my storage yard and I replaced the pump. After priming the fuel system it would start up, but not always instantaneously like it used to. It would have to crank a little bit.

I tend to let the vehicle sit as it is not my primary vehicle. I bought it for camping. Since purchasing it I've only driven around 700 miles total.

After sitting for a while it would start to take longer and longer to start, and I'd have to prime the system five or six times before cranking and it often still would not start up. It would fire the initial prime and that would be it, just keep cranking. I replaced the fuel filter about fifty miles ago as it appeared to be original.

When I got it out of storage a few weeks ago I had to dump a little bit of fuel system cleaner down the throttle body in order to fire it over. Once it fired up it ran fine. Drove it home and it's been parked in my driveway since then.

Two days ago I tried to take it out for my daily errands, but it would not fire over. Since I was running late I hopped in my other car instead. When I got back I tried to start the Suburban and it fired up immediately.

Yesterday I took it out to run some errands and had to prime it a few times before it started. When getting on the freeway it had very low power. I drove it around on surface streets and didn't notice a loss of power, but when I was going up and over a long freeway overpass I went wide open throttle and the engine would not rev over 3000 RPM. I parked it once I got home and have not driven it since.

This morning I busted out my Maddox-brand fuel pressure gauge from Harbor Freight and my volt meter to check the pressure and voltages at the pump. See the chart below:

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The "LOADED" test is a brake stand in drive up to 2000 RPM. The pressure drops back down to 51 PSI immediately when letting off the throttle back to idle.

I have a copy of the shop manual and spec for fuel pressure is 60-66 PSI. So going off specs alone I have low pressure. I do not have my vacuum gauge here at home so I do not know what engine vacuum is. I do not hear any vacuum leaks but I know that's not an accurate test.

I have not yet attempted to block off the fuel return line.

One issue I know I need to resolve is a bad fuel pump-to-fuel tank seal. The new pump was an absolute witch to install and it took tremendous force to get the pump to seat in the tank with a new o-ring. When I fill the tank to maximum fuel leaks out around the pump during turns, so I think I rolled or tore the o-ring during installation. Would a leak in this area cause a hard start concern due to a "vacuum leak" into the tank? Same question for the locking fuel cap. I do not have an adapter to test the cap.

The fuel pump voltage is also odd to me. I'm measuring 13.85 volts at the battery with the engine running, but only a steady 12.66 volts at the pump connector. I am not familiar with how GMC powers the pump, is it a regulated voltage down to 12.66?

I am not against doing a new regulator and spider assembly due to the age of the vehicle. The truck needs a new radiator and complete front end rebuild so I'm already getting parts together.

 

[Schurkey]

1. Verify that your pressure gauge is showing accurate pressures. If the guage is 5 or 10 psi "off", you could have problems and not know it from the gauge reading.

2. The running fuel pressure seems fairly reasonable. Pump prime pressure may be a little low, but not horrible.

3. 5-minute leakage seems a little high, but again not horrible.

Fuel pump voltage is better than I'd expected. GM is known for using undersized wire in the fuel pump wire harness, so some voltage drop is normal. You've got almost no voltage on the ground side, so again that's good.

I'm not convinced you've got a fuel pressure problem.

THANK YOU for supplying such a complete list of fuel pressures and voltages. I wish everyone with suspected fuel system problems could be so thorough.

How about the rest of the "tune-up" items and procedures? Spark plugs, plug wires, cap 'n' rotor? throttle-body cleaning? PCV, EGR, EVAP, and heated-air intake systems working as intended?

Connect a scan tool, verify every sensor and computer output. Check fuel trims, make sure there's an RPM signal when cranking.

 

[Pollo Dustino]

I have not done any tune-up services to the vehicle since buying it. I'll need to check the service history at work to see if it ever had plugs, wires, or cap and rotor done. The most I've done to it is a front and rear diff service, upgrading the battery to a group 65, and the aforementioned fuel pump and filter. The throttle body looked pretty clean when I poured the fuel system cleaner down it to get the engine started, but I have not had the throttle body off and disassembled for a proper service. I have plugs, wires, and a cap and rotor in the box of parts I'm going to replace when I get the truck in for its major overhaul.

One thing I have done is put fuel system cleaner in the tank at fill ups. I'm hesitant to say that's caused some issues, but who knows? This vehicle ran on unbranded pump gas from our on-site tanks its entire career, so maybe a little extra zing in the tank after twenty years has done something.

I have considered tearing into the engine to replace the gaskets that are known to cause issues, like manifold gaskets. It does have some mild oil leaks around the valve covers. I also occasionally get a code for EGR circuit performance, so I picked up an EGR valve at the junkyard and was going to replace it, along with the AC suction and liquid line.

I think I'll let it sit for a few days to bleed off, then hook my gauge back up to it and try to start it without priming to see what it reads. I think the gauge is accurate, it's essentially brand new. Bought it specifically for this concern when it started rearing its ugly head a few months ago. The problem only started after replacing the pump, so that's why I started looking in that direction. Especially with the 3,000 RPM max at WOT. Whatever got touched last, you know?

That wire gauge to the pump puzzled me when I saw it, I would have thought it would be 12 or 10 gauge. I'm considering running an upgraded circuit now. I know my splice job was good, I crimped and soldered with Mopar splice bands.

THANK YOU for supplying such a complete list of fuel pressures and voltages. I wish everyone with suspected fuel system problems could be so thorough.

I was a Chrysler electrical tech for ten years, I hated wasting our tech line's time when I had a problem. Of course, they tended to waste mine by telling me to do yet another battery and capacitor reset.

I'll bring my scan tool VCI pod home this week and check the live data.

Could a leaking pump-to-tank seal or leaking fuel cap cause fuel delivery concerns on this vehicle? I've never really encountered that on Chrysler but anything's possible in the repair world.

 

[Pollo Dustino]

Oh, just remembered. It did occasionally throw a Bank 1 system lean code, but I left some lights on while the vehicle was in storage so it completely killed the last battery and took the memory with it. I have not been able to drive it enough since then for it to really run all the monitors. I was hypothesizing that the code could have been caused by low fuel pressure, but it could also be due to an undetected vacuum leak, which is why I'm considering just tearing in and replacing gaskets while doing a new spider and regulator.

As a tech I hate shotgunning parts, but since it's my own car and it needs upgrades anyway I can rationalize away the parts cannon.

 

[Pollo Dustino]

Okay, I managed to get back on diagnosing this truck.

The truck has been sitting for at the last two weeks as I've been busy with work, the gym, and going out of town. I went out this morning and hooked up my fuel pressure gauge and read 0 PSI. I then cranked the engine without priming the system and it did not start on the first attempt, just cranked over until I released the key. It started on the second attempt. I did not have to crank it very much until it started, maybe one second.

I took the following readings:

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As you can see the readings are very similar to what I recorded on my last diagnostic session. The "no prime" crank pressure was very low in my opinion, especially considering I have the factory poppet-style injector spider installed.

I saw a good crank signal, and the camshaft retard was listed at -6 degrees. Not entirely familiar with this engine but since it's based on the old 350 I'm assuming it has no camshaft phaser and therefore no camshaft timing adjustment.

The long term fuel trim is a bit concerning. The PCM is really trying to add fuel to both banks. There were no codes in the PCM for a lean condition, but I do remember that there were codes for a lean condition prior to the battery being drained.

I also ran a 12 gauge jumper wire straight from the battery positive to the fuel pump with a back probe to get full system voltage to the pump, but it did not result in any increase in fuel pressure.

Looking at the O2 sensors made me a little concerned. Bank 1 Sensor 2 is bouncing up and down a bit too much for my liking, indicating I may have a failing catalytic converter.

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Bank 2 for comparison:

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Ignoring the converter issue, I'm leaning toward a failing injector pack or regulator. Perhaps the fresh new pump I installed last year overwhelmed the factory spider. Since this vehicle is going to be an upgraded camping vehicle that I intend to take out into the boonies I went ahead and ordered the upgraded MPFI injector spider with a new bracket and a complete gasket set. The vehicle also has a leaking radiator, leaking A/C line, and a worn out steering box so I'm going to do everything all at one time once I get a big project off my hoist at work.

 

[Schurkey]

the camshaft retard was listed at -6 degrees. Not entirely familiar with this engine but since it's based on the old 350 I'm assuming it has no camshaft phaser and therefore no camshaft timing adjustment.

Camshaft retard is adjusted by turning the distributor.

Looking at the O2 sensors made me a little concerned. Bank 1 Sensor 2 is bouncing up and down a bit too much for my liking, indicating I may have a failing catalytic converter.

I tend to agree.

I went ahead and ordered the upgraded MPFI injector spider with a new bracket and a complete gasket set.

Seems wise even if that's not a direct fix.

The vehicle also has a leaking radiator, leaking A/C line, and a worn out steering box so I'm going to do everything all at one time once I get a big project off my hoist at work.

Oh, no. "Everything" is going to expand once you start digging into it.

 

[Pollo Dustino]

Oh I'm well aware. Project cars just get more and more projectier as repairs go on. Not to mention all the other projects I have been neglecting. I didn't even buy the steering linkage parts knowing that it would just make the job even bigger for the day.

I was reading the shop manual earlier and on page 6-1739 it talks about prolonged lean conditions and fuel trim counts. Both banks on this engine are getting ramped up on fuel trim, leading me to think there is either a vacuum leak somewhere or some other source of unmetered air. It is possible this may be causing the strange O2 readings, but I won't know for sure until I get the spider and gaskets replaced, and any leaks plugged up. Might try to adjust the camshaft too, but it doesn't seem to be a huge problem even though it's four degrees out of spec.

And I have to say, these GM manuals are magnificent in their thoroughness and detail. I used Chrysler manuals for ten years and now use Ford service information, and neither of them hold a candle to the GM stuff, at least anything made in the last thirty years. The Ford stuff actively makes me angry enough to drive to Dearborn and hurt somebody because it's organized so poorly.

 

[Schurkey]

Project cars just get more and more projectier as repairs go on.

Truth.

Both banks on this engine are getting ramped up on fuel trim, leading me to think there is either a vacuum leak somewhere or some other source of unmetered air.

Or low fuel pressure/plugging fuel filter.

Might try to adjust the camshaft too, but it doesn't seem to be a huge problem even though it's four degrees out of spec.

The CAM is 4 degrees out of spec, or the CAM SENSOR needs adjustment via turning the distributor?

And I have to say, these GM manuals are magnificent in their thoroughness and detail.

And yet, there are some errors that get through Quality Assurance. They ARE good. They are not infallible.

The '88 manual (Published in '86!) had so many errors that they had to publish a "Service Manual Supplement" to correct most of them.

The Ford stuff actively makes me angry enough to drive to Dearborn and hurt somebody because it's organized so poorly.

Decades ago, the Ford manuals fell on their faces when the final suggestion for diagnosis was to "substitute a known-good part" instead of providing proper part-testing procedures and specifications. Parts substitution works in the dealership, that has a fully-stocked parts department, AND an acre or three of vehicles that parts can be temporarily robbed-from. Doesn't work for independent shops, or D-I-Y repairs.

 

[Pollo Dustino]

A quick update for now, I intend on doing a post with updated fuel pressure readings as well but I haven't gotten around to busting out the gauge.

I replaced the injection spider with the upgraded MPI version, as well as replaced the upper and lower intake gaskets. I replaced the PCV valve and grommet. Hot tanked all the hard parts to clean them thoroughly. Also replaced the spark plugs, plug wires, and cap and rotor because I found heavy corrosion on all the interior terminals. It looked like it had never been changed once since the truck was bought. I kept the original coil but may change it out at some point.

The truck now fires right up with minimal cranking and idles decently, a little roughness at times but nothing bad. It feels like it has good power and does not fall on its face at 3,000 RPM. I don't know how well it will affect fuel economy, I haven't gone through a whole tank yet.

I could only time the distributor to a max of +2 degrees because the new plug wire boots interfere with the intake manifold. I didn't want to force it into the zero position and cause rubbing. So far it does not seem to be an issue.

I'll get fuel pressure readings up eventually. Currently very busy prepping for a camping weekend.