98 Suburban Fuel Issues

[98 Suburban]

Hey everyone, I have been trying to pin down a fuel issue with my 98 Suburban 350 for almost a year and cannot seem to get it. I've replaced/repaired a number of things:

Bought the truck with a cylinder 3 & 5 misfire condition, diagnosed as a couple leaking poppet valves and leaking lower intake manifold gaskets. Pulled the intake and replaced the gaskets, and replaced the old spider assembly with a new MPI setup. misfires went away and the truck ran fine for a few months.

Then had a issue with at highway speeds (70mph+) the truck would occasionally stumble (like it was running out of fuel.) Over the next 2 months this degenerated into the truck stumbling at any speed (particularly around 1700 RPM & 2200 RPM) and developing a hard start condition on cold start-ups. Eventually it would barely start when cold (after multiple tires cranking) and it would stumble and die at idle. Tested fuel pressure and found it to be 46psi key on-engine off, 42psi - 52psi engine on, and a rapid loss of pressure on key off. This was diagnosed as a bad fuel pump, so that was replaced with a new AC Delco unit.

Just finished installing the new pump and have a slightly different set of issues. The truck starts fine (cold & hot), but it will still stumble at idle (though it will usually catch itself and stay running, only once did it actually die, and when it did it restarted fine) and stumbles badly throughout the RPM range (holding the engine at 2000 RPM results in every 10-20 seconds the engine stumbling down to 1700 RPM before catching itself and coming back to 2000 RPM.) Checked the fuel pressure again and it holds at 50psi (which I know is still low) in all conditions (bear in mind that I am using a cheap Harbor Freight tester) and no longer exhibits the rapid loss of pressure on key off (holds 50psi for almost 60 seconds before falling to 10psi after 5+ minutes.)

Also for reference the following has been replaced since I bought the truck about a year ago: plugs, wires, cap, rotor, distributor, thermostat, oil pressure sender, engine & gauge coolant temperature sensor (the gauge sensor is still wrong, reads 30 degrees higher then engine sensor according to scan tool), alternator, battery, lower/upper intake gaskets, coolant flush/fill, & shocks. I can tell that recently before I bought the truck someone did a hub bearing on the drivers front, and installed a new A/C compressor.

After reading all day yesterday and this morning I am looking for guidance as to next steps. I do have a new fuel filter to install, but I don't believe that a clogged fuel filter can cause a 10psi - 15psi drop in pressure. With both the pump and pressure regulator being new OE equipment I'm really not sure either of them are bad too, but I know it could be a possibility.

Any and all help or ideas would be appreciated! I love this truck and desperately want to get it back on the road!

 

[johnnyboy]

Might have a solution

Hey everyone, I have been trying to pin down a fuel issue with my 98 Suburban 350 for almost a year and cannot seem to get it. I've replaced/repaired a number of things:

Bought the truck with a cylinder 3 & 5 misfire condition, diagnosed as a couple leaking poppet valves and leaking lower intake manifold gaskets. Pulled the intake and replaced the gaskets, and replaced the old spider assembly with a new MPI setup. misfires went away and the truck ran fine for a few months.

Then had a issue with at highway speeds (70mph+) the truck would occasionally stumble (like it was running out of fuel.) Over the next 2 months this degenerated into the truck stumbling at any speed (particularly around 1700 RPM & 2200 RPM) and developing a hard start condition on cold start-ups. Eventually it would barely start when cold (after multiple tires cranking) and it would stumble and die at idle. Tested fuel pressure and found it to be 46psi key on-engine off, 42psi - 52psi engine on, and a rapid loss of pressure on key off. This was diagnosed as a bad fuel pump, so that was replaced with a new AC Delco unit.

Just finished installing the new pump and have a slightly different set of issues. The truck starts fine (cold & hot), but it will still stumble at idle (though it will usually catch itself and stay running, only once did it actually die, and when it did it restarted fine) and stumbles badly throughout the RPM range (holding the engine at 2000 RPM results in every 10-20 seconds the engine stumbling down to 1700 RPM before catching itself and coming back to 2000 RPM.) Checked the fuel pressure again and it holds at 50psi (which I know is still low) in all conditions (bear in mind that I am using a cheap Harbor Freight tester) and no longer exhibits the rapid loss of pressure on key off (holds 50psi for almost 60 seconds before falling to 10psi after 5+ minutes.)

Also for reference the following has been replaced since I bought the truck about a year ago: plugs, wires, cap, rotor, distributor, thermostat, oil pressure sender, engine & gauge coolant temperature sensor (the gauge sensor is still wrong, reads 30 degrees higher then engine sensor according to scan tool), alternator, battery, lower/upper intake gaskets, coolant flush/fill, & shocks. I can tell that recently before I bought the truck someone did a hub bearing on the drivers front, and installed a new A/C compressor.

After reading all day yesterday and this morning I am looking for guidance as to next steps. I do have a new fuel filter to install, but I don't believe that a clogged fuel filter can cause a 10psi - 15psi drop in pressure. With both the pump and pressure regulator being new OE equipment I'm really not sure either of them are bad too, but I know it could be a possibility.

Any and all help or ideas would be appreciated! I love this truck and desperately want to get it back on the road!

Have you considered the possibility that the throttle position sensor could be on the fritz?

 

[johnnyboy]

A couple of years back I had a similar problem with a Dodge of mine...

 

[98 Suburban]

A couple of years back I had a similar problem with a Dodge of mine...

Thanks for the suggestion! Last time I checked the TPS was reading correctly according to my scanner, but I haven't re-checked it since it started acting up again.

 

[98 Suburban]

Update to close out the issue/thread:

So I did go ahead and replace the fuel filter, but the problem persisted. From other suggestions I went ahead and re-torqued the upper intake manifold and that actually made the problem worse! It was so bad that it did trigger a CEL for a #8 misfire, which prompted me to finally find the culprit. Upon checking the cap & rotor I found that both were extremely corroded (so much that if I didn't just replace them both a year ago myself I would have sworn they were the originals.)

Luckily I had new ones in my garage, and after applying some dielectric grease to them and installing I am happy to say the truck runs great!

I guess the moral of my story is just because you recently replaced something, you shouldn't rule it out when troubleshooting. I could have saved myself hours of work and hundreds of dollars if I just started with the tune up items. Oh well, the silver lining in all this is the old fuel pump had a bad gauge sender, so at least I have a gas gauge again!