98 GMC Suburban - Multiple O2 codes/hard starting

[PackLeader]

Greetings all,

Am seeking help diagnosing my 98 GMC Suburban "issues". Details below. Thanks!

Vehicle: Pampered, well-maintained 98 GMC Suburban - 5.7L, 187K miles, 1/2 ton, 2WD, one owner, no wrecks. Location: Houston, TX.

Background: Runs/idles silky-smooth. No missing, surging, or stalling. Power/acceleration is normal. Within the past year, this vehicle has received a new fuel pump, alternator, battery, timing set, cam and crank position sensors, throttle position sensor, map sensor and four O2 sensors.

Problem: For the past several months, it''s been increasingly harder to start and the SES light is on and so fails state inspection. Also (but rarely), backfires when cranking upon failed start. Sets O2 codes for all four O2 sensors. (P0137, P0151, P0157, P0160). Have checked harness and connectors for obvious signs of stress, damage, corosion, broken grounds, etc. Unable to detect any obvious vacuum leaks. Ocasionally also sets code P0102, but only after I''ve had connectors, sensors, EGR valve etc., off for inspection. Also self-clears the P0102 code over time.

Is there anything considered "typical" or "common" for 5.7L Vortec that''s known to cause these symptoms in the absense of anything obvious? This one has me scratching my head. Would sincerely appreciate any info to help resolve. Thanks in advance.

Mike

[maxwedge]

Welcome to AF, this is the wrong forum I am moving this to the Chev Suburban forum.

[777stickman]

P0137-151-157 all say the 02 sensors are detecting a "too lean" condition.

P0160 indicates a possible heater problem with that 02 sensor.

"to lean" can be caused by low fuel pressure, contaminated fuel, exhaust leaks, vacuum leaks, PCV valve, etc.

[old_master]

A quick, simple test to eliminate a lot of possibilities is to check the fuel pressure with the ignition on, engine off, and the fuel pump running. Should be 60psi to 66psi and remain above 55psi for 3 to 5 minutes after the fuel pump shuts off. Post your results.

[PackLeader]

Originally Posted by old_master

A quick, simple test to eliminate a lot of possibilities is to check the fuel pressure with the ignition on, engine off, and the fuel pump running. Should be 60psi to 66psi and remain above 55psi for 3 to 5 minutes after the fuel pump shuts off. Post your results.

Hi,

Funny you should say that, because I just replaced the fuel pump because of another (unrelated) problem. I started experiencing very poor acceleration at wide open throttle, and/or near wide open throttle, for instance when accelerating onto the on-ramp of a highway. Moderate acceleration was fine though. I put a guage on it and at idle it was about 55 psi, mild to moderate acceleration it's about 58 psi, but it dropped to about 52 psi at WOT. The pump wasn't even 2 years old, but was unfortunately a crappy AutoZone one. I replaced it with an OEM AC Delco and now it's fine. I put a guage on it and now it stays around 62.5 psi at an idle and about 65.5 psi at WOT.

I'm kinda suspicious of a vacuum leak somewhere but haven't been able to find one. Then again, I'd expect problems idling if that were the case. I actually considered pulling the intake and replacing the gaskets since I've heard about occasional problems with vacuum leaks on the Vortecs. Could a vacuum leak in the lower intake area cause something like this? Thanks.

Mike

[old_master]

It's unlikely a vacuum leak would give symptoms like you're experiencing. GM does not publish ANY engine running fuel pressure specs, for this system. What is the pressure and leakdown rate when the engine of off? Ignition on, engine off, and the fuel pump running. Should be 60psi to 66psi and remain above 55psi for 3 to 5 minutes after the fuel pump shuts off. Make sure the fuel filter is not restricted.

[PackLeader]

Originally Posted by old_master

It's unlikely a vacuum leak would give symptoms like you're experiencing. GM does not publish ANY engine running fuel pressure specs, for this system. What is the pressure and leakdown rate when the engine of off? Ignition on, engine off, and the fuel pump running. Should be 60psi to 66psi and remain above 55psi for 3 to 5 minutes after the fuel pump shuts off. Make sure the fuel filter is not restricted.

Hi,

Appreciate your reply. I think you may be on to something here. I'm on my third fuel pump on this vehicle. A few years ago, the original pump died and was replaced with a crappy one from AZ. Then, a few months ago, I started having acceleration problems when trying to enter hi-way on-ramps, etc... although moderate acceleration was fine. I put a guage on it found that it had about 55 psi at idle, 58 psi during moderate accelleration but dropped to about 52 psi at WOT. So, I put another pump in it, but this time used an OEM AC-Delco model. Also replaced the filter. All driveability problems ceased, but I still had these O2 codes.

So last night I checked the pressure again and still have good running pressure, (about 62.5 psi at an idle and about 65.5 psi at WOT). But, the leakdown is horrible and I don't know why.

With ignition on, engine off and pump running, I have between 50-54 psi. As soon as I turn the ignition off the pressure bleeds off to just under 9 psi in under 5 minutes. Could this be a bad regulator? Is that built into the pump, or....??

Thanks again.

-Mike

[old_master]

With ignition on, engine off and pump running, I have between 50-54 psi. As soon as I turn the ignition off the pressure bleeds off to just under 9 psi in under 5 minutes. Could this be a bad regulator? Is that built into the pump, or....??

There is the problem! There are 3 common things that can cause excessive leakdown. The check valve in the fuel pump, the fuel pressure regulator and the fuel lines under the plenum, referred to as the "nut kit" http://shop.advanceautoparts.com/Pro...rtNumber=55163 To determine if the leak is under the plenum or in the tank, the fuel pump needs to be isolated from the system just after it pressureizes the system. To accomplish this, a temporary valve needs to be installed between the pump and the fuel pressure test port. Open the valve, allow the pump to pressureize the system and immediately shut the valve off. If pressure drops, the problem is under the plenum. If the pressure holds, the problem is in the tank.

[PackLeader]

Originally Posted by old_master

With ignition on, engine off and pump running, I have between 50-54 psi. As soon as I turn the ignition off the pressure bleeds off to just under 9 psi in under 5 minutes. Could this be a bad regulator? Is that built into the pump, or....??

There is the problem! There are 3 common things that can cause excessive leakdown. The check valve in the fuel pump, the fuel pressure regulator and the fuel lines under the plenum, referred to as the "nut kit" http://shop.advanceautoparts.com/Pro...rtNumber=55163 To determine if the leak is under the plenum or in the tank, the fuel pump needs to be isolated from the system just after it pressureizes the system. To accomplish this, a temporary valve needs to be installed between the pump and the fuel pressure test port. Open the valve, allow the pump to pressureize the system and immediately shut the valve off. If pressure drops, the problem is under the plenum. If the pressure holds, the problem is in the tank.

Awesome. Thanks very much for the great info. FYI, I don't have the fuel lines under the plenum. Mine's a later version with the supply and return over the top. Regardless, I pulled the upper intake and found a leaking regulator, and a really nasty carbon-caked and brittle spider. So, I tore it down further, pulled the lower intake, cleaned everything, replaced the spider (ouch!), regulator, and gaskets. Now it runs great and starts almost instantly (like it used to) and the leak-down problem is gone.

My only challenge now is to find out why the SES light came back on after 11 miles. I didn't put a scan tool back on it yet, but I'll bet it stored the same O2 codes as before. Still won't pass inspection, but at least it starts now and runs fine again. I'll pull the codes this evening and see if anything's changed. Thanks again.

--Mike

[PackLeader]

Originally Posted by PackLeader

My only challenge now is to find out why the SES light came back on after 11 miles. I didn't put a scan tool back on it yet, but I'll bet it stored the same O2 codes as before. Still won't pass inspection, but at least it starts now and runs fine again. I'll pull the codes this evening and see if anything's changed. Thanks again.

--Mike

Ok, I pulled the codes. P0157 & P0160 returned but the other three are gone. The ones that returned are:

P0157....O2 Sensor Circuit Low Voltage (Bank 2 Sensor 2)
P0160....O2 Sensor Circuit No Activity Detected (Bank 2 Sensor 2)

If I'm not mistaken, I believe Bank 2 Sensor 2 is the one on the passenger side, after the cat. All four O2 sensors have less than 5K miles on them, otherwise I'd suspect a bad sensor. Research continues.....

[old_master]

Bank 2 is on the passenger side, (even number cylinders). Sensor 2 is the sensor after the converter. Sounds like a problerm with a wire or connector going to the oxygen sensor, or a faulty sensor.... it happens.

[PackLeader]

Originally Posted by old_master

Bank 2 is on the passenger side, (even number cylinders). Sensor 2 is the sensor after the converter. Sounds like a problerm with a wire or connector going to the oxygen sensor, or a faulty sensor.... it happens.

Hi,

Well fortunately the sensor wasn't very old and was still under warranty. I replaced it (again) and cleared the DTCs and have driven about 300 miles since then with no codes. Things are back to normal again.

Rock-n-roll..... all systems go.....

Thanks to all for your help.

--mike

[old_master]

Good job, glad you got her going. Probably just a "lazy" oxygen sensor, post back if you have any more trouble.