Vehicle: 1997 Chevy Suburban with 5.7l

Issue:
-Engine runs great with throttle under 1/2 and will rev to 5000+ RPM. Push throttle past 1/2 and engine will loose power, start to surge and feel like it is running super lean.

-Engine stumbles sporadically while cruising

-Popping in exhaust if transmission is downshifted or if engine is revved freely

-Normally there are no engine codes, but if vehicle is operated over half throttle for any decent period of time, check engine light temporarily turns on and displays error codes for the EGR valve and low voltage for O2 sensors in bank 1.

-When engine light turns on, performance suffers more and the vehicle has a hard time getting out of its own way. Shut it off, turn it on, check light is off and it runs normal under 1/2 throttle.



Do I need to replace the EGR valve? Or could this be another issue that is causing the computer to throw codes for the EGR valve/O2 sensors?

I guess I don't too much about the EGR system on that vehicle, but it definitely sounds like a lean condition.

I would suspect that you have low fuel pressure caused by a clogged filter or failing fuel pump. I would definitely check the fuel pressure at the rail. Could also be clogged injectors or a clogged fuel rail.

I thought that was the problem at first and replaced the fuel filter, ran injector cleaner through, and checked the fuel pressure/ checked volume by unhooking the fuel line and seeing how much squirted out(it didn't slow down.) The pump only has about 15,000 miles on it.

I also replaced the upstream O2 sensors, cleaned the MAF sensor, and replaced intake gaskets.

If the fuel pressure and injectors are good then you could very well have a lean condition caused by the EGR or something else.

I'm assuming you already checked for loose/cracked hoses hooked to the intake under the hood when you replaced the intake gaskets.

If you haven't a would still try and hook a pressure gauge up and check the fuel pressure while driving and under a load. Last summer I replaced the fuel pump on an Xterra that was showing the same symptoms that you have. The pump would pump fuel fine when the hose was unhooked but it didn't build sufficient pressure when under a load.

EGR valve could be a freebie fix. My son's van with 4.3 V6 used to get chunks of carbon caught in the EGR. Remove and clean out took about a 1/2 hour. Never threw codes either but ran like crap. Try it.

That's what I was gonna suggest. Had a EGR code come up on a GM, removed it, cleaned it with a tooth brush, reinstalled, problem solved.

its is very common on those vehicles to get chunks of carbon in the EGR pintel causeing it to stick open however a stuck open EGR will only cause problem at idle and light acceleration. Vacuum leaks at the intake plenum are common also but this should set a P0171 and/or a P0174 lean code. What code did you have exactly P0400? 406? 1404?

Another common problem is plugged catalytic converter(s). There are 2 ways to check for this
1. Install back pressure guage inplace of upstream o2 sensor and should have 0-3 psi under throttle blip.
2. Remove y-pipe secure with wire and drive and if condition improves or disappears the converter is plugged

I'm sure your neighbors will appreciate you driving with the cat-back disconnected. I know mine did. I did the same test. However, when my cat was done, the car wouldn't drive right all the time no matter how much throttle was used.

You can buy an EGR gasket with a screen filter to keep carbon particles out of the pintle, but this does not sound like your issue. Vacuum leak, possibly internal, low fuel pressure/volume or a plugged exhaust are likely causes.

Allright, thanks for the responses so far.

-I guess I am going start out by picking up a fuel pressure guage on Monday with a long hose so I can stick it under the windshield wipers and drive it. I'll let you know what I find.

If you have any more ideas, let me know.

in case you dont have the spec the pressure should be 60-66 psi on those central port injection engines. Even 5 psi can make a difference and they do tend to eat fuel pumps

I didn't get a chance to get a fuel pressure guage, but after some free way driving I got some new codes. Here is the list:

P0131 - O2 Sensor Circuit Low Voltage (Bank 1 Sensor 1)
P0151 - O2 Sensor Circuit Low Voltage (Bank 2 Sensor 1)
P0300 - Random/Multiple Cylinder Misfire Detected
P1406 - Exhaust Gas Recirculation (EGR) Position Sensor Performance
P1408 - EGR Flow Out Of Self Test Range

Correct me if I am wrong, but it now seems obvious to me that I need to replace the EGR valve.

Actually sounds like you have 2 problems to me. I just performed a database search on your 02 codes. And the most common problem is fuel pump. Even pressure drops down to 55 psi can cause some lean issues causing low o2 sensor voltage. EGR codes seem like a seperate problem. I would go ahead and test the fuel pressure and if it drops even 5 psi or so under wot while driving I would suspect the pump.

Did you use OEM fuel pump 15,000 miles ago on replacement?

I have about same vehicle, and based on what I heard and read, I replaced fuel pump with OEM unit at significantly higher cost.

Fuel pumps are big hassle to replace on these vehicles so I got what I hope is the longest lasting - OEM.



BB

Honestly, I'd start at the top of the list when dealing with driveability/runnability concerns.

Inspect the plugs, wires, cap, rotor, and connectors.

Then, look at fuel pressure/volume, age of fuel filter, air filter cleanliness, etc.

Now start reading/deciphering codes.

chickenhauler said:

I did so previously, but it is definitely a good suggestion.

It has new plugs, plug wires, clean dist cap, properly timed, replaced fuel filter, ran a tank of gas with injector cleaner through, replaced air filter(old was still clean), cleaned MAF sensor, and replaced the intake manifold gasket.

The only reason I'm dragging along on checking the fuel pressure is because I don't want to go spend $50 on a guage, so I'm trying to find one I can borrow.


This is actually not a new problem, this Suburban is my parents old one and its been running pretty crappy for the last 5 years including when the pump had just been replaced. My dad just changed the oil and ran it. When I got it last year, the plugs were so old that the electrodes were completely burned off and it ran really rough at idle. The fuel filter was also so clogged it could barely pull 65mph. I've been working the kinks out on it since I've got it and it actually runs excellent compared to when I got it, but I won't settle for any engine to run less than optimal.

The J-ROD said:

You may be able to rent one from your local auto parts store or tool rental place.

Ok, one thing down! I figured out the EGR valve issue; I went and pulled it off and discovered that somebody put it on 180 degrees off. :lol:

So currently:
-0 Service codes
-Engine idles much better
-Runs smoother with higher throttle openings and makes better power too
-No longer surges or stumbles
-Still seems like its only making about 80% of the HP it should be though
-Still backfires if I stomp on the gas and let off in neutral

Now, to find a fuel pressure gauge...

Your 'Burb is pretty heavy for the 350 so don't expect a whole bunch of power out of it. You might try checking the exhaust mani bolts to make sure they're all tight. EGR valves can make a lot of trouble, glad it was a cheap fix.

The J-ROD said:

If this is truely to blame, your fuel pump is probably toast. A fuel filter clogged that badly will make the pump work so hard that it will self destruct.