56.3 runs rough when cold & WOT

[dmarty]

My '99 5.3 pops and farts when cold and WOT with a load. Starts fine and runs well when warm. Runs fine cold too if I don't open the throttle too far. Has 304k miles and is mostly original. The intake gasket was replaced at 100k when I replaced a noisy lifter. I also replaced the fuel press reg at 200k. Everything else is original including the O2 sensors. Any ideas? O2 sensors? Injectors? EGR? I just replaced the plugs, wires and fuel filter as they had 120k on them, but it didn't help. Thanks!

[dmarty]

To add a little more info to my own post. I did a water test on the intake and didn't get any steam out the exhaust, so I think the intake gaskets are ok. I only use dealer OEM parts including plugs and wires. I replaced the fuel pump at 150k so that should be ok. This truck is completely stock. Any ideas? Thanks.

[usmcchevy]

Is it throwing any codes?

[dmarty]

Nope. No codes.

[454stepside]

i had a customers 96 burban in the shop cause it would run find at start up and after a while whenever you stopped at a stop light and try to take off again it wouls spit sputter and die would not stat again till it cooled of turned out to be water temp sending unit and the gauge never showed it was running hot or cold replaced it and all was fine just an idea of something to check quick simple easy cheap part just to replace

[dmarty]

i'll check it out. thanks.

[russgt]

I would be interested in what a compression test shows with that many miles on it.

[dmarty]

I hooked up an auto-ingenuity OBD II PC to it and found that the ECT readings are as expected. Up stream O2 sensors on both banks are cycling, but I'm not sure how many counts there should be. At idle I'm geting about a half count per second and 1 to 1.5 counts per sec at 1500 rpm. Both banks are the same. Long term fuel trim is -3.9% #1 and -4.68% #2. Are these numbers reasonable? Is there a good manual out here for these trucks that would give the normal values. I have the official three volume GM manual from Helms, but it is of little use.

Never checked compression, but it runs fine and gets 15 MPG at 75 MPH so it can't be too bad.

[454stepside]

you might also check into an injector pack also i will be doing one tomorrow gm had a bad design on them and the check balls tend to stick

[bighaas79]

Originally Posted by 454stepside

you might also check into an injector pack also i will be doing one tomorrow gm had a bad design on them and the check balls tend to stick

Wrong engine. You are off a generation.

WIth that many miles on it and the amount of miles on all of the components you have listed since they were replaced any of them could very well be bad again. Stuff doesn't stay new forever. I mean take your fuel pump for example you have as many miles on it now as when you replaced the first one. Your fuel trim #'s aren't unreasonable, but they indicate running a hair rich, as the negative number means the pcm is trying to take away fuel.
Checking the fuel pres. regulator, injector leak down and fuel pressure is fairly quick and easy to do, and the basics should never be overlooked, i'd start there just to make sure.

[dmarty]

You're right. I plan to borrow a fuel press gauge from a friend today and check it. I really don't feel bad throwing parts at it either. With 304k miles on the o2 sensors they don't owe me anything so I plan to change them eventhough the numbers are not too bad. I found a thread on another site that had the same problem with a '99 5.3 and it went on forever, but was never resolved. He had good fuel press (don't know about flow) and changed plugs, wires, fuel fileter, egr valve and o2 sensors. Everything but the injectors, so maybe the injectors were the problem. Any other suggestions???? With no codes set and decent fuel economy, it's a hard one to figure out. Thanks.

[BBC Chevy Man]

Did you clean your mass air flow sensor? If not that's were I would start. I've got a 2000 with the 5.3 and it was doing all sorts of weird things until I cleaned the sensor.

[dmarty]

Not yet, but that's a good idea. I was planning to do it tonight.

[dmarty]

Tonight I pulled the MAF and gave it a good cleaning with MAF cleaner. Also pulled the TB and cleaned it as well. Put a fuel pressure gauge on it and I get 53-55 PSI key on, eng off and 50 PSI at idle. Rigged up a long hose so I could read pressure with gage taped to outside of windshield when I drive it and get 50 PSI cruising and 57-58 PSI at WOT. And, yes, it still pops, stutters and farts at WOT when not fully warmed up. Still no codes set. The fuel gauge has a bleed port, so I opened it with the engine running at idle and the pressure went down to 38 PSI and it flowed 16 oz in 19.2 sec for what it’s worth. Can't find any test spec's for a true flow test in the GM manual. I had to order the O2 sensors from the local Chevy dealer (a common part like that not in stock?????) and will put them in when they come in tomorrow, but based on the scanner data, I don’t expect much change. The GM manual spec’s 55-62 PSI for fuel pressure with key on, eng off, so my pressure is a little low, but it doesn’t seem that bad, does it? I checked the press reg and the vacuum line is dry. As I said, I replaced it at about 225k, so I think it’s ok. The pressure went down from 50 to 40 PSI when it sat for 1.5 hour. I'll see what it shows in the morning, but that doesn't seem too bad, does it?

Giving this a little more thought (a scary proposition with a guy like me) it seems to be a lean condition under cold, WOT, high load conditions. Just the kind of conditions that demand the most fuel. Since the fuel pressure seems fairly good, I’m thinking maybe the injectors are dirty and not flowing as much fuel as they should under high demand conditions. BTW, when I changed the plugs, they all were a nice light tan color, all very even with no difference between any of the plugs. This makes me think that it’s not one specific cylinder having a problem, but a random misfire. I picked up some Sea Foam and will try cleaning the injectors to test my theory. Or maybe I should just throw new ones in and see what happens….

Any other ideas out there?????

Thanks!

[bighaas79]

I would love to see the readings from the O2 sensors and short term fuel trims when cold vs. when warm. It would maybe give a bit of insight as to if it is running rich or leaning out as you think. This may be a better test to run after you have changed your O2's just to know you should have very accurate readings from the fresh O2's. misfire counter data would be helpful also to back up your theory of random misfire. Watch counters while its doing it to see which cylinders are affected. Your pressure and flow rates are acceptable i don't know that i'd be concerned with the readings you have. It's sounding more like a air/fuel control issue to me.

[dmarty]

I kept the fuel press gauge on it overnight and it had 14 psi yet the next morning. That doesn't seem too bad, does it? I'll change the O2's tomorrow and take some readings as you suggest. BTW, I tried 2 different press gauges and they both read the same, so the low reading isn't due to a gauge that's a little off. Thanks for the help.

[bighaas79]

It should hold the pressure higher, but how quickly does it pump up when the key is turned?

[dmarty]

When I turn the key on, the press gauge pops up to 50 PSI immediately.

[dmarty]

I learned a little more today. I replaced the O2’s and as expected, the problem remains. I was able to use the PC to record the problem, but with my inexperience with the software, only recorded about two thirds of what I should have. I found that for a period of about 4.8 seconds when the popping and backfiring (sounds like a string of firecrackers going off under hood) was going on, the short term trim on 1&2 went to 0.0 and the long term on 1&2 went to 9.36 and the O2 trim on 1&2 went to 0.0 and all held constant for the full 4.8 seconds. Normally was getting 0.0 to -4% ST and -1.5 to +1.5 LT and 0.0 to +4% O2. I should have recorded RPM and throttle position, but failed to.

The new O2’s have a nice pattern and healthy cross count and the downstream O2’s have a nice flat line if I let things settle down for a few seconds after moving the throttle.

It did set a code today. P0336 for the crank sensor, but I thinking it’s a result of the backfiring and may have jerked the crank around enough to set the code. I was stupid and erased the code without checking to see if the PC could read a snapshot of it. Hope I can set another one tomorrow.

Had to quit today as the truck warmed up and would not repeat the problem and the PC battery went dead. It was so warm today, that the truck never really cooled off. I’ll try again in the morning with cold engine and charged PC battery.

I did notice the PC read an ECM test report and it said something about the EGR failed the test. Still no codes though other than the crank position. I tried to repeat the test, but it wouldn’t do it and erased the data. Maybe the engine has to run for a longer time? I’ll try again tomorrow.

Found a thread on another site today where someone with a similar problem found that it was weak exhaust valve springs on 3 and 5. Hope that’s not it. I’m still thinking it’s bad injectors or bad EGR.

Any thoughts? Thanks!

[bighaas79]

Just a thought, when you cleaned the maf sensor did you pull it apart, so you could see the side that would normally be covered by the screen? Only reason I ask is i just happened to do a 99 today that the side you could see looked perfect but the side toward the screen and the air filter was absolutely filthy. I make it a habit to take them as far apart as i can to make sure they get totally clean, and it just happened to make me think of yours. Almost identical symptoms. Only way i caught it was at idle the trims were say -8% to -10%, right at the very limit i like to see them, and when you would bring the rpm up it would flop to +8% to +10%. Went right to the maf after that and sure nuff that was it. ran like a top after a good cleaning. Not sayin this is it but like i said before it still sounds like air/fuel control to me. It could be an egr problem, can you do a egr test with the program you have? Usually they will set a code but it wouldn't hurt to check it.

[BBC Chevy Man]

I agree with bighaas79 when cleaning the maf sensor. What I did when I cleaned my was wet a q-tip with the cleaner and gently rubbed the 3 wires with it and the q-tip really got black so I know that it was dirty.

[dmarty]

Yes, I removed the MAF completely and used a whole can of CRC MAF cleaner on it. It was dull when I pulled it out and shiny when I was done. I didn’t use a Q-tip, but pulled it out again today and used a Q-tip on the 3 little resistor things on the crossbars, but the Q-tip stayed clean and the problem remains. I pulled the EGR and it’s free and opens when I apply power to it. Also it’s very clean and the tubes also are very clean. The only test I can do with the PC is an “EVAP System Leak, Test” and when I do it, I get several faults like, EGR decel test exceed range, and MAF above expected value during no EGR off idle test. I don’t have a lot of faith in this test as I’m not sure if the ECM is doing the test as I drive or if the PC is doing it on command. The test numbers are always identical every time I run the test, which seems unlikely. Anyway, I pulled the purge valve and tested it and it’s fine. There are no obvious vacuum leaks and the idle is fine. The MAF pattern looks good on the PC and the numbers are in line with the GM manual. The ECT values reported agree with the ambient temp when cold and are 194 to 196 F when warm, so it looks good.

The problem is very repeatable. It stumbles, pops and backfires under WOT when cold every time, but runs fine as soon as it reaches operating temp. When the problem is active, the trim values flat-line, and a crank sensor code is set, although the check engine light never comes on and the code seems to disappear when I drive the truck and then check the codes again with the PC. Unfortunately, it appears that the PC does not have the ability to test individual sensors or actuators, but I can read the output of most of the sensors and all seem to be in range. I put some Sea Foam in the gas tank and will see if that makes any difference in the next few days.

Any other suggestions? Thanks!

[bighaas79]

Just so we are clear i am saying to pull the 4 torx bolts out of the maf and pull it apart into its 3 parts. And I don't believe i've seen it mentioned but what about IAT readings? MAP reading? How dirty is your throttle body, iac pintle and iac bore? could have enuf gunk in it to be hanging up a bit.

[dmarty]

No, I didn’t pull the torx bolts out and go that far. I’ll try it. I pulled the TB and cleaned it very well including the IAC and IAC port. The IAT seemed a little high at 134 F with ambient about 85 F. I was a little surprised at that, but that may be why cold air intakes give a power boost. Not sure on the MAP reading. I’ll check it out and see what the manual says should be normal. Thanks.

[bighaas79]

I got to thinking about it they might not have the bolts in em that was the older style. But the newer ones had a snap ring to hold the screen in. taking the snap ring out and removing the screen to clean would work just as well. IAT seems to high. If that reading is when engine is cold that may be part of the issue. You will get higher intake temps as the engine bay heats up but that seems too high to me. The higher temps would cause the pcm to lean out the mixture, which on start up would require a higher amount of fuel until it warmed up to closed loop. This may explain your poor running condition when cold. We gotta be gettin close we are runnin out of things to check lol.