99 Tahoe LT 350 V8, stock P300 Code

[tahoesteve666]

99 Tahoe LT, 350, V8, stock with 145k miles. So I have a P300 Code and the vehicle is running really rough, and seems excess fuel getting into the exhaust as I can hear popping and such. No visible evidence of oil leaks that I can see.I do think I notice the fuel gauge needle vibrating more than usual, but not sure.

History: over last 18 months, replaced all sorts of thing:
Fuel Pump, PCV Valve, Tranny Solenoids, Belt, Belt Tensioner, A/C Comp and Dryer. Replaced all fluids including Brake, Power Steering, Tranny, Coolant and Engine Oil. Replaced filters: tranny, oil, air. I dont recall specifically, but I either used a high mileage oil, or supplemented with an sealing additive. We replaced A/C as it seemed to seize slightly one day when we were right there working on it and that seemed tocause alot of back pressure on the engine, enough that the engine jolted and we saw oil vapor come out of the pcv valve connection. Replaced, plugs, wires, cap/rotor also.

Current: Anyway, after all the changes, it ran great for many months, not that it gets used that much though...I probably only put on 1000 miles since all the repairs. But now I have the code and very rough performace, so much so I dont drive it. Engine light had even flashed before on the way getting it home, but now its generally steady light.

Debug:
I checked fuel pressure, was 50-53 psi range.
I believe the fluids are all pure, but have not confirmed. I will check that tomorrow.
I sprayed carb cleaner around to try to check for vacuum leaks, but no apparent effect. but note that the vacuum level seems low, 35kPa = 10 in hg according to the scan tool below.
I pulled plugs, seems #2 had alot of oily buildup, maybe #4 also, but otherwise they seemed ok.
I measured cold plug resistance, they ranged from about 6-9 kOhm, with i think plugs #2,4 being highest, so was a little concerned there. So I swapped out plugs with new but they ranged from 4-6kOhm out of the box, cold, so I dont think that was the issue.
I did dry compression only on 1,3,5,7 and 2, 4 before the hose burst (300 psi rating my ass, cheap chinese crap, replaced hose with same and it burst too, complete junk). I believe 2 and 4 might have been a lower, maybe 150 or even lower, but not positive due to hose issue. I gave up on compression, replaced the plugs and then picked up a scan tool, induction light, inline plug tester, but so far, have only used the scan tool.

I used an ELM327 device w/ pcmscan tool software v2.4.12 software, cold engine, idle only and got the following results using only about 1 minute capture (and I think i might have hit the gas slightly for a few seconds of it toward the end). I attached the data export in case someone wants to see the details. I have not graphed anything yet. The summary is here.

Intake Manifold Pressure = 35 kPA
Mass Air Flow Rate = 7.72 g/s
Ignition timing advanced for cyliner 1 = 21.5 deg
Boost Pressure = .34 Bar
Fuel Flow Rate = 2.662 l/hr

Oxygen Sensor Voltage (Bank 1 Sensor 1,2, Bank 2, Sensor 1,2)
average= .408, .707, .483, .426 very wide ranges for each, but did not graph yet.

Oxygen Sensor Short Term Fuel Trim Bank (Bank 1 Sensor 1, Bank 2, Sensor 1)
average= -.2, 0.9, fairly stable, little fluctuation/variance

Long Term Fuel Trim % (Bank 1,2,3,4) = -12.5, 15.6, -36.7, -6.3
bank 4 has some variation, but others were table.

Short Term Fuel Trim (Bank 1,2,3,4) ave = -.2, 0.8, 8.0, 16.4
1,2 table but 3, 4 wide variation to both + and - sides

So anyway, I am looking for some help.
Can someone tell me what Bank 1,2,3,4 mapping is from the Fuel Trim indications above. I assume for the oxygen sensors, it is just drive1 and pass2, pre1 and post2 cat.

I am suspecting gaskets, possibly plugged cat, but dont know. I dont see any oil, so leaning against valve cover, but hoping its not head gaskets.

Any insight, or maybe direction I should pursue to continue the debug?
I am NOT a car guru by any means, but seems with the code, scan diagnostics and some simply manual testing, a troubleshooting guide should be readily available to pin point this cause.. Software developers?

Any help appreciated, I am dead in the water now..

 

[richamos]

In the GM service manual for this vehicle look for the strategy based diagnostics for the P0300 trouble code they have a step by step process for determining the root cause of this problem. It is basically a trouble tree that works excellent I used it many times when I worked as a GM tech in the 90s.

 

[richamos]

One more thing to add... If you are getting similar readings on sensors for sensors 1 and 2 (the o2 sensor after the catalytic converter showing the same changes in voltage as the o2 sensor before the catalytic converter) this tells me the catalytic converter is not doing its job. This could be from the catalyst bed being coated by carbon or additive deposits. This could be causing a restriction in the exhaust system and causing an excessive load on the engine which would cause a misfire. There are many scenarios but this one is fathomable. This is what happened on my 01 Monte Carlo SS.

 

[tahoesteve666]

Oxygen sensors reasons.

One more thing to add... If you are getting similar readings on sensors for sensors 1 and 2 (the o2 sensor after the catalytic converter showing the same changes in voltage as the o2 sensor before the catalytic converter) this tells me the catalytic converter is not doing its job. This could be from the catalyst bed being coated by carbon or additive deposits. This could be causing a restriction in the exhaust system and causing an excessive load on the engine which would cause a misfire. There are many scenarios but this one is fathomable. This is what happened on my 01 Monte Carlo SS.

Currently, I am using a scan tool and graphing the output. Since I have a very rough idle, it seems I should not have to collect data for anything other than idle, just to simplify things. I plotted the data collected for 1 minute, but only seemed to have about 50 datapoints per sensor, so effectively less than 1 data point/sec per sensor. Is this high enough resolution to see the effect? Do I need to perform a better data collection? I have attached the graph of the results.

From what I can tell, Bank 1, Sensor 2 collects first, then B1S1, then B2S1, then B2S2 and then the cycle repeats and there is a fraction of a second between each collection, about 0.25 -0.3 sec specifically on average.

Looking at the graphs, it seems B1S1 is mostly low and B1S2 is mostly high with occasional deviations. I dont see any real correlation between the two.

In contrast, B2S1 and B2S1 do seem to have a moderate correlation. The values for B2S1 seem to be a little more extreme (lower lows and higher highs) than B2S2, but they do seem to track to significant degree

thanks.

 

[tahoesteve666]

Service Manual

Unfortunately.. dont have one of those, been looking online..but so far, have not found it.

 

[richamos]

Trying to make sense of the readings from the oxygen sensors from a scan tool is futile. The readings are buffered by the scan tool software, so basically what you are looking for is sensor one should switch between a high and low reading and sensor 2 should not switch between high and low or infrequently. To make it simple if the o2 sensors 1 and 2 are switching at the same rate you have a bad catalytic converter which could be partially clogged. You could also hit the converter with an infrared thermometer and check the temp of the converter in 3 places from front to rear. The temps should be close if you see it much hotter in the front then its plugged.

 

[tahoesteve666]

Ok, so I just came in from messing with it some more while you were responding. And thank you for responding first off.

I do have an IR Gun, so I will check that.

For the oxygen sensors, do I need to scope them? No scope here unfortunately.

Thin morning, I finally checked fluids and did inductive probe on the plug wires.

Coolant was very clean. Oil was overfilled and I pumped out 1/2 quart through the dip tube. It was dirty, and seemed to have a dispersion of small copper colored particles or small air bubbles. Not sure which. I think I had used an additive previously. And when I first started the tahoe , the oil pressure was about 50 psi? and then after about 10 minutes of playing as described below, it seemed to come down to a little below 40. I will confirm.

I then started checking the timing light response. 1,3,5,7 seemed fairly steady, occasional perturbations, but not too frequently. This is first time doing this, so not really sure what to expect. The other side seemed to have some peculiar response. Particularly 8, which was either flashing really fast, and not that intensely (maybe because it was fast) or maybe it was just steady, but it did seem different. I would note that previously, I had changed wires, cap, rotor, but as I am pretty much broke, I used cheap ass ebay parts. I will have to check 2,4,6 also, I think 2,4 were ok, but not sure about 6.

My approach was to start at cyl 1, hook up probe, turn on ignition, hit timing light button and watch response, turn off vehicle, move to next cyl and repeat. At some point during this process, the check engine light actually turned off, but by the time I was finished, it came back again. I would also note that there was a few instances of backfire, so it seems fuel is still getting through to the exhaust. I am suspicious about the timing light response and will probably want to revisit this.

I will post Temp measurements soon.

If any of the above triggers some thought though, I would certainly welcome that feedback. Thanks

 

[richamos]

Have you ever done a professional fuel injector service on your truck? This is the one shops do with a 3m kit or similar that has a pressurized can that connects to the service port on the fuel rail. You disconnect the fuel pump relay and run the truck off the can. I would try this as a next step since you have done just about everything else.

If you are seeing negative fuel trim numbers this tells you that the computer is having to cut back the injector pulse width (time the injector is open) to maintain stochiometric (the ideal fuel air ratio of 14.7 parts air to 1 part fuel). What this means is it's likely you either have a very dirty air filter or obstruction or a leaky/faulty fuel injector.

Oh I forgot to say to clean the mass air flow sensor try this first. Only use a cleaner specifically designed for cleaning mass air flow sensors or you will destroy it. You take the MAF out of the truck and spray the little components on the wire, let them dry and repeat a few times. Let them air dry don't use compressed air to dry them they are very easy to bend and destroy. After cleaning clear codes and take for a drive. If you haven't cleaned it before you will notice a big difference guaranteed.

 

[tahoesteve666]

So I played with the IR gun. It may have still been warming up, but the temp gauge was at about 170 or 180, I forget which. I think I have stock thermostat in there, but not positive. Anyway, the temperature profiles of both sides seemed the same for the most part. There was about a 10 F delta across the cat itself, which grew to maybe 20 F once it was up to temp. What was a little weird to me was that the pipe after the cat seemed hotter than the cat itself. I will have to check that also. But the cat temp seemed to be about 180 F in the front, so about 100 F lower than the incoming pipe. Do these #s seems reasonable?

I do know what you are referring to regarding the injector service, mass air flow and air filter. Im pretty sure the air filter is very clean, but will check. I also have mass air flow cleaner here, so will hit that momentarily. I dont know where the fuel relay is or the injector port, but will research that. Hopefully there is a youtube video.

Some feedback on the cat temp data would be helpful. I still suspicious about the inductive probe response though. And just because I go through the motions here, does not mean Im competent. Im totally winging most of this. Not my cup of tea for the most part.

 

[tahoesteve666]

I am having a little trouble locating a cleaner that connects to the fuel rail. I see a 3M product and Seafoam, that seem to be intended to use on the throttle body, which seems pretty straight forward.

I have found a pressurized cannister system, one from Fit Tool out of Taiwan for about $70 USD and one from OTC, which is more expensive. But would like to avoid this cost and that does not include the chemicals either.

Is there a product which just comes in a pressurized cannister itself, (similar to some of the HVAC refrigerants)??

Also, this is for a 99 Tahoe 5.7 Vortec, does this vehicle even have a fuel rail? I looked around, but didnt see it. Ive seen videos that seems to show this as a spider injector setup, so I dont know of there is some accessible port to introduce chemicals. I do know where the fuel pressure port is, but dont know if I can use that..

 

[richamos]

If you go to NAPA they can sell you the kit just ask for the 3m fuel injector cleaner kit they will know what you need. Or you may just want to take it to a competent repair shop in your area they can perform the service for you. Check with the Chevy dealer they may not be too bad on their price for this service and they have done many so you know it's being done right.

The temps you are stating are they in farenheit they seem awful low for a vehicle up to operating temperature.

If you don't want to spend a lot of money on an injector service you could try running seafoam in the tank and see if you have improvement. With our big tanks I would put 2 cans in a full tank or 1 on 1/2 tank etc. You can also clean the combustion chamber by drawing seafoam through the vacuum hose that goes to the vacuum booster for the brakes. Take the seafoam can open it and have it ready sitting on the fuse box or somewhere within reach of the vacuum booster. Start the engine then pull the vacuum line off the booster but put your thumb over the plastic valve to keep the engine from stalling. Now take the open seafoam can and hold it up to the vacuum line and tilt it enough where you hear it slurp the seafoam in slightly. Do this until the can is about 2/3 gone then let the vacuum suck in the seafoam fast enough to stall the engine. Turn key off and let engine sit at least 20 minutes and then start the engine and be prepared to take your truck out and drive it in low accelerating quickly up to the redline and back off abruptly repeating this until you see no smoke coming from the back of your truck.

You want to do this in an area where you won't be offending the public (abandoned parking lot warehouse area would be good lol). If you don't you will put an entire neighborhood in a cloud of smoke!

 

[richamos]

Yes this is the spider system inside the upper intake, but there is a service port near where the fuel line attaches to the spider injector system. I would just try the seafoam treatment first before looking into the professional treatment.

 

[tahoesteve666]

Also, I checked the air filters.. like new. And I believe we cleaned the mass air flow sensor before, but I did it again this morning. Did not improve performance.

 

[tahoesteve666]

I think I saw a youtube video using the vacuum booster for the brakes.. I will consider that.

But the fuel port you are talking about, is this the same fuel port where I connected the gauge to take a pressure measurement to make sure the fuel pump and filter were behaving properly, or is this a different port?

And does the 3M kit require a separate pressurized cannister, or is it an all in one product?

I will remeasure the temperature after making sure the vehicle is up to temp. I also have alot of rust on this vehicle. I dont know how much that is affecting the IR signal. I dont think there were shields on the botton either, but Im not positive.

 

[richamos]

Yes the port you connected to when you took the pressure reading is the same port to connect the cleaner to.

Another thing you may want to consider doing is what people on the forums call the "big 3" which is replacing the grounds from the battery to engine, battery to frame and battery to body. I do this on all vehicles I own and I think it makes a difference especially with rusty vehicles. Make sure you clean up the metal where you attach the new grounds. Also don't use bare grounds straps make sure they are insulated use battery cables.

 

[lesterl]

My thoughts on your P0300...

1. Your fuel pressure looks low, but your PCM is pulling fuel, possible leaking FPR.

2. Distributor Drive Gear is known to cause these issues.

I have had both of these happen.

Also the fuel guage needle vibration (hummingbird effect), I have recently repaired mine with a needleless syringe and some RC Car differential fluid (thick Silicone), there is a writeup somewhere on how to do this and it is pretty easy...

 

[tahoesteve666]

My thoughts on your P0300...

1. Your fuel pressure looks low, but your PCM is pulling fuel, possible leaking FPR.

2. Distributor Drive Gear is known to cause these issues.

I have had both of these happen.

Also the fuel gauge needle vibration (hummingbird effect), I have recently repaired mine with a needleless syringe and some RC Car differential fluid (thick Silicone), there is a writeup somewhere on how to do this and it is pretty easy...

I have seen something online regarding the needle vibration issue..One of these days I will address, assuming I ever figure out current P300 issue.

Lester, I think my fuel pressure is about 50+ psi, can you tell me why you think its low? I was under the impression that 50 psi was right on.

The distributor drive gear suggestion is a new one. Have not seen a reference to that online anywhere.

 

[tahoesteve666]

I tried just cleaning through the vacuum line, wasnt really expecting too much out of it, and other than polluting the neighborhood, didnt seem to help resolve the issue. I broke down and ordered a fuel injection canister to try to address the injectors directly, but it wont show up for a few days. Im sure I could get it done cheaper, but at least Ill have to equipment now.

I will revisit the cat temps and the plug wire induction response in the mean time. I will also clean the intake butterfly assembly just for the hell of it, but dont think thats the issue either. And probably will order a quality hose for the compression tester to revisit that as well as a graphing code scanner. Not sure what else to do at this point.

 

[lesterl]

Fuel Pressure Spec is 60PSI (IIRC)

Drive gear on the distributor wears on the Vortecs and allows the timing to jump all over the place. It gets really thin teeth, and they don't have to be worn very far to throw the timing off. I have had this happen before and you can buy just the gear (or buy a NEW Billet Aluminum one (I paid $50 for a billet one on Ebay))...

 

[tahoesteve666]

Fuel Pressure Spec is 60PSI (IIRC)

Drive gear on the distributor wears on the Vortecs and allows the timing to jump all over the place. It gets really thin teeth, and they don't have to be worn very far to throw the timing off. I have had this happen before and you can buy just the gear (or buy a NEW Billet Aluminum one (I paid $50 for a billet one on Ebay))...

ok thks, I will remeasure fuel pressure tomorrow and investigate the drive gear.