Scanner Readings

I am back to diagnosing my 1998 C1500 {350 VORTEC} 109700 miles
It starts missing very bad after pulling up a hill, I did purchase a scanner and have been monitoring the fuel trims as well as some other points, When I got to the top of the hill the engine had no power and was shaking like it was running out of gas and engine all most died out, I then looked at both short term fuel trims they both about a negative -18, after 10 secs the engine goes back to running smooth and normal after letting off the throttle, I need help reading the graphs to determine what is causing the fuel trims to go into negative when I would expect them to be in the positive when pulling up a hill

So my question is could someone please list the points I need to record and post here to diagnose negative fuel times under this condition.
Thanks for your help!
Dean

 

If it goes negative like that on a hill I would look at the spider injector assembly under the intake manifold odds are it's leaking does your oil smell like gas?

An exhaust restriction will cause rich O2 readings, resulting in Negative fuel trim. It will, also, cause a loss of power. Check the exhaust back pressure @ 2500 RPM. The closer to zero psi, the better. 1 - 1 1/2 psi is good. 2 psi is nothing to sweat. 3 psi and you need to be concerned. Over 3 psi, you have a problem.





A volumetric efficiency test will also reveal a restricted exhaust.


 

Dean,
Viewed your back pressure test. At 3k RPM, engine has no restrictions.
Post some baseline data at idle. Both STFT and LTFT. MAF in g/s.and RPM.
Make sure your engine is at operating temp for readings.
Do you have graphing on your scanner and do you know how to read 02 voltage?
Forgot to mention. Those STFT and LTFT should be for B1S1 and B2S1. Don't worry about S2 data for now.

There is no gas smell in the oil and there is no black smoke coming out of the tail pipe when this happens, I also did a leak down test and the fuel pressure held for over 5 Min.

Dino

Yes those fuel trims were bank 1 & 2 Sensor-1, Yes I can graft the o2 sensor voltages and post them,
What's interesting on the back pressure test I did "See Link below" is how much the needle bounces into the negative on the lower RPM,  and smoothens out on the higher RPM, That test was done in front of the CAT, with the muffler disconnected. What are your thoughts on the high bouncing negative?


 

The irractic needle is due to cylinder misfire and other factors. Going into vacuum at 2500 RPM I think has to do with the muffler off. Important to me is the 3k RPM reading at less than 1psi. This shows no problem with back pressure.
I don't see any trim data other than the -18 STFT in your initial post. If you have more, I'd like to see it.

I agree with scannerjohn. The negative/vacuum swing of the needle is due to a misfire. During the intake stroke, the cylinder is in a state of negative pressure when the valves close, and the compression stroke begins. When the cylinder fires, the explosion increases the pressure and forces the piston down. When the exhaust valve opens just before BDC, there is still a positive pressure, due to the explosion. However, when a cylinder misfires, there is no explosion. The piston is not FORCED down, it is DRAGGED down by the crankshaft. The piston has simply compressed, and decompressed, the contents of the cylinder. When it reaches BDC, it has returned to the state of negative pressure that it began the compression stroke with. When the exhaust valve opens, the exhaust steam is exposed to this negative pressure. That is why the needle swings negative. That is, also, why a dollar bill will be sucked into the tailpipe during a misfire.

Since  exhaust back-pressure looks good, I would take a close look at the Mass Air Flow, O2 sensors, accelerator and throttle position, Temp Sensors...
I, still, think a Volumetric Efficiency test would be a good idea, too.

BY chance have you checked your air filter? Long shot but worth a look.

Does this do this only on hills which could be indicative of a possible wire rubbing, something getting more wet, soaked in fluid, etc?
.
Fuel tank level? Thinking maybe could be picking up a little air if all fuel isn't getting sucked up properly?

I'm sure there's more. I wouldn't throw the parts canon at it though.

Going to lay down and take a nap but could lookup any test procedures you may need later if you need to know what wires to test for what part. LMK..

I'm thinking that it does it on hills mainly because that's the only time its under a load for a period of time, I have replaced the air filter along with my other parts " I hate to admit that" it's interesting how well it runs under normal city driving and on the freeway, the only time it may through a P300 on the freeway is if when getting on the freeway I stand hard on the throttle and don't let up until I reach around 70+.  As a side note, if when pulling the hill and I keep it out of overdrive and keep the speed around 40 MPH it seems to have no noticeable issues.

Here is the capture I got when I got to the top of the hill and the engin would barely run, shaking like it was running out of gas, and got the P300

 
 

Your O2's are dead lean. B2S1 @.035mv is dead and B1S1 @.125mv is barely alive. You can't have a rich condition and a lean command with 02 voltages dead lean. If you're at WOT the 02's go full rich 800mv by default to give you power. Being that lean will set the PO300 misfire. This is why I was asking for baseline O2 data to see what your 02's are doing.

601 RPM's... I believe, we are looking at de-celleration fuel cut-off? 

9.41 g/s @ 601 RPM's ?

He says his capture is when engine is barely running when he reached the top. Engine should be full rich if he is still WOT to keep going? I did not take it as he stopped.

Thank you for pointing that out, scannerjohn. I missed that. I, still, don't like 9.41 g/s @ 601 RPM's. It would be good to see scan data graphed during the whole hill climb, and before.

Just throwing guesses out there but it appears you're in California? IF so is this vehicle already smog compliant, are you trying to get it smog compliant or does any of this not matter?

PCV valve, fuel filter?

I 600rpm I saw it said as you topped the hill. Is that as in you topped the hill, pulled off to the side and got the reading on the side of the road or was it a punch of a button as you were creating the hill while still driving as those 2 things will make a difference especially in the grams per second reading.

No air leaks, venting issues or anything like that or does that need to be tested for?

Almost acts as if it's either sucking air or starving for fuel which both can cause these issues.

Is this the TBI injection system? Maybe I missed that part.

Yes those readings were when I had just got to the top of the hill and let off the throttle, because it was missing and shaking so bad, I have replaced the fuel filter and the PCV valve, I even monitored the fuel pressure when going up the hill and it stayed around 55-57 PSI. Its fuel injection but does have the stock 1998 throttle body on it.
At an idle it has about 22 inch of vacuum not sure what it has when it starts missing.

Dino

Here is the captured data as I started up the hill, before it starts missing, I did track it all the way up the hill but I am not sure I can extract the entire data file to upload here.

 
 

So this morning I got on the freeway and doing just over 70 it threw code P300 even though the truck seemed to be running smooth, here is the data the ECM recoded when it took a snap shot  when it got the code.
 

Thanks for everyones input!
I really need to figured this thing out...!
Dino

Yes I am in So Cal. the last smog test I did passed with no issues, I am trying to solve this issue because I do drive the truck every day to work, the truck runs smooth and powerful until I put a load on it, then it slowly starts with a light miss, then as I continue up the hill the engine miss progressively gets MUCH worse, but all I have to do is let off the throttle for about 10-15 seconds, then the engin is back to smooth and powerful, then I then normally reach the top of the hill and I then can can continue driving to work like nothing happened. "But the rest of the way is flat roads. "Until I come home" and of course it does it again going back up the hill. I found IF I pull the trans down into 3rd gear and keep my speed around 40 or below it runs just fine up the hill.  

Dino

I see two things that I think are problems. I believe your fuel pressure should be 60-66. Check your fuel pressure regulator as well.
Second is the MAF readings. As Chad previously pointed out. Your MAF at idle is to high. Should be around 5.5 to 6 g/s. I notice that the readings go down with increase in RPM. As you replaced the MAF it could be bad. Check your MAF for powers and ground and 5v reference. Unless you have a scope, you're not going to be able to see if it's producing a good signal. Try unplugging the MAF. GM will run a default when unplugged. See if you get improvement.
LTFT's tell alot. Idle threw WOT.