99 Sierra 4.8 driveability issues no codes

[Dogluvr]

It hesitates a little on almost every take off from a stop. Much more pronounced if I throttle it more. I.E. it is impossible to just launch out from a light most of the time.  It's just sluggish until it kicks in after a second. The worst is around 45-50 in OD under partial load just trying to maintain speed... It misses enough to shake the whole truck and I have to punch it to get out of OD so I don't feel it anymore. I'm sure it's missing a little at other times but I can't feel near as much if the torque clutch is not engaged. I got a new MAF free from my neighbor so I threw it on today but it made no difference. My long term fuel trims are near zero at idle but jump up near 20 under acceleration and while driving they maintain around 15 or more. Do I just need new injectors? I have gotten a lean code both banks before but it's been a few months since that came up. New fuel pump only about a month old. Made no difference at all. I also did intake manifold gaskets this year. No change really. Need more ideas. Thanks!! 

[jrow8162]

Did you clean the throttle body when you changed the MAF? No stored codes when you scan it?

[riverbanks]

The lean code a while back,makes me think 02 sensors,be nice if you could watch them, see how high they are reaching,they do get lazy

[Dogluvr]

Thanks. No stored codes. I've had this truck 4 years and the only codes ever were the lean both banks. I smoked it earlier this year and found a slight leak at the intake gaskets so I changed them as well as the EGR pipe seal and throttle body gasket. Throttle was cleaned then (about to months ago). This issue is very consistent and has been happening for like a year I've just been dealing with it. Mileage is 290k by the way. Update on the fuel trims: I forgot I had the battery out just yesterday so my fuel trims were not accurate. Today they actually look good only peaking around 9. Mostly staying in the 0-5 range. I will be watching the O2's today. I took a couple screenshots of their percentages and the waves they are making. Maybe you guys can see something I don't. I'm still learning how to interpret this info myself!!  

 

To summarize, new MAF, no vacuum leaks, good fuel trims, fuel pressure stays steady at 47-49 under throttle, I've watched the TPS on the scanner and it is very responsive,  whatever the problem is, it's very very consistent. No codes. Never had a misfire code.  

 

In the photo, it's idling and then I rev it up to 2k and you can see it shoot up then level out. It's running at 2k that whole time after the peak. 

[riverbanks]

Fuel pressure too low,55 to 61 is normal

[Dogluvr]

Ok maybe the regulator then? I don't remember ever changing it. Thanks! I'll do it today and report back

[riverbanks]

Take the vacuum line off regulator and block line,see if it runs higher,that usually tells you who is culprit

[Dogluvr]

I was just coming out of the gym thinking that exact thing! I'm about to go home and do it now. Thanks bro

[Dogluvr]

Remove vacuum line, instantly goes to 55. Vacuum line back on drops to 47.  Problem is with the new FPR installed and vacuum line connected it still only runs at about 49.  No real change in driving. I feel a little more steady power cruising the highway esp. when I drove back with the FPR vacuum removed. But even then it still misses and pops under slight load while in OD. something else I just thought about. I'm running the smaller of the two batteries offered by Advance. They say the size 75 and 78 both work for this truck. How much can that affect driveability? System runs at 13.7 volts when I connect the scanner

[Dogluvr]

Before and after new FPR idling pressure 

[riverbanks]

Pump,sorry, but it ain't enough pressure,and battery, alternator does the work while in use, battery size is for different climates, just the start,I'm in upstate NY,I go for the higher cca,gets pretty cold up here,if I need beer,I need a good battery

[Dogluvr]

Even with the vacuum removed from the FPR, and pressure at 55, it still stumbles if I give it half throttle from stop. And it still misses when under partial load in OD.

[Dogluvr]

6 minutes ago, riverbanks said:

Pump,sorry, but it ain't enough pressure,and battery, alternator does the work while in use, battery size is for different climates, just the start,I'm in upstate NY,I go for the higher cca,gets pretty cold up here,if I need beer,I need a good battery

Yeah I figured about the battery. I'll have to watch the pressure while I drive. Maybe it's not keeping up. But it will shoot straight to 55 with key on and vacuum removed from FPR. 

[dtnel78]

Man your throwing the parts cannon at this thing. Back up and develop a approach of diagnostics you want to take for this. Before replacing any parts you want to check all parts. 

 

290k miles is quite a bit. What's the compression like? Spark plugs good? Checked the spark plug wires, coil conditions? I've had this same issue for awhile but only with 115 to 127k miles so far. Mine is the knock sensor or so the computer says. I have new knock sensors, ks wire harness to replace after I verify the ks's are bad. 

 

Whichever way you go I'd stop throwing money and spend some time verifying things such as compression, plugs, coils, wires, checking the O2 sensors on a bench with a propane torch and a multimeter. 

 

Have you pressure tested the cooling system?

 

Get a small oil drain  container and drain some oil into a rag to strain the oil and check for any bearing issues. 

 

Check resistance in injectors and make sure they're within spec. 

 

p.m. if you have any questions and I can see what I have for information to help you. Best information is to post year, make, model, mileage, engine size. IF you pm send the VIN# as I can get that information about the vehicle from the VIN# unless things have been changed. 

 

Time is money. Save your money and get a correct diagnosis. Getting a correct diagnosis allows you to save money and in the end you'll have more money for tools, diagnostic equipment, etc. Tool toys are fun once you get use to them. 

[Dogluvr]

Time IS money!! So I really appreciate you giving me some of yours for free. You're absolutely right, I need to run a few more tests before going forward. 

 

Good thing is I haven't spent any money except $40 on a discounted FPR. And the intake gaskets I did were from a lean code and a verified smoke machine test I did myself. The fuel pump was a warranty replacement earlier this year and that was only because I'm friends with the manager at Advance who gave me a Delphi in exchange for the Carquest I bought in a pinch back in '16 when it failed while I was out of town. 

 

But the point is I'm being lazy about it, you're right. I'll return with test results hopefully tonight if I have time.