Jump to content

99 Sierra 4.8 driveability issues no codes

Dogluvr

Recommended Posts

Dogluvr Enthusiast

Dogluvr

Posted
Posted

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 Enthusiast

jrow8162

Posted
Posted

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

riverbanks Senior Enthusiast

riverbanks

Posted
Posted

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 Enthusiast

Dogluvr

Posted
  • Author
Posted

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. 

D5E63E72-A48A-40FF-9437-957312194E21.jpeg

Dogluvr Enthusiast

Dogluvr

Posted
  • Author
Posted

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

riverbanks Senior Enthusiast

riverbanks

Posted
Posted

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

Dogluvr Enthusiast

Dogluvr

Posted
  • Author
Posted

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 Enthusiast

Dogluvr

Posted
  • Author
Posted

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

riverbanks Senior Enthusiast

riverbanks

Posted
Posted

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 Enthusiast

Dogluvr

Posted
  • Author
Posted

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 Enthusiast

Dogluvr

Posted
  • Author
Posted
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 Enthusiast

dtnel78

Posted
Posted

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 Enthusiast

Dogluvr

Posted
  • Author
Posted

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. 

Archived

This topic is now archived and is closed to further replies.

Go to topic listing

  • Latest Articles

  • Posts

    • KARNUT
      Trading 7.3 (Godzilla) for G.M.C gas

      By KARNUT · Posted

      My nephews 7.3 got noisy right around 100K miles. While replacing the engine he drove his old 6.0 Chevy that had over 200K miles on it. It an old reliable backup. I haven’t been around the old shop lately. I’m curious to find out if there using the new GM heavy duty gas.
    • Atlas
      Wow I'm in a pickle. (Broke down truck) and a story.

      By Atlas · Posted

      I don't know if you or anyone enjoys it or not but, why not share it. It's GM "truck" related and we're in the right place to discuss that 😉 I'm also working on some creative writing skills, I'm not quitting my day job to pursue that, either!   As I said, I've hit a wall because I'm not making any progress, and the progress is the fun part for me.   The idler pulley and EGR valve came in. Against better judgement, I went out there last night and put both of them in. I was able to get a nut on the back of the bolt on the side with the broken flange. There's enough flange still there that I can get that side of the EGR to snug down. It's not perfect, it's not "right", but this is the fix I have for now. If I decide to keep the rig, I talked to a guy locally who will sell me a complete intake for $100. If it's my truck, a broken flange like that would bug me too much. Yes, I'm crazy. But I embrace it.   Pulley on, EGR installed, started the truck. Great...still have a pulley squeaking like mad. That was the first disappointment. I thought I had pinpointed the problem. Before I replaced it, I sprayed the back of that pulley with PTFE and it would shut up for about 30 seconds. Is the squeak actually the belt? I'm frustrated.   SO then I decided why not disappoint myself twice, so I took it for a spin and sure enough, the CEL set for P0401 insufficient EGR flow again.   I had some extra time, so when I arrived home, I decided to ask AI about my EGR code. We had an interesting conversation.   I asked how to tell if my MAP sensor is bad and if that could be setting an EGR code since a bad MAP may not be sensing the requisite drop in manifold pressure when the EGR activates. I gave it the MAP value at idle and it told me to blip the throttle hard and watch the MAP reading. It said no, the values appear normal and it's reactive to pressure changes, and then instructed me to test voltage at the harness. I don't think it's the MAP sensor. Interesting, though, I gave it MAP reading values in inHg, and it thought I was using PSI, so I had to correct it. AI is a very dangerous tool if you aren't treating it like the machine that it is.   Then it walked me through testing the EGR and harness. It took me down an interesting path. It was referencing a gray wire and a white wire so I told it I can't tell the difference on my harness because it's dirty and old and both could be white or gray, I'm not sure. So it gave me a more accurate description of the ABCDE pins on the connector and started referencing pins instead of colors. Helpful.   The EGR harness has 12v power, and it also has 5v reference power from the PCM. What I'm unclear on is there's a ground wire and I thought AI had said that one of my tests revealed the ground in the harness was likely bad and it instructed me to wire a test ground. I got distracted and navigated away from the AI conversation, and when I went back and asked for clarification it said I must have misunderstood. (how rude- next time I'm saving the conversation!)   But I think it was onto something. I recall NOT getting 12v when I tested the power wire to the ground wire, but I did when I tested the power wire, grounding the voltmeter to the engine block. AI said no, the ground wire is actually for the PCM, that's how it commands the EGR on by grounding it and so the circuit should be open with the key on (i.e. EGR off). But I think there's also a static/overall ground that should always be grounded, and if that's bad, then the EGR isn't activating when the PCM commands it on, and thus setting the code.   There's a bunch of other tests it suggested, back-probing with the EGR plugged in and engine running. I need to get some time to focus, and not just tinkering after a long day.  
    • kylant
      Trading 7.3 (Godzilla) for G.M.C gas

      By kylant · Posted

      the 7.3 lifter issue is real, not few and far between.  my 6.6 gas does not use oil.  the 7.3 with 4.30 gears pulled hills better and felt better off the line. the 6.6 gets the job done though. mpg sucks
    • kylant
      Remote Start Noise 2024 2500 HD Gas

      By kylant · Posted

      are you talking about the loud whooshing, grinding type noise? mine does it too
    • Donstar
      Slate Electric Truck

      By Donstar · Posted

      I am a huge fan of the Slate.  (SUV for her, base pickup for me.)  Unfortunately, there are no plans to sell in Canada but maybe this will change.  I think if/when Ford offered a high range. EV Maverick, it would be my first choice.

  • GM-Trucks.com Clubs

  • Popular Contributors

×
×
  • Create New...