Sluggish/hesitation, especially when cold

Discussion in 'GM Powertrain' started by TJbear, Jan 23, 2013.

  1. TJbear

    TJbear Rockstar 100 Posts

    2000 Sierra, 5.3L, 103K miles. Having an ongoing issue, driving me NUTS. Any tips appreciated.

    I replaced the fuel pump module with a Delphi unit due to a bad sending unit this past March and also replaced the intake/valley pan gaskets/knock sensors and harness, and also valve cover gaskets in April.......

    A few months ago, I started experiencing some light to moderate pinging on acceleration. Within a day or so, the pinging was gone. Perhaps some bad fuel or something. But I decided to run some sea foam through it anyways just in case. I had done this before with no ill effects. I also cleaned the MAF sensor with the proper cleaner at that time......

    Within a few days, the truck started behaving a bit strangely. Seemed a bit sluggish on acceleration, especially when warming up. Idle seemed maybe a little low. Shifting characteristics also changed. It seemed slow to downshift when going around a corner and was just acting wierd in general. Also seemed like the truck was slowing down faster when coasting. Thinking I had reinstalled the MAF wrong and/or neglected to tighten something down, I removed it, cleaned it again, and reset the computer. No change. Replaced fuel filter. No change. With the hesitation and crappy shifting, I was POSITIVE I had screwed the MAF up somehow when I had cleaned I bought a new one. No change of course.

    Thinking about the seafoam, I pulled all the plugs (AC Delco Iridiums), which only had 30K miles on them. They looked alright. Beginning to fall into the trap of throwing parts at it and after some more research, I decided to replace the front O2's, which were original anyways. No change. Bought new plugs. No change. :grrrrrr: At that point, I stopped the parts changing madness before it got out of hand and haven't done anything since then (couple months now).

    I also checked the fuel pressure. Was within spec for the bleed down test. Running at idle, it was at just over 50 IIRC, but the needle bounced around/vibrated when I would rev it up. Not sure if that matters or not. No fuel visible in the pressure regulator hose but I think I could smell a very slight trace of fuel odor when sniffing around the hose port. Again, not sure if that matters.

    It's totally driveable, never misfires or runs rough, no codes, fuel mileage unchanged. It's just a PIG. Certainly not the peppy truck I was used to. With the very cold weather recently, it feels like an old carb engine with a sticky choke when I drive it away in the morning. Colder weather definetlly makes it worse. I highly doubt its the intake gaskets I replaced. I've replaced intake gaskets on various vehicles over the years and never screwed it up. Feels like I'm hauling something all the time.

    I also notice a very slight hissing or whistle sound under moderate to heavy acceleration. I thought vacuum leak at first but it actually sounds more like air being forced OUT somewhere rather than in. Pehaps I partially clogged the cats when I did the seafoam? IDK, all these symptoms could be a total coincidence and not related to the seafoam.
  2. Skippy

    Skippy Member 2 Years 100 Posts

    Have you cleaned the throttle body, and replaced the PCV valve? You might simply have an air intake problem. Dirty throttle bodies can have a significant impact on throttle response.

    Here's an article I've written on how to clean the TB, if you're unfamiliar:

    Another thing I can think of would be your rear diff. When was the last time it was drained/filled? Water in the diff can cause all kinds of problems as things start to deteriorate. Poor power transfer is one of them. It'd feel sluggish, but it would be because the engine can't get the power to the back end properly. The same would go with a transmission service, though it's HIGHLY unlikely this is the problem unless you've got a really bad tranny. :)

    If you're catalytic converter is truly clogged, you might find that it's going to show up in the rear O2 sensor readings. If you've just replaced them, it's unlikely you've a problem, though a live-data scantool can show you what's going on.

    Personally, I'd start with the TB. Also check for a restriction in the path of the airflow INTO the air box. If you've got an obstruction, you'll get a whistle, and insufficient power. This assumes you've got a decent air filter in the box and it's not 100K miles old. :)


  3. TJbear

    TJbear Rockstar 100 Posts

    Thanks for replying.Sorry, forgot to mention a few other things I replaced/checked:

    Yep, cleaned the TB up when I did the intake gaskets.

    Replaced deteriorating PCV hose with a GM replacement that also came with a new fixed orifice PCV valve.

    The air filter is half a year old.

    Rear diff fluid is only a couple years old.

    Trans fluid is bright red and smells OK but I'd still like to change it at some point.

    I only replaced the front O2 sensors. The rears appear original.

    I will take the air intake apart again and check things out. Maybe I have a crack in one of the folds of the flexible tube portion.

  4. Skippy

    Skippy Member 2 Years 100 Posts

    Hmmm... Off the wall thought here. Do you have a MAF sensor PLUG problem? Changing the MAF sensor wouldn't alleviate that. You might consider grabbing a multimeter and checking each post. Unlikely, but possible the data is being corrupted at the electrical connection.

    The only other thing I can think of is another bad fuel pump. Recently, I had to do a fuel pump three times. The first two replacements were bad. The first one just didn't work after 2 days, the second it was 10+ months, and all of a sudden it didn't have power going up hills. On level ground, it wasn't a problem to get the fuel pressure readings that made it look normal, under any load though it was sluggish. Driving up 6% grades was not possible faster than 35mph (a 6.0L engine unloaded should have NO problem with that.. and hadn't had up until then). Basically, the fuel system just couldn't keep up under load. We spent forever looking at other things (plugs, wires, etc), before we came back to the fuel pump. Changed it out (under warranty still) and BINGO, the problem went away.

    Just a couple of more thoughts....

  5. TJbear

    TJbear Rockstar 100 Posts

    I did some more checking. Removed the air intake to check for any holes or cracks. Put it back on. The rubber boot where it connects to the throttle body seemed sorta floppy and stretched. I tightened the hell out of the hose clamps at both ends when I put the intake back on. Early in the evening, I drove it to school.......and it seemed like it had some pep again. The whistle noise under acceleration was gone. When I was almost there, it started to be a pig again and the noise was back. Same crap on the way home. Maybe I'm onto something here. Gonna pull the intake again and have a closer look. I also noticed the oil seems to be getting dark faster than normal the last few months.............maybe I need new rubber boots at both ends......
  6. Toymyster

    Toymyster Rockstar 100 Posts

    Also check your coolant and intake air temp sensor. If those are reading out of range, you will have issues. Sometimes they read out of range just enough to give you problems, but not enough to give you a code. Another bad fuel pump would not surprise me one bit. Hope this helps.
  7. Skippy

    Skippy Member 2 Years 100 Posts

    Major air intake leak after the MAF would most certainly create those conditions! Good find...
  8. TJbear

    TJbear Rockstar 100 Posts

    If I confirm this is indeed the problem, rather than buy a new boot I thought about perhaps just pulling the whole stock intake and upgrading to a CAI setup. I don't have any other mods and the exhaust is stock. Will it sound weird this way? Is there any benefit to doing this?
  9. TJbear

    TJbear Rockstar 100 Posts

    After some research, I ended up buying an Airaid MIT rather than just replace the rubber boot for the stock intake at the TB.............and now the damn thing is hesitating again. I tried loosening, and re-positioning all the connections but to no avail. Feels like a dog off the line now. WTF? The added sound is nice and it seems like it maybe has a little more power at higher RPMS.......but the low speed hesitation sucks. Any thoughts on this? About to send the damn thing back.
  10. Skippy

    Skippy Member 2 Years 100 Posts

    Not a whole bunch of benefit to a CAI unless your whole engine needs to breathe better. Without any other upgrades, you're probably looking at 1-2 HP at the rear wheels, which isn't generally felt, other than with an occasional increase in throttle response. Given your current experience, I'd first take a look to make sure your MAF sensor (MAF = "Mass Air Flow") is connected and cleaned. A filmy coating over the wire can result in an insulating effect on the MAF (it operates by reporting how much voltage is required to keep the wire at a given temperature, when air moves across it it lowers the temp, requiring an increase in the voltage and then the MAF knows air is increasing... that's the quick version anyway. coated wire can give inaccurate readings (both high and/or low) Make sure it's been cleaned with MAF sensor cleaner (you can get a can at any auto parts store), anything stronger and it's likely to be damaged. Don't use things like reak parts cleaner on it!). If you've actually got a broken sensor, this can account for your current failures, too. (Damage can and does occur when messing with it) Hopefully a broken MAF sensor is not the case.

    If you're sluggish on the response, it actually sounds like you're in a restricted state. Essentially not getting enough air (which is backward from what you'd expect from a CAI. Check your air filter and make sure you don't have something screwy going on like an unremoved film. Beyond that, did you clean your throttle body?

    Also as a quick and dirty check to ensure it's not a failed MAF... you could always replace the old filter setup and confirm things are better, then put the CAI back on and note if it gets worse again. If that's the case, it's definitely the CAI!


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