Hesitation and low power. I have done EVERYTHING!

Discussion in 'General Chevy & GM Tech Questions' started by moogvo, Aug 22, 2012.

  1. moogvo

    moogvo Moderator

    Ok, so I have a 1995 K1500 5-speed, 4.3 V6. It has low power, particularly in 3rd and 4th gears. It is somewhat noticeable in 2nd gear from time to time.

    So far, I have replaced the following: fuel pump, fuel pressure regulator, dismantled and cleaned the TBI. Fuel filter, plugs, wires, cap, button, knock sensor, coolant temp sensor, distributor, cleaned all electrical contacts, checked TPS on a scope.

    It doesn't matter if it is in closed loop mode or not. It is MUCH more noticeable when running the A/C. I also notice the presence of gas on the inside of the air cleaner lid. I have replaced the MAP sensor, EGR valve, EGR solenoid, coil...

    I have no check engine light warnings. I am totally stumped on this one.
  2. Big_Mike

    Big_Mike New Member

    Before I attempt to give any advice, let me ask... How many miles does the engine have on it?
  3. moogvo

    moogvo Moderator

    177k miles.
  4. Big_Mike

    Big_Mike New Member

    Let me just say, I'm no master mechanic, but I know a few things about automobiles... I'll try to give some decent advice. Remember that the internal combustion needs three things to run optimally: spark, compression/air, and fuel. A problem with one will cause poor operation (just put that out there for the record). I am wondering if there is adequate fuel pressure. I read that you replaced the regulator, but without enough pressure being supplied, the engine could perform poorly. You may want to check that to be sure (restrictions in fuel lines like dirt can be culprits). Next is compression as a possibility. Not enough can cause operating issues as well. It sounds like you went over the ignition system fairly well, but just to make sure, did you check/set the plug gap to spec? Hope that helps... Maybe it will give you something to think about.
  5. Big_Mike

    Big_Mike New Member

  6. moogvo

    moogvo Moderator

    Thanks mike. I have a new fuel pump on there and the pressure is within spec... 13 to 15 psi. (remember that this is a TBI unit). I checked the lines while installing the pump and they were clear of obstruction.

    I have not done a compression check, only because the problem is intermittent. I am beginning to suspect the cat.

    Thanks for the ideas. Please don't give up!
  7. MTM

    MTM New Member

    Does yours have a MAF sensor?
  8. moogvo

    moogvo Moderator

    No. It has MAP sensor... A nice, shiny NEW one, I might add. :money:
  9. MTM

    MTM New Member

    I didn't think so. Well I had a similar problem with mine where it just had no go what so ever and it ended up being the mass air flow sensor. Only other thing i could think of would be a plugged cat.....or slightly plugged cat
  10. Blazer454

    Blazer454 New Member

    So when you say "low power" what does it feel like? Missfire, engine tries to die, or engine seems to be struggling to move along? You say it is more noticable in higher gears, is this an auto tranny?
  11. moogvo

    moogvo Moderator

    Imagine what it might feel like if the power wire to the ignition coil were dirty and not making a good connection. Imagine the voltage dropping off under load. I cant exactly say that it feels like a misfire. It feels like a weak spark condition in that it fires, but not with all of the "oomph" that it should. I have advanced the base timing to about 4*, which has helped a bit but has cut some of my take off power.

    It is a 5 speed manual gearbox.
  12. Blazer454

    Blazer454 New Member

    So the motor struggles a lot almost as if you were towing a HUGE load? I see you have changed almost everything related to fuel and spark. Have you watched the fuel injectors in the tbi unit spray fuel while it is running? I would make sure that they arent gummed up. If you have access to an exhaust backpressure gauge you can take out the upstream O2 and check your back pressure before the cat to see if it is too high. Or just cut out the cat. Since its a 95 with tbi this exhaust only has the upstream O2's that are before the cat right? If so then you dont actually need a cat
  13. moogvo

    moogvo Moderator

    I took the downpipes and cat off today to see if that was a problem. Turns out that the cat has already been hollowed out by a previous owner. I can see from the rear of the pipe all the way to the o2 sensor.

    All was not lost, though; I needed to replace the donut seals at the manifold anyway. The injectors seem to be spraying fine, although I know that you really can't tell by looking at them. I am going to disconnect them one at a time to see if the truck acts differently using one of them or the other.

    Yes, it feels like there is a big load I am trying to pull along. A friend of mine says "Dude... It's not a 350. I understand that it is not a 350, and I realistically don't expect V8 power... So this isn't a case of unrealistic expectations...

    Computer maybe?!

    Also, the inside of the exhaust pipe is heavily blackened and sooty. The truck runs rich (or has incomplete combustion maybe). I know this already because I can smell it. I just confirmed it today. Maybe another clue.
    Last edited: Aug 27, 2012
  14. Big_Mike

    Big_Mike New Member

    Yeah buddy, no problem man! Hope you can get something figured out. It's a bummer having vehicle trouble... Diagnosis can often prove to be difficult. Best of luck man.
  15. Crawdaddy

    Crawdaddy Moderator Staff Member

    Have you considered that it might be something later on in your drivetrain? Perhaps you're having transmission issues or something that's putting excess load on the engine.
  16. moogvo

    moogvo Moderator

    No... I will have to sit on this one and scratch my head... I have considered that it misses Craigslist and wants to be on there again...
  17. dpeter

    dpeter New Member

    Is the air intake unobstructed and a fresh filter in place? If it was carbureted I'd say look for a stuck choke.
  18. moogvo

    moogvo Moderator

    Clean filter and no obstructions.

    Thanks for chiming in. :)
  19. My4xburbanrocks

    My4xburbanrocks New Member

    If the cats were hollowed out did the previous owner re tune ? Exhaust changes such as a hollowed out cat can change your a/f mapping. Could be running to rich in certain areas of the power band.
  20. moogvo

    moogvo Moderator

    Well... it would have been doing that when I bought it if that was the case. This popped up a few months later. I have now replaced the distributor with a brand new unit. It didn't fix anything really, but when I move the timing around a bit, it seems to have an effect on the problem. If I turn it up to where it spark knocks and then turn it back until it quits, the mid range power and the hesitation feel goes away but I then lose take off power and have to ride the clutch to get going.

    I have found a happy middle point with the base timing that seems to give me good take off power and minimizes the low power issue in the mid range, although I can still feel it bucking a bit. First and second gear, it runs like a raped ape. 3rd and 4th it does accelerate fine, but that's when the bucking begins. I can hold my speed up and even accelerate in some cases on small inclines without shifting out of 5th gear.

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