1994 GMC 5.7 1500 Strange Noise

Discussion in 'GMC Truck Forum' started by timw128, Oct 5, 2016.

  1. timw128

    timw128 Full Member

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    Hello-

    Well, just got my 4L60E repaired from the 'Limp Home' condition. No more than got that problem solved and another gremlin popped up.

    We took the truck for a test drive to validate trans repairs and all was well. Parked the truck for a bit and came back out and hour later, started it up, and this god awful loud squeal is going off under the hood. It diminished somewhat when blipping the throttle. It is NOT the alternator, water pump, etc. We took the truck for a drive and noticed that when opening the throttle it goes away until the vacuum starts rising again. So obviously, it's throttle/vacuum related.

    Anyone have any ideas as to what's going on, or may be going on?

    Thanks in advance, and if you need more info just holler.

    Regards-

    tim
     
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  2. gmctrucks

    gmctrucks Full Member

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    Check the tensioner pulley or air conditioner compressor or flywheel since it was at the Trans shop also if you take the belt off you can see if the noise stops or just listen closely.



    You could also use like a piece of PVC pipe or a stick from a broom and so on.
     
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  3. timw128

    timw128 Full Member

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    Thanks for the reply... it wasn't in the Trans shop. I do all of my own work.

    It's not in the engine accessory drive- checked all of that, even removing the serpentine belt, and running engine.

    Yes, we're aware of the acoustic properties of screwdrivers, dowels, pvc tubing, etc. We have a diagnostic stethoscope we use for that.

    Let me re-phrase from OP... the high pitched squeal is clearly vacuum related. It is worse at idle, when intake vacuum is high. When accelerating down the road, the noise all but goes away when opening the throttle. When vehicle speed catches up to engine rpm, and the vacuum rises again, the noise comes back. When the engine was first started this last time, it went into a fast idle state and gradually slowed back down to normal. It had started doing this even before the trouble started with the 4L60E.
    I've thought for a few months that I may have an IAC problem, because coming to a stop sign the vehicle is slow to return to normal idle once stopped, AND when cold starting the engine it SOMETIMES misses for a few seconds and then clears up. I've also suspected some anomalies with the TPS. Both the IAC valve and the TPS are original to the vehicle. The truck is 22 yrs. old and has 279K on the clock. I've taken good care of it- oil changes @ 3K, trans filter and fluid @ 50K, sparks plugs every year, cap and rotor every 2-3 yrs.

    I'm leaning toward the issue possibly being the IAC, maybe the EGR, or possibly the TBI base gasket. I do know that the intake gaskets are going to need replacing not too far down the road, although they are not the problem with this issue.

    Tomorrow, we'll pull the air cleaner and check the vacuum lines, pull the IAC and check for deposits, measure impedance at A-B and C-D terminals, and pintle offset. I do know that the TBI PVC hoses are notorious for developing leaks, too- especially on the GM vehicles with braided line. We'll disconnect battery before IAC diagnostics, and reset IAC if the resistance checks out on the DVOM.

    Regards-

    tim
     
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  4. MrMarty51

    MrMarty51 Full Member

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    try snugging down the bolts holding down the throttle body. If that seems to change the pitch then try installing a new gasket under the T~Body. Might just change it anyways.
    If You can have it squeeling, try spot shotting a little ether around where it seems to be coming from. You have a bad vacuum leak.
     
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  5. timw128

    timw128 Full Member

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    Yessir, I do!... The noise sounds like a dog whistle on steroids and it reverberates around the engine compartment so bad it's hard to tell where it's coming from, although it's definitely not mechanical. I'm going to run a couple of tests tomorro...errr!...later today. I'm out of ether, but have some other stuff that'll work, Marty. We'll see how the IAC diagnoses and if it's messed up we'll replace that and the TBI base gasket at the same time. If it's not, and is just a bit dirty, we'll replace the TBI gskt. The goal is to NOT make needless trips to the AP store. Parts chasin's NOT me forte! :rofl::rofl::rofl:

    My intake gaskets need a changin', as my plugs get a bit frosty...not too bad, though. It's the typical GM dissimilar intake/cyl head material issue. Dealt with it on other GM products. When they get up there in age and mileage the gasket surface on the intake manifold starts deteriorating at the coolant crossover ports. We fixed many of them with a decent mill bastard file and some Devcon aluminum epoxy. That stuff works like a charm!...it's not a head gskt. issue, and the compression is relatively good and even for the mileage, and there's no cross contamination in the oil.
    [​IMG]
    Can whack this stuff down a bit thinner with acetone, too. JB Weld works, too, and costs a helluva lot less!

    Thanks!

    tim
     
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  6. gmctrucks

    gmctrucks Full Member

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    Maybe check your intake gaskets with carb spray hopefully that will help find a vacuum leak for a easy fix maybe even check the hard plastic lines that pull vacuum and TBI gasket as well I wish you luck.

    It reminds me of a Firebird I had I changed out the cam and heads and intake and something else messed up that wasn't even messed with I think it was my battery or coil I'm not to sure it was a long....time ago.
     
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  7. MrMarty51

    MrMarty51 Full Member

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    Hi Tim.
    Yup, My 93 K1500 has 280,000 miles on it. I have replaced the intake gaskets twice. Book said to use sealer around the crossover coolant ports, caused it to leak every time. The last time I used no sealer, put it together with dry gaskets, used JB weld on the ate out areas and sanded them flat. maybe 20,000 miles on it since then and still no leak.
    The oil pressure had dropped at about 200,000 so I installed a timing chain/sprocket set and an oil pump, sealed up the leaking oil cooler hoses and it is still dry. The oil pressure went back to normal, about 55lbs at OP temp and cruising on the Hiway. about 40 to 45 lbs hot idle.
    The TBI gasket on it also started a real light squeal, when coasting at lower speeds. Replaced that gasket and it is all good for now.
     
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  8. timw128

    timw128 Full Member

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    Yeah, the only sealer I use where pressure, vacuum and hydraulics are present is this product from Mercury Marine...never an issue with it. It's chemical/oil resistant and is used for sealing 2 cycle outboard crankcases. Good stuff! The intake gskts. are going to have to wait until next spring- Hopefully!
    [​IMG]
    We'll pull the air cleaner and nose around a bit and spray around TBI base and intake gaskets, check bolts. New gasket's no big deal.

    I'm getting 40-45 psi going down the road hot, and 25-30 psi idle hot, in gear. On a cold morning a rod will lightly knock for a split second. I use a quart of Lucas Stabilizer every OC, too. I don't pay attention to the mileage at this point, just do OC twice a year as the truck doesn't see 2500 miles a year. Trying to find an engine in the bone yard to rebuild but SBC's have become about non existent.

    Well, was going to fiddle with it today, but weather's looking gnarly out, with rain-AGAIN!:rofl::rofl::rofl:, but really :mad:.

    Thanks for input, Marty!

    tim
     
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  9. timw128

    timw128 Full Member

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    UPDATE:

    Went out and started truck this morning. No dog whistle on steroids noise. Been thinking about this a bit and come to the conclusion that it's more than likely IAC related.
    Here's why- I had the battery disconnected during transmission diagnostics and repair. When you reconnect 12v reference to the ECM the IAC has to reset, or relearn. There's a sequence that has to be followed with the ignition on to reset the pintle against the seat- Ign. ON for 10 secs., Ign. OFF for 10 secs., Start engine and let idle for 5 mins., Eng. OFF for 30 secs. Restart and use as normal, noting idle quality. This is what I did and all seems fine. Apparently, the pintle had defaulted to the off seat position and stayed there and the manifold vacuum was constantly pulling air at a very high rate through a small source opening, causing dog whistle effect. I can not observe the cycle rate on the IAC without a decent scanner. There's also the possibly the cycle rate is not within spec and the pintle shaft is a bit dirty.
    So, with all that said, I think I'll pull the IAC, check the resistance with DVOM, clean it and go from there.

    Like I mentioned earlier, there have been classic signs that the IAC isn't function correctly with high idle at unexpected times. A non ported vacuum leak would not behave like that.

    Regards-

    tim
     
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  10. MrMarty51

    MrMarty51 Full Member

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    Sounds to Me like You got it nailed.
    On the O.P.
    A friends K2500, 5.7, His O.P. was at 15psi at 2000 rpmz. That is the bare minimum according to the book.
    When He`d start it cold, a slight knock.
    I had told Him that I have an oil pump and timing chain/sprocket set with less than 20 miles on them, Told Him I`d buy gaskets if He`d want to help install them. He had to be in Williston N.D; about 200 miles to the n.east. Before He got back to town here, the rod bearings shucked. Not good. He doesn`t have much money and now needs the engine rebuilt or a remanned enjun.
    He has an 02 Dodge Dakota with a wheel bearing out, We took My 2000 K3500 and the trailer and went to Broadus and hauled it back to town. His 92 K2500`ll set in Mine yard until He can get back to work and put together enough to get the enjun that He wants. He is a networking specialist and makes very good wages whilst He is working, Just got laid offt from the oil patch over at Williston is all. He`ll getertogether.
     
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