KFtransam

Hey guys, long time no post! Sold my lt1 and miss it!

Now I've got a 96 gmc k1500 and I can't find any answers!

Has an obnoxious vibration and I can't find the source. I'm hoping someone can help me diagnose this. It's only present starting at 2200rpm and ending at 2900.

I've looked at the fan/clutch, all is good there. I don't see any bouncing in the harmonic balancer.

I would imagine this could be stemming from the TC or flywheel, but I'm not really sure how to test that.

Are the flywheels weighted?

Any help is appreciated,
Thanks, Kyle

SS RRR

iTrader: (2)

Not sure what vortech you have, but found this thread in this very forum.

http://www.google.com/url?sa=t&rct=j...,d.cGU&cad=rja

KFtransam

Hmm.. Thank you. I've heard of this problem before and honestly considered making the switch from the notorious "central sequential injection" to gm's upgrade "multi-port"

Though, my hesitance being that the MPFI replacement assy. Is fairly pricey and until I can narrow down my options. I'm looking elsewhere.
Not to completely rule this out, but in my opinion if my 5.7 we're starving for fuel at 2200 rpm (causing the vibe) than it would only continue up the board. Not clear up at 3000 or so like it is..

I suppose, I could test the fuel pressure, though, all was good after replacing the fuel pressure reg. and pump less than 5k miles ago.

bowtienut

iTrader: (1)

Unless your engine is over ~200k miles, don't expect the MPFI upgrade to do any miracles for you. The stock system really doesn't display any shortcomings other than the typical syndromes of deposits on the injector nozzles that are typical of any injectors after hundreds of thousands of miles.

KFtransam

I'm on the fence about it. Heard a lot of mixed reviews...

But in the time I've owned this truck, I've replaced just about all the basics.

I bought it two years ago with bad intake gaskets. After replacing those, it's been a bit of a "step by step" dealing with a thousand neglected issues. I figured it's worth it at 130k miles.

The only ignition/fuel system parts I haven't replaced are the injectors and the plug wires.. Plug wires were already new delcos..

So tomorrow I'll be taking care of the cheaper fix

zraffz

iTrader: (1)

Just FYI, the flexplates are weighted in one spot. The flywheels don't physically have any weights on it but are drilled to certain depths in certain spots.

How is the rubber in the harmonic balancer?

KFtransam

Just finnished my plug wires, I guess the old ones have been in and out of there to many times... Deffunately brought some power back, but I still have this vibration..

Whether or not it's a fuel or spark issue, i dont know. There is a loss of power during the vibration, though it could be a physical vibe causing false knock..

I'm going to double check the rubber on the balancer now, not really sure how to check it aside from looking for physical damage or dry rot.

KFtransam

Well.. My cheap-o handheld scanner is showing the timing at -23 BTDC at an idle.. And -30 at 2200 where the vibe starts.. Nothing seems odd, no drastic changes in timing

KFtransam

Well.. I just threw my fuel pressure gauge on there, the pressure is dropping rapidly after it's primed before a start.

Ruling out the fpr, which I changed a few months back and was holding pressure after the change. I'm lead to blelieve it's a popit stuck open...

She starts and runs okay, holds 52psi while running, jumps to about 60-65 when you goose it. Which I think is normal. Which again leads to the problematic "central sequential injection" being the problem.

Looks like I'm just going to bite the bullet and spend the cash

guppymech

iTrader: (7)

KFtransam, Maybe just hold on a minute before you change out the spider. I've had a '98 C1500 since new and have had great luck keeping the fuel system clean by running a bottle of Seafoam through the tank twice a year. Maybe try some, it may clean out the popets. FYI, Menards sells Seafoam for less than 7 bucks a can.

Ed Wright

iTrader: (1)

If it is an injector issue your fuel trims should be screwed up. I'm not sure why you thought the timing would be an issue. A big split in fuel trims side-to-side would indicate an issue. Both sides equal, but too far from zero also indicates a problem. Like Optisparks, the "spider" injection deal gets blamed for too much.

KFtransam

So much to learn! Thanks guys, when I get home I'll throw the scanner back on it and Check fuel trims.

As far as the seafoam, I've tried that a few times, in fact earlier yesterday to no avail

Ed Wright

iTrader: (1)

Watch for misfires too. Nice thing about OBD2, if you have a misfire problem, it will even tell you which cylinder(s).

KFtransam

So I checked it out today, my fuel trims jump around a bit, but it seems as if they stay fairly close together from bank to bank. Ranging anywhere from 0.00 to 6.50 at an idle.


Unfortunately, my cheap scanner doesn't read missfire counts. Maybe I'll try pinching off my return line to completely rule out the fpr. I don't know what else it could be

Ed Wright

iTrader: (1)

A vibration and misfiring is not the really same thing. Can you not tell if it is missing by just listening?

KFtransam

That's just it, it's a vibration, not a missfire. I just don't know where to look. Now that I've found this problem with fuel pressure I'm inclined to believe that's the root of the problem.

Ed Wright

iTrader: (1)

The only way fuel pressure won't cause a vibration. Low enough it could cause a misfire.

BIGsmokey10

iTrader: (3)

has anyone ever tried running a vacuum line off the intake or cold air intake and tapping the distributor cap to vent the distributor? seems like getting corroded points on the cap is really prevalent on these motors.

KFtransam

I've never heard of anyone trying. But let me tell you, it's worth it to spring for the delco cap. The brass points hold up much longer

KFtransam

Okay, so I can consider the drop in fuel pressure a problem. But not a cause for vibration.

So, I'm back to physical imbalances?

I still have yet to take a look at the flex plate or TC.
Mostly because I don't know what to look for, aside from a missing weight, what's to see under there?

This has been a headache from day one