To start, I know this question has been beat to death but of everything I've read here and elsewhere, the other issues and resolutions don't seem to add up. My apologies for that. Here's the scoop: 86 GMC C2500 LWB factory original with the original M Code 350 (A.I.R. removed) and QJet (say what you want but mine gets 17mpg hiway/12 town so zero complaints here...lol) Truck has been owned and driven daily the last 8 years by yours truly with no problems or so much as a rattle. 2 weeks ago I noticed a slight vibration, rpm specific but nothing major so I chalked it up to a bad load of fuel. Last week, hell came to visit. IN PARK, idle was perfect but as soon as I touch the pedal, heavy vibration. When under load, the vibration was even more pronounced. Swapped with a fresh set of R45TS plugs just to be on the safe side. No change and nothing other than perfect on the outgoing set. Noticed the balancer had a little dryness to the rubber so swapped that out as well. No change. Inspected flywheel for cracks, looseness, etc...nada. (Sidebar: I've noticed it repeated over and over that the 350 flywheel shouldn't have any weights on it, yet when I look at mine AND consulted with every other part supplier I could find including the bowtie house, the M code 350 CAME WITH A WEIGHTED PLATE and a weighted plate is shown to be direct replacement...how is that possible??? thus my confusion) Again, this truck has run perfectly for just shy of a decade so my doubts lie on the internally balanced argument as the weighted flexplate would have vibrated immediately if it were incorrect for this motor...correct? That said, checked TC bolts for correct torque and all was good. Dropped oil, inspected for glitter, and ran a magnet down the pan...you guessed it, nothing. Present condition: Very hard, very violent starting...once started, 25" vacuum stable, 700rpm stable/fast 1100, no vibration unless you tap the pedal...in drive headed to the highway, vibe present all the way through acceleration until you get to desired speed then all is well...repeat for getting on the highway but vibe disappears and runs smooth as silk at cruising speed. Putting up for the night, diesels like a Honda with turets then dies only to faintly backfire about 5 seconds later. Timing at 8 BTDC reset to 4 today as called for originally still no change. Good plugs, wires, cap, rotor, and coil... Hopefully that description helps...I'm at a loss. Besides maybe a bent rod/pin or broke skirt, what, short of a bent crank would cause such a selective vibration?? Thanks in advance for any and all suggestions and my apologies for the novel...just figured I'd save us all days of back and forths over all the usual data
Actually its the phrase, " very violent starting ", we are a very peaceful fun loving bunch, and quite startled by such aggressive wording and behavior.
You think you are at a loss ? Iam .....HUH.... Iam going to get drunk . "AND THE BOWTIE ROLLS ON" nuttin personal, it was a woman,not a chevy ,that drove me to drinkin
sounds like a ballancer is getting ready to let go and mess up your cooling system and other grill parts as well as maybe even a hood but its stock who cares Lol sorry look at the rubber on the ballancer
OK here ya go getting all touchy feely on me. Lets pretend that this engine is in a deuce roadster. All ya got to do it replace it wioth a hard charging hell bent for death Oldsmobile backed by a B&M hydrostick and you are good to go. or you may think about the flex plate.
Thanks traditions, I didn't word that correctly at all...that was intended as starting of the vehicle in the morning and is in no way an implication of me wanting to harm any of the great advisors/mechanics on this forum LMAO... plymouth, the balancer was changed a couple of days ago due to dry rot of the original rubber...all to no avail And before I am officially beheaded, I failed to notice that this forum was geared toward the earlier detroit iron so again, I apologize. Either way, a very perplexing situation the ol truck is in
Balance weights on the flexplate could be for "zero balance" but then again some knuckle dragger coulda put the wrong one in some time in the past, or grabbed the wrong one off of the shelf if you replaced it yourself.
thank you Mr. Lawless, and welcome to the HAMB sir, now lets all see if we can get you going. Just a little friendly fun as we are generally like 1962 and older here, but we will do our best to help. Again, Welcome. TR
Good point Highlander...and on that note, given the mul***ude of experience and things broken of the mechanics here, what is the possibility that the flexplate could be broken closer to the center and not in the vicinity of the TC bolts as one (ie, me) would suspect? I had a ****** vacuum actuator issue some years ago resulting in a hard shift to reverse unless you waited a few seconds coming out of neutral or park. Got me to thinking that maybe there was some undue stress during that time period that has finally reared its ugly head?
he's lucky. i asked a question about a 1987 frame i had and it got deleted. timing chain ever been changed? i had one coming apart it ran like **** till it qiut running.
sickboy...ya know, it hasn't...and I REALLY wish I'd have thought to go ahead and replace it when I was replacing the balancer LOL... my ADD strikes again
I had a Ford pickup years ago that I chased down every lead on and finally changed the engine(390 FE)The new engine shook just the same . Humm eventually found out it had thrown one fan blade. Hard to believe it never hit anything on the way out. Hope this helps.
SBC flex plates normally break in the center where they mount to the crank flange. You may also look for a va***m leak at least 75% of all vibration issues I have looked at were va***m issues. I did trouble shoot a 2nd gen camero for a friends kid back in the '90s that had developed a vibration issue. For whatever reason i decided to tap the balancer with a hammer and it went all crooked on me. Dropped the pan and the front main cap and discovered that the crank snout was snapped off from the crank. probably just cracked until I tapped it with the hammer. But that was a once in a lifetime thing.
I'm with pork-n-******, failure/cracks around the crank flange or bolts not torqued and lock tight not used. Also, check for thrust/main bearing worn. Watch for movement of crank, { walking }, for and aft while reving up engine and idling back down.
only thing that makes me want to second guess myself is hard starting. Is that what he means by violent starts?
Lawless, Pull your t/c cover and look real hard at the flex plate hub area. IF there is any rust, more then likely the plate is cracked. Other than that you do not have a "knock" noise that would indicate a worn pin hole or loose pin, worn rod bearing. You may have a bent rod, broken crankshaft (mostly on the 305) don't laugh, I had a Corvette owner drive in the shop with a broken crankshaft. The engine had a severe vibration and a wierd noise.
I agree with the aforementioned. I'll pull the flexplate and see what kind of stories it has to tell. Per the hammer treatment, I tried that too lol...now I don't feel so bad about doing so... the little angel in my head was screaming explitives the whole time I was tapping on it so I actually felt guilty for about .2 seconds lol I spray tested for vacuum and intake leaks but mainly concentrated on the carb phenolic spacer as Qjets are quite notorious for warping base plates. I'll test the vacuum again more thoroughly just to be sure...thanks for suggestion.
P-N-B, the O/P mentioned 25" vacuum and stable. If that flex plate is cracked and or loose, that could very well make for some violent cranking. He mentions " at the desired speed, all is well ", maybe thats when the converter locks up and pulls things tight and cancells the vibration.
VERY hard starts porkn******... ricks garage, you won't catch me laughing on that one...never would've believed it until I actually saw someone pull in with one too. Crank had broken diagonal on a rod so it was still able to spin lol...this isn't quite the same vibration that I remember noticing on that car tho. Being that the consensus a**** everyone seems to lean to the flexplate crack issue, I'm going to go ahead and pull it and have a new one on hand. I'll either have a new flexplate or a really cool new shop clock when the old one hits the ground lol
That sounds sensable. My beasty snapped the center out of the flex plate right away when I first got it running. It had a vibration that I chalked up to the build but I had to pull the ****** to replace a sof plug that it blew out of the back of the block and shazaaam there was the busted flex plate. It didn't vibrate at all on the highway just idleing and first pulling away.
That was my line of thinking. I've seen a couple of them that had the cracks right around the bolt circle and you couldn't see it by just pulling the cover and looking. Also, check the bellhousing bolts. they will create a similar vibration if they are a bit loose. At least the ones in the bellhousing of my daily driver 71 did. And did the new balancer match the old one? The truck may not be traditional but I bet it has hauled a part or two for a traditional ride.
To that note Traditions, I failed to mention that oil pressure is also stable and normal. Vibration + oil pressure loss is usually indicative of a broken crank also so I should have made note of that in my earlier post...
Dont know if this has been mentioned,,,,use a marker or chalk,& mark the relationship of flexplate & converter.Take all the flexplate to converter bolts out & start it up.(Slide the converter back!),,Give it a few revs & see what it does. Try bolting the converter & flexplate back up in a different spot. Dont ask me why,but I remember doing this a few times while in the trade,& it fixed a vibration problem. Cant figger out why it would just start vibrating out of the blue though. Good luck.
Mr48Chev...actually it has haulled quite a few...my company actually designs, fabs, and supplies custom parts for several rat/resto-mod/customs shops around this area. The balancer was a spot on for the old one and no I didn't check the trans bolts yet. I think yall have pretty well nailed down the flexplate possibility for me. Everything else I've read, heard, etc simply said "prolly the flexplate or sumthin"...which doesn't tell me enough to go tearing my truck apart. Again, my sincerest thanks for not only helping me identify the probable cause but also explaining what factors could have led to such as well. I'll definitely let y'all know what I find
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