Hello,
I have a 1990 GMC Jimmy S15 4x4 with 4.3 vortec FI
Had it in for service for intake manifold gasket (coolant leak), transmission service, tuneup ( plugs, wires cap rotor). When I picked it up, ran horribly with no acceleration and hesitation. Brought it back and after some head scratching they were able to figure out the ignition timing. Runs fine but this is when I started hearing a knock.
No knocking when cold. When warm going down highway at 65 mph, no knock. Let off gas slightly, rather loud knock. Let up more and knock goes away. Slight knock when engine idles warm and pronounced when revved.
I checked ignition timing with EST unplugged and it's at 0 deg during idle warm. When revved, timing advances and knocking starts. Stethoscope seems to indicate bottom end or tranny. I put Lucas treatment and thicker oil. No difference. Brought it back to service place and was told (just by listening, no checks) spun rod bearing or such and engine needs overhaul. (Well I wouldn't have just put 2 grand into it if I knew that!). I think they were tired of dealing with it and there was zero knocking prior.
Any suggestions? Knock sensor? Only codes I pulled were o2 sensor and EST (from the bypass perhaps?).
Thanks.

 

start it and warm it up - put it in gear and hold the brake - if standard ease out on the clutch - if it's a rod usually noise will change - also you can pull one ing. wire off the dist. one at a time listen for a tone change then reinstall on dist. - if the rod is bad on that cylinder the knock should change in tone

 

Thanks odat. I will try this when I get home tomorrow.
I read where pulling wires one at a time is not good for EI, so I didn't do it.
Perhaps wishfull thinking hoping it's not rebuild time (170,000 miles) but I would think a knock wouldn't just happen suddenly and curiously after having work done on her.
Thanks

 

IMO you shouldnt hurt the EI by doing the plug wires I've done it for years with no ill effects however it will prob throw a miss fire code but that can be erased - the cheap a$$ bearings in the newer engines won't hold up to antifreeze anytime at all so it could possably be a bearing - we did a 1997 4.3 in a ls last fall, no matter what we did could not get the knock to change in tone, pulled the engine all 6 rod bearings bad and two mains too boot - installed a reground crank and bearings in her and she runs like a top - I think all in all we had under $400.00 invested in the lower end rebuild - what ever you do if you drop the oil pan don't do as the instructions say about installing the gaskets dry and do the old school way and a light coat of RTV - installed dry they will leak {don't ask me how I know}

 

OK to follow up on this issue, I warmed the engine and put truck in drive and held the brake (auto trans) as odat suggested. There is no discernable difference in the knocking when I rev holding the brake. Under load it will not knock and when I let up it will knock.
Then I pulled the plugs of the distributor one at a time. Other than a slight drop in rpm there was no difference UNTIL I pulled the #6 wire (right rear facing forward). When it was pulled there was a noticible increase in knocking, even at idle.
So, it is sounding to me I may have a spun bearing on #6. Would anybody like to chime in?
That being the case, I may just run it until it goes, then decide whether to do a long block swap. I have put much into the truck already and hate to just junk it. Just not sure of my mechanical abilities to do such. It needs new tires, a/c, paint, and then there is the transmission and transfer case which runs fine but has same number of miles, etc, etc.

 

Could somebody please comment on my last post?
It seems that my manual suggests that by doing the test of pulling one plug wire at a time, a spun bearing would be indicated by knocking that goes away when that wire is pulled. Instead, when I pull the no. 6 wire, the slight knocking heard at idle seems to be more pronounced. Does this indicate a possible spun bearing?
Thanks

 

did you find a solution? I have the same problem

 

Bearings and pistons and rods can do strange things. One thing for certain is that #6 cylinder has a big problem. I'm sorry but if you continue to drive it, it will not be long for this world and the engine will end up being complete junk. I would either fix it before it is driven more, or find another vehicle to drive.

FYI - Intake coolant leaks can get into the oil and take out bearings.

good luck