Engine knocks

egarcia528

I have a 96 2WD Suburban with 72K miles on it. It has been well maintained and driven very carefully. Even though Chevy claims that it does not need a tune-up before 100K miles, I brought it in for a tune-up because it was feeling sluggish. My mechanic told me that the engine "knocks" and he thinks that the engine is damaged and needs an overhaul! This mechanic is a trusted friend and wouldn't suggest anything just for the money. Does anyone have any suggestions?

rayt2

Go to this website, http://www.angelfire.com/tx5/gmpistonslap/ to learn all about the notorious "knocking" Chevy's. Also try the pick-ups discussion board, there is a topic there (GM Engine Knock)dedicated to the enging knock issue. The Piston Slap website has a lot of info on this and may explain your woes. GM says the knock is normal and causes no damage.

Ray T.

zueslewis

That site pertains to the '99 and up 4.8, 5.3 and 6.0. The '96 Suburban has a 350 and shouldn't be knocking, tapping or making any other percussion noises.

obyone

I had posted this in answer to egarcia528's post on the GM knock topic. There is a tsb for the knock.

99-06-01-003 APR 99 Engine Bearing Knock Noise

zueslewis

V-8s is different than piston slap. It's sounds like the Suburban just needs TLC or engine rebuild/replacement.

rayt2

Sounds like it was rode hard and not well maintained then if you have a knock at 72k, that's not a lot of miles for an engine nowadays.
I know the piston slap site addresses present day Chevy Knockers but it also touched on some of the older engines too. Either way GM's response was "it's normal".

Ray T.

swschrad

then Chevy should never be able to sell a car again in this country, as the public needs to rise up and demand better.

if the dealer and zone are just avoiding saying, "this car was beat to hell and we sure ain't paying for that < ozzy > and his problems," then it's understandable.

you buy used, you can't tell which is which. I suspect we all know which is which on this forum, but no evidence, no conviction........

faireyfly70

will a 351 egine bolt up to a transmission that has been bolted up to a 302

zueslewis

the bellhousing from a 351, it shouldn't be a problem.

a_l_hubcaps

My '86 Pontiac Parisienne (305 V8, carburetor, 48,000 miles) has had a spark knock problem for the entire time I have owned it; 2.5 years now. It only happens when going uphill and accelerating at 35-40 mph. If I leave the trans in D rather than OD when driving below 50 mph (of course, I always put it in OD on the highway), it doesn't knock nearly as badly. I switched from 87 to 89 octane fuel, but that doesn't seem to have fixed the problem. What is the most likely cause of knocking on this particular engine? I'm afraid this problem will eventually destroy the engine, which would be a shame because it's a low mileage engine that doesn't burn or leak oil, so perhaps I should do something about it before it turns into a $2000 problem.

-Andrew L

zueslewis

isn't advanced. It should be set by the tune-up specs on your engine compartment sticker. If the timing is advanced, it'll ping like crazy, especially under load.

a_l_hubcaps

zueslewis-

I actually tried that this summer; my dad and I got a timing light and attempted to check the timing. However, we looked all over the place and we couldn't find the marks with which to check it. I know we were looking in approximately the right places, as we adjusted the timing on my brother's '77 Celica the same day, and we were able to find the marks on that car. My Chilton manual depicts a small hash-marked strip like a ruler as being in there somewhere, but I couldn't find anything that looked like that. If anyone knows where to find the timing marks on this engine, please enlighten me.

-Andrew L

0patience

The timing mark is on the crankshaft harmonic balancer. It is a single cut in the balancer.

There is a timing tab. Look for the small tube just above the balancer, the timing tab is to the left of it. May need to clean it to see it.

Looks like this.

timing procedure

Set timing at slow idle and Before Top Dead Center, unless otherwise specified.

Disconnect and plug distributor vacuum hose at distributor.

1981 4.3L (265), 4.9L (301), ground distributor bypass pigtail (blue connector).

1982-88 1.8L (112) FI, 2.0L (122) OHC, 2.5L (151) FI, 3.8L (229), ground diagnostic test terminal under dash.

1983-86 2.0L (122) FI, 2.8L (173) FI, 5.0L (305) CFI, disconnect bypass connector (tan/black wire).

1987-89 All FI W/distributors, disconnect timing bypass connector, single lead in every harness between distributor and ECM.

1981-89 Others, carburetted, disconnect 4-wire connector at distributor.

Timing MUST be set to specifications on emission label if different from setting listed. If "Check Engine" light comes on during procedure, remove ECM fuse to clear.

jc1973

dont hook up a 351 engine those engines are crap especially the windsors

a_l_hubcaps

0patience-

Thanks for the info. I'll have to take another look at it. I'm at college right now so I don't have access to the equipment, but I'll try setting the timing again next time I go home.

-Andrew L

Mr_Shiftright

Alcan, how is timing advanced on this vehicle? Distributor? If so, that's where I'd look next after you check the timing?

obyone

who's Alcan?

Mr_Shiftright

A very helpful gent who visits us often and provides tecnical suggestions.

But if anyone else knows, great. I just don't have the books on this car and I don't recall how timing is controlled.

a_l_hubcaps

Shiftright-

I have the generic Chilton manual for 77-90 Buick Oldsmobile Pontiac full-size, which is not a terribly good book (I really should get an official shop manual). But anyway, the directions in there state the following:

"Using a timing light or meter, set the timing at the specified RPM by loosening the distributor hold-down clamp and rotating the distributor until the specified timing is obtained at the timing marks on the crankshaft pulley."

I'm not exactly sure how to do that, but I'll figure that out once I've figured out how to check the timing in the first place :-)

-Andrew L

swschrad

there are two half-inch hex head bolts, if I remember the size correctly, that hold a plate that surrounds the distributor shaft. you loosen one just a titch, and loosen the other one a titch plus maybe a half turn, and then,

WATCH OUT FOR BELTS AND MOVING PARTS !!!!

you can turn the distributor shaft, housing, and cap assembly while the engine is running. I would advise tightening up the back bolt to just a titch loose when starting, then crank it back until you can just turn the distributor system, while shooting the timing light at the mark. with the appropriate wires yanked and vacuum lines plugged, set the distributor at measured idle = book value (adjust the carb idle screw first if need be) with no accessories on and the a/c disabled. then tighten up the back bolt again and see if things shifted... if so, loosen slightly and reset, then don't wiggle the distributor this time when tightening the back screw. if it stays in place, torque both of them. if it stays in place, congratulations, and shut down to hook the rest of the geegaws up.

I am glad that I haven't had to adjust one with the distributor right up front, where access to the front of the two bolts that hold down that clamp is prevented by the bouncing fan belt over it. that means on such engines, you don't want to be putting your fingers anyplace near the front half of the distributor. might be a nice idea to chalk up the belt before starting the car to make a white danger line.

oh, yes, that third arm God gave you should come out for this one to hold the timing light :-D

Mr_Shiftright

Okay, if that's how you time it then the distributors internals are probably advancing and retarding the timing during acceleration. So if the car is timed correctly and it still pings, the timing advance could be screwed up. Having exhausted all alternatives, a rebuilt distributor is not a bad idea on any older car and not terribly expensive.

I hate to suggest guessing of course, and you should be able to check advance by revving the engine with the timing light applied; however, I don't know the specs of how much advance you should see. What you would look for, of course, is TOO MUCH advance curve.

More modern cars can't really vary their timing except from internal wear, which is also a possibility on your car with a sloppy timing chain.

a_l_hubcaps

swschrad and Shiftright-

Thanks for all the info. I plan to do the timing check/adjustment when I'm home for a week in October, so I'll let you know how it goes.

-Andrew L

swschrad

I had a time way back with my 76 buick, couldn't find out why it would die on being put into gear. until the paycheck came in, rigged a ton of light angle aluminum to hold the choke plate from either going rich or lean, held it in the middle of its range, until I figured the issue out.

turned out the distributor advance motor, vacuum pulloff operation, was shot. couldn't verify it by taking the hose off, pushing the operator in, and covering the hose connection and seeing if it stayed in. finally made a magic-marker band at engine-off position, and diagnosed it by discovering nothing I could do with vacuum made any change. bought a new one on payday, somehow avoided dropping the two short-barrel screws on the distributor baseplate into the works during replacement, and fixed 'er right up.

if I had dropped one or both of the screws that held that vacuum pulloff into the distributor, it would have been Very Bad News, and I would have had to pull the distributor and shake 'em out, or tear it down to get to them. be so advised that you don't want to screw that up.

so if you're statically-timed and it didn't matter, check the pulloff if so equipped. if not so equipped, it's probably dependent on the engine computer operationally delaying the spark impulse, and that could be caused by failure of several different sensors... but there should be a $$$ light lit, and codes to be pulled, on newfangled modern engines.

but for your late 70s GM, I betcha that distributor pulloff is wayoff.