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'82 Chevy S10 p'up 1.9L 4cyl Isuzu 4 speed 2bbl ported vacuum advance 2WD 140K miles (very basic)

A month ago I did a valve job on this engine. It had breached the headgasket between cylinders 1&2 and burned intake valves 2&3.  The machine shop said they took off 12 thou of the aluminum head but all else looked good.   This was the second valve job on this truck, the first was done about 60K miles ago. That one was done by a local mechanic so I don't have details.

Since the valve job, the engine has a low load (very low)medium and high speed knock.  At medium speeds, the I hear the knock only when I feather the gas. Any sort of load at medium speed and the knock goes away. But it persists at high speeds. I re-checked the timing and it's dead-on factory spec at 6BTDC.  If I disconnect the vacuum advance the knock is audible at high RPM only.  There is no knock when the engine is lugging with or without vacuum advance.  I vacuum checked the EGR (pulled vacuum at idle - engine idle deteriorated to the point of stalling) and it looks as it is not blocked.  Checked "smoothness" of distributor mechanical and vacuum advance and they operate smoothly. Checked compression and appears to be OK at 110-120 on all four. Checked the timing chain and there is very little slack (feels like latch is between teeth on tensioner)Otherwise, engine is quite, idles well, power is OK.

I was thinking that maybe I skipped a tooth or two on the timing chain when I put the head together (SOHC). I checked the TDC mark against intake and exhaust valves just closing/opening on #4 cylinder method and valve timing appeared to be OK.  But I went ahead and retarded the valves by 2 teeth anyway. On checking with TDC and valve timing, it looked like the #4 exhaust was closing a little after TDC. I lost some pick-up-and-go, but the knock seemed to go away except after 75mph when it started but very lightly. I opened the valve cover again and put the chain on the original setting.  The knock came back.

As long as I keep the vacuum advance disconnected and the speed below 65mph, engine does not knock. With vacuum advance connected, I can get it to knock even at low speeds (35+mph) as long as I keep load off the engine (feather the gas like in cruising).

All this, forces me to think that it's a valve timing issue. But without dismantling the top of the engine and the front (including oil pan) I can't really check it for certain.

Anyone out there have experienced the same?  Any suggestions?  HELP!!!!

bulltoad  ???

Hello John,

It's driving me up the wall.  I have tried Sunoco 94 without much reduction in high speed knock.  It happens under very light load cruising conditions with the vac advance connected.  At medium speeds the knock goes away when I lay on the gas and load up the engine. I don't get knock going up the hill!  I can summarize the problem as follows:

Vacuum advance connected
low mechanical advance + high vacuum (low speed cruise)  = no knock
medium mechanical advance + high vacuum (medium speed cruising) = knock
high mechanical advance + high vacuum  (high speed cruise) = severe knock
high mecahanical advance + low vacuum (high speed load) = knock

Vacuum advance disconnected
low mechanical advance (low speed cruise)  = no knock
medium mechanical (medium speed cruising) = no knock
high mechanical advance (high speed cruise) = some knock
high mecahanical advance (high speed load) = some knock

Factory compression ratio for this engine is 8.4:1 .  Even if 30 thou total was taken off the head, it sould still be below 10:1 no?  From all the calculations, I agree with you that it's probably compression related.  The cylinders are between 110-120psi.  If I only knew where to find the factory compression it would help to check.

I'm running without vac-advance now.

I guess plugs are cheap and I could try a lower temperature plug.  This is a 49 state engine.  Would there be any type of sensor that would cause this?

bulltoad

John,

You make very valid points about age and costs.  Sometimes we are driven by emotion rather than good common sense.  Also, by the challenge!

There are two ported vacuum sources on the carb. One for the advance the other for the EGR. I followed the diagrams and they are plugged in the correct places. Just out of curiocity I reversed them and the engine went down hill. The EGR port has vacuum coming in later than the advance port.

Through ALLDATA I checked the factory compression limits. They listed as 119-170psi with all cylinders to be within 8.5psi from ach other.  I'm running on the low end of this at 115psi or so.  When I retarded valve timing by two teeth, compression was up near 170psi.  But decided to bring it forward again because the engine was knocking badly.  It's why I thought it may be related to valve timing. But to check it correctly I have to dismantle the top and front ends.

At this point, it has become a mental challenge for me. I don't really want to dismantle the engine and I can live with what I have.  I can change valve timing rather easily by removing the valve cover and also being careful to check for interference when I do this.  My question at this point is, which way to skip teeth on the sprocket and theoretically, what effect that would have on the "knock".

I tried retarding valve timing by moving the link toward engine rotation (taking up chain slack from the tensioner side) and hand rotating the engine to bring the chain taught again. The knock was reduced but didn't go away.  Would going the other way reduce it or intensify it?

I mean theoretically, if an engine's valve timing is correct. Would retarding or advancing valve timing increase knock?

bulltoad

P.S. I did check the link you posted.  There is a shop around the corner from me, but I don't really want to waste their time because as you pointed out, the costs outweigh the benefits.

John,

I really appreciate your input on this.  Another question I have is on piston slap.  Would piston slap necessarily result in excessive oil consumption?  I've run this engine for 3K miles now and it has not burned any oil.

bulltoad

John,

Took your advice and had Damon in Bridgewater NJ listen to the "knock". His conclusion was that it was probably a wrist pin and not knock.  Key was that we could make the engine make the noise sitting in the shop at idle and reving it up and holding it there with no load. Thanks. Saved alot of time and busted knuckles. I'll ride it out as is.

bulltoad