I have a almost completely stock 79 (L48) with about 95K on it. I put on about 2K in summers only and store it over the MN winters. About a month ago, I was taking part in a Fall Color Corvette Run when my engine started to knock pretty loudly and lost a lot of power when it was under any load over 3K RPMs. I had driven it over 200 miles earlier in the day when this started - pretty suddenly. About a half an hour prior to this, it "hiccuped" onnce or twice. I ended up limping home at about 40mph which seemed to keep the knocking from occuring. I haven't had a chance to take a look at it since then but I'm not much of a mechanic at all but am hoping it is something simple like a plug or spark plug wire that failed. If it's not that, I'm hoping something in the distributor/timing and not more serious involving a full tear-appart. Even for something like a vacumme advance line or distributor/rotor cap replacement - I'll probably have a professional diagnose and repair it.

My question is, what should I be looking for when I crawl under it? I plan to pull each plug to see if they're is a specific cylinder (or 2) that has a fouled plug which would lead me to believe it's a plug/wire/distributor issue. If they all look the same, could it be rotor cap/timing/vacumme advance? Any advice would be appreciated. Thanks in advance!

 

Drain the oil through a coffee filter and check for metal.

 

Bent pushrod?Broken rocker?

 

Good call. Remove the valve covers and look for any rockers off the valves, cracks in the rockers, broken springs, or bent push rods.

 

first, check oil for coolant, then check coolant for oil.

then use an heavy duty test light, one with an heavy gauge ground wire. then with eng. running. kill each cyclinder, by poking test light though the top of the plug wire at dist. cap. find out which cyclinder(s) is dead. you'll notice the drop of rpm's when an good is killed. no rpm drop on dead cyclinder. this will narrow down to which bank or each cyclinder is not firing.

also you can listen for the knock, use an long extenion, or screw driver. place the end on various places of the eng. listening for the knock. stay away from the pullies and fan(big boo boo might happen)

what was before, and after, oil pressure reading?

 

Thanks for the quick replies. The oil looks good and coolent is still very green. The oil pressure is still steady right where it always has been. When I change the oil this week, I'll drain it through a filter and check the inside of the oil filter to see if there are any metal fragements mixed in. HOPEFULLY there are none.

I will take a look at rockers and springs to see if I can see anything broken or cracked. I think with the articles and how-to's posted on the internet mixed with the help of this great message board, I could probably replace the rockers and springs. When I replaced the valve cover gaskets a couple years ago, they seemed to be original so if one were to crack, I wouldn't be surprised.

I'll be sure to post back with what i find. If the football games get off to a bad start, I'll probably move out to my garage and start playing detective.

 

I would NEVER poke a hole in a plug wire to test for spark!!! Why ruin a good wire????
If you need to check for spark,remove the plug wire from the distributer or plug a listen for a change.
For the OP,I think you lost a rocker,too!
Sly

 

If you have a temp gun on a commercial run out and shoot each exhaust port.If you have a plug or rocker issue one will show cooler than the others.
If you dont have a temp gun "spit" works pretty good.

 

Well, I removed the valve covers and checked things out. The rocker arms and springs and rods were all solid - not broken, and in tact. Everything seemed normal with no evidence of metal fragments on top of the heads and there was no damage to anything I could see. I'll be putting the car up and taking a look at the oil/filter after I change it and examine the spark plugs.

 

Could a plug wire be grounding itself to something in the engine compartment and causing a miss at high RPM. This may make it seem as though it were knocking.

 

Aren't ignition misfire situations harder to detect at higher RPMs than they are at lower RPMs? Seems I recall at lower RPMs they are more apparent and at higher RPMs everything evens out.

 

Can you describe the knocking???
Could be a rod knock or spark knock......see where I'm going with this.
Very difficult to diagnose sounds on the internet.
Sly

 

Hey
I am new to the corvette world but not to chevy's. I had a similar problem in my boat this summer everytime i got on the throttle it sounded like a piston was going to come through the head. I had good compression in all cylinders when i took the motor apart i found 2 lifters that were shot, the cam was fine. I replaced the lifters on #7 cylinder and was back to pulling skiers.

Hamilton

 

But he says that his valve train is intact.
Sly

 

Hi all, well I'm back on the site again after a long MN winter with an update from a post I started in October. I just had a mechanic take a look at it since I am definitely NOT a mechanic. He said his diagnosis is that the cam has gone flat.

Is this possible? fixable? and if so, what's a fair price to replace a cam shaft? He thought it might total $1K with parts & labor. I trust the guy but that seems kind of high in my opinion. I was thinking about selling it anyway and am wondering if I'll get my money out of this fix or not?

Thoughts? Suggestions?

 

1K does seem kinda high. I'd be curious as to how many hours he's estimating to do the job.

 

Is it possible? Yes.

Is it fixable? Yes, by replacing the cam.

But, my face went a little screwy on this one. I have been open about the fact that most of my engine internal experience is with VW engines which are solid lifter engines. Wiped cam lobes can happen in a solid lifter engine seemingly without warning, and for all kinds of reasons. If I was diagnosing a bug with this problem, and the rockers checked out (which they probably wouldn't), I would be thinking of checking the cam next.

However, the L-48 is a hydraulic lifter engine. It is my understanding (though again, not truly my area of expertise) that wiping a cam lobe with hydraulic lifters is highly unusual. That said, if this is a decent mechanic that you have used before, he has had the advantage of seeing the car and hearing the sound, so I would guess he knows better than I.

How much will it cost to fix? I dunno, I haven't priced any cams in at least 20 years, but I would think a mechanic would charge somewhere in the neighborhood of 4-6 hours to do the repair. That's a WAG though as I have never done that job myself.

 

Flat cam in a 79 is pretty common and 1k isnt bad. I doubt once he gets in there that he can keep it that low. Have him detail what he is going to do before he starts. He will most likely pull the engine.

 

how did he checked the cam did he remove any lifters ?? sound fishy

 

You can see a flat cam by just watching the rocker arms but the OP said when it stated the engine knocked.A bad cam doesnt knock unless he was referring to it popping back through the carb as a knock which a flat exhaust cam lobe will do.