Stingydonut

So i have a 81 with the original motor in it. 85k miles (i know that doesn't matter when the car is 40 years old). so basically the car runs like crap. it shakes and seems to missfire. two mechanics told me it was missing. one thinks it might be the cam, the other thinks it might be either the cam or a stuck valve. a list of things ive done so far, new belts, distributor, plugs and wires, oil change, fixed all vac leaks, at least any major ones, i sprayed around with starter fluid to check for idle changes, so id assume its mostly free of leaks (though the headlights dont really lift up on their own, and when they do they are lazy they close fine though). there are no knocks or valve ticks heard throughout the rpm range. but at idle and low speed i get these little thuds, or skips every few seconds. it seems to go away when at cruising speed. i dont remember it doing this clunking thing as bad before my mechanic did the distributor and plugs and wires (though i dont remember for sure), my mechanic was testing the vacuum on the engine and said it was only pulling about 12-13 pounds max, an these motors should be getting about 15-16. i bought a tester to do a compression test. im going to do it wet and dry. if anyone can give me some numbers of what i should be expecting as far as normal that would be awesome. how would i diagnose it if it as valve issue or a cam issue (i tried the dollar bill test and i think it failed as it flaps back and fourth)? my mechanic said if i go change the cam on an old engine like this, theirs a good chance it starts burning oil. i def do not have enough money to swap the motor right now so im hoping this can be fixed without thousands of dollars spent. if the compression is good does that mean the issue will resolve with new heads/a cam swap? and would heads and a cam swap be within the realm of something a novice can pull off over this coming winter? thanks!!!

Kevova

If you can see it It's not missing. Find a real mechanic with an "Sun Scope" (oscilloscope). It should be a big deal to diagnose. 81 should have computer controlled carburetor is it still present and functional. Check engine light? Do you have factory manuals?

Stingydonut

thanks for the reply. yes it still has the factory carb. and once in a while the check engine will flash on. my mechanic also said i should consider replacing the carb too. i have the year specific manual for it yes.

Duane4238

Find a new mechanic who knows what he is doing. Most younger "TECHS" just like to replace parts.
Duane

Kevova

Codes? MAP sensor should be a plastic rectangular box opposed to a round pot metal type. Looking at plug wires in the dark may indicate condition of wires. If it looks like Christmas lights wires should be replaced. Condition of cap and rotor....last tune up was? Disconnect egr valve. Make sure valve isn't stuck open. Condition of coolant sensor connector ? Original round type used to deteriorate and were **** to shorting. Redesigned sensor and connector was the fix. What is the carb dwell? Section 6E is mostly about driveablity. Sub section B has symptoms. You could pull valve cover and check operation while engine idles valves should all be moving about the same. Worn cam valve isn't moving or not as much as the rest. 81 was pre scanner so DVOM and a couple jumper wires a point type dwell meter on 6 cylinder scale or a dvom with duty cycle is what you need.

derekderek

Mechanic did distributor? Computer controlled, or vacuum-centrifugal? That computer controlled carb-dist setup was okay...30 years ago. Does new distrib have a vacuum advance can on it? You would be better off ditching the whole setup for 80-older style no computer setup. Do compression test. Take off valve covers and crank engine watching rockers. Do all 16 move roughly the same amount?

Kerz

Some very good advice has been posted. I really like the ones about finding a good mechanic.
Compression check is always a first step in proper diagnostics. I don't believe there is a definite cylinder pressure, rather the numbers should be within ~10 % of each other. Usually compression tests are performed with all spark plugs removed and throttle wide open.
Vic

Bikespace

Crossed ignition wires? Do you have a digital timing light? Email Lars for his timing papers, follow them, and see if it improves. I can't say that it's not an ignition problem, but despite throwing parts at it, it still might be.

Like what derek derek said, a 1980 or earlier HEI dist with an adjustable or set vacuum can might solve it.

lars

You need to do some correct troubleshooting. You are not going about this correctly, and you're dealing with idiots for mechanics. If the engine has a miss, there is an obvious, reason for that, and you don't need a new engine or new heads because of a miss. The '81/'82 engines are known for having "soft" cams that would go bad. Do a compression test to identify any low cylinders, and then do a leakdown test as well. Pull the valve covers off and observe the rocker arms while cranking to visually identify any valve with less movement than the others. If the tests and visual observation show that you have a defective cam, just replace the cam and lifters. Replacing the cam does not cause oil consumption - that's complete nonsense. If the rest of the engine is tight (as will be shown by the leakdown test), just fix the problem. If the leakdown test, compression test, and the visual check of the valves shows no problem, then you have an ignition system problem. At that point, simply identify the cylinder that is missing and identify the component that is at fault (wire, plug, cap, etc.). Ignition timing won't cause a miss.

Lars

OldCarBum

Excellent advice from all.
Buy good testers and research the proper use,
If money is an issue many parts stores will rent you the needed testers and equipment for huge savings.
You will be surprised how easy it is to do the tests yourself and it is not rocket science on how an engine works.
Do the tests and post your results.
The people giving you advice here, know what they are talking about.

Stingydonut

thanks alot guys, im going to go to work on diagnosing this, i too think this guys a crap mechanic, it really shouldn't be that difficult to diagnose an issue on a small block chevy hes probably just looking for a payday on a engine rebuild for which he quoted me $6,000 LOL... for that price it better have 600 horsepower lol. i dont really understand this "id really have to get into the engine to figure out whats going on" bullcrap... your only job is fixing cars.... im not paying you to tell me its hard to figure **** out.... this advice is better than the 1500 dollars i had to spend to have him tinker around with my car and it still runs like it has a potato stuck in the engine.

derekderek

So where you at? One of us is probably close enough to drink your beer. Post a pic of your engine. I am curious what distributor he put in there. I have never seen a 1500 dollar dizzy before. Better have a gold cap on it!

Stingydonut

hahaha about an hour north of NYC, list of things included in that 1500 dollars, tune up, air filter, new belts, new fluids, new wires, cap and plugs, new headlight switch (apparently thats a real bitch to replace), rag joint replacement (i included my own rag joint), oil sending switch replaced, few missing bulbs replaced.... still 1500 is a stretch for that... and actually it was more like 1650.

Stingydonut

for some reason its not letting me post pics i hit upload and they dont show up on here, weird

Stingydonut

Stingydonut

and heres one of the whole car in case anyone was curious

jb78L-82

This issue should not be a big issue to isolate from an HONEST mechanic. Either fuel issues (fuel pump/carb) or ignition which $150 new distributor would eliminate all likely components in one shot.

My guess on an 80-82, wiped cam from soft cams back then and low ZDDP additive oils available today courtesy of our beloved EPA...................

Stingydonut

what would a honest mechanic charge for a cam swap? i know you can get a new cam for a few hundred bucks. or should i just put on my big boy pants and try to install it myself?

drwet

That depends more on your ambition and willingness to learn and get your hands dirty than it does on your current level of skill. I swapped my first cam when I was eighteen years old with a knowledge level that I consider a little scary now. (I hate to admit that was 42 years ago.) The point is I was willing to try, I wasn't in a huge hurry, nor was I unwilling to ask for help and advice. If you have the time and the interest I would recommend you go for it. I don't know many guys who own one of these cars who can afford to pay someone to do the work on it. They just need to much too often.

lars

Flat rate for replacing camshaft and lifters in that car is 7.7 hours. Add .4 hours for power steering. Shop rates vary across the country, but the average is at about $100/hour. That puts labor at a bit over $800, plus parts. If the cam is at fault. As noted, that's an easy thing to troubleshoot.

Lars