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Rough Idle help

Great info, like always diesel4me.

I think Ill give em one more day and check the compression. Then Ill limp it back home and take some good meds and attempt to get back in there myself. The only thing I didn't change was the distributor (don't know why) but this was a crate motor with only 40k miles on it. I presume the old owner swapped over the old common parts which would be the norm so distributor then and then Ill hit the engine. I hate this theory of throwing parts at it but the diagnosing is giving me hell. I can only hold my 16yo daughter off with driving my 7.3 for a few more days, she wants her blazer back now that school has started (she was the cool kid with that ride). LOL
 
I wish I could see the engine in person--though I'm not as limber as I was,I probably could at least diagnose the problem for you..its climbing up into the engine compartment that kills me,and having to lie on the engine to get to the carb or distributor..

Its possible they did not adjust the valves right,or they tightened the rocker arm nuts too much and now the lifters are holding some valves open--you should hear it firing back thru the intake or out the exhaust if that was the case..but it could be ignition related..even 3 or 4 bad plug wires maybe..

You can hook up a vacuum gauge to see how many inches of vacuum it has too,that gives a good way of telling if the compression is good,the valves adjusted right,and the timing is set right...

Sounds like you daughter is a "chevy girl"...smart,not liking the Ford!..:D...
 
So here is the latest. Picked up the beast from the mechanic, who never called to tell me anything and never said a word when I told them I needed it back so I can get it running. They didn't question it and charged me for valve adjustment and handed me the keys back. What shop gives a vehicle back not fixed and just shrugs it off. I don't think they wanted to bother with it.

Anyway, sorry just venting. I got the #1 TDC and zeroed it out and put a new distributor in. Started it up and it immediately sounded better. I timed it at idle to just a hair above 8 degrees and took it out for a ride. It unfortunately sounded rough idling at a stop sign and shook pretty bad. I went back and re timed it and it was the same at idle. I should say I initially timed it with the vacuum advance lines both plugged and it didn't make a difference when they were connected after re checking.

The PCV valve was knocking pretty good as it idled rough. I did noticed that when I disconnected the vacuum line coming out of the booster pump it increased the idle and the pcv valve smoothed out. As soon as I plugged the line it idled rough.

I don't have a vacuum pressure tester, but I'm sure I can rent one. If that is what I need to check, how do I go about doing that. If there is something different I can look into by all means suggest. THX
 
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haven't read, but start with the basics.. make sure you have a healthy breathing rotating assembly.. comp test.... if a prob is found a leakdown... it should run fine at that point with proper fuel and fire.. make sure your dizzy weights aren't fugged up and it's advancing, retarding properly... i always check idle and total...
 
The PCV valve shouldn't make any noise,or at most a slight buzzing type noise with it removed from the valve cover..

If the PCV was making a knocking type noise at idle,I'd guess that might be a good indication the valves are not seating fully in one or more cylinders,and that is letting compression back up into the intake and make the vacuum unsteady due to some cylinders having lower compression..one misfiring cylinder can affect others also..

When you say "booster pump" are you referring to the power brake booster.or does this have a smog pump too ?..

Its been decades since I had to buy a vacuum gauge that also doubles as a fuel pump tester,but they were around 10-15 bucks back then--well worth having one to test engine vacuum,set the timing,and check things operated by vacuum..
 
Ok diesel4me I think Ive got it narrowed down for you so enlighten me now :bow: or any other guru's who got it figured out.

So I finally, yeah I know, hold your comments.... finally took a compression test and voila, #8 cylinder had ZERO compression. I wet tested it and ZERO compression. I took the valve cover off and while running, it "appeared" that the rocker arms were moving as all the other cylinders just fine. Sooooooooo do I have a bad piston, can all three rings be bad ? Or do I have a valve issue?
 
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Or did the mechanic who only did one thing, valve adjustment, tighten the valve lash too tight and the valves aren't closing all the way ????
 
Since you already have the cover off, I would do a quick and dirty valve adjustment on that one cylinder and recheck the compression. But, your present results combined with the rattling PCV valve, leads me toward a piston with a hole in it.
Of all the piston problems, that is the best. Ring problems will usually trash the cylinder walls. But, I have seen low mileage engines last a normal life after the one bad piston was replaced.
Assuming it was a simple piston failure. If something got dropped into the cylinder, like a piece of valve or the end of a spark plug, then there may be other problems.

If the valve adjustment doesn't change things, then the next best move is to pull that head and see what is what. However borescopes have come way down in price lately due to cheap video cameras you might find one for a good price, or some of the parts houses may rent one.
Quicker and easier than pulling the head.
If you can't find one for rent, there is always this outfit.

https://www.harborfreight.com/digital-inspection-camera-61839.html
 
I would think there would be a ton of blowby huffing out of the breather(s) if a piston had a hole in it,ditto for bad rings (you'd be unlikely to have zero compression with bad rings,you'd get at least 20+ lbs)...

My guess is a valve was set too tight and is not closing,or a lifter has pumped up too far and got stuck,and is not allowing one to close fully..also it could be a burnt valve,usually you will hear misfiring thru the intake or exhaust if a valve burns enough to result in zero compression though..

I have seen a few engines that had so much crud build up under the head of the valve the valve couldn't close fully,or let enough air enter the cylinder to make good compression (you need air TO compress,to have good compression)..

Its possible the head gasket may have failed on the side facing the intake,but that is a long shot,and the compression would pressurize the crank case and make a ton of blowby,and oil ooze from all the seals..a head gasket can fail between two cylinders and let compression escape into the other cylinder,but you'd have two cylinders low on compression right next to each other..

The fact adding oil to check compression wet did not increase the compression points towards the valves being at fault--or a piston failure,as Fordum suggested,but again,usually a holed piston makes the engine smoke so bad and run so poor you cant even drive the vehicle..
 
Just want to point out first that you guys are all awesome and gracious to take so much time out to try and help.

So I was up to midnight last night scratchin and scratchin and decided to put together a make shift leak test tool with my compression tester. With the #8 cylinder closed off I shot down air and nothing came out of valves or intake area including carb. My daughter, yeah I had her a$$ up at midnight to help confirmed it was heard out of the exhaust pipe.

So I would conclude that I have a burnt exhaust valve ????

Now when I get the head off and confirm this, the big question is why. Am I still evading a timing chain replacement ? I haven't used the above tip of rotating crank backwards to check if rotor lags more than 12 degrees when moved.

THX
 
Don't pull the head yet. If you still have the air setup, back off the exhaust valve adjustment all the way and try again. I would be suspicious of a lifter problem anyway, but you have also had some unknown ability "mechanics" adjusting the valves.
Its entirely possible that the valve is good, just being held open.
 
Hey Fordum, So prior to the leak down test, I thought exactly, the idiots tightened the rockers down to zero lash. So WITH the compression tester in the #8 I started the beast and loosened both the ex. and comp. until they chattered pretty good and got NO increase while turning each on the gauge.

Question is, what I did above was wrong and should have adjusted valves the regular way ??????
 
No, if you were actually adjusting them, that would have been a good start, but you would go from there.
Given what you were testing, you did good.
The only other thing I would do at this point before pulling the head, would be to loosen the exhaust a little more and see if you can wiggle the valve. In other words, if there is something screwy with the spring tension.
Unless the spring has failed and its not noticeable, then its either burned badly, or there is a big chunk of carbon or other trash holding it open. You could also compare the heights of the exhaust valve stem to the other valve or other exhaust valves.
If it lower than the others, its probably trash like a chunk of carbon or varnish holding open.
Many times, depending on how the valve is burned, a burned valve will often be recessed into the head and will cause the stem to be higher than the rest. If just one side is burned off, then it will look normal height.
 
Not that your all on pins and needles, but here is the diagnosis.

After zero compression in #8 cylinder and a leak down test, I realized the compressed air was leaking out of sealed (theoretically) cylinder into the exhaust. After many meds and MFers, got the intake and heads off. Culprit was a bent exhaust valve. Very little damage to piston which I plan to smooth out. My question is to all before I go in a shove a fist or foot somewhere, Would a reputable mechanic/shop been able to catch a bent valve while doing a "valve adjustment". I'm leaning on YES but I want to be reassured before I go into shop and raise hell.

20170828_043720506_iOS.jpg 20170828_043810378_iOS.jpg
 
It is possible to make the valves contact the pistons during a running valve adjustment--you must stop and wait several seconds after tightening the rocker arm nut a 1/4 turn at a time,to allow the lifter to bleed and center the little piston in its travel,otherwise the valve can touch the piston...so being in a hurry,often allows this to happen..
Usually a push rod will bend first--but not every time..

You are lucky you did tear it apart--that valve is now weak where it bent,maybe even cracked,and eventually its head would have likely dropped into the combustion chamber and destroyed the block & piston,and several other parts..
And yeah,I would think any good mechanic would have at least done a compression test first or a leakdown test to confirm there was indeed a valve leaking after they were adjusted..
 
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