#5 & #8 not firing? need help.

[mako_22]

I just recently bought an 80 corvette and I’m having a problem, #5 & #8 cylinders are not firing. I changed the plugs, wires, cap, and rotor and still the same. Checked compression and vacuum and both were good. I replaced intake gasket just to rule out, still there. I am getting a good spark on those wires and I made sure timing is set at 8 before TDC. What could this be, what should I check?

 

[69_coupe]

First thing I would have checked is the engines mechanical condition. Worn camshaft is a common problem. Take of the valve covers and see if the valves are moving on this cylinders.

Vidar

 

[mako_22]

Allready adjusted valves and all seemed to be moving same. Compresion was 115 on both 5 & 8, and vac. off of intake was between 15 & 20. Car runs out strong but a little vibration. At idle it runs pretty rough and they dont make any difference in rpm or smooth when i take 5 & 8 wires off & on. Feels like vacuum leak, but I took all vacuum hoses off and pluged and it still was there.

 

[brusso]

How good was the compression? I don't recall what a good psi reading on the guage should be, but all of the cylinders should be the same with minimal varriances among cylinders. I mean something like 5% max or something. It may be more or less I can't remember. Like 69_coupe said, poor valve operation caused by worn cam will give you varried compression across the bank. Post your compression readings and I'm sure someone more insightful could help.

PS I would think that you would have a bouncing vaccum gauge needle if 2 cylinders were not firing? Just a thought before you tear the thing down.

 

[dman]

check fireing order. #5 & 8 pos crossed @ cap ????

 

[dman]

ps.. comp for #5&8 .little low .. others cyls are ??????????? psi . high mile car ??????? 100k + .. pos time to rebild .

 

[mako_22]

Fireing order is good. Is that low comp., have not checked other cylinders yet been really buisy will check as soon as i can. Well supposed to be 76,000, car in real good shape does not look like it would be 176,000. Runs out strong feels like there fireing when reved, but not at idle.

 

[dman]

comp test. 8 cranks per cyl. throtle wide open. fresh bat . insp all 8 like this .if comp still 115 #5 & 8 leak down test is next . a cyl @ 100 psi will missfire @ idle all day long .off idle 1 4 3 6 7 2 cyls helping 5 & 8 make more wind .................

 

[Jacko]

things to check:

fuel pressure check
compression check
running compression check
leak down test
pull valve cover to look for any valve train problems
spark plugs
coil
spark plug wires

Start the car in the dark and look for shorting plugwires. The high voltage in modern ignition systems and wide plug gaps combine to find the smallest electrical leak in plug wire insulation. If one bad wire is causing a misfire, you can be sure the others are not far behind.
Also try swapping sparkplugs and plugwires around.

 

[The73vetteman]

Compresion was 115 on both 5 & 8, .

This seems low. My '73 L48 tests at around 150psi on every cylinder (and that's with 115,000 on the clock).

It is important to compression test every cylinder. That way you spot one or two that differ from the rest.

'73

 

[mako_22]

OK scrach the 115 got a new comp. guage, checked them all and heres what i got. #1-160, #2-155, #3-170, #4-160, #5-165, #6-160, #7-165, & #8-165.

 

[Vettehead Mikey]

How do know for sure it's #5 and 8 not firing?

 

[brusso]

Comp looks good. It seems relatively even. I think that you said that you had spark at those cylinders. Is it posible that the spark plug gap could be off? I think that the gap would have to be damn near nothing but it could happen. A gap too wide would be unlikely and, with an HEI, the plug would probably fire if the gap was .020 or .030 over normal gap. If the gap is closed then you have no spark and the plug would be black from not firing. Good luck and please keep us updated.

 

[mako_22]

I know its 5 & 8 because at idle I can take 5&8 wires off at the same time and it wont make any difernce in the idle. The gap is good on the plugs & I just put them in, it also did it with the set I replaced. It acts like vac. leak, the car runs out strong so there fireing some when revved. Is between 15 & 20 good vacuum? About 17,18?

 

[dman]

all cyls look ok @ 155 & up . maybe carb to lean. causeing lean missfire ??????????? carb driping ??????? this is tough one. no scope. no infrared, no hands on ???? spray around intake & carb base with carb clener . fog top of carb .@ IDLE SEE IF IT SMOOOOOOOTH OUT ??????

 

[drags1998]

Well Ill Give it a shot, double check timing,(with vacume NOT connected)2.recheck cap-rotor-wires,recheckfiring order,1 8 4 3 6 5 7 2 IF &ONLY IF ALL THIS IS OK!!!! Check & make sure you dont have a jet or a needle problem in the carb. PS If you have the compression you say you do, Its not internal problems If all this fails PUT OUT A CRY FOR C4C5specialist He is very very Good at helping out. GOOD LUCK

 

[drags1998]

Go for a jet or needle in the carb, If all else fails Put a cry out to C4c5specialest If he cant fix it IT AINT BROKE

 

[brusso]

I haven't put a gauge on a 350 in a long time but I want to say that you should have about 20-22 at idle and no bouncing of the needle. What you have is in the ball park of what I remember and if it is steady 17 or 18 or 19 and not bouncing 17-20, I think you are ok. If you don't have the carb cleaner, you can use a rag, in the palm of your hand, and try to snuff out the carb partially while engine running. If idle smoothes out, then definetly a vaccum leak or lean idle mixture.

 

[brusso]

Are you also able to check EGR valve operation? If you can get your fingers to access the underside of the valve housing and accuate the valve. First off, while engine is idling, there should be no vaccum to the EGR. There should only be vaccum to the EGR at part throttle (ported) and engine hot. Now, with engine idling, accuate the EGR. The engine should stumble. If no change, or if you can't move the EGR diaphram, you may have found your problem.

 

[dman]

good call brusso. didnot think about egr.........! . pos not seating