GOT_CAMARO?

I'm having trouble tracking down why cylinder 4 is not contributing to the greater good.

It started one day several weeks ago on my way home for the weekend, the car started missing pretty badly. The mis is more pronounced at low RPMs under load. But, as I accelerate the mis gets less pronounced above 3,000RPMs, but there is a noticeable lack of power.

I knew I had a mis but not sure which cylinder(s) so I did a power balance test and found cylinder 4 did not cause a drop in RPM.

So far I've checked spark and got a good hot snap. I swapped the plug in the offending cylinder with its neigboring cylinder hoping the mis would move, it didn't.

The plug wire measures 71ohms (MSD superconductor) and a spare good wire measures 70ohms. Just for kicks I swapped the wires anyway, it didn't fix the mis.

The entire ignition is a little over a year old (coil, ICM, pickup coil, wires, plugs, cap and rotor)

Since the ignition was functioning well I did a leakdown test on the cylinder and got 12% leakage, the neighboring firing cylinder also had 12% leakage. So mechanically the engine seems fine.

Also, I pulled the valve cover off to verify the valves were opening and closing while the engine was running, they do.

Since the car is carbed I don't have an injector to check, but after talking with my mechanic he suggested the intake manifold gasket could be could be leaking under the manifold allowing the cylinder to suck in crankcase vapors thus diluting the air/fuel mixture. So, I pulled the intake off and found oil on the sealing surface and in the intake runner. Maybe this was the cause? Nope, still misses.

Now I'm pretty much out of ideas, I'm going to talk to my mechanic again on Monday, but until then, maybe the mass of collective thoughts here may be able to spur some new ideas

jtsk

iTrader: (3)

could be a bent valve or a burnt valve but not that likely. Also, the distributor it's self could be bad. How many miles on the motor? Had to replace my entire distributor at about 140k because of a dead miss.

GOT_CAMARO?

The motor has around 25k on it. I don't think it could be valve related because it would have shown up when I hooked the vacuum gauge up. With the revs around 2500 the needle was dead steady. The distributor has more than than 25k, but I'm not sure exactly how much more. It's an Accel billet aluminum, I believe I put it in 5 years ago.

jtsk

iTrader: (3)

do you happen to have another distributor you could drop in and see if it fixes the problem?

regal301

it takes fuel,spark and compression to fire.

i would do a compression test on all cyls.

then check spark at all cyls.

also make sure the egr valve is not partialy stuck open.

GOT_CAMARO?

jtsk, unfortunately I do not have another distributor to drop in.

regal, I do not have the EGR valve on, just a block off plate because the diaphram was leaking. I have spark at all cylinders, I didn't check the compression though because I knew cylinder 4 is dead, the others passed the power balance test.

regal301

i would compression check all the cylinders anyway,so you can rule out any internal engine problems.you can also read all the plugs.it could be a defective plug,i have seen that a lot lately.

if that cylinder has spark the leave the distributor alone.

if you have the large cap distributor,check the rotor button,if the ground strap inside with the coil is missing it tends to burn the rotor button and center of the cap.

just because the parts arent that old dont rule them out to be fine.i pull plenty of bad brand new parts out of the box all the time.

i dont know you or your knowledge of cars so dont take any insult to what i say,just trying to help you find the problem.

regal301

also if you have headers i cant help but say its a wire,also how did you do the balance test? hopefully it was by pulling each wire one by one,not grounding out each cylinder with a test probe through the boots.

chevymec

iTrader: (5)

Do a compression and leak down test. That will tell you what you need to know.

GOT_CAMARO?

Alright, did a compression test and got 160psi on all cylinders. I had previously done a leak down test, but only on the dead cylinder (#4) and cylinders #2 and #6, each had 12% leakage.

regal: I did the balance test using a test light connected to a sparkplug that was grounded to the block. The sparkplug was to simulate the approximate resistance caused by firing the plug so as not to fry the coil. I probed the test light between the sparkplug wire and boot, I did not pierce the boot. I listened for a change in RPM as I probed each cylinder with the engine warm and idling. There was a distinctive drop all but #4. This is how I came to the conclusion #4 is not contributing.

Oh, and I don't take any insult about what you say. With regard to my automotive knowledge: I'm finishing up a 4 year automotive degree at Penn College and as far as my abilities, everything in my sig. was done by me, I'm fairly competant, but I'm always learning!

I talked to my mechanic today and he gave me a few new ideas such as checking for vacuum leaks because I remembered my vacuum was not stable at idle, I thought it may have been due to the misfire. He also suggested loosening the valves a bit with the engine running to see if the cylinder starts firing. We shall see what that brings tomorrow...

Grampa

I had this problem with a Z28 305 some years ago. It turned out to be a pulled rocker arm stud causing the problems. Actually had 2 that were kind of bad, one which was marginal.

Good luck!

rgarcia63

I know it isn't definitive, but you should be using a vacuum gauge to diagnose the problem, at least to narrow it down.

'87IROCZOWNER69

iTrader: (1)

Try asking your professor from college, he might enjoy a good challenge!

GOT_CAMARO?

Yep, I've been using a vacuum gauge. Vacuum is low (13inHg) and not rock solid, but it isn't varying a whole heck of a lot, it drops a max of 3inHg at most than rises to back up at idle above idle it pulls about 22inHg and is rock solid. I figured this is due to the dead cylinder. If I had a burned valve it would vay wildly at and above idle. But the compression and leakdown tests don't show any adverse valve operation.

Oh, and as far as asking my profs at school, I'll have to wait until after Christmas break..

ASE doc

This is a perplexing problem. Since your compression numbers look good, I would expect the issue to be valve train related. Also, because the problem started while driving. Ive seen a few cases of a bent rod caused by hydrolock. The leak down numbers were good but there was a notable drop in compression. The one thing I can think of that would cause power loss without affecting cranking compression is a valve not opening completely. Someone mentioned a pulled out rocker stud. Im thinkiing more of a partially collapsed lifter, since the miss seems to go away above 3,000 rpm. This could be the lifter pumping up.

faster1k

i had a similar problem but it ran fine sometimes (but DRANK coolant)and went to do compression test and #8 was 10 psi lower than the rest.. decided for other reasons to pull it anyway while i was in there and i looked in the cylinder (cracked sleve and half the piston was gone above the second ring) and it was still holding up to 60* shot on a bone stock motor lol.. you could be in the same boat

rgarcia63

If the problem isn't mechanical then it's electrical.

87irocz383

I had the same exact problem, did all then tests ( leak down, compression test, vaccum also) and found the problem was a wiped out cam lobe. I see you have a roller cam so maybe unlikely, but anything is possible. Mine happened due to improper break in on a flat-tappet cam, I know there is no "break in " for a roller, but check out the lobes and see whats up with them, just trying to help

rgarcia63

Which test(s) where conclusive, or no help at all?
Which cylinder & cam lobe was damaged, intake, or exhaust?
What do you mean by improper?

87irocz383

All of the tests I did cam back acceptable, good compression ( 150-155 all cylinders) vacuum test needle moved around a but at idle, but would be steady from about 1500 RPM and up, and leak down test showed 6-10% on all cylinders. When I found a wiped out lobe or lobes, it was the intake and exhaust lobes on cylinder 4 and the intake lobe on cylinder 3 was starting to take a crap also. What I mean by "improper" is when the guy before me installed the cam, he did not run the engine at 2000-2500 RPM for 20-25 mins at first start up to break in the cam, when I got the motor, he told me the engine only had about 1000 miles on it since a mild rebuild.

rgarcia63

So the damage was found by pulling the cam and inspecting it?

87irocz383

I took the lifters out because of the suspected problem and found the ones i mentioned before trashed, therefore yes the cam was pulled and seen the lobes were wiped out.

ASE doc

Breaking in a flat tappet cam is critical. Since oil producers have reduced or eliminated zinc content in their motor oils, due to catalytic converter damage, zinc additives must also be used along with a good assembly lube. The beeak in process itself is not good for a brand new motor either, a new rotating assembly should be warmed up at idle while coolant temps and oil pressure are monitored. Firing up a new motor and revving it at 2,500 rpm for 20 minutes has always been unnerving for me. This is why I insist on roller cams in any motor I build. The extra cost is minimal when compared to the benefits. Be aware however that roller lifters can fail and that the devices that prevent them from rotating can also fail. Generally you'll get some valve train noise when this happens but not always.

GOT_CAMARO?

It's been a while, but I thought I'd wrap this up. My dead cylinder is now fixed. I got fed up with it over spring break and took the car to my mechanic who did a formal power balance test and found more than no. 4 cylinder dead. So he suspected a valvetrain problem as well, but since compression and leak down numbers were good, he reset the valves with less preload. Turns out that fixed the issue, probably as ASE doc pointed out, a collapsed lifter or two, or three... Anywho, the car runs stronger than ever. Now I just have an annoying vibration coming somewhere... Stay tuned. Off to the search bar!