my girlfriends car is a 91 camaro with the lo3. she recently failed emissions HORRIBLY! we narrowed it down to a misfire in cylinder #4. the idle does not change when that plug is pulled. we are pretty sure that the plug is fireing because we pulled it and grounded it and watched the spark. so what are the other possible causes for the misfire in that cylinder? the valve stem seals are worn and it smokes when it starts up, but i dont think that has anything to do with it.

 

Check the entire ignition system. Check for corrosion under the cap where the rotor makes contact with it. In fact just replace the cap, rotor, plugs and wires if they have never been changed. The car probably needs a fresh tune up and simple ignition woes can cause an incomplete burn.

 

we already replaced the cap rotor and have a new plug wire there, we are pretty sure its not ignition related

 

Check for vacuum leak. If good check compression, if low do a leak down test.

 

if doing horribly in emissions... you might just need a new catalytic converter

 

as far as the catalytic converter would you mind explaining how that would help him out?

if he has a mis in the #4 cylinder then he is doing nothing more then dumping raw fuel into the exhuast. a cat converter might help but will not solve his problem and even if it did it is nothing more then a bandaid being the new converter will soon become plugged up due to all the unburnt fuel



not trying to be *** but come on man read the whole post

 

try a leak down test as someone else has said or at the very least a compression check.

what you might be able to do just to kinda jury rig it for a small moment is pour a little oil in the cylinder which should help it seal up a little better and see if the mis goes away.
again not much just a LITTLE.
least I'm taking this from my experience with my rotary. when I can't get it started due to flooding (which washes the oil off the seals and makes it not seal very good) I put a little oil in there and she starts right up and runs fairly well. as well as how some ppl do a wet compression check.

it's worth a shot

anothe thing you might be able to do is if you have one of those car stephescope thingies is to put it on your #4 injector and see if it is even firing. or maybe even just take it out. no fuel means nice mis though you prolly wouldn't fail emissions in that case




few qu4stions for you though?

hows the oil look?
does it smell like gas?
bubbles in the oil?

what about the coolant?
looks nice no gas smell or oil in it?

 

the oil looks fine, however the coolant had looked very rusty looking (possible head gasket prob?) so we flushed it out. tomorrow ill do a compression test to see what that is. should it be somewhere around 150? by the way, its throttle body injection so there is no individual injector for each cylinder. i was thinking of putting some seafoam down it, perhaps the valve have some deposits on them, the car does have 160k miles on it.

 

I had a similar problem on my 92 TBI engine with 200K miles on it. A very slight miss (almost unnoticable) and emissions were horrible. I didn't plan on keeping the motor in the car so I spent $200 installing a new cat just to get an emissions waiver. It made no difference in actual tailpipe emissions, BTW. When I tore the motor down it was obvious that #8 wasn't firing for whatever reason (exhaust valve was dark, as was the plug's center electrode).

If I had to guess, I probably just had a problem with the plug/wire on that cylinder (which would have saved me $200, if I had taken the time to diagnose it!). Valves and seats were good, no cracks in the head and the piston/rings looked just fine, too, when I tore it down a few weeks ago just out of curiosity. Someboday had put in platinum plugs (the wrong ones for the motor) and apparently one of them or the wire going to it just didn't want to play ball. One misfiring cylinder can send emissions to the moon.

I highly recommend doing a compression test on the offending cylinder and comparing it to a couple of known-good cylinders. If they're about even then it's probably either a vacuum leak or an ignition related problem.

 

Don't over complicate things. Don't throw parts at it. Its a very simple engine. Check for a vacuum leak. Check compression on a few cylinders, then do a leak down if required. Read the plug. If you start with a new plug, read it after 10 mins of driving. White like when you installed it means no fuel, black & wet is fuel, no spark or a vacuum leak or not enough compression to support good combustion.

 

sorry, I made the assumption that he had already done a full tune-up

 

vacuum leak may be possible, but would that cause a specific cylinder not to fire? when i check the compression tomorrow, if it is low is there anything i can do to fix that?

 

Did you gap the plugs correctly? Are you sure that plug is still good. I would just changes the plugs since they are so cheap. Also, get your hands on an ohm meter to check your wires.

 

On what part of the emissions did it fail?
A misfire would do it.
So would a non-functioning EGR valve.
Also coolant temp sensors as well.

 

it failed everything except the gas cap test. ill recheck the gap on it, but it had the misfire before we changed the plugs.

 

well i did the compression test, the bad cylinder had 100 psi and the rest were around 150. now i just gotta figure out why the compression is so low. without doing a leakdown test, how can i figure this out? oh and the spark plug for that one reaked of gas. maybe the walls are washed? i guess im gonna go put a LITTLE oil in it and see what that does.

 

I doubt this is it, but maybe you should make sure the plug wires go to the right cylinders.

 

well with oil in it the psi went up to 125, which is still kinda low

 

but it indicates that there's a sealing problem. I dont know how that would cause you to fail emissions though... maybe with a busted head gasket, letting coolant in and coolant is bad for emission?

edit: nevermind, with a busted head gasket chances are the whole bank of cylinders would be low on the compression test too

 

did you check the O2 sensor? That is the problem with my car right now, i have to find it and have it replaced.

 

guys those little things might help but first hing to take a look at is why ONE cylinder isn't firing

o2 will cause issues through all 8
cat will cause issues through all 8
plug wires wired wrong will cause issues through at least 2 not just one


we have one main issue which is one cylinder not firing
take care of that then we can take care of the other things


sorry just me doing tech support has gotten to my head


if the oil helped the comression a little you might want to check to see if the injector is leaky
not sure how hard it is to take the injector out but if you can take it try to set soemthing up that will alow some gas to pressurize in there and see if it is dropping a nice amount of fuel to leak in there
maybe when it sprays it just squirts out in a straight stream rather then a fine mist which will cause the fuel not to ignite and can cause washed down walls

 

I think we're missing some important info here... is the car fuel injected or carbed? Is it modded any? When exactly you remember the misfire happening, and what events surrounding it

 

Maybe pull the valve cover and start it up, see if the valves are opening and closing on that cylinder. If the plug, wires, cap, and rotor are good something has to be keeping it from firing.

 

high HC means a missfire
High Co running rich
High NOx running lean or bad EGR

If you had high NOx then your EGR is probly screwed you can check it by pushing the diaphram up and plugging the vacuum line and if it drops back down it`s bad

since your car is TBI and only 1 cylinder is missing the it`s NOT an injector Your cyliner pressure is not too low but it is alittle low (all cylinders should be within 25% of eachother) so your rings are probly good, so it comes down to a blown head gasket, bent valve OR weak valve spring. A bent intake valve will cause air to enter the intake and you`ll hear it comming out the breather or a bent exhust valve will cause air to go out the exhust pipes or a blown head gasket will make the coolant bubble (most of the time not all the time) screww an airline into the plug hole to do these tests and set the psi to around 50. if thats not it pull the rockers off cyl 4 and 5 and push down on top of the spring to open the valve on cyl 5 and then do the same on cyl 4 and see if it take the same pressure to open the intake and exhust valves on 4 as it does on 5, hope this helps it`s all I can think of.

P.S. to those who care I`ll tell you little bit about a cat.

To check a cat make your car run rich, if you do the cat will not work very good probly less then 25% and then make your car miss your cat should kick into high gear and work 90-100%, this is because cats NEEDS oxygen to lite off, NO oxygen your cat can`t lite off. So to those of you that think you need a cat you probly don`t and putting a cat on a car that runs rich it won`t do you ANY good. but to test the cat you need to be on the dyno because you need to measure your emissions.

 

i have her sheet on the results from the test.


HC 4 g/m limit .8 g/m 1432% greater
CO 23.16 g/m limit 15 g/m 538% greater
NOx 2.47 g/m limit 2 g/m 95.9% greater


as you can see the failed them all horribly. to band aid the situation can i pull the intake/exhaust rocker on that cylinder so that the unburned fuel does not get into the exhaust and she will pass? the only problem i can see with that is the lifter will be sitting in there with nothing keeping it firmly on the cam and a lobe could come around and smack it.

 

You got a dead cylinder. You mainly failed on HC- unburnt fuel going out the exhaust. Classic dead cylinder symptoms. Given that the compression is also low in that cylinder I'd say that something is MECHANICALLY wrong in that cylinder. Rings or valves. On an older engine with a flat tappet cam I'd also add a cam lobe going flat to the list of possibilities, but on a later model engine with a roller cam, like yours, that is unlikely. I think you've got a hurt engine, pure and simple.

BTW- on a roller cam engine like that one you can remove the rockers/pushrods on that cylinder and try again, like you describe. The lifters can't come out of their bores becuase of the lifter retaining plate and "dogbone" that restrains them. However, I wouldn't do that just becuase it makes the hairs on the back of my neck to stand on end, not becuase I've ever actually tried it.