1981 SC cyl 3 not firing - HELP!
 

Hey all, it's been a long time since i've posted...

My 81 SC (US spec) has an issue I can't figure out.

Cylinder 3 won't fire.
- No air leak
- Intake runners are good
- Injector has good spray pattern
- Compression good
- Leak down good
- Tried different spark plugs
- tried different plug wires
- tried different cap, rotor
- has spark
- tried different injector
- tried different ignition control module
- new fuel injector o-ring and sleeve
- scoped the cylinder, and the combustion chamber and pistons look great
- tried different fuel line

Runs smooth on highway, misses obviously at idle (slightly lumpy w/ intermittent popping from exhaust), poor gas mileage, especially in stop and go traffic.

I don't know where else to go...

It's getting fuel, air, and spark... wtf?

Thanks in advance, everyone...

edit: Also, idle is usually good, but after idling for a long while, or after a warm start, it will rise to about 1400-1500rpm

Leakdown, provide numbers and done hot or cold.

Compression, provide same and include answer to did you block throttle open and have a battery charger on car?

I don't have that information. The car is at a shop in the area.... i'm just hoping for some off-the-wall ideas to throw at them...

Data. Gotta have it.

Leakdown: <5%
Compression: 170-175

tested both hot and cold, throttle open, on charger and off

apparently the compression is significantly lower, on the low end of spec, in cylinder 2, but that is firing just fine.

They tried swapping the fuel lines from cylinder 2 to 3, no change
They tried swapping distributor, no change

What do the plugs look like? Is the #3 significantly different? Looks leaner? Looks richer?

Do you have an infra red thermometer gun? After (or while) running the engine, can you take temperatures on the headers right at the exhaust ports (before they enter the heat exchanger) to see if the #3 exhaust is cooler (which it will be if that hole doesn't fire).

You might run it on a chassis dyno, and take air/fuel ratios both at idle, and at high RPM. You might be able to differentiate between an idle issue in #3, or whether it just isn't pulling its weight on the highway (these cars are not slugs running on only five cylinders).

Have you pulled the intake valve cover off and checked to see that both the intake and the exhaust (you can spot a bit of its rocker where it is on the cam) are behaving as they should?

This is a real head-scratcher.

This may be scraping the bottom of the idea barrel, but the spark plugs are grounded by the case. Is there some way that the plug in cylinder #3 has become electrically isolated? I can't think of how off hand... I'm just spitballin'.

I'll get more information tomorrow evening when i talk to them again...

I'm just having these horrible visions of needing to rebuild the engine or something... that is terrifying to me as i just can't afford to do that...

could something like a valve adjustment be a possible culprit?

they said that the plug was dry when they checked it, and have tried multiple plugs... also they said the threads in the head looked ok, so it SHOULD be grounding properly...

im getting freaked out because these guys have been in business for a long while and are almost out of ideas...

Out of adjustment valves should show up on leak down/compression test if they are so out as to cause a complete no fire. Same with flat cam lobe.

If the shop has no IR gun have them put their finger literally right on the exhaust pipe where it exits the port but before heat exchanger on a cold start. They should all be untouchable in a minute or less. Obviously the IR gun is best but I've done the above many times over the years with good accuracy if the cylinder is a dead hole.

I'm sure they have a heat gun. That was how they were able to tell that the cylinder wasn't firing. They said that it was cold...

And won't running the engine with a dead cylinder cause damage? What keeps the cylinder from hydrolocking from the fuel?

How certain are they the cyl is not firing ?

Off the wall.... considered another shop?

Just in case, it would be good to double check that the nuts from the intake runner to the heads are tight.

Tip of the day........
 

pmax,

This is what I would do to determine if the questionable cylinder is really not firing(?). Forget compression, leak down test, air leak, etc. for the meantime and focus on this test. Remove the FP relay, have a fully charged battery, and hook up an inductive timing light or a spark plug tester to #3 and crank the starter several times. Try to test cylinders #1, #2, and #3 to compare the individual intensity of the strobe light. Keep us posted.

Tony

+1.. Seems to me someone is making a mountain out of a molehill.. Should not be that hard to track down..

If your valves are opening you have no issue.

Do this.

How so (cam lobe)? Flat cam would not open the valve. Let's say exhaust valve does not open. How would that impact compression?

I'm thinking that worn lobes wouldn't be so flat that valve wouldn't open some. I do know that there will be a difference compared to other cylinders if they are healthy. I would also do a dynamic compression test as well, looking for anomalies.