91 zeee

91 z28, 305 tpi/auto. 117k mis. I have a miss at idle; pretty bad. I replaced the cap, rotor, module (tach reads incorrect; I thought the module might be the problem), coil, spark plugs, and plug wires. I used all GM parts. Miss is still there, not changed at all. Car also has a new fuel pump and fuel filter. Any idea where I should look next? Could it be a fuel problem? What would I look for? Thanks

Vader

Replaced everything? Including injectors? Intake gaskets? Injector O rings? Valves? Head gaskets? Camshaft? Rocker studs? Vacuum hoses?

In other words, there are many things which can cause rough idle. If the cylinder leakage test or compression test doesn't reveal any mechanical problems, start looking at the fuel system. Injectors can dribble or stream fuel instead of atomizing it, causing poor idle but reasonably good operation at part throttle and above. Small vacuum leaks can also cause misfire/rough idle, including gaskets, O rings, hoses, the brake booster, etcetera.

Supervisor42

Is it an intermittent miss (it hiccups) or a regular miss (one cylinder never fires at idle)? It could still be many things.
Regular miss:
Fuel injector bad (clogged or burned out)
Valve train problem (broken/cracked rocker, lobe flat on cam)
Burned valve in cylinder head
Piston/ring problem
Vacuum leak

Intermittent miss:
Vacuum leak
Injector stuck open or partially clogged
Any sensor on the engine
Excessive play in distributor shaft
Excessive slack in timing chain
EGR valve not closing completely (sticking on carbon)

We need to take the 50/50 (regular or intermittent miss) and we can troubleshoot it down step-by-step without changing a bunch of parts.
At least you got the usual suspects out of the way first...
Did all of the spark plugs you took out look pretty much the same?

91 zeee

ok, ur right, I should have said I replaced everything that was likely 'ignition related'. I hadn't thought about the injectors or a vacuum leak until now.
Can I check the injectors and o rings myself or do I need some special tool to do it? Also, is there a simple way for me to look for a possible vacuum leak? ( I think the vacuum switch on my heater may be leaking; could that be part of the problem?)
Thanks

91 zeee

yes, all the plugs looked the same (one did have a strong fuel smell to it). i'd say the miss is all the time (it warms up at high idle without the miss and anytime I have the throttle appplied there isn't a miss, but at idle it's constant).
I hooked a GM scan tool to it today and there was nothing out of the ordinary (I can list what it logged if it would help)

91 zeee

any ideas on how/what else I need to look for?

hilts

I would start the car and with it at an idle pull the plug wire that goes to the plug you said had strong fuel smell if the idle does't change I would change that plug and wire.The best way I found to check for vacuum leaks is to spray aroud vacuum lines with carb cleaner or ether, just be careful not to do it on a hot motor,the idle would increase where a leak is present.

ZZ28ZZ

Post the data log. It can't hurt.

You've got some good replies for things to look for.

Ign problems usually get worse when eng is under a load.

I'd do a compression test first. If all is well, I'd start looking for vacuum leak.
Plain water in a squirt bottle is good for finding med-large size vacuum leaks.

If you can get out of the wind, an UN-LIT propane torch can be used to find vacuum leaks.

A 2ft piece of vacuum line can be used like a stethoscope by placing one end in your ear and "probing" suspect areas with the other end. A short section of tubing bent at a 90 deg angle on the "probe'in" end of hose works well to get into tight spots (like the bottom side of runner/plenum mating surfaces). Works better than you might think.

You can squeeze shut vac lines with smooth jawed pliers to see if there's excessive air entering intake from something connected to that hose, like power brake booster, PCV, A/C controls, etc.

Injectors are hard to test accurately. If you suspect them, it's best to send them out for inj service, or replace them, or both and wind up with an extra set for quick swap-outs