TABBOO

I have an 87 TA with a 305 TPI. I have a slight glitch in my idle. It idles at the correct rpm (about 550 in drive) but every 20 seconds or so it acts like it misses a beat or something. I had a new pickup coil put in the distributor last summer and the guy advanced the timing to give me more off the line power. My question is if he went too far since from what I understand the timing setting is only used at idle could the computer be trying to compensate and causing the blips in the idle? The car runs fine except for at idle. This is a 30,000 mile car in as new condition with new plug wires, distributor cap & rotor. The ignition module is new also ( AC Delco).
Any ideas that could help will be appreciated. I haven't been able to check how far the timing is advanced cause the car is stored for the winter, we have about 24 inches of snow in Ohio right now!!!

Supervisor42

Yeah, it's like a case of the hiccups. If it's not a plug wire leaking voltage at the boot, it's something called "uneven fuel distribution". This means that one cylinder is idling so lean that it misfires at idle every now and then.
2 primary causes:
A vacuum leak causing too much air to enter one cylinder at idle, leaning it out.
Or a clogged injector that is "dribbling" instead of spraying. Try a bottle of injector cleaner or you may be able to use a non-contact thermometer to measure the exhaust port temperatures and isolate which cylinder it is. It'll be a little colder than the rest.
(p.s. the handheld is also really usefull in backyard grilling!)

TABBOO

I'll check the plug boots & for a vacuum leak. I know the injectors are OK because I pulled them all last spring & had them professionally cleaned & flow tested. I can get a noncontact thermometer so I will try that too. Thanks!!!
Any ideas on best way to check for vacuum leaks would really help me. Is starting fluid the best way?

Supervisor42

Not if you value your eyebrows. Most folks use carb cleaner, some use WD-40. Starting fluid catches fire too easily and goes "whoop!", no eyebrows.
Also don't forget intake gaskets can leak air in from the lifter gallery, which the spray won't detect.

gcgarner

Bringing this back from the dead because I have the exact same problem.

Except I have all new plugs, wires, dist. cap, injectors (Accel 24# w/ custom tune to work with injectors). Just because my injectors are new, does that usually mean they are all good?

How do you check for a plug wire leaking voltage at the boot?

Where are the most common places for a vacuum leak that only affects one cylinder?

Oh ... one more thing. I have set minimum air and reset IAC. I can't adjust my TPS since it's a 92. I idle at 550 in drive and around 650 in park/neutral. But when I was setting minimum air, with the IAC it unplugged, I couldn't get my car to idle lower than 1000? When I plugged it back up and started, it sat right at 550 in drive and idled great except for the "hiccups". P.S. It only hiccups once warm. And one more tidbit ... when I step on or turn the throttle, my revs come down a little slower than I think they should after the throttle is released. Cable is not hanging up and blades are closing right. Thought that might be typical TPI but didn't know if this might give some more insight.

Supervisor42

Run it night where it is pitch black dark and look for flashes and/or;
Steal the wife's Windex bottle. Pour the Windex into a jar.
Fill the Windex bottle with tap water and mist away.
At each end of the ram tubes there is a gasket. Since this is bolting rigid pieces of metal together (plenum and manifold) it is common for there to be a mismatch.
A single cylinder miss caused by a vacuum leak (too much air makes it too lean to fire) will always be worse the lower the rpms are.
You can isolate the limping cylinder by lowering the idle speed with the trans in drive (blocks&PB) until it is at the lowest stable idle.
Wait a while with it like this and then check the exhaust port temps with an IR gun ($29.00)
The limping cylinder will be 50°F cooler than the rest.

gcgarner

Great info.

Are you saying to spray water onto my plug wire boots?

Also, once I have located a vacuum leak, how do you go about fixing it? New gaskets?

Supervisor42

Yeah, just mist them. If it's dark, you'll see a light show.
If there is a vacuum leak at a gasket, a new gasket and some anerobic sealant (AKA loctite gasket maker) not silicone will fix it right up.
Read the previous posts about keeping your eyebrows.