rough idle

[cul8tr]

1991 Legacy L
At idle it's rough. It shakes like it's missing on at least 2 cylinders. I have pulled all the plug wires. 3 of them have an effect on idle, more rough idle. 1 has no affect at all when the plug wire is removed.
When the rpms are raised from idle the motor runs smooth.

Dealership tells me that it's carbon build on the valves. It's RHD I'm a letter carrier.

[Flatfourced]

Sounds like a vaccum leak.

Get a can of carb cleaner or starter fluid.

Spray it around the engine compartment, aim at hoses and around the base of the intake manifold, avoid the exhaust obviously.

What you are testing is to see if extra unmetered air is being pulled into the engine from a loose hose or bad gasket. If the car idles normaly for a second or 2 while you are spraying a certain spot, you have found the culprit.

Engines pull alot more vaccum at idle, little at WoT.

If this test shows no results it's easy to take the shops advice and clean the top of the valves. Get a can of seafoam and a pcv valve sized hose. With the hose attached to the pcv valve, pour about 2 tbsp at a time in then plug the hose with your thumb. It will create a vaccum and open the valve sucking the seafoam evenly onto the intake manifold. Get ready for alot of smoke. Repeat for about 1/4 of the bottle.

Now you should also test your spark. I would start by buying a new set of plugs, if not done within the last 10-20k. Then you have the old plugs to safely test spark if that dosent fix your problem. Simply put one of the old plugs (or a screwdriver) into the spark plug wire, place it on the block or head (or 1/4 in from in the screwdrivers case), and see if you have a spark. (careful the screwdriver bit may zap ya)

It still could be the spark, it is even less noticable at high rpms. So, brings up, any power loss at the higher rpms?

I feel like it's plugs or a vac leak.

good luck

[cul8tr]

I've used a vacuum hose and listened around the motor and all hoses. Nothing, I'll try the spray method. I have fire on all plugs.

It's only that 1 cylinder that does it.

Thanks

[Legacy777]

You found your problem, you've got a dead cylinder. You just now need to verify whether it's an issue with spark or fuel. Pull the spark plug for the cylinder that is dead (didn't affect idle). Look to see what it looks like. You can also connect it to the spark plug boot and rest it against the block. I'd suggest disconnecting all the other spark plug wires and have someone crank the motor. You're looking for spark between the spark plug gap. You can also use a noid light or spark tester that fits inline between the spark plug and wire.

If you have spark, you've likely got a dead injector. You can confirm this by using a multimeter to check the resistance of the injector and compare it against the others.

[cul8tr]

Hey Josh thanks for the info. I didn't think about a dead injector. The plug is the exact same color as the rest but I'll double check it tomorrow and test the injector. Thanks

[Specialchowmein]

Any update on your progress? I'm having the exact issue. I pull the plug on one cylinder and nothing happens, also pulled the injector wire, still nothing. Compared output going to the injector on the bad cylinder against the others and they're all the same. Also, I got a remanufactured injector, put it in, and didn't see any change. The first thing I tried was new spark plugs. The old ones all looked the same with the proper light brown color. I haven't pulled the spark plugs again yet, but I'm going to be doing a compression test, we will see what that says.

[Legacy777]

Welcome to the BBS.

Try swapping spark plug wires and see if the dead cylinder follows the swapped wires or stays the same.

[cul8tr]

Sorry about not keeping in touch with this. Life and work have been hectic.
I going to get a injector for that cylinder.
Sometimes it actually idles fine but most of the time it's rough at idle. In the upper rpm range it seems to run just fine. Makes me think that at higher rpms it has more suction to possibly pull the fuel thru the injector.

[Legacy777]

There is actually less engine vacuum created at higher rpms than at lower rpms, so I'm not so sure that theory holds up.

As I mentioned above, you could try checking the resistance of the injector to see how it compares to the others and the specs.