90 Toyota 22R engine low idle, shaking, extremely jerky acceleration

Question

I have a 1990 Toyota pickup with the 22R engine (carbureted), and recently it started idling very low and shaking at idle. Work done in the last year, other than brakes and oil:

• Carb rebuild
• New plugs
• New plug wires
• New fuel filter
• Fuel pressure tested, all good
• Compression tested, also good
• Ignition coil tested, good spark to all leads out of the distributer cap

I know the idle mixture screw is set to the correct factory setting, and the idle screw is adjusted properly to 700 rpms.

Some observations:

• When started cold the engine will usually die within 3-4 seconds and then upon restarting I have to hold my foot on the gas for 15-20 seconds to keep it from dying.
• It will occasionally idle at 700, but will also drop to 600 and even go up as high as 1250 sometimes.
• The shaking goes away if I hold the rpms above 700.
• When it is idling low, getting moving in first gear will cause horrible jerking, that feels as if I'm repeatedly stepping on the brake, but once I get up above 15 mph or so it will drive nice and smooth, with no misfires or jerking/hesitation. Edit: -New observation, every couple seconds I can hear a ZAP sound, I checked and there's no spark arcing at the boots of the wires on the cap or from the wires to the engine, it's got to be inside the distributor cap right? If it sometimes arcs to the wrong cylinder would that account for the backfiring thru the exhaust and the terrible vibrations at idle?

Answer 1

From your description, I'd suggest you have a vacuum leak. If it were an automatic it would be a little more easily diagnosed. Anyway, you need to discover where the leak is at, but would suggest you check around the base of the carburetor, any hoses going to a vacuum source (like for the brakes booster or PCV), and the around the base of the intake manifold where the manifold meets the head. One way to do this is by using a spray bottle with water, spraying these areas and listening for a change in pitch with how the engine is running. Usually it will be a short episode of it running slower, the pick up in speed. You can also use carb cleaner in the same manner, but it will usually pick up speed then slow back down when you've found the leak.

Answer 2

The Paulster is most likely correct on the vacuum issue.... but.. Lets now forget spark. Its also possible that you've got an intermittent defect in the spark plug system. Possibly a grounded or burn wire somewhere (possible for most of us.. but I see your stuff is new), a crack in a distributor cap, or a worn out distributor cap / rotor. I'd sure pull the cap and look inside. Verify the cap is secure on the distributor.

You might also perform the cylinder disconnect test. Run the engine at idle, remove a spark plug wire one at a time, verify the idle drops when you remove each wire. You will need an accurate tachometer for this test, either in the instrument cluster or via a separate tool. If you find one wire that when removed has no effect on idle speed, inspect that plug system for a defect. Note: you will need an set of insulated pliers for this task. DO NOT USE YOUR HANDS to remove a high ignition wire. You will get shocked. Please be careful with what you touch on an engine while its running. (I still smart from getting my hand caught in an alternator fan thwack! and run to the emergency room.)