4AF erratic idle - Toyota Nation Forum : Toyota Car and Truck Forums

kileak · 01-31-2009, 11:48 AM

I have an '88 4af carbed. It will bounce between 1000 and 1200 rpm at about 1 second intervals. Any one know what might cause this problem? Thanks.

ae924d · 01-31-2009, 07:54 PM

mine is doing that right now too, only it's actually missing here and there, check your O2 im pretty sure thats what mine is...

kileak · 01-31-2009, 08:11 PM

I'm pretty sure my car doesn't have an 02 sensor. Unless I'm very wrong. Where would it be? In the exhaust system somewhere yes, on the CAT right?, but where exactly? But I'm almost positive it doesn't have one

Rolla88 · 01-31-2009, 08:57 PM

I have a 4AF, check where the manifold meets the exhaust, also check your plugs and clean them a bit and the vacuum hoses as well

verlaryder · 01-31-2009, 09:03 PM

It's almost impossible to say what could be causing the wierdness because we don't know anything about the preventive maintenance of your car and modifications that have been made. Assume the engine is totally stock, check for a disconnected hose(s) that should be connected to fittings on the underside of the air cleaner. Adjust the ignition timing to 13 degrees BTDC with nothing disconnected or 5 degrees BTDC with the hose to the distributor subdiaphragm disconnected and plugged. Adjust the warm engine idle speed to 650 RPM (manual trans) or 750 RPM (auto trans). Adjust the throttle positioner setting speed to 900 RPM, Inspect the throttle position switch. Procedures can be found in the factory repair manual for the '88 and '89 Corolla which can be purchased at ebay.com for $15-40 or so.

The right thing to do is tune up the engine to factory specs using Toyota parts (plugs, distributor cap, ignition timing. idle speed, etc.) following factory manual procedures and then come back here if your still have wierdness. A worn oxygen sensor would not cause an eratic idle speed. But high idle speed means even the base idle is not set correctly and setting idle speed is part of a tune up. Erratic idle speed might be faulty tune up + maladjusted throttle position sensor

A 4AF shouldn't be turning 1000 rpm to begin with, so even the basic tune up items

freakinbox · 02-01-2009, 01:45 PM

yeah your looking for 750 rpm at idle, I like it better at 1000. I find the alternator runs better and the volt meter doesn't drop as much at 1000 rpm. But I am having similar issues, I think mine is mostly my plugs and wires needing to be replaced.

Like also said check the vacume lines. Get the timing done. What I think might be the issue on alot of our cars is the vacume advances might be going.

kileak · 02-01-2009, 03:14 PM

I just put new plugs and wires rotor and all that stuff not long ago. I was thinking maybe it was a vacuum hose issue. I'll check the vacuum hoses.

90 GP · 02-01-2009, 03:31 PM

Check the vac hoses on the throttle positioner. It's a dual vac unit with two hoses on the left side of carb when you are facing it from the front of car. The dual vac unit has two linkages and separate adjustment screws. You will need to check a manual for the specific settings. Also check to be sure that the choke butterfly is opening all the way. Spray it with carb cleaner to be sure it operates freely.

ae924d · 02-03-2009, 02:58 PM

Well i fixed my problem last night, we went for a new O2 sensor and i started hearing a hissing noise, turns out the vacuum line came off of my MAP sensor, plugged it back in and it runs brand new again..