Rough idle can be caused by many things. Here's what you can try with any rough-running 3VZE:
1. Run a compression test. Leaking head gaskets and burnt exhaust valves are common with this motor, and if your engine has issues with compression, that must be addressed first - or all money and time spent will be wasted. A compression test is easy to do and a compression tester is inexpensive and can be found in any auto parts store or on Amazon or ebay. (
http://www.amazon.com/Actron-CP7826-Compression-Tester/dp/B000CPJM66/ ) The 3VZE plug threads are 14mm, a standard size that most testers will work with. Specs and instructions are in the factory service manual. If you don't have one, this may be of help:
http://groups.google.com/group/alt....ta.trucks/browse_thread/thread/fb4090219d075dc0/2c29fe9529394fb3?lnk=raot&pli=1 Look in the Engine - Mechanical - Compression section.
2. Check to see if the computer has stored any codes - it doesn't always light the check engine light. Instructions in the Engine - MFI System -- Diagnostics section. Address any issues highlighted by the codes.
3. Clean the throttle body with a sensor- and catalyst- safe throttle body cleaner. Make sure the back of the butterfly and the small air passage in front of and at the base of the butterfly receive a few shots - that's your cold idle air bypass ("air valve"). Be very careful not to get any cleaner on the black plastic throttle position sensor - it does not like solvents. Dump a bottle of Red Line's excellent SI-1 Complete Fuel System Cleaner in the tank - it's one of the very few injector cleaners that still contains PEA (PolyEther Amine) - the super effective injector cleaner invented by Chevron that Chevron's own Techron doesn't even contain anymore.
4. You can check that the cold idle air bypass is working correctly by screwing the idle adjustment screw (the big front-facing screw on the top of the throttle body) all the way in when the motor is
cold. The idle speed should go down. Unscrew it to previous position and wait for motor to get hot. Then screw it all the way in again. The motor should stall, or very nearly stall - noticeably slower than when the motor was cold. If no difference between hot and cold, your air valve either needs better cleaning or you'll need a new throttle body. (The valve is located below the butterfly and it is coolant driven - when the hot coolant hits it, the valve moves - much like a thermostat.)
5. Perform a tune-up, if you haven't in a while. Plugs, cap, rotor, air filter. The best performing plugs for the 3VZE are the factory spec Denso K16R-U. Autozone or rockauto.com have them. If plug wires are cracked, very dirty, or no longer soft and flexible, replace them with Toyota wires - they blow the aftermarkets away. Be sure your tune-up includes setting the timing - with terminals TE1 and E1 in the check connector jumped with a paperclip - to the factory spec 10 deg BTDC or, for a bit more snap, 12 deg. Instructions in the IGNITION - Distributor section of the fsm. Timing lights are also inexpensive. Don't need a fancy one, just so long as it has an inductive pickup it'll work. Sears, parts stores, Amazon or ebay will fix you up. Set the idle speed beforehand to 800 by adjusting the front-facing air bypass screw on the top of the throttle body, when the motor is hot.
6. If the O2 sensor has over 90k miles, replace it with a Denso. Good prices from rockauto.com.
7. If the motor is still running poorly, you'll need to start checking sensors with an ohmmeter. First place to look is the TPS - throttle position sensor. There are two terminals that are supposed to have continuity when the throttle is closed (in the idle position): the E2 and IDL. Check in the ENGINE - MFI System -- Throttle Body section of the manual for specs. If these terminals don't have continuity when throttle is closed, that is probably your main problem - the computer thinks the throttle is open and it's injecting too much fuel. You should first try adjusting the tps, but if you can't get continuity on those terminals no matter the position of the tps, you'll need a new one. (Good prices from 1sttoyotaparts.com) Be sure to follow the instructions for positioning it exactly. This was the case on my 89, and when I replaced it, it began purring at idle. Check that the resistance between E2 and VTA is in spec at idle and when full open throttle, and be sure the resistance between them changes smoothly as you open the throttle, with no spikes or dropouts. It appears some year manuals have a typo for the idle specs - they show the upper limit as 6.1k ohms when I believe the proper spec is .61k Some years have .8 or .81 as the upper limit. The fsm says you'll need a feeler gauge to test the idle signal, but that's only really necessary if you are adjusting the position of the tps. Otherwise, it's enough to know that E2 & IDL have continuity when at idle, and do NOT have continuity when the throttle is cracked open a mm (or more). (If you DO need to adjust the position of the TPS, about the only way to get a feeler gauge in position is to remove the throttle body.)
8. If the tps checks out, check two other sensors with your ohmmeter: the ECT (engine coolant temp sensor), which tells the computer when the engine has reached operating temperature. Before the ECT sends the signal that operating temp has been reached, the computer runs in "open loop" mode which ignores the O2 sensor signal and runs mixture off a preset value. If the ECT is bad, the motor will never run in "closed loop" and so it will never run "right". The other sensor which does sometimes go bad with our age motors is the VAFM - volume air flow meter (sometimes called the "vane" air flow meter). That is actually two sensors and a switch in one box. The switch is the fuel pump switch - the left two pins on the terminal. Those should have no continuity when the vane is closed and should have continuity when the vane is in any other position. That grounds the circuit opening relay which powers the fuel pump. Then there's the intake air temperature sensor, and, of course, the air flow signal. Specs for all these are in the ENGINE - MFI System -- VAF Meter section.
9. Still no dice? You need to make sure both circuits of the coil are good by checking it with your ohmmeter. Also verify that the crankshaft position signals going to the ECU are good - the ECU uses those to determine spark timing and advance. Those signals are generated by the distributor: check that the signal rotor clearance and signal coil resistance values are within spec. See the IGNITION - ON VEHICLE INSPECTION section of the fsm. If you like playing with fire, you can pull the coil wire off of the distributor and hold it a half inch or so from an engine ground while someone cranks the motor. You should see bright blue sparks. You might want to be holding that wire with a REALLY good electrical insulator, if you don't want first hand knowledge of what 50-60 thousand volts feels like(!)
10. Another system to check is the EGR. Stuck open or stuck closed both cause problems. Check the EGR valve itself, as well as the vacuum modulator and the VSV. Procedures are in the manual.
11. Still running roughly? Need to make sure there are no vacuum leaks by spraying starting fluid around all the vacuum hoses (end-to-end) and the throttle body and plenum, while listening for an increase in idle speed. You can also use an unlit propane torch with the valve cracked open. I've heard of people spraying brake cleaner and listening for a
decrease in idle speed, but I've never tried it.
12. Another possibility is the cat is clogged. The only way to be sure is either a pressure test before and after the cat (small holes have to be drilled in the pipe to do that) or you need to remove the cat and visually inspect. A quick and dirty check is to feel the exhaust at the tailpipe while someone gives it a few revs. You should feel distinct blasts of exhaust when the motor is revved, but delayed a second or so. A clogged cat will tend to smooth things out and won't give distinct "blasts" of exhaust. An alternative is to remove the O2 sensor and see if the motor is more peppy or if it seems like all the exhaust is trying to come out of the O2 sensor hole. BE CAREFUL! The exhaust is hot enough to burn you that far forward.
13. If still not running right, a fuel pressure test may be in order, but low fuel pressure usually causes more trouble at higher engine speeds than at idle. This test requires a pressure gauge with the appropriate size banjo connector.
Still running poorly? :headbang: Try shaking a chicken's foot around the motor - it might scare the evil spirits away...
.
