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?
  • 1
    Welcome to the site, great first question.
    – Zaid
    Commented Jan 16, 2017 at 3:28
  • 2
    Leaving this as a comment rather than an answer since my knowledge of carbureted systems is somewhat limited. The symptoms seem to suggest that you have an issue with air bypassing the carburetor and making its way into the engine. You've ruled out spark and compression as being possible causes. You've also partly ruled out fuel as an issue since fuel pressure is OK and the idle mixture/speed screws are where they should be. Bear in mind that lack of fuel at low loads can also explain what you're seeing
    – Zaid
    Commented Jan 16, 2017 at 3:38
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    I know nothing about carb systems, but my 2002 Mazda Protege once had similar symptoms. Turned out my air intake hose had a leak, I replaced and it was fixed. Couple years later the problem returned, this time I gave the throttle body a good cleaning and it mostly went away again. Also check your idle air control (IAC) valve and replace if necessary. I would have done but it was rusted in.
    – Dan A.
    Commented Jan 16, 2017 at 6:39
  • I also originally thought lack of fuel, but after driving it hard when it was running crappy, I immediately shut it off and checked the carb and the bowl was still full so it definitely was getting enough gas. Thanks for the idea tho Zaid Commented Jan 18, 2017 at 1:43
  • Dan, I believe that this old vehicle doesn't have an IAC, I think it's just got the little mixture screw in the carb. I'm definitely gonna be checking the whole dang thing for air leaks however. Thank you for the suggestions Commented Jan 18, 2017 at 1:44

2 Answers 2


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.

  • Thanks I'll give that a shot, my manual says to use a stethoscope to check for vacuum leaks, I'll do the water or carb cleaner first and then give the stethoscope a go Commented Jan 18, 2017 at 0:21
  • @CameronFloyd - A stethoscope is going to give you internal noises really well, but I'm not sure you'd hear a vacuum leak using one. Don't get me wrong, I'm a big proponent of the automotive stethoscope, I just don't think it's going to work very well in this situation. Commented Jan 18, 2017 at 0:23
  • Just by dumb luck, I think I found the problem. While listening to the vacuum lines, I bumped one and almost killed the engine. upon closer inspection, the tube was completely bent in half and letting no air through. I took a magic marker and cut both ends off, and slid it over the tube to keep it straight and now it runs much better. It has started backfiring now though when let off WOT. I think its backfiring, that is. Its making several popping sounds as it goes from ~3000 rpms to 700 Edit: essentially I'm asking If the popping is bad or not Commented Jan 18, 2017 at 17:03
  • Don't forget that on some carburetors, they use engine vacuum to control spark timing. If that is off, you'll see lots of backfiring. Also, does this vehicle require the use of a timing light to align the distributor cap to the cam shaft? I wouldn't think so, but I don't remember the 1990's (its all a haze)
    – zipzit
    Commented Jan 18, 2017 at 17:47
  • Just checked the manual and it says to use a timing light and a tach to set it/check it. I have one of each buy I have never used them before, I'll try to figure it out and report back here, thanks for the suggestion Commented Jan 18, 2017 at 19:11

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.)

  • The current distributer and cap works, I fitted a new one on and went for a test drive and it drive the same so I returned that to AutoZone. There was just a little green corrosion from copper (I assume) inside and I took some 1500 grit sandpaper and got rid of that. I'll get that cylinder test done ASAP, I tried to do that one time with my bare hand and that went fantastic. I got zapped no less than 10 times per wire. That was tried for a different uses though so I will do it again. Thanks for the suggestions Commented Jan 18, 2017 at 0:25
  • I did the cylinder disconnect test a few minutes ago and each one made it run significantly worse when unplugged, so they're all firing Commented Jan 18, 2017 at 16:13
  • Concur. So you are okay on spark! One question, mentioned in my comment above. Does this vehicle require a timing light to align the distributor to the camshaft? Is that set up correctly? If you are off the vehicle will run poorly.
    – zipzit
    Commented Jan 18, 2017 at 17:48
  • Theres a hissing noise coming up thru the carb, could that be the problem? What should I do to fix this Commented Jan 20, 2017 at 18:19
  • It turned out to be a mulfunctioning egr. Not sure how it was malfunctioning, but it was. Took me 1 golf tee to plug the vacuum line to it and it runs like a dream now Commented Jan 29, 2017 at 1:26

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