Alllan20

Hello, this is my first post here to hopefully find a solution.
I currently was gifted a 1990 4Runner with the v6. For a number of years, it has had a problem with shaking at idle. Allow me to explain. When in park, it feels smooth as it should around 1000 RPMs when warm. However, once in any driving gear, it drops to the 500-800 RPM range and shakes unbearably. This fixes itself once you get going to normal driving speeds past that rev range. But it never ceases. Things to note are that from the exhaust you can smell hints of raw fuel as well as the inconsistent pulsation. Interestingly enough, when testing for spark from the distributor, cylinders 3 and 6 do not make a difference when unplugged. They have good compression and wires but still don’t work. Other than that, the things replaced are the injectors, spark plugs, and ignition coil. All of which made no difference. The questionable mechanic shop that had worked on it before hadn’t even bothered to correctly aligned the distributor, but it will only run correctly at 30 degrees before TDC. I have my speculations as to why this is, but still we are left absolutely stumped. I would appreciate the help.

rattlewagon

Am I reading that right? cylinder 3 and 6 are not firing? That would explain the gas smell and running rough.

Sounds like the vibration is from the engine not running well. But could it be worn out engine mounts? If you let off the brakes does the shaking go away?

The dist being off that much could be that the timing belt is off a tooth. I would check timing, also a good idea to check the throttle position sensor. Have you checked for check engine codes? Even if the light isnt on there could be codes stored.

You said "for a number of years" How long has it been like this? Gas mileage ok (for a 3.0 4runner lol) ? Loss of oil or coolant?

maxvp01

Check for carbon tracking on your distributor cap, it will look like a white line and it makes a path from one of the contacts to ground pretty much.

Also check if any of the molded in contacts in the cap are missing, i’ve seen on cheaper distributor caps that some have gone MIA or have been molded over by plastic (bad quality control)

Inspect rotor button for corrosion, if it looks slightly dirty try lightly sanding it to expose some fresh metal and try.

You are for sure getting spark to the distributor, and to 4 plugs. so it’s possible spark isn’t an issue, but check those things i mentioned above just to confirm. they are easy to check.

If you did skip a tooth, doing a compression test might be beneficial to figure out if your valve timing is where it should be. Other than pulling the upper timing cover.

Are your injectors aftermarket or reman? I had gotten a set of aftermarket ones and the tips melted off 4 of the 6 and it caused all sorts of problems. I didn’t think that was the problem because they were new. That mistake made me take months to figure out my issue. I also advanced a LOT to I think 26 degrees to compensate for that issue.

Alllan20

To begin answering, the shaking does stop as you get up to speed, but not at a standstill or just off of a stop. Not sure how exactly to check the throttle position sensor or how that would effect it. As for timing we have yet to determine but I suppose we will once we decide to go with replacing it with the kit we have. It is possible that it could be off or something, the belt has been on there for 110,000 miles. I’ve just checked for codes despite no light and interestingly found there was one stored. It blinks code 51 which is connected to the troubles areas of A/C switch circuit, A/C switch, A/C amplifier, TPS circuit, the TPS, and ECU. Continuing down the list, I say “number of years” because I couldn’t get a definitive answer from my grandfather or the others who drove it other than yes it did in fact shake. As for fuel consumption is slightly down. Not sure by how much because I have yet to check. Although I could be misjudging just by figuring it’s a 17 gallon tank. No loss of oil or coolant other than a the passenger side valve cover gasket is leaking but still remains full. Albeit over full. Also no Coolant in the oil despite being the original head gaskets. And the oil does not smell of fuel. When looking at the distributor cap and points, I see nothing out of the ordinary or nothing at all. On the rotor button, it looked as it should but lightly sanded it and the rotor to be safe. Not too sure how weird the spark is, but if you pull off plug 6 and compare it to plug 5, they seem to be running about the same. Yet to check the others on this set of plugs. As for a compression test, they seemed to be in very healthy condition for their prolonged use. For my fuel injectors, they’re the Ultra Powers. They seem fine but showed not very substantial enough of a difference from the original ones. And that’s about as far as I’ve gotten with this.