Hey all 1994 4Runner, 3VZE, A/T, 4WD

I am experiencing engine rpms drop under certain conditions. I can recreate the situation while driving and at stand still.

This happens while driving at ~45mph as soon as I take my foot off the gas. Say I'm cresting a hill at 45mph and I get to the top, take my foot off the gas. In this scenario the engine rpms will drop down to 1.5k once...twice...sometimes three times before they figure themselves out. It's not just the tach - I can feel the loss of power. Anyways, thats the anecdotal story - here is the easier way to recreate at standstill:

- Put in neutral
- Engine is at operating temp (NOTE: I do not experience any issue with a cold engine)
- Put foot on gas and bring the rpms up to 2k.

In this laboratory setting - I also experience the engine faltering and dropping to 1.5k 2-3 times. For this reason I have ruled out the transmission since the 4runner is is neutral the whole time. I have checked and tested the fuel injectors and spark plugs and they are good. I am thinking that this is some kind of vacuum leak, maybe in the EVAP system? Or fuel pressure issue?

Does anyone have any advice on how to proceed debugging? Interesting that the issue is not there when the engine is cold. Is that a clue?

 

Ha I think I replied on the 2nd gen subreddit too, had some weird dejavu when reading this again. The major differences when the motor is cold is open vs closed loop and the cold start injector is adding extra fuel initially.

1.) Vacuum leaks are abound with the spaghetti mess of vacuum lines on these motors, use starting fluid or brake clean to spray around the lines and listen for engine pitch changes
2.) TPS, you can find tons of articles on this site on how to test/troubleshoot possible TPS issues
3.) When is the last tuneup the ignition system had?

I don't know if these trucks have a limp mode but check for CEL's, specifically thinking of a possible knock sensor code but anything might help point you in the right direction.

 

So I had the TPS calibrated about 6 months ago at the dealership. Was having a funny misfire and went to Toyota and asked them to calibrate it since its such a finicky thing to calibrate. I believe I can rule that out. I'll try looking for a vacuum leak. I know that my fuel filler neck has acrack in it somewhere so I was wondering if a vacuum leak in the EVAP system could be the cause.

What exactly is an engine tune up? I'm actually quite new to all things automotive so still learning. @COMTB

 

I would suspect an ignition issue. In your "laboratory," try putting the inductive pickup of your timing light on each plug wire. If you have a plug misfire, you should see the flashing stop for a moment on the offending plug. A possible explanation for this is a cracked plug wire, so try wiggling the wire during your test. (Be careful; if you have an insulation crack and get your hand near it, you could get quite a wallop.)

 

Scope brings up a good point if you have a timing light that would quickly tell you if you have an ignition issue. Either as mentioned using the clamp on each plug wire or even just checking your timing at idle to see if it is constant or it jumps around (signs of a misfire)

For a tuneup, usually that means checking/replacing Plugs, Plug Wires, distributor cap, and rotor at a minimum. Additional things to check are your air filter and all your fluids (these are not necessarily related to your issue but worth mentioning). You said you tested the plugs and they were good how did you test them? Just verifying spark isn't always enough because a very weak spark can cause problems. You can use an ohmmeter to verify the plug wires (you should be able to find a manufacturer spec for your wires online), cap/rotor is something I replaced when I first got my truck just for piece of mind and knowing when it was done but if there is significant wear, corrosion, or buildup on the contacts this can also cause problems with the ignition system.