rivets

This has just been happening recently. The truck ('97 4WD Limited / 159k) is completely stock. I've already replaced the plugs/wires/fuel filter/timing belt/acc belt/water pump/thermostat. I have yet to change out the tranny fluid or the brakes which are currently vibrating a lot during stops.

At highway speeds, around 70mph (2200-2600rpm), while keeping a steady foot on the accelerator, the RPM will suddenly drop and rise within a 200-300 rpm range. It is as if the engine is trying to shift or re-position itself. This will happen for a brief 2-3 seconds.

On a 45 mile commute, this happens maybe once or twice AND happens every time in the past two weeks.

Possible related issue:
Sometimes, when the truck is coming to a stop or leaving a full stop, there's a "thunk" coming from the front/under the car. Seems like a hard downshift. On rare occasions, it will slightly hesitate when leaving the stop.

When I went for a alignment, the shop had said that the battery is on its way out with a tested 400 CCA. Don't think it's related but I figured I'd throw it out there anyways.

Any ideas on what's causing this strange bogging? Tranny going out? TPS? Seized calipers? Fuel pump? Bad fuel?

BigBallsMcFalls

check for strawberry milkshake


clean the maf

250000_yota

definitely check the trans fluid. anything but nice, clean red is old or an issue. if you've never changed it, chances are it is in need. also smell it. if it has a nice burnt smell, then your definitely in need of new fluid.

and on the battery, what is the CCA rating on the battery? 400 sounds like a pretty good battery to me. at our shop, we load test the battery to 1/2 the CCA at 9.6 volts for 15 seconds. it should be able to hold half or higher. any lower, and we warn the customer, drastically lower or dropping from half, we recommend a battery replacement.

SkidPalace

Don't worry about the tranny thump. It is a known issue with the slip yoke on your drive shaft. When you come to a stop, your rear end gets "pulled" to the back to the truck, extending the shaft. This slip yoke can get stuck in this extended position with old/bad/dry/dirty grease. When you let off the brakes, the axle tries to return to its normal position and you hear a thump when the slip yoke unsticks.
You would have found MANY posts reagrding this if you searched for it. But I am not here to lecture you on using the search button.
Find the zerk on the dive shaft and pump a ton of grease into it. Get all eight zerks on your two drive shafts & u-joints while you are down there. Some have said you need use to a grease with Moly in it or the thump will come back after a while. I just use regular all-purpose grease as I don't mind greasing all of my fittings on a regular basis.

As for the rpm fluctuation, I can only guess throttle position sensor? It's a cheap part and probably needs to changed/cleaned anyway.

ScottyC

iTrader: (1)

Torque converter is likely going out. It would explain the RPM fluctuation and thunk coming to a stop.

But, I would do a fluid flush and filter first to see if something is sticking. Do you have any check engine codes?

SkidPalace

As for the battery, the shop was probably just trying to upsell you. You should know if the battery is bad. Does your truck struggle to start? How old is it? ~6 years is about as much as you can expect from a decent battery. If in doubt and you live in a cold weather environment, bite the bullet and put a new one in yourself. ~$60 at Costco.

SkidPalace

Dang, after re-reading your symptoms, it could be something like Scotty says. The drive shaft thump occurs almost always AFTER coming to a stop, and it will never cause a hesitation.
Good luck.

gdutch

I had similar symptoms when I was experiencing a rear axle seal leak (a known problem on this generation of 4Runner / Taco). The seals go bad letting diff fluid leak out, which can run down the axle and get in your rear brakes causing them to stick. When I was having this problem, it would sometimes cause the truck to shake when taking off from a stop.
Also, if your brakes are dragging it would make sense that your engine is having to rev more at highway speed. When you're going that fast, the diff fluid is going to heat up and become more viscous, making it easier for it to run down and get in the brakes. I would check this immediately, it should be pretty obvious if you're leaking. If that's your problem, here's how to fix it:
https://www.yotatech.com/forums/f128...4runner-76339/

After that I would suggest you look into a rear brake job / do it yourself, as dragging brakes like that will burn up pretty quickly.

rivets

SOLVED! - for now.

Thanks for all your help, guys.

So, since my last post I've changed the motor oil/filter, ran a bottle of fuel system cleaner, and did a full transmission flush. No indication of strawberry milkshake. Coolant looked fine and the tranny fluid seemed fine as well albeit a faint burnt smell.

The problem still existed although the transmission was shifting more smoothly. I still experienced hesitations at 69-70mph. Seemed to be getting progressively worse, too.

Today, on my morning commute, the RPM surge started again and this time instead of fighting the engine, I decided to let the RPM dip. It fell from 2500rpm to 1800rpm. That's when the CEL came on (finally). I pulled over and ran the code on my trusty $30 harbor freight OBDII scanner. Pulled a P0120 code. Throttle position sensor.

Called the nearest Kragen and they had one in stock. $50 later and 5 minutes in the parking lot and the car is running fine again.

Littleladyjen

So I just bought a toyota sequoia 2002 and I'm having the same issue. I already replaced the radiator and now I'm pretty sure my tps needs to be replaced but I can't for the life of me find it. I know the general area it would be in and have the replacement part. If anyone can help me out that would be awesome!