I have a 1998 4Runner with 416000 miles. Compression is at 145-150 is it worth doing another timing belt kit. The motor sounds and runs great. No oil leaks and doesn’t burn any either. Curious how long I should expect it to last and if I do the belt kit should the heads be done as well. Any input would be greatly appreciated.

 

That reading sounds a bit low, but it could be the gauge reading low, could be the way you're testing it. Do you have all the plugs pulled, and the throttle held wide open when cranking?

If so, try squirting a small amount of oil in the cylinders and retesting, see if the number jumps up. If not, perhaps a leakdown test to see where the compression is going?

And if it has actual lower compression, but it's not burning/using oil, then you might be losing a bit of HP as crankcase blowby, but it will probably keep on soldiering on for a long while. An consistent reading across all the cylinders is almost more important than the number itself, it indicates there are no emerging issues in any particular cylinder(s).

While the timing belt/front end of the engine is halfway to having the heads off, it's still only half way. I wouldn't be pulling the heads speculatively if it was my truck.

 

since you can do a timing belt without removing the intake it takes way more work to do the heads.
what's the rest of the truck like? frame rust?
if you're doing the work yourself slap a timing belt and water pump in it and go for a record!
it you're paying someone else then you should consider cost compared to you truck's value.
If it's a P.O.S. to the M.A.X. run it til it fails and find a quiet resting place for it.

 

@aowRS is approaching (if not surpassing) 500k miles. Well maintained these should be near million mile motors. JDM engineering is built to last in the jungles and deserts of the world far from dealership parts counters.

As far as your 140psi readings, if they are all within ~10psi, I would change the belt and pump and keep on running. That reading could indicate a misadjusted gauge.

At 400k miles you wouldn’t be crazy for a motor refresh, pulling the heads to see the crosshatch and maybe new rings and valve seals, but that is all going to be contingent on the condition of the rest of the vehicle. No sense spending $1500 on a motor refresh on a truck that’s worth $500 as a rolling rust scab.

 

I'm just shy of 497,000 miles - original owner. The engine & transmission have never been out and I do every last bit of maintenance myself. The first timing belt job was at 171,000 miles, the second one at 350,000 miles. So using that extended-interval logic, I'll tear into it again for timing belt job #3 around 520,000 miles.

My truck still looks very nice, has the original paint, is very clean on the inside, and has nothing worrisome with regard to frame rust.

So, I'd say do a timing belt job and carry on. When was your last one? BTW, I've never checked compression on this engine, and drive it pretty hard.



Andreas