Ok so this problem has been plaguing me for some time on my 99 4Runner. I recently finished rebuilding the engine, having the car parked for about a year. I went to start it up the first time and it just turned slowly but wouldn’t start. There was an occasional boom some times while trying but that was it. I replaced the battery and the ancient starter but nothing changed. The alternator is relatively new (less than a year old when I parked it) so I’m completely out of ideas here. The terminals look good and tight, all connections on the starter and alternator are good and everything was cleaned with electrical part cleaner. I’m totally lost at this point and just want to get this thing started!! ANY help is GREATLY appreciated..

What's the resting voltage on the battery? Even a new one might be discharged... Then check all grounds (including the one behind the engine to the firewall), battery connections, etc.

-Charlie

It reads 12.7 volts resting. I checked the ground to the firewall and that is good, and the ground on the drivers side is grounded to the engine mount. Would this perhaps make any difference versus being grounded to the engine block?

Engine ground is right under the oil filter just above the sealing surface of the oil pan. Clean that spot and tighten the bolt.

How long was the rebuilt engine sitting for?

The block was completed about 6 or 7 months ago and everything was put back together a little over a month ago.

Did the engine ever run properly after the rebuild?

Maybe take the belts off and take the plugs out and

rotate the engine by the crank bolt and see how it feels.

I would then be taking the timing covers off and verify timing.

Sounds like it could be out of time.

No I haven’t gotten it to start since the rebuild. Moving the ground didn’t help either. I tried turning it by hand before, and it moves fine up to a point, then moves fine again. I’m not having to lay on the wrench but it’s definitely more stiff in a certain spot. This is with all the belts and stuff still on.

With spark plugs in the engine, there will be stiffer spots at each compression cycle (so, 3 per engine rotation). You will hear sucking/blowing sounds as you force air past the rings.

-Charlie

Hate to say it, but rebuilding engines is an expertise. It's not a simple job of just replacing parts. Precise machining and measuring needs to take place for the job to be done well and experience in this sort of work comes from working as an apprentice under a journeyman engine builder to learn all the tricks of the trade. There's a reason why even the most experienced DIY Mechanics will pay a reputable shop to perform an engine rebuild for them. Same goes for setting up gears in differentials. Sure, you could get it right the first time, but there's also a very good chance you don't and now you're back to square one after spending a lot on parts and labor at a machine shop that hopefully knew what they were doing.

Maybe I'm wrong. I've been wrong plenty of times. But then again, maybe I'm right and the engine needs to be gone through again.

Oh yeah, that’s for sure. I’m no expert by any means, I only do this stuff on my 4Runners, usually referencing your videos for some support as well. I had the bottom end done by a machine shop I trust (they did a 3.4 block for me once before) and everything there seemed good. The only thing I did not replace mechanically was inside the heads, so no camshafts, no valves or valve springs/stem seals. Would this cause slow cranking? I can’t get a clear answer researching whether or not worn or defective valves would cause slow cranking. I wouldn’t be against replacing them if that’s the issues since I’ve done valves on another 4Runner head rebuild, I just don’t know if that’s the issue. At this point I’d be happy if this was the case, so I can get new valves in and get it going.

For more reference on where I’m at now, timing looked good when I checked again, and it seems to spin normally by hand. Just spins very slow, with an occasional misfire.

I had someone hold a multimeter while I started the engine to measure the voltage drop. How low should the voltage drop when trying to start? Denso’s page says anywhere within the range of 0.2-0.4, while I’ve seen other places mention a drop to 9.8 volts on the starter is normal? Mine read 9.6 volts when trying to start.

Worn valves wouldn't cause slow cranking. It could affect your cylinder compression though.

Welp. For any future wrenchers stumbling upon this… here’s the summary:
When you go online to look up “slow cranking” and you see “bad starter” ……
ITS A BAD STARTER ….
And yes, even your rebuilt Denso starter.

Thanks everyone for your input, as well. It really helped me narrow this down until I quite literally had no other choice other than to look at what everything points to online.. A bad starter. Specifically (to my understanding) the solenoid.