Engine Mechanical problem
1990 Toyota Pickup 4 cyl Four Wheel Drive Manual 178000 miles
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My truck always made a slight knocking noise that got louder as i let off the accelerator. I always assumed it was a badly adjusted valve. Recently, while climbing it got a lot louder. I pulled the valve cover off expecting to find a loose valve stem nut. They were all still tight. I drove another 15 miles when I heard a huge bang and the wheels locked up for a second. The knocking noise was now gone. I started heading for home when the car did the same thing a few more times. I pulled over and called for a tow truck. I tore the engine apart and cannot find any damage. The top end seems find and the rods are not broken the bearings on the cam shaft also seem ok. When I crank the engine by hand the pistons move freely with no strange noises. Any ideas?

Hi there,

Thank you for the donation.

Reading your question, i find one comment interesting, you say the wheels locked up for a second, if this was the case and the engine was still running i feel the problem is not in the engine, as you have already stripped the engine further testing and diagnosis is not possible, but if as you have, we have to think long and hard about what is going on. You say there is no indication as to a problem in the engine, with the sort of noise you were describing i feel that it would be fairly obvious if there was something wrong. I feel that with the knocking and the lock up, i would be interested in what the diff oil looks like, i would drain that and check for metal fragments first. have a look here and report your findings. Also as you have the engine down, pull the pistons and make sure that you don't have a cracked skirt as this will cause a knocking noise and drop off the main bearing caps and check that all is well there as well.

Mark (mhpautos)

I will check the differential oil and and pull the pistons and get back to you.

When I crank the engine by hand I feel some resistance at certain points in the stroke but no grating, grinding or clunking. I did notice some very deep grooves worn by the timing chain.

I cannot believe such a violent noise has not left any obvious signs. After the first bang, I coasted the truck down a long hill in third gear and it made no noise. When I got to the flat and started to accelerate it happened again but the noise stopped as soon as I let off the accelerator.

When I got home and the truck was loaded off the tow truck, I started the truck and nosed it into the garage. It sounded like I had rocks in the crank case the whole time until I shut it off. I tore it down before trying to diagnose the noise because the noise was so violently loud I expected pieces to literally fall out of the engine when I tore it down.

Hi there,

Just curious if you have found the problem yet?

Mark (mhpautos)

mhpautos wrote:

Hi there, Just curious if you have found the problem yet? Mark (mhpautos)

The loud bang was the timing chain guide being ripped off and hanging up on its way to the oil pan. The banging noise after that, sounding like a broken rod, was the guides bouncing around in the engine. The problem is the guides are plastic,but that is also the good thing in that the palstic did not cause any extra damage bouncing around inside there. A bit of research showed this to be a fairly common problem with Toyota truckes of this era. As a matter of fact, there are many after market steel versions of these guides to remedy the problem.

Also, without the guides, the chain wore a hole through my timing cover.

Hope this helps future posters.

Mike