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Engine / mechanic trouble, 1998 Toyota Tacoma

My truck was recently “missing” the mechanic stated it needed a valve job. After completing the valve job ($1500) I drove the truck 40 miles then it died. The mechanic now says the increased engine pressure must have bent a rod and is now recommending replacing the crank and rod or replacing the “short block”. Is this a legitimate diagnosis and could the problem be attributed to the mechanics inital work? Am I being scamed?

What you are being told about the increased engine pressure bending a rod sounds suspicious at best.
Normal engine pressures will not bend a rod and if the crankshaft and rod must be replaced this means a total engine rebuild or replacement.

There are a lot of details not known about this incident but here’s a theory.
A valve job means removal of the cylinder heads and head gasket replacement of course.
If the job was not performed properly it’s possible for a head gasket to give up in one area or the other and dump coolant into a cylinder, or cylinders.

This in turn could cause a hydrolock situation. Hydrolock means fluid pours into the cylinder faster than it can be expelled past the exhaust valve. Since the fluid won’t compress this means a piston rising in the cylinder will slam to a stop and the engine will quit running instantly. This can very easily cause a bent rod and is not really that rare a thing.

Another possibility is a valve dropping into the cylinder and this will do the same thing.
The only difference is that the head gasket scenario may be a fairly quiet occurrence but the valve drop would likely be a catastrophic bang.

Without examination of the truck I can’t tell you for sure what happened but I can tell you that no normal engine pressure will bend a rod. Hope that helps you out some.

Thanks for the comment it is helpful. Here is a little more information if anyone is interested, when I picked up the truck from the valve job there was a quite knocking sound that was not present when I dropped it off. The knock became louder over the 40 miles I drove it before it failed. When the engine stalled it was like it lost pressure and died, it started again then knocked very loudly. Is there anything I can look for that would help determine if this was pre-existing or if it was due to the mechanics initial repair, for example damage to the piston, bent rod, or spark plug damage etc?

How many miles on this truck?? Was a compression test done at the very beginning to verify the need for a “valve job”??

The only plausible explanation is that the low compression cylinder was not making any noise because no load was being placed on it…After the valve job restored compression, the failed rod bearing started pounding now that it was working again… If this is a 22R engine, these have a reputation of lasting a long time if maintained…But nothing lasts forever…

Thanks for the replies. The engine has 160K on it. Pre-valve job it was probably at low compression (it had trouble starting, and sounded like it was missing sort of a muffled sound when running, which the mechanic attributed to a bad valve and low compression). The engine is the 4 cylinder, not sure what 22R is. So from Caddyman’s post it sounds like the mechanics explanation is plausible? Thanks again.

I think it will take someone who is very mechanically astute and who will have to spend some time eyeballing closely the piston tops, head gaskets, and valve train parts.

It is possible that coolant leaking into a cylinder can cause a knocking sound as it dilutes the fuel/air charge (in essence, lowering the octane a lot) and after a few miles this small leak may have increased to the point the engine hydrolocked.
If (still theorizing here) this is the case, the piston top and cylinder head combustion chamber may appear to be much cleaner than the others or it may even have a shiny finish.
Hot coolant will perform a pretty major cleaning on engine parts.

Another possibility is a screwup on the rocker arm setup leading to a valve being dropped. This would be very apparent by a damaged piston, mangled valve, etc.

Just offhand, based on the comment about increased engine pressure due to a valve job being the reason behind a bent rod and a knock from the get-go I have to say the first is pure BS and the last is an indicator of a screwup by even allowing that truck out the door in that condition.
Really, you should have turned it right around then and there instead of moving on but that’s beside the point.
Hope that helps anyway.