I've been having an intermittent overheating problem on my '97 4 liter ohv. It acts like there's air trapped in the coolant system: 1 minute it gets almost all the way to 'H' on the gauge then a minute later after taking a sharp turn or heading up a steep hill it'll drop back down to normal. I am also having a slight loss of of coolant in the system. No leaks on the ground or noticeable smoke from the exhaust.

I addition to this, I have some stumbling on start up. Doesn't happen all the time but does so more when engine is warm. Feels like a cylinder misfire. When I bought the truck not long ago, it was only running on 5 cylinders and traced it down to a cracked plug (porcelain broke). I did check for vacuum leaks which, if large enough can give the feeling of a misfire as well, but found none.

The other day I decided to test each cylinder and pressurized each to around 80psi at TDC with rad cap off. Coolant spilled out on two cylinders, driver's side closest to radiator and passenger side closest to firewall.

the worse offender being the same cyl where the plug had broken.

I'm wondering if it's safe to assume at this point that the stumbling, cracked plug and overheating all point to a head gasket failure or crack(s) in the head somewhere.

Any thoughts?

 

With regard to the temperature spikes/dips, I initially was going to suggest something simple like changing out the t-stat, but then I read the part about coolant spilling out of cylinders during the compression test and, well, that's a game-changer.

It's interesting that coolant is getting into cylinders on both sides of the engine, which would seem to indicate that both head gaskets have failed. That is possible, especially during a severe overheat situation (or several). This is probably a "worst case" type of scenario because of possible damage to the heads themselves.

On the other hand, a failed lower intake manifold gasket can also allow coolant to get into the cylinders. It's notable that, in your case the #3 cylinder (passenger side closest to firewall) and #4 cylinder (driver's side closest to radiator), happen to be the cylinders that are closest to coolant ports.

In any case, if you are going to be tearing down the engine to get at the heads, the lower intake manifold has to be removed first, and the LIM gasket can be inspected at that time.

 

Thnx for the reply, Rockledge.

From what I've been reading, it's not uncommon for this engine to develop cracks in the head. Was watching a utube vid last night where the guy found one on his (cyl #3 like mine) which allowed coolant to enter. Maybe I've got that going on, too, in addition to gasket failure.

Anyway, I have a spare engine from a '95 I might try dropping in there instead of messing with a tear down. That's if it'll work in a '97.