This is just like the old coffee shop, which was like the old coffee shop down the street. Pull up a chair, sit down and enjoy some good discussions. Please, no political, religious or racial posts.
Maybe someone can steer me on this one
My Son's well maintained 92 Chevy 1/2 ton has suddenly developed this atrocious detonation knock at driving speed. The truck is a 350CI five speed manual.
Temperature and oil pressure are normal, idle is very smooth, and today he changed out the spark plugs that after 25,000 miles were just fine for their age. Engine is not lacking in power.
I'm thinking about changing the engine knock sensor located toward the bottom of the engine block next before sending it off to the garage if that does not cure the issue.
Are there any other things that I might take a look at along the way before handing it off to a tech?
Help he has my Sierra and I want it back
I totally agree with that Paul however I went ahead and changed it anyway. There was no change. That was probably all for the better anyhow because there was a wall of rust between the sensor head and inside the of the water jacket not allowing the coolant to directly contact the sensor. I poked a small file into the hole until coolant gushed out and installed the new one, the old one looked like a rusted hunk anyway so that's that.
My mechanic suspects a defective intake manifold gasket and he is going to do an O2 test and take the computer readout first even tough no check engine light shows. It has the earlier OBD 1 system. He tells me the issue is common after he head to intake miter angle was altered during the 80's
I have an engine show coming up at Bernardston, MA. this weekend and need my now borrowed towing rig back by Thursday 6PM sharp
Exhaust detection in the coolant system is not a good thing
Most likely a leaky head gasket, the top end will be getting pulled off by the garage.
In the meantime, I'm balls to the walls getting this spare set of heads together but I still need to pick up valve seals.
after grinding the valves the seats lapped down real nice.
Looks like the engine show is gonna get rained on anyhow
Ken I dont think the knock sensor goes in a coolant passage. Without doing anything else try to use a better grade of gas. Maybe high test to see it the knocking does away. You should be able to set the base timing. How many miles and what weight oil are you using. Do you have a manual way to check oil pressure?
Mike, oh there's a internal coolant leak going on alright- that's one thing for sure. there are traces of antifreeze on the newly installed #2 spark plug.
I want to get a set of fresh heads prepped before the mechanic tears it down. Used Chevy 191 truck heads are cheap and easy to find. I cleaned the spare head cores and lapped in the valves last night. The valves had been reground some years back stored away in a box and the valve seats cleaned up very nicely during the lapping, they did not require grinding. The valve seals will be in this Afternoon. I'm left with plenty of time- it will be Tuesday or Wednesday before the mechanic gets into it because of the holiday.
The knock sensor is located at the bottom of the engine block just above the oil pan directly in front of the starter and that is the very bottom of the coolant galley, hence rust accumulated in that area blocking the threaded port. Replacing it was not the solution but a $29. chance worth taking.
We both stick with Sunoco gasoline and the first thing he tried was a fill up with 93 octane, that did nothing.
Update, well just like everybody else he had to wait his turn at the garage. They got the heads off yesterday. A half inch breach in the gasket between #5 and #7.
A dyecheck showed a crack in the head at the same spot. So on go the fresh heads. Good news is the block shows little piston ridge wear and the cam galley was nice and clean. Should be done Friday, Maybe I can post a pic of the gasket later, it's an OMG fur sure
The gasket got discarded but I thought I'd post a pic of the other culprit.
Crack is directly opposite the exhaust gas recirculation port.
The crack extends right through into the exhaust valve seat! Probably pretty lucky that it did not fall out
After 236 thousand miles this was the first serious problem
She runs like a top now
he has my Sierra and I want it back 
