Hello all. I just purchased this blazer from the depths of craigslist for $1500. 1998 blazer 4 door 4x4 with 148k miles on it. I looked over this blazer extensively for multiple days, along with multiple startups, cold starts and whatnot and didn't hear a thing.

Bought it, brought it home and went to start, and heard what appeared to be a rod knock for about one second and then it started. Once it starts there's no knocking, no ticks, nothing, a quiet, good running motor and I did drive it around a couple times the last few days and it runs perfect with plenty of power.

So today I was driving it and my oil pressure would go to 0 at idle, and would go to around 40 PSI cruising at 40 mph, and raise and drop respectively, but would drop to 0 at idle and the "check gauges" light would come on.

I checked the oil and it looked okay (im changing it tomorrow) and would like to know what might help this situation. Could it be low fuel pressure from a bad oil pump? Could it be a bad oil pressure sensor? Could it be worn crank bearings or piston slap? Old oil that lost viscocity? Bad oil filter?

I have knowledge on motorcycles but I know nothing about cars. I heard the gauges on these blazers are junk anyway but doesn't answer why there's a rod knock for one second. Any answers will help.

 

With 148k miles, the engine should still have plenty of life in it provided it was maintained fairly well in its former years.

The oil pressure gauge in the dash is only an indicator, with a level of accuracy well below that of a dedicated mechanical gauge. You should look into hooking up a mechanical gauge under the hood and see if it shows the same.

With the information you provided though, I am leaning towards excessive bearing clearances leading towards low oil pressure at idle.

Depending on where you live, you could try a higher weight oil like 10w40 and see how that goes. Worst case scenario, you are out the cost of the oil if it doesn't help and you want to switch back to the standard 5w30.

My K5 has some not so nice noises that were quelled by using 10w40. It is a bit of a bear to turn over when it's cold out, but I am shooting for whatever keeps the engine together the longest. I do have two other 4-bolt main 350's that could be dumped in with a little work.

 

I live in ohio and there is an upcoming brutal winter so what do you suggest?

ALSO, I read places where these GM motors will build up excessive carbon and put 1 pint of seafoam into a full tank of gas and drive it like you stole it for 20 miles and it cured a knock. I've seen it done on a 4.0 jeep. Worth a try?

 

I know a guy that has a motor with 100k miles that runs still in the junker for $400. I'm thinking about buying it and putting it away until this motor cans if that's the issue.

 

For the heavier weight oil, you could try just a cheap bottle of oil from Walmart, running it for a week to see if it helps. If it does, it points to either a failing oil pump (not likely, but possible) or excessive bearing clearances.

Carbon buildup typically results in a knocking noise once fully warmed up and you get on it going down the road. You could try running seafoam through the motor, but I would recommend running it through the throttle body. With a vertically oriented throttle body like the 4.3L engine has use the following process:

• Remove intake duct work from the throttle body
• Disconnect MAF sensor (electrical connector in duct work near air filter housing)
• With the engine running, dribble 2/3rds of the bottle into the throttle body, using your hand on the throttle linkage to keep the engine running at a slightly elevated idle.
• Pour the last 1/3rd of the bottle into the throttle body and let the engine stall.
• Let the engine sit for at least 30 minutes
• Drive it like you stole it until the smoke clears!!!

It is best to change your oil after this process as well as to at least pull the spark plugs to check for any chunks of carbon stuck in the plugs that might reduce their performance and/or life expectancy.


If you can hear the motor run and can verify that it is working properly, it wouldn't be a bad idea to pick it up. If you can't hear it run, you may want to open it up to inspect the internals prior to dropping it in. It would suck to find a new problem or the exact same problem after putting the replacement in.

 

I'll also try putting some into the oil and running it for for 5-10 mins and see if it's got a clogged lifter or filter, then I'll take it somewhere to get the oil changed and put heavier weight oil in. Good idea?

 

Could be the oil pump bypass valve is stuck open. A broken spring would prevent the oil pump from building sufficient pressure. Cheap, easy fix without having to remove the oil pan. The diagram below shows how the bypass valve works. The valve on the 4.3L is pressed into the block. The oil filter adapter must be removed to access it. If I remember right, the valve is under $10. Might be worth a try

 

How do I go about changing it? Never messed with this stuff.

 

Have you hooked up a mech gauge to see if the pressure is actually low? I would do that before changing any parts.

 

I have not done that......