78 Blazer, sbc 400 knocking sound while in park..Updated.

[MuzachMT]

Hello,
I will start with useful information that may help in a diagnosis.
sbc 400, rebuilt 8k ago (not by me), bored 30 over, edelbrock 1406, edelbrock intake, hooker headers. Auto tranny, Idles at 1k in park, 6-700 in drive.

Problem: Recently I have noticed a knocking noise which seems to be coming from lower in the motor. Weird thing is that it is most noticeable in park, but also around 2k right before it shifts. It is pretty much unheard at idle while in drive. I bought this truck a couple of months ago already rebuilt so I don't know what cam, etc was used. All I have done to it is installed a fuel pressure regulator set at 5.5 psi, as that is what edelbrock says for the 1406 carb.

Another symptom that may or may not be related is a hesitation slightly after take-off (made a little better after the regulator).

I daily drive this thing about 2 miles each day, and need it to keep running ;P so any help would be greatly appreciated.

My mechanical knowledge and ability to do any major work is very limited, so I can't do any major troubleshooting, but any simple engine-in-truck ideas would be awesome!

**Pulled the inspection cover and the flexplate is fine, all bolts tight and not a single even hairline crack. Dammit. I was hoping that was it because now I can only expect it's a bearing..8k on a rebuild..must have been a lie or shoddy work!

 

[Deuling]

Put a pry bar between the harmonic balancer thats attached to the crank on the very bottom (will also have a pully on in front of balancer) and between the block. You should only be able to pull the crank out a few thousands of an inch. If its a real noticable movement, you have problems in the lower end. My 305 did this and i noticed i could move it out more than i shouldve been able too. It did the same thing. Still ran fine though.

 

[MTChevy]

Nothing to add except i miss belgrade

 

[MuzachMT]

It does still run good, but from what I just learned, that would lead to a lot of stress on the crankshaft if it is loose and allowing too much vibration? Is that something I can get to right from underneath without pulling anything? It's gotten to dark and cold to go check it out right now, but I will in the morning and probably answer my own question . Thank you

 

[MuzachMT]

Nothing to add except i miss belgrade

It's a nice area, just waiting now for flathead pass to open up so I can see what my truck can do (Assuming I don't throw a rod or tweak my crankshaft by then )

 

[MuzachMT]

Oh, would my timing being slightly off be a culprit to the knocking? I assume it could be for the hesitation problem, but no timing light atm.

 

[NorCal_Chris]

Check the flexplate while your at it for cracks

 

[toomany]

As said, check the flexplate for cracks. Also check for a loose torque converter. I've had exhaust leaks that sounded like a rod knock before too.

Having said that...I just went through this on an 02 Ford van. Knocked while idling and a little before each shift (when the r's got up there). Noise went away when you put it into gear. Turned out to be in the lower end. Bad main bearing was the culprit.

 

[MuzachMT]

I'd hope it isn't a bearing issue with only 8k on the motor.

Is the flexplate accessible on a 78 Blazer (turbo 350 tranny) any way besides pulling the tranny back or completely dropping it? Lack of money and mechanical ability will lead to a sale if not lol.

 

[NorCal_Chris]

You can pull the inspection cover and look at the flexplate pretty easy. Grab a flashlight and look around were other is attached to the crank it likes to crack there.

 

[TommyM]

Ditto flexplate. . . It's happened to me twice and the symptoms were EXACTLY as you described. Do a Google image search for "cracked flexplate" and you'll get a whole bunch of photos of what you're looking for.

Tommy