manifold vacuum

[tfret]

5.7L MCM260, I'm only pulling about 10 ~ 12 inches of vacuum at the port in the top of the manifold. The manual says 18~20 is normal. I have leak tested with Propane and WD40 and can't find anything. I also checked that the manifold bolts are torqued at 30lbs. Any suggestions on what to look for? Also, there are two threaded holes on the left (port?) side of the manifold that have nothing in them (see attached). What are they for and could they be leaking? Thanks for any help/suggestions.

 

[bomar76]

Re: manifold vacuum

5.7L MCM260, I'm only pulling about 10 ~ 12 inches of vacuum at the port in the top of the manifold. The manual says 18~20 is normal. I have leak tested with Propane and WD40 and can't find anything. I also checked that the manifold bolts are torqued at 30lbs. Any suggestions on what to look for? Also, there are two threaded holes on the left (port?) side of the manifold that have nothing in them (see attached). What are they for and could they be leaking? Thanks for any help/suggestions.

Nothing goes in those holes, pretend you don't see them.

Some Low STEADY vacuum causes:

1. Retarded (Late) ignition timing.....have you checked that?
2. Late valve timing...did some one have the engine apart and screw up the valve timing?
3. Intake leak....Sometimes the propane method does not find them...and try replacing the carb to manifold gasket...did you check there?

 

[bomar76]

Re: manifold vacuum

In looking at your pic I can see someone has had the carb off for whatever reason.....and I would bet they never replaced the carb to intake gasket...gaskets are one time use.

 

[tfret]

Re: manifold vacuum

Yes, I have checked the timing and it's right on the mark.
I replaced the carb last year and used a new gasket.
I determined by turning the crank pully by hand while watching the distributor rotor, that I have 6 degrees of "slack" in the timing chain. Other than that I am assuming the valves are timed correctly. It is an 84' model that I bought used 4 years ago. The compression is around 150 on all 8. Something must be wrong somewhere. I really appreciate any tips you can share.

 

[Fishermark]

Re: manifold vacuum

Is there a reason you are checking your manifold vacuum? In other words, is the engine running rough or soemthing? Hard to start? Or is it just a case of you doing some routine checking?

If it isn't running bad I would leave it alone.

 

[tfret]

Re: manifold vacuum

Is there a reason you are checking your manifold vacuum? In other words, is the engine running rough or soemthing? Hard to start? Or is it just a case of you doing some routine checking?

If it isn't running bad I would leave it alone.

As far as I can tell it runs good. The only thing is the idle screws on the carb must be set rich (out 3.5 turns) in order to get it to idle. Otherwise it will stall. That's what got me checking things.