intake manifold leak?

[nakoni]

Hello from a "newbie"

I just joined iRV2, this being my first post

I own a 1984 Holiday Rambler, 34', class A, P30, with a 454 engine. Under moderate and up acceleration I hear an engine knock which I thought was a timing problem. And though I just had new plugs, dist cap & rotor Installed I still have a very slight (and about every 5 seconds) engine mis which is only noticed if standing outside by the exhaust pipe. A mobile mechanic said these problems are caused from an intake manifold leak. I do not hear any "hiss" of air intake and the engine feels very smooth at idle (again that pesky mis is only noticed if I'm standing outside by the tailpipe).

I am hoping someone reading this may have knowlenge of, or advise, before I put out the $$$ for this work. I have had very bad luck with the few mechanics that will work on motorhomes in my area.

I happened upon this site by accident and feel very fortunate in doing so.

Thank you for any advice you may have

Nakoni

[gauto5150]

Originally Posted by nakoni

Hello from a "newbie"

I just joined iRV2, this being my first post

I own a 1984 Holiday Rambler, 34', class A, P30, with a 454 engine. Under moderate and up acceleration I hear an engine knock which I thought was a timing problem. And though I just had new plugs, dist cap & rotor Installed I still have a very slight (and about every 5 seconds) engine mis which is only noticed if standing outside by the exhaust pipe. A mobile mechanic said these problems are caused from an intake manifold leak. I do not hear any "hiss" of air intake and the engine feels very smooth at idle (again that pesky mis is only noticed if I'm standing outside by the tailpipe).

I am hoping someone reading this may have knowlenge of, or advise, before I put out the $$$ for this work. I have had very bad luck with the few mechanics that will work on motorhomes in my area.

I happened upon this site by accident and feel very fortunate in doing so.

Thank you for any advice you may have

Nakoni

Your engine should have an EGR valve. When the EGR valve is activated, the timing is advanced to compensated for the "bog" the engine receives after the initial activation of the EGR valve.
If the valve isn't opening or disconnected, the advance will cause a pre-detonation, or spark knock.
Now I don't know if that's what you're hearing but I've repaired cars and trucks for a long time and that was always a common problem on the older engines.
The advance system for the timing is operated by vacuum or electronically, off the top of my head I think yours is electronic but I could be mistaken.
I hope this helps...

[jfran304]

Hi Nakoni,

GM has had a problem with intake manifold gasket leaks. If you can get to the intake manifold it isn't that hard to replace the gaskets. They make an aftermarket gasket that isn't supposed to leak like the OEM ones. I've had to replace the ones on my 88 5.7L Silverado and on our 98 5.7L GMC Yukon.

The way I found out it was the intake manifold gaskets leaking was on the 88 I sprayed the top of the engine with soapy water while the motor was running and I could see the bubbles. On the 98 I started getting oil in the radiator. I never noticed the engine missing on either vehicle.

Good Luck,
Jon

[Midniteoyl]

Use WD40 to spray the intake manifold gasket. If its leaking, the RPMs will increase.. Use the same trick on the carb gaskets too..

[leadman]

The advance on your engine is mechanical. If your are hearing a knock or ping this can be caused by several things, with some related to your intermident misfire at idle.
An inspection of the mechanical weights and springs under the distributor rotor in the distributor cap is a good thing to check as this is usually neglected by techs during a tune-up. If you see alot of rust and the springs are loose and weights loose on the pins a rebuild or replacement might be in order.
The EGR valve can be carboned up an sticking causing a vac leak that results in a lean fuel mixture, thus a ping under load and intermittent misfire at idle. Stalling may also occur.
A carb base gasket or intake gasket can cause much of the same issues. Also inspect for a stuck PCV valve or any cracked or broken vacum lines. There are normally a couple lines going up to the bottom of the air filter housing that can fall off. Some rigs have a vacuum controlled heater valve in line on a heater hose that can also leak, along with the dash control mechanism.
I have also seen misadjustment of the carb idle screws causing a misfire like you have at idle. Most techs nowdays aren't familar with how to properly set a carb so they back the screws out to get the highest idle then turn the idle screw on the butterflys out to lower the idle. Here in AZ we have emissions testing so casrbs have to set correctly to pass. If you have questions on this PM me.
The advice on testing with water or WD40 is sound advice but use caution with the wd40 as it is flammable, being mostly solvent.

[nakoni]

I want to thank those who offered advice in helping me solve my problems. I learned that the company (nation wide and well known) who claimed to have tuned my engine and replace my master cylinder with a "new" one did neither but charged me just over $1500. I'll be suing.

QUUSTION: jfran304 and leadman mentioned the EGR valve but I could not see one. Would a 1984 454 have one? If yes where would it be? Yesterday a mobile mech came out and replaced NEW plugs, dist cap, and rotor. The engine knock is still present though not as pronounced and needing greater acceleration to hear.

Thanks again folks. Glad to be a part of this community of RV'ers. Definitely a lucky find.

Nakoni

[leadman]

If it has an EGR valve it will be on the intake manifold close to the carb. Can say positively your rig will have one but if you call a Chevy dealer with the VIN they wil be able to tell you.