351M warped intake manifold ??? - Ford Forum - Enthusiast Forums for Ford Owners


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Old 10-19-2016, 11:15 AM
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Default 351M warped intake manifold ???

Hello,I am installing a new intake manifold on my father 78 Grand Marquis,very clean car,with a 351M motor. I spray brake cleaner near the intake,front of the motor near the intake on the passenger side of the engine. Upon spraying the cleaner,the motor quit.The vacuum was stable at around 18,but as the engine warmed,the vacuum was all over from 18 to 20,and the motor idle fluctuated and would die out.
Three times now,installing a intake gasket,the end seals walked out on me,and I broke 2 intake bolts,number 9 and 10 on the torque,these are the 5/16 near the carb that are close together,as I could not get close to the lower number on the FT pounds of torque.
I used epoxy on the underside of the rubber end seals,solved that slippage problem,but installing the intake bolts.I can get 1 through 8,the larger bolts to torque,but after breaking 3 intake bolts,taking everything apart,drilling and removing,and replaced the bolts from the hardware store,grade 5 hardness.
Does it seem that maybe I may have a warped intake as I cannot get the 4 bolts around the carburetor to even get to the lowest torque settings?? I am going to try to tighten them by hand and see if they will work??Is this a good idea?? Thank you,Mark B.,Cedar Lake IN....Thank you in advance for any input or ideas,,Have a great day,,I am trying to????
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Old 10-19-2016, 12:02 PM
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Welcome to the site.

One thing that could be issue is if the heads to block surfaces have been cut too much. Checking the aftemarket (new meaning aftermarket?) for straight, good insurance, but doubt it would be bent unless used or prior installations. You never know though. If a used OE, could have already been cut for another application.

What that does is it changes not only the clearances tighter on the end seals since the manifold affectively sits lower in the valley, but can change surface angles of the intake manifold to heads and gaskets themselves. Generally using any sealer on the end seals (other then the seal gap at the intake gasket ends) decreases the available end seal clearance. Some with slip out, others may split the end seal laterally. If a contact type sealer is used, it should be very thin and only on the block side.

Silicons always seem to spit out the seal, even if allowed to set. What does the manifold look like if dry set on the block? If any noticeable uneven gaps, then you may have to have the manifold cut.

With any vacuum leak, you should see the vacuum value decrease. I wouldn't use hardware store bolts, most would tend to be too soft. Go with grade 8 if need be. That said, if the manifold wasn't a match to the block, or close, when trying to torque the manifold down, it may be a softt reading until any gap was pulled out or closed up..

Years ago, I worked in fleet applications, and back in the day when the ' M' had factory valve/spring issues, and later valve jobs etc., I still had some end seals split or lip out. Could have been the seals (depending on supplier) since these were still stock uncut manifolds with maybe one head surface, so I started making my own end seals, wider and a bit softer and had great results...

Last edited by Hayapower; 10-19-2016 at 12:22 PM.
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Old 10-20-2016, 11:32 AM
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Hello Hayapower,THANK YOU for the input,I went and used a grade 8 bolt,but it still seems like it want s snap even at a setting of 18 FT pounds on the torque wrench. So I stopped when I thought that they would be tight enough for a seal,but when I am done with the carburetor and start the car up,hoping it will seal,if not,I may squeeze them down a bit,but I think they are tight enough,as when taking them out,they seem to not be super tight anyway. I have my fingers crossed,but I think it should be OK,but if not and any more tightening,I will have to pull it again and check the heads,as I put a straight edge on the intake and it seemed to be A OK,. I will keep you up to date when I get her running. Thanks again for you reply,Mark B. Have a great day.
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Old 11-07-2016, 11:09 AM
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Found a burnt up Vapor Canister Purge Valve,waiting on finding one to get rid of the vacuum leak,cannot get the idle set with the leak.having a hard time finding a 4 port purge valve for a 1978 Grand Marquis with a 5.8. Still looking.
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