My old thread was starting to get a little long and wander around, so I thought I start this one and focus on the current issue.

3 months ago I had my 390 re-built, it was bored 0.040 over, all the usual rebuilt stuff was done (new cam, oil pump rockers, etc etc) but my old heads were to far gone to rebuild, and he got me a different set and rebuilt them instead. They put it all together for me and I had a 500 CFM two barrel on a stock intake manifold on top. A month later it started misfiring really bad, so bad you couldn't drive it.

Long story short I pulled the intake and it was warped. So I got a old Offy intake and had that worked over (two cracks welded and gasket surfaces milled) and stuck it all back together with a holley 570 4bbl on top this time. I had the carb re-jetted (it now has #65s instead of 54s) And it still won't run right, it misses bad at idle and randomly around the RPM range.

I've checked and re-checked all the basics like timing, plugs and wires, cap and rotor, checked for vacuum leaks (found a few and fixed them) I've checked compression, done a leak-down test, and everything checks out good, except one thing: the plugs on cylinders #2, 3, 4 ad 5 all look lean. Also, exhaust manifold temps are much higher on those cylinders (other cylinders are about 300-315*, these ones are about 410-430*) To me that says leaking intake, but is there anything else I could have missed?

Sorry for being so long winded, but I'm getting to my wit's end with this motor. I've even caught myself daydreaming about replacing it with a diesel or straight six.

Sam

 

Sam, is that a dual-plane intake? If memory serves, 2, 3, 5 and 8 are all on the same plane on a dual-plane intake. Is #8 a little lighter than 1, 6 and 7? But then, #4 wouldn't be lean too...

Definitely sounds like an intake leak. Either the carb base, or the intake gaskets themselves.

What bolts are you using, and did you check the depths of the holes versus the length of the bolts? And how are the ends of the manifold sealed, cork, or did you use RTV?

 

I think read somewhere this manifold is a split single plane, (here's the same manifold for a 351: Offenhauser 360 Degree Single Quad High-Rise Intake Manifold 5883 Ford 351W on eBay!)

We checked the bolts and they are ok, and we used RTV on the ends of the manifold.

Sam

 

Well you claim ? that you had the same problem with both intakes ?any damp pulgs on the other side ? depending on how much you take off the intake, you should take half as much off the deck side of each head .. If all of this has been done..then it tells me maybe thew block deck is not in square ? But the list could go on and on..as I do not know your motor... If it is indeed a vacuum leak, or to elimenate that ...fired the motor..and spary some WD40 slowly around the intake where it meets the heads..and if the rpm picks up..part of the problem is there..mis-gasket match...Intake and heads arent meeting properly.. Also check that when you torqued doen the intake..you did it in proper order...Or one side is note equal with the other side and is being picked up off the suface of the intake side of the heads..Try that first and I'll go onto some other things... Good luck and keep us posted ..

 

Yeah, two intakes. On the other one (it was a stock 2bbl intake) it was also warped, so I assumed that was it. It was only out by .005 though, my machinist said the gasket crush should make up for about .020. Either way it was leaking bad, when we pulled it apart you could see where it had been sucking oil from the valley.

I torqued the current one down in the proper sequence, so I'm pretty sure that isn't it.

One more thing: there definitely is a vacuum leak, I used a smoke tester and we got smoke out the back of the intake, right around the port for the #8 cylinder, which is strange, because that one doesn't seem to be running lean.

The heads and block were shaved, but I don't know how much they took off. I wonder, my heads are replacements that he got for me, is it possible they aren't shaved to the same height?

One more thing, the problem keeps showing up on #3 and #4 the worst, could there be a low spot on the head right there?

EDIT: One more thing, the longer I drive the truck, the lower the oil pressure gets, (the lowest it ever got was 15 lbs at idle, I saw that and shut it off quick) But once it cools off, pressure returns to normal. Funny thing is, the temp gauge never gets above 205*, with a 195* thermostat.

Thanks Russ, I appreciate the help!

Sam

 

I had this once. The guy who finally fixed it said that FE engines tend to cross fire if the # 7 and #8 plug wires are too close. He set mine about 3 inches apart and it stopped

 

Wow, .005" could make that big a difference depending on how far it goes across the head tho..and if it runs toward the combustion chamber..into a cylinder..and you say you changed the coil, plugs..altho I have got a bad plug or even two in a pack, and also have put on new plug wires and had one leak..So I 'd revisit that also before digging back in to the motor, and as far as being down to 15 psi of oil on an FE motor at Idle..thats fine..I had some that would idle at 8 to 10 psi.. Just figure around 10 psi per 1000rpm's of the motor..As in a stock build...

I'll think about this a little later but now I need to run...

 

Those Fel-Pro Printo-Seals are known to break down over time (and you did mention the problem after the rebuild showed up about a month later). Did you use them?

A well respected FE builder has recently posted in another forum, the current preferred gasket is a 1247-S3 with steel liner - or an MS90145 passenger car gasket.

I know when my manifold was warped (or the gasket went bad...or both...never was quite 100% sure which it was), I got a lot of smoke out of the exhaust under firm acceleration. Something to look for anyway.

ps: And although everything points to an intake leak (lean running, warped manifold, etc.) never under-estimate the problem old ignition wires can cause when it comes to misfires and in general, poor running. I see you already dealt with the wires, but for anybody doing a Search and finding this thread... change your wires before tearing the engine apart.

 

The shop assembled it with the print-o-seals the first time, but I currently have victor reinz gaskets in it.

To be honest, I don't think it sealed right from day one, looking back I can see a lot of little things that were warning signs, but I just didn't know what was causing them till it finally blew on the freeway.

Another detail I noticed: I had the exhaust manifolds off and there was an incredible amount of carbon in the ports for #1, 2, 5, 6, 7, 8 (about a 1/16 of an inch thick), but the ports for #3 and #4 were so clean you could eat off them.

EDIT: What are the tolerances for head warpage? (intake side)

Sam

 

In my 1966 Ford Manual, it only states one tolerance for any of the head or block surfaces, and that is no more than 0.003" in any 6 inches or 0.006" overall.

It's easy to check with a machinist quality straight-edge and some feeler gauges. Only thing I wonder about is "what if both mating surfaces have the same problem?" The gap could potentially widen out to 0.012", and that's a lot for a gasket to seal. Ford doesn't address this.

My memory fails me at times, but I know I also had intake gaskets distort and leak that were another brand/type vs the PrintoSeal. They were black and had a blue stripe around the ports, although the stripes didn't stand proud like those on PrintoSeals.

In my case, the intake was so warped, it would seem any brand of gasket was going to fail.

Besides my engine smoking when I accerated hard, I had another symptom. Once the engine was warmed up, it would whistle at idle. Probably at any time the vacuum was high, but could only hear it at idle due to my noisy exhaust.

But I never noticed any cylinders running lean, or misfires.

One might think that an air leak might be very small compared to the size of the port, except during periods of high vacuum, and that means low engine load, so misfires may not be related to leaky intake gaskets. Of course, a massive leak might cause misfires, but I havn't seen any of my gaskets fail in that massive way.

Now here I am just guessing, but could it be those carboned-up cylinders are actually sucking and burning oil, and the "lean" cylinders arn't really lean, but the plugs appear bone white due to the ignition timing being too far advanced (or they're just still new)?

In other words, the problem cylinders might not be the ones you suspect, but the oppposite. And end cylinders do seem to leak before the center ones when warpage is involved.

Just thowing out some ideas, but basically wanted to give you the tolerance info.

 

Funny, I remember mine having that same whistle at idle, I never was able to figure out what it was!

I never thought of it, but it makes sense, if it's eating oil those cylinders would be spitting carbon like crazy. I remember the last time the gasket blew there was the classic oil burning smell, I'll have to see if that shows up again.

We'll see if that head is warped, I've got an appointment with a shop tomorrow.

Sam

 

As it turns out, the intake leak might be the worst of my worries. While I was under the truck the other day, I noticed coolant leaking out of the bell-housing (along with a bunch of oil) but I don't remember any freeze plugs back there, or is my memory just failing me? (the block is a regular old 68 352 block with a casting date of 71, if that makes any difference)

Sam

 

I can't say with first hand knowledge but I did read somewhere that one of the bellhousing bolts enters the water jacket.

 

I got the truck back from the shop today and I am thrilled with how it turned out! The shop searched for the coolant leak, and discovered it was a leak at the front of the motor that was blowing back along the oil pan rail and some of it was working it's way into the bell-housing. So once that was solved they checked out the intake and decided to pull it, they re-sealed it and dialed in the carb and vacuum advance and the engine runs much better! The idle was a bit rough at first, but the more I drove it the more it smoothed out and anything above idle is smooth as silk, not even a hint of a misfire. To top it off the hydrocarbons are even from both cyl banks and they smoke tested it and it held all the smoke.

The general conclusion we've come up with is that I must've messed up when I put it back together, not sure what I did, but all the parts checked out good, so it must be me. They did say that I had used way too much copper coat when I put it together, I wonder if that was part of the problem.

Anyways, I'm glad to finally have a solution for it! Now I can finally get some real exhaust put on it, the way it is it's deafening, kinda fun though, it'll set off car alarms no sweat.

Sam

 

Thanks for reporting back, Sam. Glad to hear it worked out well for you. Another FE on the road