View Full Version : Exhaust crossover in the intake manifold (PCM 351)?

07-11-2008, 01:26 PM
Does anyone know if there is an exhaust passage thru the intake manifold on a PCM/Ford 351W? I'm trying to rectify what I'm pretty sure is a carburator heat-soak issue. Carb is a Holley 4160. Engine runs perfectly if started from a cold start. Idles, launches, and will keep running great as long as it's not turned off for more than a minute or two. If turned off while hot for ~10 minutes it's really hard to start and runs like poo - won't idle / floods / dies under 1200rpm. Many have posted with these same symptoms, few have diagnosed it correctly and even less have offered sound advice for solving it.
I spilled some fuel on the intake manifold and it boiled away vigorously. I can picture the same happening inside the float bowls, thereby causing the floats to "sink" in the bubbling fuel causing really rich running and fuel trickling into the carb throat even when not running.

I've seen suggestions to lower the float level, insulate the fuel line (pump to carb), relocate the fuel filter outside the engine box (near the tank) and add a phenolic riser under the carb. Those are all bandaids on the root cause that the intake manifold it hot. One or a combination of them may improve things enough so as to not have a problem most of the time, but it's not really a solid fix.

If there's an exhaust crossover, it seems logical that blocking it off or reducing it's flow would reduce the temperature of the intake manifold, thereby the temp of the carb. I thought I saw a post somewhere that marine intake manifolds are different than automotive intake manifolds... could this be the difference? Thoughts? Experience? Suggestions?

Thanks kindly!

07-11-2008, 02:15 PM
I could be wrong but on a '78 boat, that would have been before they started using EGR, since boats tend to be behind automotive technology. Also, if the exhaust gas doesn't have anywhere to go, it would have limited ability to heat up the intake. I guess it's possible but I would look into the raw water impeller and thermostat first. If they did a crossover, I would think it would be outside of the manifold.

Maybe you could call PCM and ask. I don't know how far back their records go, but it's worth a shot.

07-12-2008, 10:27 PM
The manifold doesn't need to have EGR to have exhaust crossover passage. The purpose of the crossover is the heat the floor of the plenum for better vaporizing of the fuel. The exhaust gasses travel back and forth in through the intake b/c of the syncopated firing order of each of the V8's banks, with the 90* crank. Incidentally, this heat source is also how older NON electric, automatic chokes are heated, and opened. Your intake does have an exhaust crossover, but MAY or may not have an intake gasket that allows for the flow of exhaust gasses through that port.

I'm with Jim, however, that the problem you're describing is not DUE TO the existence of a functioning exhaust crossover passage. Whether you have one or not, your intake is too hot, and IMO, that has to be a result of too high and operating temp for a boat. I would also recommend double checking the cooling system and your engines operating temps. After running my engine as hard as one can run it, the temp is ~150*. You can ALMOST hold your hand on the intake.

07-13-2008, 12:36 AM
Is there any chance that the intake manifold has been removed and either the original gasket was reused or the wrong replacement was used?

07-13-2008, 12:43 AM
I pulled the intake manifold last night to have a look, got new gaskets, then made some restrictor plates that fit in the gasket openings to partially block the exhaust crossvover flow (skidim mentions in their intake manifold gasket description that they provide a restrictor to eliminate hot start problems).

I took the boat out today and it seemed much better, but I also didn't shut if off after a hard run and wait awhile. I idled it continuously while trying to teach my 10yr old daughter to ski, so wasn't quite the same conditions as the other outings where I'd shut off while getting geared up. It did start and run fine while loading on the trailer after sitting off at the dock while I hiked up to get the car and trailer (10 min?). I'm gonna consider it fixed... with my fingers crossed.

Operating temp comes up to about 170 whether running slow or fast and never goes higher indicating the thermostat is working, but I'm not certain it's the right temperature. The impeller is new, and seems to be working just fine.

07-13-2008, 12:59 PM
170* is a bit high, compared to MY boat....but definitely not too hot. That same engine would run 200*+, in a car.

07-13-2008, 01:51 PM
I have a 160 tstat and rarely see above that in the summer. It will creep up after a run, but will settle down to about 150ish. 170 seems high in my opinion also. Just because the RWP impeller is new doesn't mean it is circulating well. How old is the Circ pump. I had cooling issues as well. Turns out the exhaust manifolds and T-stat housing was so corroded that the flow was terrible, and heat transfer was terrible. I opted for a new t-stat housing, t-stat, manifolds, risers, circ pump, and impelloer. No problems yet and she runs cool. I NEVER shut my boat off while skiing. This keeps the motor at a relatively constant temp. After a run, shut the motor off for ten minutes. Notice the temp is VERY high (maybe 180+) from sitting. Now you start the boat and suck 70 degree water and cool the motor rapidly to 130-140 maybe. This is called shock cooling and is bad for engines. This is what is attributed to bad head gaskets and/or mainifold gasket leaks. On aircraft, repeated shock cooling cracks cylinders. I have seen a cylinder depart the engine in midflight and punch right through the cowling. No fun!! Not saying dont do it, but if you are in the habit and do it every time and often, look for problems down the road to happen. Just my opinion.

07-13-2008, 02:57 PM
Just because the RWP impeller is new doesn't mean it is circulating well.

I agree. a good inspection might be in order