Vacuum leak only when engine is “cold”?

[MericaMopar]

I have a 361 with a weiand 8008 aluminum intake manifold. I have checked my brake booster for leaks, even disconnected every vacuum line from the carb and plugged them. I seem to have a vacuum leak i cannot find still but only when the engine is relatively cold.

Stone cold, start the engine, let it run for 30 seconds, put in drive theres a good chance it stalls immediately, afr goes to 20+. The warmer the engine gets the less severe the issue. Once the engine is up to temp and the thermostat opens, the issue goes away, car runs and drives great. The exhaust crossover is blocked off by the valley pan.

Here is the kicker, it cannot be a fuel or carb issue because i replaced my holley 600cfm with a holley sniper system and it exhibits the exact same behavior.

I have sprayed brake clean around the intake base and carb several times and the engine doesn’t pick up rpm. Could it be the intake manifold is pulling air from the crankcase and as the engine heats up the manifold expands enough being aluminum and seals itself? Is there any way to test for vac leaks from the crankcase?

 

[CBODY67]

One problem is that you have the exhaust heat blocked. The exhaust heat to the bottom of the intake manifold plenum is there to help the fuel air mixture vaporize, with an aluminum intake manifold, that vaporization will usually happen sooner than with a cast iron intake.

On one of my non-Mopars, I recently had a rebuilt engine installed, which I added some factory aluminum from a later EFI modle engine. With EFI, no heat crossover provisions.

Compared to the prior engine (with an aluminum intake and heat riser valve blocked open), the engine takes more time for drivability to be right. The intake manifold plenum has to get heat from transfer from the coolant and cyl head temperature. Takes a few more blocks of travel for that to happen. But the heater gets warm quicker with the aluminum heads.

As to the Holley EFI system, I would be concerned why the computer is not apparently trying to keep the engine running? IAC activity and A/F ratio adjustment, for example.

IF you want to seal up the intake manifold valley pan gasket better, get the pair of paper gaskets to fit a Chrysler 440 6bbl motor (which had an aluminum intake its first model year). Then get some high-heat black silicone sealer. Put the sealer on the bottom side of the gasket, inside the beads, scrape it flat, let it cure overnight, then re-install with the paper gaskets (on the top and bottom of the intake gasket). My experience in doing this with an Edelbrock aluminum in take on my '67 Newport 383 is that made enough difference in the way the engine sounded that it was worth it. Not a huge difference, but enough I could hear it. FWIW

Contact Holley Tech Assistance and explain your issue and tell them you'd like a bit more "Cold Start Drivability" than what the system now offers. Also check the condition of the ignition system, plug wires, and spark plugs, too.

Keep us posted,
CBODY67

 

[Big_John]

I have a 361 with a weiand 8008 aluminum intake manifold. I have checked my brake booster for leaks, even disconnected every vacuum line from the carb and plugged them. I seem to have a vacuum leak i cannot find still but only when the engine is relatively cold.

Stone cold, start the engine, let it run for 30 seconds, put in drive theres a good chance it stalls immediately, afr goes to 20+. The warmer the engine gets the less severe the issue. Once the engine is up to temp and the thermostat opens, the issue goes away, car runs and drives great. The exhaust crossover is blocked off by the valley pan.

Here is the kicker, it cannot be a fuel or carb issue because i replaced my holley 600cfm with a holley sniper system and it exhibits the exact same behavior.

I have sprayed brake clean around the intake base and carb several times and the engine doesn’t pick up rpm. Could it be the intake manifold is pulling air from the crankcase and as the engine heats up the manifold expands enough being aluminum and seals itself? Is there any way to test for vac leaks from the crankcase?

I'd say the intake manifold isn't sealing.

Aluminum heats up, expands and seals.

 

[MericaMopar]

As to the Holley EFI system, I would be concerned why the computer is not apparently trying to keep the engine running? IAC activity and A/F ratio adjustment, for example.

it does try to compensate and catch it, it can recover, it just happens so fast when it’s cold it doesn’t have time to catch it, it is better than the carb. With the carb it would instantly die.

 

[MericaMopar]

I'd say the intake manifold isn't sealing.

Aluminum heats up, expands and seals.

That’s what I’m thinking to, it will just be the third time now I’m installing and resealing that manifold. Maybe I’ll get it right this time!

 

[MericaMopar]

IF you want to seal up the intake manifold valley pan gasket better, get the pair of paper gaskets to fit a Chrysler 440 6bbl motor (which had an aluminum intake its first model year). Then get some high-heat black silicone sealer. Put the sealer on the bottom side of the gasket, inside the beads, scrape it flat, let it cure overnight, then re-install with the paper gaskets (on the top and bottom of the intake gasket). My experience in doing this with an Edelbrock aluminum in take on my '67 Newport 383 is that made enough difference in the way the engine sounded that it was worth it. Not a huge difference, but enough I could hear it. FWIW

I think the last time i installed the manifold i used paper gaskets on the top side of the valley pan, but not under and I didn’t use any sealer other than in the corners where the heads meet the block. I’ll try your method out. Thanks

 

[Big_John]

Thinking about it... You could have a crack in the manifold that would cause the same issue.

 

[MericaMopar]

Thinking about it... You could have a crack in the manifold that would cause the same issue.

That makes sense but would be a bummer. I’ll inspect it really well when I have it off.

 

[Big_John]

That makes sense but would be a bummer. I’ll inspect it really well when I have it off.

Small invisible cracks can be checked with a penetrating dye and UV light. If you have a machine shop that does Magnaflux, they probably do the dye too.

 

[rags]

I'd say the intake manifold isn't sealing.

what if he were to pull and plug the pcv and shoot some propane into the pcv hole in the valve cover. would an O2 sensor pick it up?

 

[Big_John]

what if he were to pull and plug the pcv and shoot some propane into the pcv hole in the valve cover. would an O2 sensor pick it up

Thats a great idea!

The manifold sits up on the valley pan, so it wouldn't pick up anything on the bottom, like a crack for instance. If it's leaking around the bottom of the intake face, it seems like that would pick that up.

I wonder if a little careful squirt of starting fluid between the manifold and the valley (if that is where the leak is) might pick up the vacuum leak by a jump in RPM. You'd have to take it easy with the stuff though so it doesn't go boom.

 

[rags]

I wonder if a little careful squirt of starting fluid between the manifold and the valley (if that is where the leak is) might pick up the vacuum leak by a jump in RPM.

i wouldn't think twice about it but with a flammable carburetor cleaner like cyclo. i saw in the first post he's used brake cleaner for this but i don't know that all brake cleaners are flammable. larger question is what data is available and how is it displayed. on a graphing scan tool for an o.e. system, the O2 sensor reading would spike instantly if a leak passes a flammable detectant even without a jump in rpm.

You'd have to take it easy with the stuff though so it doesn't go boom.

all things in moderation.

 

[Big_John]

all things in moderation.

This past Saturday... Mounting new tires for 300L

 

[Justin Plant]

I use a Weiand intake on my Dodge 400. The felpro intake set came with the paper inserts. It leaked like a sieve. I pulled the bolts and slid the inserts in place. No more leak. Put the inserts in and go from there

 

[MericaMopar]

I took the manifold off yesterday. I did try flammable brake clean between the manifold and valley pan and all around but didn’t see an increase in rpm. I wasn’t looking at the o2 gauge while doing this, and I’m not sure what the polling rate is on it to display the value if it would capture a spike or not.

When i took it off i saw i had used paper gaskets behind and on top of the valley pan the last time i installed it but no sealer other than spray copper gasket adhesive at the ports. I could maybe convince myself if i tried hard enough to see at least less of an impression on a couple runners where maybe it was leaking, but nothing obvious. The front and rear hold down bars had oil on the bottom of them, is that normal for oil to seep around the bolts or something? I don’t really see how oil would get around the valley pan at the front and rear.

When i put it back on i used the paper gaskets and some sealer around the ports on all layers. I let it sit hand tight for an hour then torqued it down. I’ll retorque it when i get home today and fire it up and see if there’s an improvement.

If it still exhibits the same behavior I’ll just deal with it until the 440 I’m putting together is ready to put in.

 

[MericaMopar]

This past Saturday... Mounting new tires for 300L

I’ve never had luck doing this, but i think I’m always to conservative on my boom juice application.

 

[live4theking]

This past Saturday... Mounting new tires for 300L

How did you break them down. I did a motorcycle tire recently and it was a royal pain. Just to get it on the rim.

 

[Big_John]

How did you break them down. I did a motorcycle tire recently and it was a royal pain. Just to get it on the rim.

These particular wheels didn't have tires, so I didn't have to break them down.

However... If you take the bumper jack out of your car, you'll see that there's a slight curve in one side of the base. You can use that to break the bead.

Where did I get that you might ask... Right out of the FSM!

I actually have an old jack that I can use with the hitch on my truck.

I

 

[Big_John]

I’ve never had luck doing this, but i think I’m always to conservative on my boom juice application.

I had a tire service guy tell me how to do this. It's a long story... He told me how to do it over the phone but he was really careful how he worded it that he wasn't suggesting I actually do it...

There's a bunch of videos on YouTube that shows it.

 

[rags]

This past Saturday... Mounting new tires for 300L

cool. we would always spray a line of the stuff away from the tire to make a wick. for your next trick, you should try an acetylene bomb!