Difficult vacuum problem - ideas ??

[DonMaximo]

This is a Crate engine that was broken in on a dyno with a carbureted setup.
It's naturally aspirated and has fewer than 10 miles on the engine. I'm using my Ford/A9L EFI setup for induction and fuel delivery now. They're light miles since I can only drive around my neighborhood.

First off, I don't have a radical cam.

.556 I / .513 E Gross Lift
273 / 282 Duration @ 0.006
108 Lobe separation.

I'm reading 10-12" vacuum at idle. I've had to turn the idle adjusting screw in almost 3 turns in order to maintain some resemblance of idle at around 725. I should only have to turn this screw 1.0 --> 1.5 turns.

- I have replaced the PCV grommet & PCV valve.

- A smaller line from manifold is plumbed to the fuel pressure regulator

- The larger manifold vacuum line goes directly into my vacuum gauge, instead of the tree

- All other manifold vacuum lines have been capped.
The intent is to minimize and isolate areas where vacuum is relevent.

- Passenger side Valve cover has a plug

- Driver side Valve cover is plumbed into TB (in stock fashion)

- I have generously sprayed carb cleaner from air filter to intake, soaked upper & lower intake, soaked PCV in the rear of intake ---->>> not a dent in the current idle.

- If I pull the larger vacuum line off manifold, vacuum drops to a steady 5"

- I have relashed the vavles -

- My next plan is to pull all spark plugs and perform a compression test and leak-down test. It's my understanding that an erratic or abnormal vacuum can result from Valve related issues. If this yields no favorable results, I may install a stock 5.0 upper/lower intake.

What have I missed ?

What would you suggest ?

This one is a bitch...

 

[84Ttop]

What exactly is the problem?

 

[DonMaximo]

Hmmm...

Does 10-12" of vacuum sound normal to you guys ???

I provided the cam specs to drive this point home...it's not normal... and that is exactly what the problem is.

************************************************

I have a game plan and it's listed in my original post.

All the low hanging fruit considerations have been made... and executed.

If someone is familiar with valve conditions adversely effecting vacuum... I'd really like to hear about your experience. While it may not solve mine... it could get me thinking about my issue in a different light.


Thanks

 

[robo]

that's about right for vacuum with the cam you have. (this is why you see vacuum canisters for sale to help with power brakes on cars with big cams)

 

[robo]

also, you will not have manifold vacuum inside the valve covers so plugging them up tight will not help you with any diagnostics.

what is the cam duration at .050?

i have a mild 306 with a 230-230 at .050 cam and it idles at 12 inches and is at 6 inches or so under load.

 

[DonMaximo]

Robo,

My cam runs 224/230 duration at 0.050

Been waiting on Mfg. to get back to me on what I should expect from vacuum... but I definitely would have lost money on this bet... that I would be <below> 15".

So, if this is the case, do you recall how you set the idle ???

Typical adjustments calls for:
#1 - Disconnect SPOUT
#2 - disconnect IAC
#3 - back out idle screw all the way...turn in until contact with TB plate
#4 - Turn another full turn in...
#5 - Start an already warmed engine.

In my case on step#4, I found that I was turning in 3 full turns before it would maintain idle. When I reconnected SPOUT & IAC, idle would race. I soon found myself adjusting the idle with everything connected...and would make note of the behavior on the next cold start up.

Again, any commentary on this subject would be appreciated.

Now... I'm anxious to speak with the Engine builders....

Thanks again

 

[robo]

have you adjusted your base timing? with a bit more cam you will want to increase your base timing towards 18-20 degrees. it will idle better with a bit more timing in it.

the engine i have with the 230 230 cam is in a carburated boat so i set the idle by adjusting the idle air bleed screws and the throttle stop.

good luck and let us know what the manufacturer tells you about all this.

 

[DonMaximo]

Ah...

Carb v.s. EFI

A rather significant difference in setting idle. There are parts of me that wish this was as simple as setting up a carb.

A carb is all mechanical...you have idle screws, choke, mechanical advance... etc.

With EFI, the EEC (computer) controls the advance. Unless a cam has been advanced/retarded upon installation, it's recommended to set base timing at 10 BTDC with the SPOUT disconnected. With the SPOUT disconnected, the computer will not have influence on timing. So for advance, you can see big differences in this area. Once base timing is set, you reinstall the SPOUT and the EEC can once again control advance.

And while the IAC (Idle Air Control) is a mechanical unit, the computer basically controls the solenoid within the IAC to affect the amount of air which bypasses the throttle body valve/butterfly. You can also think of this unit as the choke for an EFI setup where it minimizes air when the Engine Coolant Temp (ECT) is quite low.

With all of this, there are settings within the computer which all work in concert with one another to control idle under a variety of conditions.

Regardless of these differences though, vacuum is generated the same way...so this much we do have in common (or could).

Nonetheless, I really need to hear back from the Engine builder before pulling more hair out... If they tell me "Yes, expect 10-12 @ idle"... I will consider myself educated once again and resume my EFI datalogging and tuning.

If not, I'll ask for advice on what to do next. Obviously, I would prefer not to have this discussion...but again... my old sorry ass really did expect more at idle. The jury is out...

Many thanks again for the exchange of thoughts.

 

[HISSIN50]

What kind of IAC DC do you see at your various settings? At 3 turns of the idle set screw, is the DC under 10%? And when you set the screw to spec (and it idles low) was your DC high? Did you change your commanded idle?

You should be able to tune all around this (if there is not a mechanical issue causing it).

FWIW, there's nothing wrong with setting the idle with the SPOUT and IAC connected. It just means that you're making relative changes - nothing that can be compared to anyone else's car. To do that, you'd have to get it to idle ok and then disconnect the IAC and SPOUT and note the base timing. Since you tune, your settings will probably not compare with anyone else's because you've changed parameters inside the EEC (this is best anyhow).

And the difference in timing with SPOUT in vs out is about 10-15*. There's most of your increase in idle (the IAC should be almost non-existant at hot idle while over your commanded idle).