EFI Conversion - Vacuum Troubles

I posted this in the 5.0 section, but zero responses, so it's up to you guys!


I've got a '67 Mustang I just recently converted to EFI using a Fox harness. Here's a listing of the parts in it:

Victor EFI Intake
Trick flow Heads
Lunati Cam - 218/228 @ .050 w/ .536 lift
Long tube headers (O2 sensors in the collectors)
BBK TB (70mm)
Ford 30 lb/hr injectors
Pro-M 76mm TB calibrated for 30 lb injectors
Stock Distributor with MSD box, initial timing at 15 degrees.
A9P computer (car is manual however)
No EGR or Canister Purge

All of the electronics are brand-new with the exception of the mass-air meter and the computer.

Problem I'm having is the vacuum at idle is only 4" and if I rev the engine, the RPM will hang around 2000-3000 RPM and settle back down to idle after a few seconds. At 3000 RPM, it's pulling 20" of vacuum. I've sprayed brake cleaner at the lower manifold/head gasket, upper plenum gasket, TB gasket, solenoid gasket and the engine doesn't rev up like any are leaking.

Idle is fairly steady at 800 RPM (+/- 100 RPM), but if I let it idle for a while it'll gradually slow down and eventually try to die. Vacuum is steady the whole time, not jumping around at at.

Codes out of the computer are:
34, 67, 81, 82, 84, 85

I've got one of the EGR simulators, but I think it may be defective since it's spitting out 34 and 84...

I'm really at a loss guys, please help!

Idling isn't really the problem. I've set the idle higher (like 900-1000), but it doesn't solve the vacuum problem.

Nope. For one going to try a different vacuum gauge to check to see if mine is accurate. Also going to run a compression check to see if maybe I'm leaking through an intake valve...

Any suggestions?

Ronstang said:

Have you readjusted the valves Jeff? You might have them a little tight.

Click to expand...

I did that when I had the intake off. If anything they were a little loose (1/4 turn after contact). They're are ~ 1/2 turn after contact now. I'm doing a compression test tonight and that should turn up any problem cylinders in that respect.

That's a possibiliy. I had the port shared with the MAP sensor.

I'll check into that this evening.

Solved!!! Well, found the problem.

Intake gasket was leaking again... Loosened up all the rocker arms so the valves were shut, pulled the TB off and replaced it with a plate and then pressurized the intake from my air compressor. Cylinder 4 had a gasket leak.


I had the heads milled when the engine was apart. I asked the machine shop if the intake needed to be cut to match (heads milled .020) and they said "no". So either they're full of it (possible) or the intake has already been cut to a different angle (also possible).

Anyone know how much angle needs to be cut into an intake for this amount of milling? Since I know the water passages aren't leaking (no water in the oil) I presume I could just RTV around all the air passages, but I don't want to do this if I can avoid it.

Thanks for all the input, I'll start a new thread for the above question.