Speed denity help!!!!

[Abbadon]

I have an 87 gt that I just put an edelbrock performer rpm2 manifold on. Now the car idles at 3500 rpm. Previous to this I put 1 3/4 long tubes, off road x-pipe and two chambers, 70mm throttle body and egr, 255 in tank pump, CAI, full tune up with MSD coil, wires, cap and rotor, underdrive pulleys, a/c and smog pump eliminated and car ran perfect. I have replaced vacuum lines and checked vacuum at 19psi, fuel pressure and timing are perfect, new map sensor, iac, tps, pcv, thermostat, and o2 sensors. No missfire, plugs are good.
I pulled codes and got a 44,94 which are for the thermactor system, but believe these to be because of smog system eliminated. Also got a code 15 for ecm rom fault. already have a new ecm on the way, just want to know if anyone has any other ideas if this doesnt fix(other than mass air conversion)
Thanks much

 

[stykthyn]

I had this same issue. I put an edelbrock intake on an 87 and couldn't get it to lower the idle. One possibility is a shifted lower intake gasket. It will cause driveability issues and be nigh undetectable.

 

[Abbadon]

Thanks, I'll check that also. Anybody else have any ideas?

 

[NIKwoaC]

What effect does unplugging the IAC valve have?

 

[Mustang5L5]

Sound like huge vacuum leak. What intakes gaskets did you use. Spray some carb cleaner where head meets intake and check for leaks that wAy.

 

[Abbadon]

Unplugged IAC does not change anything, used edelbrock part #7720 gaskets.
thanks

 

[Abbadon]

wouldn't gasket leak show on vacuum test? Thanks

 

[aar0s]

changed the gaskets on my cobra intake last summer and had the same problem, turned out that teh heater tube was pushed over on the valve cover and the egr space was on top of it causing the upper not to seat on the lower. Im going to say you have air comming in somewhere around the intake.

 

[Boostedpimp]

It happened to me before with a holley systemax intake.. I installed it and the gasket shifted somehow as I installed and torqued down the lower intake. I used the carb cleaner spray trick all around everything and couldn't find a vac leak and eventually gave up and yank it to see half the gasket slipped down out of position.

 

[Mustang5L5]

wouldn't gasket leak show on vacuum test? Thanks

No because it's technically not a vacuum leak into the internals of the engine.

You are leaking air into the combustion chambers through the intake somehow. Carb cleaner test will confirm. Spray around the lower manifold and if your 3000RPM idle jumps to 3500RPM, your intake isn't sealed properly

 

[jrichker]

There is no easy way to find vacuum leaks. It is a time consuming job that requires close inspection of each and very hose and connection.

Small vacuum leaks may not show much change using a vacuum gauge. The range of "good readings" varies so much from engine to engine that it may be difficult to detect small leaks. The engine in my first Mustang pulled about 16.5" of vacuum at 650-7250 RPM, which I consider rather low. It was a mass market remanufactured rebuild, so no telling what kind of camshaft it had. Average readings seem to run 16"-18" inches at idle and 18"-21" at 1000 RPM. The only sure comparison is a reading taken when your car was performing at its best through all the RPM ranges and what it is doing now. Use one of the spare ports on the vacuum tree that is mounted on the firewall near the windshield wiper motor.

Use a squirt can of motor oil to squirt around the mating surfaces of the manifold & TB. The oil will be sucked into the leaking area and the engine will change speed. Avoid using flammable substitutes for the oil such as propane or throttle body cleaner. Fire is an excellent hair removal agent, and no eyebrows is not cool...

The vacuum line plumbing is old and brittle on many of these cars, so replacing the lines with new hose is a good plan. The common 1/8” and ¼” vacuum hose works well and isn’t expensive.

Diagram courtesy of Tmoss & Stang&2birds

Vacuum leak due to slipped lower intake manifold gasket...

Ask Nicoleb3x3 about the intake gasket that slipped out of place and caused idle and vacuum leak problems that could not be seen or found by external examination. Spay everything with anything you have, and you won't find the leak...

See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host) for help on 88-95 wiring Mustang FAQ - Wiring & Engine Info Everyone should bookmark this site.

Ignition switch wiring
http://www.veryuseful.com/mustang/tech/engine/images/IgnitionSwitchWiring.gif

Fuel, alternator, A/C and ignition wiring
http://www.veryuseful.com/mustang/tech/engine/images/fuel-alt-links-ign-ac.gif

Complete computer, actuator & sensor wiring diagram for 88-91 Mass Air Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/88-91_5.0_EEC_Wiring_Diagram.gif

Vacuum diagram 89-93 Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/mustangFoxFordVacuumDiagram.jpg

HVAC vacuum diagram
http://www.veryuseful.com/mustang/tech/engine/images/Mustang_AC_heat_vacuum_controls.gif

TFI module differences & pinout
http://www.veryuseful.com/mustang/tech/engine/images/TFI_5.0_comparison.gif

Fuse box layout
http://www.veryuseful.com/mustang/tech/engine/images/MustangFuseBox.gif