Low Idle Vacuum on 94' TBI Suburban

I have a 94’ Suburban with a 5.7 liter TBI engine. This is basically stock setup. I purchased the vehicle with 184k miles on it and it now has 236k. Ever since day one, I noticed that I only get 12” vacuum in idle (park). I have read that steady, low vacuum readings indicate possible vacuum leaks at the throttle body and/or intake gaskets, or perhaps late timing. I set the base timing to the specified 0* BTDC with the connector unplugged.

I replaced the following components with new pieces: O2 sensor, MAP sensor, TPS (set at stock .60 volts), IAC, knock sensor, EGR, PCV, fuel pump, filter, air cleaner, plugs, distributor (cap & rotor and ignition module), plug wires. I did not change the water temp sensor(s) because both the dash and computer (scanned with an Auto-X-Ray) read the same and are normally about 200*-205* when at operating temperature.

When it hit 235k miles, I decided to replace the engine. I figured maybe the timing chain and/or cam was worn causing the low vacuum. I installed a new GM replacement engine made specifically for this truck. I also installed Edelbrock’s MPFI conversion kit. Therefore, I have a new intake on it as well. The fuel pressure is good. This swap also means that all new gaskets and hoses were installed. I also installed a new Edelbrock TES system with the new engine, but I had already installed a new catalytic converter and Dynomax cat back system at about 200k miles.

The low vacuum was (is) present with both engines. With the original engine, the plugs always burned clean, with a slight tan/grey color. The engine only runs with about 20* timing BTDC when warm and idling in park. Is this normal? This is with the base timing set per the specifications. I have tried advancing the timing manually a few degress, but it makes no difference with idle vacuum, other than detonation under load. I also noticed that the MAP sensor reads about 1.45 volts when idling in park or drive, which is a little high due to the low vacuum reading (I think?). This probably adds to the problem by running a little rich at idle. I do not believe I have an intake or throttle body leak, only because I changed the intake and TBI gaskets once on the original engine (Fel-Pro), and again with the new engine and intake (stock GM per Edelbrock recommendation). I always use Edelbrock’s Gasgacinch when installing intake gaskets.

The engine runs fine, other than the low idle vacuum. When it idles (about 550-600 RPM in gear), it makes you think that there is an aftermarket cam in it. It gets between 18-20 mpg on the highway, so I really can’t complain. But I would like for it to idle a little smoother, with higher vacuum (normal for this engine should be at least 17”).

I am stumped. Any suggestions would be greatly appreciated. Thanks, Ed.

Vacuum leaks on MFI usually cause a higher then normal idle or fluctuating idle. The computer responds to the lean mixture by riching the system and idle goes up. The hunting begins when the computer tries to bring down the idle but when it does the vacuum doesn't leak as bad and the computer has to adjust again. This constant adjustments to keep things in order reeks havoc with idle control.

Have you double checked you guage against a known good one?

Funny how you gauge read the same with a engine with 238k as with a engine with little to no miles.

Wow! You are very thorough.
I would check that the computer is actually trying to make the IAC move. Maybe the driver in computer is bad or wiring is open somewhere.
Screw your old IAC back in then hook up the new one to the wire connector. Create a small vacuum leak to cause rpm to increase now watch and see if the pintle starts to extend which would indicate the computer is trying to lower rpm by shutting idle air passage.

You know you could probably just slowly unscrew the old IAC a little bit instead of making a vacuum leak too.

I did not save the old IAC, but will swing by the parts store and pick up another one. For $30, it worth the test. I will also get another gauge to verify that the vacuum is actually low. I will report the results later. Thanks, Ed.

Greg,
I tried your IAC test and everything appears fine.

Johnsongrass1,
I did not buy a new gauge, but checked mine against a brand new Tundra. I only got 14" at idle in park! Looks like I have a bad gauge. I also noticed that when disconnected, the needle falls back to about 3 psi on the pressure side. My guess is about 5" off.

If it were a snake, I'd be in big trouble . It's amazing sometimes how the most obvious things to check are the last you think of. Thank God for this message board and a little common sense. Thanks for the replies. Ed. www.edgesz28.com