TBI information: FWIW

I posted the following over on PN and thought someone here might find it helpful.

Still doing research on the PCM/Holley TBI setup. Here is what I have learned (remember, I am trying to solve my rich running condition).

The system is basically 100% a Holley ProJection 4D(i), 900 CFM progressive system. The ECU gets input from MAP, BARO, TPS, CTS, DTS sensors. The main fuel delivery map is obtained from the MAP sensor, with the backup being the TPS. A rich run condition can be caused by one or more of the MAP, BARO, TPS or CTS sensors failing and by excessive fuel pressure. I also understand, after talking to several people, that the system idles rich period, always has, always will. The idle speed is controlled by the IAC which opens or closes (via a pintle?) a vacuum bypass port. The MAP sensor then adjusts injector pulses to compensate for the gain or loss in manifold pressure. So, the EEPROM needs to be rewritten to provide less fuel at idle for any given manifold pressure (way beyond my scope). It may also have to do with the four huge injectors and the giant throttle plates....

So, to fix my problem I have purchased a new BARO and MAP sensors (identical parts...GM P/N 16137039 or equivalent aftermarket units) and today I am going to purchase a CTS (coolant temperature sensor) from my local CC dealer. I cross referenced the Holley sensor (P/N 534-10) to a Borg Warner sensor at less than half the cost of the PCM unit, but I am a bit nervous to install for one main reason: The Holley CTS sensor, designed for cars, is set to turn the "choke" setting, via the ECU, off when the coolant temperature reaches 180 degress. FWIW, the CTS is just a switch for cold starting (causes extra injector pulses). Now, we all know that our PCM engines like to run at a normal temperature of between 160 and 180 degrees. So, I am afraid that the automotive CTS will stay "on" even after the engine has warmed and thereby cause it to run rich ALL the time (mostly noticable at idle). I am going on the assumption that PCM specified a CTS with a lower temperature set point...or at least I hope they did as this could be part of the chronic rich idle condition for the TBI setup. I plan to test the set point in a pot of boiling water once I have the new sensor. If it triggers at 180 degrees, then maybe a resistor or something else can be done to "fool" the ECU into turning the "choke" off sooner. But, if the ECU is looking for voltage or no voltage, I am not sure what to do.

After I replace the three sensors this evening, I will test. If still running rich, I will test the TPS sensor and adjust or replace as necessary...a faulty TPS can cause the rich run condition. It is interesting that most sensor failures manifest themselves via a rich condition. I guess better rich than lean to protect the engines and give the astute operator an indication that something is wrong.

I will also check fuel pressure to ensure it is not above 15psi at any time. I fear that my return fuel line connection to the tank (no fitting on my '93 tank) is causing a restriction that is thereby increasing fuel pressure. One thing that PCM apparantly specified on the Holley setup is a fuel pressure (Schraeder type) port on the TB unit...very handy and I believe not normally fitted to the automotive versions. The fuel pressure can be adjusted via an allen head screw in the middle of the pressure regulator fitted to the rear of the TB unit.

I know this was wordy, but maybe someone else will find it handy.

BTW, I thought/think the TPS voltage should be 0.63 volts at idle.

TPS is usually .5 to 5 volts. Do a sweep and you will see a gradual; gain in voltage.A really good tool is an old volt meter with the swinging arm.

What size are the threads on the CTS? If it happens to be 3/4" NPT, I wonder if you could use one of these:

www.koboldusa.com

Perhaps you could try one that switches at 140 or 160.

No O2 sensor on marine engines....wish there was, it would help compensate for normal wear and tear of the TB unit.

Well, I took the boat out for the first time yesterday and the engine idled poorly and had a terrible mis-fire problem at speed. Idle was unstable and rich. Mis was severe. Other than that, the engine pulled very hard and had incredible power and smoothness upon acceleration.

I need to test the TPS.

Mostly it is the Holley Marine Projection 4D(i) system with, what I believe, a customized control unit (ECU).

So, yes a Marine unit.

That's OK David, some of us carb-guys find it interesting and educational anyway. You should be a shop teacher at a technical school. ...then again, I guess construction management pays better.

I'm looking forward to your final glowing report on that project