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CFM Tech's GM TBI Tuning Tips
For GM Cars and Trucks

 From the basics that we all should read to the "Wow I didn't know that!

Note: If the plain language rambling below starts to bother you  - Read it anyway.  If anything it will make you a better reader!

Proper State of Tune

We all think our engine is in the proper state of tune, but the real question to ask yourself is," Is it?" When was the last time you:

  1. Replaced you fuel filter? We see them literally clog up between 15k-20k miles.  Yours may be fine - then again, it may not be! Because of the shear amount of times we've seen this happen we religiously change them every 15-20k miles.

  2. Replaced or even checked your Spark Plugs READ THE FOLLOWING WORDS - Always purchase and install the recommended factory AC Delco spark plugs. Again, we can't tell you how many running problems or lack of power symptoms we see because of using the wrong spark plugs and, yes, especially from the aftermarket and expensive "Go Go gadget" split or multi electrode spark plugs. This is definitely a place where you can save your money! Why do we say this? Proper heat range is 5 billion times more important than anything else a spark plug offers!

  3. Checked or replaced your spark plug wires? We praised AC Delco with plugs, but here is a different story. Although their  (AC Delco) wires are decent quality when new, they tend to start having problems after a few years of use.  We've seen them last anywhere from 20K miles to an average of about 40,000 or so miles. What do we like? MSD and Accell 300+.  Do not get any generic 'that's all we carry - yeh their fine' auto parts store wires!!!  If in doubt, get the AC Delco's - they are good out of the box.  We don't care to remember how many times we've seen brand new wires from other manufacturers causing major drivability problems.   Oh, and if you are using an MSD, Crane, Jacobs, or etc. ignition system - high quality wires become even more important! Remember,  electricity takes the path of least resistance.  The more energy an ignition system has, the larger the chance the electricity finds an easier path to take than to the spark plugs!

  4. Checked or replaced the distributor cap and rotor?  As with wires, AC Delco's cap/rotor is good out of the box but will only last 20k miles or so. The center electrode in the cap and the rotor wear out quickly.  This wear will multiply if using an aftermarket ignition! Why? This is where the electricity is forced to jump a small gap to get to the next component and only where spark energy is present EVERY time any of the spark plugs 'fire.' Think about it, this part of the ignition (coil wire too) gets used 6 times more on a V6 and 8 times more on a V8 than any spark plug or spark plug wire. Also remember, anywhere there is an air gap there is high resistance - the spark jumping these gaps has more energy than elsewhere is the system, thus more wear. This is why spark plugs, distributor cap, and rotor wear quickly. What do we recommend - Accell cap and rotor. If not available in your area, get the AC Delco - just remember to check/change it more often!  READ these words - NO AFTERMARKET DISCOUNT PARTS HERE!!!

  5. Checked your timing? Go buy a timing gun or at least borrow one. This can change over time so make sure to check it! Here, we are interested in the initial timing since the computer assumes that the base timing is set where it is supposed to be - usually 0 with the computer/distributor timing wire disconnected. Get a manual (haynes is okay) to find out exactly where yours is. Okay, now we are assuming you have checked the timing and it is now exactly at 0 degrees with the wire disconnected. Re-connect it and see if the timing jumps advanced.  here we are concerned, not in the advanced reading because it is computer controlled, but just in the fact it advances.  If it stays at 0 degrees you have a problem! We've seen this a good amount of times and it usually ends up being the distributor itself.

  6. Checked for air leaks in the exhaust ? Yes, this is a big deal! Any air entering the exhaust before the 02 sensor will cause the 02 sensor to 'read' the extra air in the exhaust system, and therefore the computer will see this and believe that the engine is running too lean. It will add extra fuel to try to compensate for this false 'lean' reading by the 02 sensor. Fix those leaks now!!!

  7. Checked for vacuum leaks? A small (or large for that matter) vacuum leak can play havoc by changing the intake manifolds vacuum level. The MAP sensor reads the intake's vacuum level and relays this info to the computer.  The GM tbi computer relies more on the information from the MAP sensor than any other sensor in the system! It is read and adjusted for at all driving levels - Open loop, Closed loop, and even WOT! Fix those leaks now!!!  Tip: other than the obvious vacuum hoses and connections  Check the throttle body base gasket and intake manifold gaskets - these are very normal areas for vacuum leaks.

Okay I think you are starting to get the point now. Check all items associated with normal engine maintenance. If you can't remember when you replaced 'such and such' REPLACE ITIf you don't know or can't remember which manufacturer is better for which part - GET THE AC DELCO PART!!!

Ok, now on to the Good Stuff!!!

Fuel Pressure

     The first performance step is  to address is the fuel pressure. GM did not set the fuel pressure exactly the same on each vehicle - they just made sure they where set between 11-13psi. A 1-2 psi difference in fuel pressure makes a drastic difference in performance! Believe us! In fact, 13psi is just enough fuel pressure for a stock tbi engine to accept one significant airflow improver (ie: our Tbi Power Plate, ported throttle body, headers, or etc.) without running into a significant lean condition.  We have found that 100% stock tbi engines enjoy 12-13psi and those with some modifications enjoy 13-14psi.

   Q:  "Why should I add fuel pressure? It's running fine."     
 Is it? Do you know what your 02 sensor is reading for an A/F ratio?  [See below for tips on tuning with the 02 sensor.] Do you know if your detonation sensor is decreasing timing to compensate?  Just because there is no drivability problem does not mean that there is no significant power increase to be had from richening the A/F ratio somewhat.  Horsepower is not always what you bolt on - the whole system must be tuned to work with each other!

   Q  Why does my vehicle pick up speed when I back off from full throttle?   
Your probably running lean! One, when you let off the gas you are cutting off the air supply and thus the A/F ratio returns closer to ideal. Two, the computer reads the 02 sensor under .75 throttle and thus adds / subtracts fuel to maintain the A/F ratio.  Over .75 throttle the computer does not read the 02 sensor! It uses it's programmed "fuel enrichment" mode! This mode is typically programmed for engine reliability not maximum power, thus, it is just rich enough that the head gaskets and pistons are not in risk of catastrophic failure. 

   Q:  When it is cold out my vehicle starts okay but stumbles when I apply the gas. Why is it okay once it warms up?    
Your computer relies on programmed values until the 02 sensor heats up enough to start reacting to the A/F ratio. Once it does, the computer reads the signal and adjusts the fuel trim to make the A/F ratio correct.  

 Is this okay? Do I have to live with it?   
   A: No, it is not okay. Plus, it is very bothersome!  Again, this is telling you that the factory tbi system is set up very lean.   Raising fuel pressure will alleviate this!

   Q:  Okay, it seems pretty obvious that I need to raise my fuel pressure but  how much do I raise it? And, how do I know how far to go?        
Get yourself a fuel pressure gauge! You can either be creative with fittings and "T" one inline on the inlet side or you can purchase a gauge kit made for the GM Tbi.  We have them and so do some parts stores.  Take notice of the stock reading before you add the adjustable pressure regulator so you have a known value and running condition. Install the Adjustable Fuel Pressure Regulator and set to 14psi.  Take the vehicle for a test drive and note it's performance, especially at WOT. Raise the pressure in increments of .25. When the performance drops off and/or the vehicle runs too rich, lower fuel pressure to the best setting.  Checking your 0-60 times with a G-Tech or 1/4 times at the drag strip will be your final indicator of which gives max performance.

Using Your Oxygen Sensor

     Although not a "miracle" instrument, your oxygen sensor can tell you a lot about your air/fuel ratio.  This is not an absolute instrument for it is only efficient at  14.7:1 air/fuel ratio plus, even exhaust heat changes the mv's by a few tenths!  We've found that the 3-wire (heated) 02 sensors are more accurate than the 1-wire (non-heated) 02 sensors. We've also found that the heated 4-wire 02 sensors are more accurate than the 3 wire 02 sensors.  Note: the 1 wire 02 sensors will not usually heat up enough from the exhaust temperature to work at idle and low rpm. If you are using headers, the 1-wire 02 sensor will sometimes not even heat up enough to work at cruise rpm's.

  At idle and under .75 throttle the 02 sensor mv's should switch over and under .45mv's (14.7:1 air/fuel ratio) several times a second. Again, at idle or under .75 throttle, if  it constantly reads over this you are  running too rich -  If it constantly reads under .45mv's you are running too lean.

     At WOT the readings should not switch back and forth. The computer does not adjust to the 02 sensor signal anymore. You should now be getting a somewhat constant reading over this.  Now, because the 02 sensor is not very efficient in reading exact lean or rich a/f ratio's the following chart should be used as a guide but not for exact information.  With that said, naturally aspirated engine's typically make the most "safe" horsepower around 13-12.5:1.  Super/turbocharged engine's "safe" horsepower is around 12:1.

 Example: we've tested some naturally aspirated vehicles that ran best with with .7--.75mv's and some others that ran best with .8-.85mv's while achieving the same air/fuel ratio of approx.12.8:1. The difference was that the oxygen sensors produced somewhat differing voltages.

Fuel Ratio






















Important information on the different types of oxygen sensors available.

1) The first type is the single wire (non-heated) type.  This relies on exhaust heat to bring it to the 650 degrees F. the materials need to produce voltage from the difference of oxygen content inside the exhaust system vs. outside of it.  When there is not enough exhaust heat to raise the 02 sensor to approx. 650 degrees - as is cold start, low cruise rpm, headers and/or cat back exhaust, absence of catalytic converter, etc - the computer will rely on it's chip programming.  This is no where as efficient as when the computer reads the 02 sensor signals and adjusts the fuel trim for proper air/fuel ratio.   This is the first generation of oxygen sensors used by manufacturers and can be found on most GM Tbi vehicles. A few models in the last 1-2 years of GM Tbi production came with the more preferable 3 wire (heated) 02 sensors which are discussed just below.

2) The second type of 02 sensor is the 3 wire (heated) that is built with it's own internal heater.  This heater typically only takes 10-20 seconds to heat the 02 sensor to it's proper operating temperature.  Since this is wired to an ignition switched circuit the 02 sensor will now be on from almost the instant you start the engine to the instant you turn it off.   Fabulous! Emission laws can work in our favor.

  • Black wire is the signal wire

  • 1 white wire is the 12v ignition switched wire

  • 1 white wire is the ground wire.

  • *** The heater is not concerned with which white wire is used as + or - , it is just concerned that 1 is used for + and the other - to complete the circuit.  This makes it very easy to retrofit a 3 wire oxygen sensor in place of the very inefficient 1 wire sensor.

3) The third type of 02 sensor is the 4 wire that also has its own heater but includes a separate ground for the signal wire.  The use of this signal ground lessens the chance of electrical noise and interference, thus enables the 02 sensor to provide a more accurate and reliable signal.


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