I have decided to create a page on trouble shooting TBI. Works with any fuel injection really since most shops really can’t properly diagnose unless computer just tells them what is wrong and unfortunately with TBI that usually doesn’t happen.  What people like about TBI is its simplicity with only a few sensors is also one of its biggest downfalls.  Pretty much all issues are going to fall into 4 catagories.

1. Idle.  Idle issues has to be one of the most common issues that I get contacted about.  Typically 2 problems.  Idle surge or idle hunting and high idle.

Lets start with the high idle. this is most often caused by a vacuum leak.  TBI being a MAP sensor based system will see the leak just like opening the throttle blades and will feed the leak and then you have a high idle.  Finding the leak can be tricky as it can be the intake gasket on the bottom side if you are using a Chinese junk carb intake or you weren’t real careful putting the intake on.  Next cause for high idle is a bad coolant temp sensor in the intake by the thermostat housing as idle speed is temp based. Next, you can have a bad IAC (idle air control) valve on the side of the throttle body.  Thing isn’t real cheap so we save changing it until last.  you can remove it can clean with WATER ONLY.  NO CARB CLEANER and some penetrating oil and work it in and out a bit and see if that helps.  Also a really rich fuel mixture will often cause a high idle. so if you plugs are black don’t expect a good idle need to figure that out under fuel issue.  Finally a faulty chip which should set a check engine light code 51 will cause a high idle because limp mode sets a high idle and a set ignition advance to allow you to limp home on the very basic of programming.

Idle surge.  No one likes it and can be bad enough to cause the motor to stall at not so great times.  This is either a too lean, too rich or jumpy timing.  Need to see what has changed since it idled correctly.  Cam change is one of THE most common causes of idle surge.  Running with stock programming with a low idle speed setting combined with the fact the chip is designed for 20-21″ of vacuum at that idle rpm with a cam upgrade which typically drops the vacuum to 18″ or less will cause the motor to run too rich which will spawn a high idle speed where the motor will then lean out and rpm drops and repeat.  Need to run a chip designed for your camshaft.  If you chose a cam that doesn’t make enough vacuum for TBI then your going to have some level of idle issue regardless of chip with TBI.  Its processor is too slow and fuel table too small to handle a big low vacuum camshaft.  stock cam is only 165/175 duration @ .050  you can run up to a 214/218 duration cam with a new chip without too much issue.  Above that very likely to have a problem.   Now if your motor is stock and your having this issue its VERY rare problem.  I recommend checking that fuel pressure in the fuel section and the timing tests on the distributor in the ignition section.  Jumpy ignition timing from a distributor problem can set off idle surge.  Worn out cam chain can also cause that timing to jump on very high mileage engines.

2. Fuel.  Fuel is the easiest to figure out so I am going to start with it.  Typically we either don’t get enough or we get too much.  Not enough is typically weak fuel pump or low fuel pressure.  So when diagnosing fuel issues.  Always good to have a pressure gauge on hand.  I use a Actron CP7817 which you can typically find on ebay very reasonable and a good diagnostic tool to have.  Stock setups can range from 9-12psi and find they run best closer to that 12 number.  If its running 9 or 10 you typically have a weak fuel pump.  Nice thing about the Actron gauge is the hose is long enough that you can watch it while you drive. If the fuel pressure drops more than 1 psi under 8-10 seconds of full throttle the pump should be changed and dont put the stock TBI pump back in even if your engine is stock.  Put the 1996 Vortec EP381 pump back in.  MUCH better pump than the EP386 TBI pump.  Now to the too much fuel.  First check the number 3 and number 4 spark plug electrodes as those cylinders always run off a different injector.  The coloring should match on these plugs.  If they don’t that is most often a bad injector although I have found a faulty chip can occasionally cause that but again most often its an injector. If both plugs are nasty black then we do have too much fuel. So how does the TBI system control fuel?  All fuel control is based of the MAP Manifold Absolute Pressure sensor and rpm readings. Map sensor measures vacuum in the engine and feeds that to the computer and the ignition module provides the RPM and the computer looks at a fuel table on the chip and sprays the amount of fuel for that vacuum level and rpm. Not the most accurate method of metering fuel into a motor but it works.  the later OBD2 systems have a map sensor for redundancy but meter their fuel based off a MAF Mass Air Flow sensor that actually measures the incoming air.  So back to the MAP sensor system, if you don’t have good vacuum signal to the MAP sensor it will run rich as that causes the computer to look in a different part of the programming because as throttle is opened and air rushes into the motor the vacuum level will drop.  But if you have a broken vacuum line, faulty map sensor or a problem with the engine where its not pulling good vacuum and not from air rushing in the motor will be too rich.  So how to figure out the cause.  Vacuum gauge.  Simple mechanical gauge.  Hook it to all ports on that throttle body.  When you get to the map port just move map to a different port on the throttle body. Typical TBI motor will idle with 20-21 inches of vacuum stock.  Pretty much any of the cams I recommend or have listed on the website will pull at least 17-18″ of vacuum at idle. If you have 10, 12 or eve 14″ of vacuum you certainly have a problem.  If just one port is low then its likely just clogged up and move map to a good vacuum source and be done with it.  if however they are all low well they wont all be clogged so need to figure out why.  Ignition timing too low is typically the main cause.  People overlooking the part in the book about disconnecting the tan wire with black strip (computer advance) wire to set the base timing to zero (4 degrees on big blocks or 6 degrees on TPI cars)  A typical SBC idles with about 16 degrees of advance which is what the chip will advance to when the wire is reconnected so if you didn’t set that timing correctly that timing will be way too low and your vacuum will be low.  Also if you needle is very jumpy at idle that is typically a sign you have a rocker arm too tight or you have a weak valve spring that the oil pressure to the lifter is enough to shove the intake valve off the seat which will screw up the vacuum.  Using high volume oil pumps are really bad to cause this problem.  Try loosening the rockers a 1/2 turn and check your vacuum again. If the vacuum level checks out and your getting at least 16″ which is typically enough that it wont foul out spark plugs and your still too rich then it can also be the coolant temp sensor.  the one in the intake manifold near the thermostat housing.  this controls the choke.  When they fail and they do.  They typically read -40 degrees which is darn cold and the computer will POUR the fuel to it.  People often think o2 sensor and while those can be a problem the computer ignores the o2 sensor for the first couple minutes that motor is running so if it runs poorly cold and good warm that is not o2 sensor.  If it runs good cold and poorly when warm that can possibly be an o2 sensor. Usually o2 sensor issues won’t foul plugs.  Just makes them run dark.  To confirm a bad o2 really takes a laptop running datalogging software that I have listed on this site or a Scanner like Snap on, Genisys or whatever to look at fuel trim data.  Listed as either Long Term or BLM Block Learn Multiplier.  these numbers can tell you how that computer is adjusting the fuel.  Short Term or Integrator numbers are how much the computer is adjusting the fuel after the BLM number change.  When in open loop when computer is ignoring that o2 sensor waiting for it to heat up, the BLM number will be 128 indicating no change.  When the number goes higher the computer is adding fuel to the programming.  Lower its subtracting.  I like my numbers to stay in the 120’s  they can briefly jump out of that range but on average should stay in there.  If you have dark plugs and the BLM numbers are high then you certainly have an o2 sensor problem but if they numbers are low like 115 or less and your plugs are still black and your vacuum level is good then its either the programming on the chip is off, fuel pressure is too high or the injectors are stuck.  For now I believe that covers the fuel issues although some of the ignition issues in the next section can sure cause fuel burning issues too.

3. Ignition. Only thing harder to diagnose than ignition problems on fuel injection is electrical (wiring)   First the only sensor in the system is the knock sensor which is pretty much worthless for diagnostics. Next problem is its the most over looked part of the engine build with most people skimping in this area causing a world of headaches.  Backfires are ignition problems.  Through the intake timing is too low, out the exhaust there is typically a problem with the distributor and the timing is jumping a lot.  Old or new distributor,I don’t care your need to run the following test to verify its working properly.  use a Timing light and leave that tan/black timing wire connected for this test as we are wanting to look at the advance.  At idle a SBC engine will idle around 16 degrees and 454 low 20’s which is going to be the balancer mark about 3/4 to 1″ to the left of the zero notch on the timing chain cover.  Then rev to about 3000 rpms and hold and timing should climb to 30-36 degrees depending on programming on the chip and low 40’s for the big block. KEY thing to look for on these is that timing should not waiver more than 2 degrees.  If its jumping around a lot you either have a bad pickup stator in the distributor or a cracked, broken or faulty magnetic reluctor ring that spins over top of that pickup stator. My personal rule is if the distributor is out of the vehicle for any reason I replace that pickup stator with a new AC DELCO D1987 pickup stator.  New motor put a new distributor in and not that chinese crap from your local auto parts store.  I find a new GM performance distributor 1103952 which you can get from Summit Racing, Jegs or probably your local dealership gives you the least amount of issues.  I have tried them all and nice new GM seems to work the best.  Even when I put the new distributors in I run the very same timing test.  Plug wires.  Stock motor the stock replacement plug wires seem to do fine but with cam upgrades or head/cam upgrades put some decent wires on the motor.  Bang for the Buck, I have not found anything that works better than the Accel 5000 series 5114K wires.  Spark plugs, again AC Delco gives the most consistent results.  R44TS for stock 1/2″ iron heads or R44LTS 3/4″ reach for Vortec iron heads or Aluminum heads.  Most of your Dart heads now use the 3/4″ reach but if your using an aftermarket head find out which reach plug you need because running a 1/2″ reach plug in a 3/4″ reach hole runs terrible and darn difficult for me to diagnose from here.  MSD boxes like a 6al. Please don’t use these.  TBI is not a race car injection system.  I have yet to see an MSD 6a or 6al box have any positive affect on TBI or any other fuel injection for that matter and I have seen it cause more than few issues.  KEEP TBI SIMPLE is my rule. Spend that money on the new distributor and good plug wires.  for coil, TBI is typically fine for the stock coil but I don’t have a problem if you use a performance coil. I typically always do.  Pertronix D3002 or DUI (Davis Unified Ignition) 31723.  With stock coil gap plugs at .035″ and with the performance coil .040″.  Knock sensors since they are part of the ignition system I will mention them here although they dont cause too much drivability issues.  They can affect performance.  use the correct sensor for the year vehicle.  the later model TBI systems used a different knock sensor that looks identical to the older ones and will definitely cause a performance problem.  Use a new knock sensor with your new motor and DO NOT over tighten the thing. Spec is like 15ft lbs which is about the same as oil filter.  I typically go hand tight and then about 1/2 to 3/4 turn with wrench to what feels good without being too tight.  go wrenching on the thing and it will be bad.

Spark, no fire is not a computer problem.  Its either going to be a bad coil or ignition module or blown fuse.   The computer controls the advance on the spark not the actual spark.  The spark is initiated from the ignition module via that pickup stator around the shaft of the distributor.  So if you have no spark its going to be one of those 3 problems.  Do not trust a new module from your local autoparts store.  I have had plenty of the things dead right out of the box.  I have even had dead Accel and MSD modules as well.  No one is immune to faulty electrics.

4. Electrical

I saved this for last as no one likes electrical issues.  These will drive you insane.  Bad enough the computer system and wiring is 25+ years old now.  all the plastic electrical connectors are brittle and locking tabs often broken.  Key thing here is don’t pinch any of these wires between the bellhousing and motor when installing motor.  That will give you a fit.  The key issues with fuel injection and wiring is the ground wires.  Poor grounds gives me more weird issues than anything else in the wiring.  Sensor plugs typically throws a code for the sensor and that can be found pretty easily.  Bad grounds and nothing.  The computer has at least 4 grounds hooked to motor.  Some up front on the intake by the thermostat and some attached to the back of the passenger side head.  Lots of wrenching over the years can booger up the wiring at the connector or it can break off there at the connector.  Either will certainly give you problems.  Check and clean them thoroughly.  Most overlooked is the main grounds.  The battery should be well grounded to the engine block which isnt’ so much a problem but the ground from the engine block to the firewall is often overlooked and often corroded in to or just plain broken. This will not only cause you computer issues but issues with all the electrical systems in the vehicle.

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GM Fuel injection for the average guy