A How To Carburetor Diagnostic Guide – Part 3

Continued from Part 2...

5. Throttle position sensor (TPS) and/or mixture control solenoid (MC Solenoid):
These of course are two components found with the electronic feedback carburetors only.  Starting with the throttle position sensor or TPS for short first because it’s the mostly likely of the two to fail.  Both of these sensors have a specific out-of-vehicle resistive value, which can be measured using an ohmmeter, but values vary with vehicle and year so one should consult the repair manual.  Not all but most carburetors also require a specific adjustment value for the throttle position sensor.  Most all carburetor rebuild kits will have adjustment value sheets inside.  If either value is off then air/fuel ration will be affected!   The mixture control solenoid or MC Solenoid for short has a resistance value that can also be measured.  Much in the same way as the TPS consult a repair manual for resistance values.  However, I’ve found the average value falls somewhere around 24 ohms as relative to ohms law.  The MC Solenoid should have a supply voltage of 12V pulsed DC.  At the bottom of the MC Solenoid there is a neoprene or steel tip that can be in some cases damaged or missing.  This will require an out-of-carburetor inspection; even then one may not see it correctly. 

6. The supply voltage to the above sensor and solenoid -
There should only be a continuous supply voltage to the throttle position sensor – please consult repair manual for values.  However, the mixture control solenoid requires a timed pulsed DC voltage, and an expert should test this only.  Possible causes of voltage failure to TPS are wires that are either broken or shorted to ground.  However, even though possible causes of voltage failure to the MC Solenoid can also be broken or shorted wires, in this case it can also be failure of the vehicle’s onboard (computer) Electronic Control Module (ECM) or Powertrain Control Module (PCM) depending.  Note: Improper testing of this system correctly can lead to computer failure. 

The above items are the most common reasons for carburetor failure or at least it appears to be a carburetor failure.  I would say that one third of the carburetors sent to us from other shops where misdiagnosed and were not carburetor related failures.  Sensor related failures tended to give similar symptoms as that of a carburetor failure. 

What if it’s not the carburetor, what are some of the other possibilities?   Below I have listed out some things of consideration.  However, if your vehicle has been worked on a lot by people who really have no clue what is wrong or how to even fix it in the first place then the possibilities of what could be wrong increase ten fold.  In this case I suggest that you continue searching for someone who can help you.  They are out there, keep looking. 

Ignition System Failure –
Your vehicle’s engine has many maintenance related components of the ignition system that must be maintained on a regular basis – please refer to your owner’s manual for recommended scheduled maintenance.  The number of components required for maintenance depends on the year of your vehicle.  However, regardless of year I will list out several most common components required for maintenance.  Maintenance usually means replacement of components.  The best way to start your diagnostics is with a visual inspection.  Any broken, burnt, cracked, etc. components warrant attention first.  Consult a repair manual for complete diagnosing of your vehicle. 

a. Ignition contact points – That’s right, contact points, but these are contact points with electrical current flowing through them, which means that life spans are rather short and require replacement especially if your vehicle has been worked on a lot over the years.  Unless perfectionist has restored your vehicle the odds are the return path of electrical current (ground) is corroded or missing one or two of the three or more required.  Why is this important?  An inadequate grounding system causes excessive current built-up to pass through the contacts thus burning the contacts and leading to premature failure.  Ignition points are among the most common reasons for breakdowns among older high mileage vehicles.  All points require a very specific adjustment.  The adjustment of the points should be established correctly before any attempts are made to adjust the timing.  Point gap affects timing, but timing does not affect point gap.  Please note this point – no pun intended. 

Disclaimer: You have been advised to read all and any repair manuals and/or consult a professional repair shop.  By no means am I the last or final word for your repair issues.  Don’t just take my word for it before attempting any diagnostics on your own, consult a professional and/or read your repair manual first. 

SEE: PART 1 | PART 2 | PART 3 | PART 4