High Idle - Rich Exhaust

[Jon_Bueno]

1989 CHEVROLET C3500
7.4l TBI
Automatic Transmission

Fuel Pressure: 11.5 psi
Manifold Pressure: 18 hg
Thermostat: 205° F
Timing: 4° BTC
Controlled Idle: 750 rpm/13 counts
Minimum Idle Air Rate: 700 rpm
TPS at Idle: 0.82 volts
MAP: with-in ranges
o2 Sensor: voltage high at idle (0.8-0.9v +/-)

New Parts and/or Less than 10k miles:

New -
Oil and Filter
Air Filter
PCV Valve
Fuel Filter
Coolant
Coolant Temp Sensor
Thermostat
o2 Sensor
Spark Plugs
IAC Valve
TPS
Vacuum Lines
Rebuilt Evap. Canister
ECM Swap

< 10k Miles -
Injectors: Serviced and Tested
Spark Plug Wires
Cap/Rotor
EGR Valve
(2) Catalytic Converters
Distributor
Rebuilt TBI
Air/Smog Pump
A.I.R. Check Valves
Fuel Lines


First off I’ll say there was a time when I was just throwing parts at this truck. But most of them have been replaced due to general maintenance and/or at the request of a service technician. I’m usually pretty proficient at diagnosing and fixing problems myself. However, I have been stumped with this truck since I purchased it.

My biggest issue is that it will NOT pass emissions due to a RICH condition, o2 voltage is HIGH at idle (800 – 900mV +/-). And lately my idle has been HIGH. I can’t seem to get it under 950+/- rpm. I have checked for vacuum leaks with WD-40 but can’t find any noticeable difference. My IAC may or may not be functioning properly. I have gone through the diagnostic procedure for the “IAC System Check” from the “Light Duty Truck Fuel and Emissions” factory service manual, and came to a problem. Test procedure calls for testing the IAC harness connectors with a test light. Which resulted in “light steady or flashing all circuits” (I believe them to be flashing). Which states “Faulty IAC connection or IAC valve, or plugged passage”. I have good continuity from the IAC harness to the ECM connection. I’ve replaced the IAC and verified the passages to be clear. I also eliminated the possibility of a “short to voltage” (for a steady light diagnosis). I have verified “Controlled Idle” and “Minimum Idle Air Rate”, both OK. I’ve noticed on the TunerPro RT scan the “Reference Signal” says “YES” but also goes away momentarily then returns. Should that signal be steady?

I’ve exhausted all of my knowledge and patience. It just seems that I am going in circles or missing a procedure at this point. Any help beyond this point would be greatly appreciated. Thank you.

 

[thinger2]

Dont use wd40 for an intake or leak test.
It just doesnt work
WD 40 is the most misused and misunderstood can of magic ever invented.
It is not a lubricant.
It is not a starting fluid.
It is a protecterant.
If you are trying to get your intake to suck something in and tell you a story?
Use MAPP gas
Gas will maybe get pulled in through the leaks.
WD40 just plugs the leak long enough too lead down the wrong path.
WD40 is not a diagnostic tool.
It is a preseritive with great PR

 

[1997]

wow, lots of parts, very frustating for sure.

what code(s) are being set?

GM manual lists possible causes which are very concise.

AC delco replacement parts?

 

[Jon_Bueno]

Dont use wd40 for an intake or leak test.
It just doesnt work
WD 40 is the most misused and misunderstood can of magic ever invented.
It is not a lubricant.
It is not a starting fluid.
It is a protecterant.
If you are trying to get your intake to suck something in and tell you a story?
Use MAPP gas
Gas will maybe get pulled in through the leaks.
WD40 just plugs the leak long enough too lead down the wrong path.
WD40 is not a diagnostic tool.
It is a preseritive with great PR

To be honest it was the first time using wd40 for vacuum leak. I agree with you about it being a protectant only. I first used propane with no results. Just trying anything at this point. I'll try MAPP tomorrow see what kind of results I have. Thanks.

 

[Jon_Bueno]

wow, lots of parts, very frustating for sure.

what code(s) are being set?

GM manual lists possible causes which are very concise.

AC delco replacement parts?

Yes lots of parts, AC Delco parts for what I have replaced. I'll admit some didn't need replacing, but for "peace of mind" it was worth it to me. I've had the truck for quite awhile, but have had the emissions problem forever, (passed maybe twice) and some times really close to passing. The high idle is new.

 

[Jon_Bueno]

wow, lots of parts, very frustating for sure.

what code(s) are being set?

GM manual lists possible causes which are very concise.

AC delco replacement parts?

Codes:
24 - VSS used to get a code 24 repeatedly, but I think that is taking care of. It hasn't come in awhile. I think odometer motor was unplugged inside the cluster.

32 - EGR this one came up recently on a road test. never had this one before. EGR Solenoid connections were gummed up. Swapped it with one from the salvage yard, it tested OK. Maybe it wasn't any good either? I double check tomorrow.

Since I have changed out the IAC and confirmed ignition timing for this post I had to reset codes. I have yet to do another road test to see if more codes have been set.

 

[Nad_Yvalhosert]

Dont use wd40 for an intake or leak test.
It just doesnt work

I wholeheartedly agree.

Use MAPP gas
Gas will maybe get pulled in through the leaks.

Question: how does the MAPP gas work when the engine is running and the engine driven radiator fan is spinning, blowing the MAPP gas away from the intake?

 

[Nad_Yvalhosert]

O.P. Use a flammable-ish propellant liquid, like carb cleaner, instead.

Questions:
What is the temp sensor reading? Via scan tool, not the gauge... Too cold of ECM temp input and itll run rich, and raise the RPM.

Which temp sensor did you replace? Head or intake?

Did you change the idle set screw when you rebuilt the carb? Did you adjust the idle set screw after getting it running?

TPS voltage at idle should be 0.5 V.
0.82 V is too high. Try adjusting the TPS, engine off, and restart the engine to see if idle RPM comes down

 

[Jon_Bueno]

O.P. Use a flammable-ish propellant liquid, like carb cleaner, instead.

Questions:
What is the temp sensor reading? Via scan tool, not the gauge... Too cold of ECM temp input and itll run rich, and raise the RPM.

Which temp sensor did you replace? Head or intake?

Did you change the idle set screw when you rebuilt the carb? Did you adjust the idle set screw after getting it running?

TPS voltage at idle should be 0.5 V.
0.82 V is too high. Try adjusting the TPS, engine off, and restart the engine to see if idle RPM comes down

Hey Nad,

Temp runs at about 202°F on the scan. Had a 195°F in originally, it wasn't getting hot enough. switched to a 205°F

I replaced the intake temp sensor, also checked for continuity of wire back to ECM and connections.

I only recently adjusted the idle set screw. I have it adjusted to 700 rpm in "Drive" for Minimum Idle Air Rate.

I would agree with the TPS. The manual says 0.6v, but also states less than 1.25v during diagnostics.
It is non-adjustable on the 7.4l However, before I replaced it, I slotted the last one with a dremel and was able to get it drop 20 points. But it didn't want to rise above 1.4v anymore either.

 

[highwaystar]

Just curious about the "rebuilt evap canister", did ya send it off to be rebuilt? Thanks.