My 95 GMC Jimmy is idling high and will not idle at the desired RPM. It happened to me after running down the highway for about 20 miles. The TPS and MAP voltages increase with RPM. The IAC count is 0 at idle and goes up to about 50 at full throttle when the engine is warm. EGR valve also seems to be functioning correctly based off of voltages. I cannot for the life of me find a vacuum leak. I’ve blocked off both vacuum ports at the back of the intake and there was not change to the idle. I’ve smoked the intake through the brake booster vacuum hose and found no significant leaks (a small leak through the opening in the intake for the throttle shaft. Any thoughts would be greatly appreciated.

 

Which ODB system do you have? What scanner is that?

Problems with the IAC valve can cause this. A guide:

https://axleaddict.com/auto-repair/I...roubleshooting

A vacuum leak sure but any unmetered air so check the air horn after the MAF, and the PCV valve. Normally fuel trims can help confirm a vacuum leak but I'll wait on the ODB and scanner questions.

How are your lower intake manifold gaskets?

A dirty throttle plate can cause this

A faulty TPS can cause this, but it looks like you are at 0% at idle which should be good.

A faulty power steering pressure switch can cause this if so equipped.

Is the alternator/charging circuit working as it should?

George

 

Its a 1995 so I have the OBD II style connector, but it is not compatible with most scanners. The scanner is a Bosch Veronix GM Tech II scanner. There is no MAF on my setup, it has the CMFI system so it only has a MAP sensor. PCV valve is working correctly and has no leaks. I hope the lower manifold intake gaskets are good, the engine is only a year old. I would have thought the smoke machine would show any leaks in the gaskets. I believe the TPS is okay, it doesn’t have a power steering switch. The charging system is working normally.

 

You will have to bear with me because you probably have the transitional ODB 1.5 system and I won't be the first member to be humbled by this system. I also don't understand the concept of counts in scan data. That said:

Since your truck uses speed density instead of mass airflow its critical that the IAT and MAP sensors are accurate. Look at their values on a cold engine not running and compare those values to ambient conditions. Do the same thing with your ECT even though that looks OK on your warm engine. Move the TPS sensor through its range of motion and make sure the reading is glitch free and makes sense.

How many O2 sensors do you have? Your display looks like a 4 banger with one pre and post cat sensor and only one set of fuel trims.

Can you see your O2 sensor output in live data graphic format or with a scope? If not can you see the voltage values in lave data to see them switching?

Does your throttle body need to be cleaned?

Have you done any removal/cleaning/inspecting/testing of your IAC?

Has your fuel mileage changed significantly?

George

 

Doing some reading I see that cross counts is the number of times that the O2 sensor transitions from low (0.15V) to high (0.85V). What is the time period for those 241 counts? Then we can compute a switching frequency and that gives us more useful info.

Can you tell me what counts signifies for the other parameters like fuel trims and IAC position?

George

 

I'm not sure about the time period on the 241 counts. I'll play around with the scanner and try to figure it out. I know if the IAC count is 0, the IAC valve is fully extended. Here is the best reference I could find. TPS and IAC. The fuel trim count of 128 is equivalent to 0% fuel trim for both short term and long term fuel trim. So on my scanner the long term fuel trim count of 137 is equal to ((137/128)*100)-100 = 7% fuel trim.

 

So the maximum count is 255 so the count resets to 0 when it hits 255. I didn’t time it yet, but it hits 255 quickly, around a minute or two. So I believe it’s flipping back and forth between rich and lean like it should.

 

The minimum should be 120 counts per minute, that's 1hz. You should be hitting around 0.15v and 0.85v each peak. Its best to watch a live graph but this is a start. Final evaluation of O2 sensors is to make sure they have not become sluggish. You force lean and then rich and watch for the reaction time.

How many O2 sensors do you have? Is there one upstream for each of 2 banks or only one upstream period? You add ST and LT together for each bank and we want that total to be < +/-10.

George

 

That IAC article that you referenced has a good set of gremlins to chase. It states that IAC counts should not be below 20. Your best bet may be to exhaust everything there. Idle screw, binding cable, air leaks, etc

George

 

I have 2 o2 sensors, one upstream and one downstream.