I'm working through various issues this truck and I'm running into an idle in the 900 rpm range when hot. I'm expecting it to be in the700-750 range. It is a little higher while in gear.

I've tested all over for vacuum leaks with carb cleaner. Nothing at any line or fitting. No leak at the TB or at the intake manifold mating surface. I've pulled the vacuum lines to the TB and plugged the ports on the TB with no change. I've cleaned the TB plate and barrel.. I've removed and cleaned the IAC. I've run the truck with and without the IAC and the idle races high if the IAC is unplugged.

I'm using DashCommand with the correct GM PIDS. MAP pressure is abut 38-43 kPa. at idle. The TPS shows -2.5 degrees (that's negative) at idle. the TPS appears to be the OEM AC Delco unit or at least has "AC" stamped on it. I cannot verify if the IAC is the OEM part. I took a look at the adjusting screw on the TB and there is no plug over it. No idea if some previous owner has turned the screw.

I'm not sure what to make of the TPS reading a negative number. Most of what I've read says it should be 0, but most people seem to be talking about voltage in different interweb posts. Since its a negative number in degrees (which will increase if you step on the gas pedal), I'm thinking the computer knows there is too much air bypassing the TB and its trying to restrict as much air flow as possible. That's just a guess. The plugs I replaced indicated it was running rich. I'm kind of suspecting either a bad TPS or an IAC that does not have enough travel in its pintle to cutoff adequate air to the intake. Or someone turned the adjustment screw and the computer cannot overcome this physical setting.

Any input on the issue would be appreciated. Other than swapping parts, I'm kind of stuck.

 

What do your fuel trims look like at idle and 2500, out of gear?

George

 

At Idle, fuel trims were 1.6% long term and 6.2% short term.

I'll have to check @2500 rpm when I get home. I've seen them go high for a second or two, but I had a post down stream O2 sensor exhaust leak that I welded yesterday.

 

Also, regarding that throttle position sensor. What’s the reading at idle and what’s the reading with key on but the engine not running?


George

 

At 2500 rpm short term fuel correction is 5.5. long term is 0.8.

Tps is -2.5 degrees key on, engine off. Same as at idle. It was around 6 degrees at 2500 rpm.

One more bit of info: the truck stumbles under acceleration when cold. Kind of bogs, then catches. Fine when warm. May be indicative of a tps issue.

 

Is the cold engine stumbling that clears up when it warms correlated to when the fuel system transitions to close loop or is it unrelated? George

 

That TPS sensor value at idle makes no sense. Do you also have that parameter as a voltage instead of percentage or degrees on your scanner? If not can we measure the voltage value at idle and full throttle (engine off)? We would first check the 5v reference, then the ref ground and then the output value in volts.

George

 

I don't know that the stumble is the vehicle going into closed loop. If I start the car back it down the driveway at idle put it into gear to go forward and give it some gas it stumbles. I wouldn't think it goes into closed loop that quickly.

I can't test voltages with the scan tool I have. I did test them with a multimeter. With the throttle closed, key on, I measure .44 vdc. At wide open throttle, I measure 4.25 vdc. There does appear to be a jump in the linear voltages when I back the throttle down from WOT slowly. This happens at about the 1.5vdc range. So I'm thinking the TPS may be causing the stumble. I don't know if it accounts for the high idle.

 

If the voltage output from the TPS is not smooth and linear as you slowly go through the travel of the throttle then yes the TPS does need to be replaced. That minimum value might be high enough to cause the high idle. Your fuel trims do not suggest an air leak Which would be the other main reason for this. What I meant about the transition to close loop is you said the stumble clears up after the truck warms up and so my question is does the stumble clear up and go away as you see the fuel system transition to close loop on your scanner? Regardless, step one is replace the TPS. George

 

I put the new TPS in. It is at negative 4.7° idle and negative 4.7° key on engine off. It is more negative than the one I took off. So, it's not the TPS that's causing the high idle. Must be either the iac or someone adjusted the throttle stop screw. That's an oddball 5 point safety bit, so I'm doubting anybody monkeyed with it because very few would have the right tool for that one. I looked around and throttle bodies on the internet and I see that this particular throttle body doesn't have a plug over the adjustment screw. So a missing plug is not the clue I thought it was.

Any other thoughts would be appreciated, but I guess swapping more parts is the next move. Is there a way to test the IAC out of the vehicle and measure the pintle's amount of travel?