Idle Troubleshooting

[tomtom72]

Glenn,

Thanks for not thinking I was trying to be a wise guy with the IAC passage.

TPS = 0.54 v at idle, hot. It's adjustable, but I don't know what the range of adjustment is, sorry. I never had to adjust mine, but the hold down screws are in slots and it looks like there is a useful bit of adjusting that can be done.

I bought a used MT-2500 off e-bay. I'd kill for a Tech1A. The real issue is finding all the software for all the different systems. Old technology & all for an antique car.
It doesn't exactly agree with my tach when I scan the car at hot idle. My tach down there appears to read low to me anyway. Also, my idle seems to fluctuate between 650 and 675 when I scan it, sometimes even as high a 700 and as low as 600. The TPS reports in at 0.54 v.


Tom

[gbmidyear66]

I'm down to about 800 RPM now,maybe I should just declare victory and move on?

The tube that connects to the dual PCV valve connector was a little loose - tie wrapped that.

Re-torqued all the plenum bolts

Re-tie wrapped the short rubber tube at the front left plenum, I could change idle speed just by wiggling it.

Re-tie wrapped MAF sensor hose.

I think my next step is to pick up a scanner and a small bottleof propane, this is getting a little frustrating.

[gbmidyear66]

I am still fighting with my high idle, and am currently out of ideas.

I bought an Innova scanner off of EBAY which I am waiting to arrive so I can check the IAC value, TPS voltage, the target idle of the ECM, and to confirm the tach RPM.

Meanwhile - I tried the binder over the air intake, it killed the engine. This leads me to believe that my efforts with tie wraps, etc to eliminate vac leaks was successful.

I manually opened the secondary throttle plates - there appear to be nice black rings (presumably DAG) in behind

My TPS sensor is installed at the max clockwise end of the adjustment range, any adjustment counter clockwise only increases my idle speed.

I alkso check for vacuum at the dipstick (which I read somewhere should exist), but did not detect vacuum there - is this a clue?

Any fresh ideas or insights much appreciated.

[tomtom72]

Glenn,

Not very sure about the "vacuum" from the stick's tube? Crankcase should be somewhat pressurized via power stroke blow-by, no?

If you use some straight pins, you could probe the TPS connector & use a DVOM to read it's voltage. Use the straight pins to go in thru the back of the engine harness connector, this way we make no holes in the wire insulation. It's a bit tedious, but it will work. I did it once using regular phone wire. The other alternative would be to make up a T-Y harness to go in between the TPS and the engine harness with an extra female connector to use as a probe point.

I'm very loath to even suggest this "option". We have a idle stop screw on the underside of the linkage. The factory adjusted it and it's to be left alone for the duration. If someone played with that before your ownership, that could be a contributing factor to this high idle issue. Most people don't even know about that screw so I would reserve that till the bitter end. It's not easy to get to with the T/Body mounted on a complete, running engine anyway. I think there was one of those special GM tools to mess with it on the car.

You covered all the usual suspects, and very well at that! I'm thinking along these lines: high idle rpms = too much idle air(?) from bad IAC(?) or bad TPS(?), but if the ECM thinks the coolant is cold, it commands IAC to open(?), then the ECM's temp sensor maybe at fault? But that only makes sense if the motor is cold, or in open loop.(?)

Closed loop high idle = too much air(?) = vacuum leak ( we don't have any!), therefore, suspect could be fueling(?). Why is fueling rich? = bad O2's(?), bad IAT sensor(?) I'm not sure if the fueling was out that a couple hundred rpms could be smelt if too rich or too lean in either open or closed loop operation.

I'm going to go look in the FSM in section 6E to see if something jumps out at me; be back later on coffee break!


Tom

[tomtom72]

Okay, break time...in section 6E under intermittents ( pg. 6E3-B-7 ) on "incorrect idle speed" they refer to TPS voltage and linear operation of the rotating element. IAC system check @ chart C-2C, ECM grounds, and alternator output & grounds, throttle linkage is not binding, PCV valves are not clogged.

As an aside on the PCV's. I have found thru my 5 yrs of ownership that this motor is very aware of how clean the PCV's are kept. By virtue of our inherent excessive oil vapor generation I have taken to replacing my PCV's every winter. I find that they become sticky after only 5k miles in my car. It's not that they don't work, but the sound is no longer that nice clean metallic click when you shake them. I've no idea if this is relevant here or not.

Okay, I looked up the IAC diagnostic chart. Do me a favor, are the PCV's okay for sure? Yea, the chart is a bit complicated and requires some special electronic gizzmos to perform. There are work arounds for those of us that do not have a Tech 1A and all the attachments....I just have to figure them out...sorry I'm not that quick with this electronic stuff

Let me know what's up with the PCV's.

Tom

[A26B]

Originally Posted by gbmidyear66

My TPS sensor is installed at the max clockwise end of the adjustment range, any adjustment counter clockwise only increases my idle speed.

I alkso check for vacuum at the dipstick (which I read somewhere should exist), but did not detect vacuum there - is this a clue?

There is something amiss with the TPS. Virtually all I have ever seen, are roughly in the center of the adjustment slots, i.e. the sensor is situated level. Your TPS may be sick.

Re: vacuum @ dipstick seal. My initial thoughts are this won't be the case with the LT5. There are 2 fresh air/filtered vents, 1 on each cam cover, for crankcase ventilation. At idle vacuum, it doesn't seem likely/feasible that there is or should be negative pressure in the crankcase that would be detectable.

[gbmidyear66]

Tom

Thanks for all your efforts on my problem, much appreciated. I'm going to try to get to NAPA before close tonight and pick a couple PCV valves up - I expect mine are original, and given their cost, I should just replace anyhow.

Yeah, I was aware of the idle stop adjustment, but just didnt want to go there until after having gone over and eliminated any faults evident with the scanner.

Jerry

Thanks for clarifying the absence of vacuum on the dipstick


I'm going to replace the PCV's and run the scanner (which I'll get early next week - have to cross the border and pickup at the parcel depot) and report back. It seems that 90% of these idle issues are vac leaks, but it looks more and more that my case is one of the other 10%.

Glenn

[tomtom72]

Glenn, you're welcome.

I have thought about what Jerry wrote about the TPS...and damn if mine isn't level and so are all of the ones I've seen. Hummmmm....maybe there is something to that?

Have a good weekend!