TBI Surging/Hunting for Idle When Warm in Park

[Bubb]

Howdy all,
Long time lurker and fan of the site. I'm having a problem that's driving me crazy and am hoping the community can help. I've searched for this exact issue with no luck yet. Here goes:

The truck:
1995 Silverado K1500 ECSB Z71, 5.7, auto, 82k miles
I bought a year and a half ago from the original owner, who hadn't driven it in years
Been going through it and getting it in good running shape again, lots of recent work

The problem:
Had trouble starting when cold, so I took it to Oreilly's and swapped the battery after they confirmed it was bad. Immediately afterward it began surging/hunting for idle (never did this before). After a couple mins it found it's groove and I suspected the ECM just had to do some learning and would be fine. A few weeks later now it's still doing it.

These are the symptoms:

• Hunts for idle, varying rpm from ~500 to ~800 and will find a steady idle after ~15-30 oscillations, sometimes starts the hunt over again afterward
• Only happens in Park or Neutral, never happens in Drive or Reverse
• Only happens once it's warmed up (closed loop), seems to get worse the warmer it gets
• No driveability concerns other than maybe a rougher than normal idle, good power, good MPG, no CEL

This is what I've checked so far:

• TPS, IAC, MAP sensor, CTS all seem good (and all replaced in the last year and a half)
• Fuel pressure is good at 13psi
• No vacuum leaks or other leaks (base of the TB, intake, etc) that I've been able to find
• Ran the procedure to reset IAC, didn't change anything
• I can get the hunting to go away if I spray carb cleaner down the TB but suspect that this is just masking the issue

Since I've owned it I've also rebuilt the TB, installed new injectors, new EGR, new O2 sensor, plugs, wires, cap, rotor, catalytic converter, PCV, vac lines, evap canister, EGR, EGR switch, evap canister switche, and many other things. But the only change I made just prior to the issue was the battery.

My thinking is that it's the IAC that's causing the hunting, but there's some other reason the ECM is commanding to IAC to do so. I don't have access to a tool that would allow me to monitor sensor readings from the OBDI ALDL while it is happening.

 

[shovelbill]

i'm gonna watch this thread and learn something....did you disconnect the O2 sensors?

 

[Bubb]

Haven't tried disconnecting. Since they're so cheap I did go ahead and throw a Delco one in there the other day, made no change.

My thinking is now that it may be unable to decide whether it's cold (open loop) or warm (closed loop), and it's bouncing back and forth between the two until it's sure it's warm. Unfortunately there doesn't seem to be any easy way to watch the sensors real time via scan tool like with OBDII.

I've checked the connectors for any melting or discoloration and tried jiggling/tugging on the wires/connectors while it's doing the hunting, but haven't found anything.

I may try and find the appropriate value resistor and plug it into the CTS connector once it's warm (unplugging kills the engine)--that way I can rule out some intermittent reading coming from the CTS at startup.

Open to any suggestions...

 

[grampadirt]

I'll give it a shot but please bare with me,it's been years since I'v set it up and have forgotten much of how I did it.There's a website called Tuner Pro.net(IIRC) that can monitor much of what our ECM sees.Here is a snippet of what I see...

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[Bubb]

Thanks grampadirt, I've been reading up on this a little bit and may have to dive into it. Unfortunately I don't have a laptop to use at home but may be able to take one home from work.

Anyone got any other ideas for handheld tools that could take sensor readings from the OBDI ALDL real-time? I've heard tell of a Snap-On "red brick" and some others--what are people around here using?

 

[Bubb]

All,
Lots of quality time spent under the hood this weekend, but still haven't solved it unfortunately. Here are some updates and new data points:

• Noticed that if I deny the computer the opportunity to surge/hunt 'til it finds its groove (by putting it into R or D right after starting), it will start doing its surge/hunt thing after putting it in Park if the drive is short enough (pulling it into the garage from the driveway). Longer drives, it won't do this.
• After it did this I thought maybe the issue was that the computer couldn't remember where idle should be. So I picked up a refurbished computer at Oreillys, but it did the same thing after installing
• Tried disconnecting the O2 sensor, made no difference
• I got an appropriately valued resistor (240ohm, correlates with ~190 deg F) and plugged it into the CTS connector to rule out some intermittent issue there, made no difference
• Got out the timing light and noticed that it was running very advanced (off the scale of the reference marker) and was retarding the timing with the surges, such that peak retard corresponds with peak surge RPM
• I found out there's a way to set "base timing" (not letting the computer control advance) by disconnecting a wire under the dash. This did take away the surging, indicating to me that there's no mechanical issue the computer is working to overcome (timing chain slipping, worn distributor gear, etc). I also noticed that it was set to 4deg advanced, where the manual and underhood label state base timing should be at 0deg. I adjusted it back to 0 and re-connected that wire, didn't make a difference in the surging behavior but it did bring down the amount of advance at steady state by about the same 4deg. No discernible difference in power or driveability since.
• The ignition coil and pickup coil pass the resistance checks, all connections/wires seem good in the distributor. Doesn't seem to be excess play in the rotor/shaft.
• I went ahead and swapped the Ignition Control Module for a new one, made no difference.

A couple questions: do you know whether the computer knows the cam position, and how it does if so (with no cam position sensor)? What would the computer do if it thought there was a knock/detonation?

 

[Tech]

Buy one of these
http://www.ebay.com/itm/12-Pin-ALDL...:g:o10AAOSw9mFWHvks&item=272141092674&vxp=mtr
and download ALDLDROID for your smartphone or use the free app and you will have all the data you need.
50+ parameters 8 times a second updates.

The ECU has a parameter that adjusts the idle pulse width via battery voltage. Higher voltage =shorter /faster pulse.

Things that you could try to manipulate the problem
Turn on the headlights to add a load to the alternator.
Tweak the idle screw up or down a little
Run a can of Seafoam through it
Look for cracked or disconnected vacuum lines

 

[Bubb]

Thanks Tech. Unfortunately I don't have an Android device to work with this or a laptop at home. I've been poring through the shop manuals and there are a few tests I can do at the PCM connectors, but nothing like what this software can do.

I've tried some of your suggestions already--tweaking the idle screw didn't help, and I've gone through and replaced all vac lines in the last year or so. The behavior doesn't change with the headlights on. I don't think it's a gunk issue, as I rebuilt the TB recently. The only change I made right before this problem started was putting in a new battery.

I'm getting familiar with the PWM method of varying fuel delivery, and I know that the computer is varying idle air, timing, and possibly fuel delivery when it surges/hunts. What I don't know is why. I think I've gone through and checked all sensors at this point and ruled out a mechanical problem.

The two things that have stopped the surging/hunting are spraying carb cleaner down the TB and disconnecting the base timing connector. So, if I don't let the computer control ignition and/or fuel it's fine. : )

 

[michael hurd]

Check the EGR, as well as the magnet in the distributor.

 

[Bubb]

EGR is a new Delco unit, and the problem persists even if I plug the vac line going to it and/or disconnect that EGR valve/switch.

Can you tell me more about the distributor magnet test? Where can I find it, and how do I test?