Intermittent Rough/Hunting Idle when warm

[90GMCZ71]

I am the original owner of a 1990 GMC Sierra K1500 5.7 with 115,700 miles. I am also the only person to work on this truck so I know everything that has ever been done to this truck.

 

I have been trying to solve a rough/hunting idle that only happens when the engine is fully warmed up. I have had this problem for a while now (over a year) and decided recently to finally fix this problem; however, I am now in need of some expert help.

 

This does not happen at every light but it seems to happen the longer I am at a light. I also notice that after a trip on the freeway (10 miles or more), it will almost always happen when I exit the freeway and stop at a light. It feels to me like the idle drops and then it catches again. It has never stalled but sometimes it feels that it will.

 

Last year, before my smog test, I gave it a complete tuneup and decided to replace the EGR valve (AC Delco). It passed smog but I still had the rough/hunting idle. Over the past Thanksgiving, I decided it was time for some repairs. Here is what I did:

 

New Intake (Edelbrock - I replaced the intake due to a leaking tstat housing) and gaskets.

Timing chain and gears (The stock chain was clearly worn).

New Coil (Stock Coil was bad)

New 02 Sensor (AC Delco)

I also rebuilt the TBI including new injectors (the old injectors did not have a good cone shape pattern); unfortunately, the AC Delco gasket kit did not come with a regulator diaphram so it was not replaced (although, it did not look like it was leaking).

New Pickup coil and shaft (the pickup coil housing was cracked and the magnet on the shaft was a little loose)

 

Timing is set to 0 with the EST wire disconnected.

Fuel Pressure is at 13 PSI (including when the problem occurs).

 

I have WinALDL and a cable so I can see some of the ECM parameters. One thing I noticed is that IAC count will be stable around 16 and then drop to 8 or 9 when the idle drops. I can post some WinALDL data if that would help. Raw MAP is usually around 90 at idle. Coolant Temp is accurate (195 degree stat).

 

Over the summer, the rear bearing on the alternator went out and I rebuilt it.

 

The exhaust including the cat, are completely stock and have never been replaced.

The heads have never been off the engine. I do have a little blue smoke at startup but nothing major.

 

I have unplugged the IAC while the problem occured, at it had no affect. I have also plugged every vacuum line and again it had no affect; except when I unplug the MAP, it will usually stall.

 

I should note that the engine runs good otherwise. For a 16 year old truck with 115,700 miles it runs pretty well (especially after the timing chain was replaced).

 

I have been reading posts for months now. At this point I am looking for advice on what I should be testing next and how.

 

Thanks,

90GMCZ71

 

UPDATE STARTS HERE: PROBLEM FIXED (Updated 10/24/2006)

After reading some posts about Misfires on the Vortec Engines due to the distributor vents being clogged, I decided to pull the distributor one last time before trying a replacement distributor. One vent was completely clogged and the other was mostly clogged (probably around 75%). I also noticed some corrosion on the terminals where the pickup coil connects to the ignition module. There was no play in the shaft and I tested the pickup coil with a meter and it looked good.

 

So I cleaned out the vents and cleaned up the terminals on the ignition module (using emory cloth), coated them with dielectric grease and dropped the distributor back in. I let the engine warm up and then set the base timing.

 

Well that was Saturday and it has been running better every day as the ECM relearns the fuel trims. The low end miss is gone and it is running a whole lot better. The idle is also much better. It will probably take a week or so for the ECM to adjust and I want to see what happens to the gas mileage. I am also due for a smog check which it should now have no problem passing the test.

 

I will report back once it passes smog and any change in gas mileage. Many thanks to all who helped me with this.

 

My guess is that the blocked vents caused the corrosion on the pickup coil terminals which gave a weak signal to the ECM.

 

//90GMCZ71

[GM-Tech]

Do you have access to fuel trims? That would be called Integrator and Block Learn back then.

[90GMCZ71]

Do you have access to fuel trims?  That would be called Integrator and Block Learn back then.

 

<{POST_SNAPBACK}>

 

 

 

Yes. INT varies between 125 and 130 during the problem. BLM is stable at 123 or 122.

 

WinALDL can output to a text file is that helps. I usually view the output in Excel.

 

//90GMCZ71

[GM-Tech]

Yes. INT varies between 125 and 130 during the problem. BLM is stable at 123 or 122.

 

That's darn good, actually. Ideal is 128. The block learn would indicate it's a tad on the rich side, but nothing really significant there. I was expecting to see some really high numbers, indicating a lean condition.

 

Sure, go ahead and post the text file. Maybe I'll see something, maybe not. You've already done everything I would have thought might have been part of the problem.

[90GMCZ71]

Yes. INT varies between 125 and 130 during the problem. BLM is stable at 123 or 122.

 

That's darn good, actually. Ideal is 128. The block learn would indicate it's a tad on the rich side, but nothing really significant there. I was expecting to see some really high numbers, indicating a lean condition.

 

Sure, go ahead and post the text file. Maybe I'll see something, maybe not. You've already done everything I would have thought might have been part of the problem.

 

<{POST_SNAPBACK}>

 

 

 

 

Like I said, it runs good except for this rough idle. Even when I had it smogged, the tech was surprized at how fast it passed (and this was with a loose timing chain).

 

I upload the spreadsheet at WinALDL Spreadsheet

 

If you do not have Excel, I can save it in a text file.

 

Since I feel that this is heat related, I need to figure out if this is ignition, fuel, electrical or exhaust.

 

The fuel pressure is good but I wonder if the regulator is leaking. How do I check or test this? I assume that there is a vaccum port internal to the TBI for this.

 

Could the ignition module be going bad when hot? Is there a test to verify this?

 

If the cat was going bad, wouldn't I see some other issues (like bad acceleration)?

 

Thanks for your help.

 

//90GMCZ71

 

PS Let me know if you have issues seeing the file

[GM-Tech]

No problem with excel. I have that. I'll have to admit I just took a brief look to see if there was anything obvious. I can see it's not something one can take a brief look at. It'll take some studying. The raw data is pretty much worthless, but I see there's also more standard types of readings for the same items.

 

The fuel pressure is good but I wonder if the regulator is leaking. How do I check or test this? I assume that there is a vaccum port internal to the TBI for this.

 

Could the ignition module be going bad when hot? Is there a test to verify this?

 

If the cat was going bad, wouldn't I see some other issues (like bad acceleration)

 

I don't recall any specific way to check the regulator for leaking. Yes, it's internally plumbed to vacuum. Been so long since I've even seen one, I don't recall much more than that. However, if it was leaking, I would expect INT/BLM to be way off.

 

The way to test the ignition module is to replace it. We used to have a tester for those but it never worked worth a hoot.

 

If the cat was clogged enough to cause an idle issue, it would certainly have a noticeable effect on performance.

[90GMCZ71]

Well, I finally found my intermittent idle issue. I am posting this so maybe it can help someone with a similar issue.

 

The vacuum hose between the TBI and MAP is a hard plastic line with 2 elbows glued on. The glue on the elbow near the TBI turned to liquid when the engine got warm and the hose would not hold a vacuum. I did the blow test on the hose and it held but with a vacuum pump it was clear it would not hold.

 

I removed the elbow and used some new vacuum hose between the hard plastic line and TBI. Truck is running great again. Wish I had found this sooner!

 

If the idle is hunting on a TBI, make sure to test the vacuum hose with a vacuum pump and also make sure the MAP holds a vacuum. Even the slightest leak will cause problems.

 

//90GMCZ71