LT5 idle issues HELP!

[YESZREE]

My 91 ZR-1 has an idle problem. When I am coming to a stop and I put the clutch in, while rolling to a stop the idle surges slowly from 800 rpm to 1100 rpm at a slow cadence. I've checked for diagnostic codes and found none. I verified the TPS air ration setting and all is ok.
What gives? Can anyone help?

:duh

 

[Wegone]

You may want to remove the IAC and clean it and the passage..

 

[8388]

If it's not the IAC, it could be the chip. Some 91 stock chips had drivability issues.

 

[Steel Half-Shafts]

Sounds like the MAP sensor hose is loose or has some oil in it. :bang

Mine did the same thing a few years ago, and I had some oil in the hose preventing the Map sensor from working correctly. Pulled the hose, cleaned it out and it's been OK since.

 

[Hib Halverson]

There are a variety of problems that can cause unstable idle.

Two have already been listed
1) calibration
2) dirty IAC pintle and/or passage. If you go to fix this DO NOT soak the IAC in solvent

Also, look for a "coked-up" throttle body. Pull off the air duct, hold the throttle wide open and look down the TB with a bright light. If you see black crusty deposits on the throttle plates or the bores, pull of the TB and clean it.

If the throttle body has been replaced, you might need to perform the "minimum air" adjustment listed in the service manual.

You also may need to perform the IAC learn procedure listed in the manual.

Look for disconnected hoses that would cause a vacuum leak.

Take a look at oxygen sensor operation with a scan tool. A "lazy" O2S can cause idle instability.

Does the engine have bad plug wires or bag plugs?

Is the engine modified? If so, to what extent? Some modifications can cause loss of idle stability.

Good luck.

 

[YESZREE]

There are a variety of problems that can cause unstable idle.


I thought the idle problem was gone but...NOT. I cleaned IAC, and it was very dirty. I also cleaned the housing which it presides in. I also cleaned the trottle body, although it wasn't very bad. My car has 20k miles and some mods. Porting and polishing of the heads and intake, degreed cams, jeal chip and headers, followed by B&B 3" SS exhaust and 4:09 gears and ram air with K&N.

I also found that there was a vacuum leak where the MAP sensor connects at the intake. The metal tube connection point for the vacuum tube came out of the intake when I was removing the tube. I epoxied it with titanium epoxy and installed a new sensor.

I have not performed the IAC learn procedure. Please explain what that entails.

P.S. Remember that this only occurs when the wheels are rolling. As soon as I stop the surging stops!

Thanks for you help, Ed

 

[Hib Halverson]

I don't think the initial post in this thread mentioned the engine was modified.

Now that I know that and I read...

P.S. Remember that this only occurs when the wheels are rolling. As soon as I stop the surging stops!

..I'm going to guess you've got a calibration problem. To know for sure, I'd need to see some scan test data taken at light throttle.

The Jeal cal.....was it custom-burned for your engine, bought off the shelf or bought used? If it was not custom-burned, do you know for what package of mods for which it's designed to work?

The TPS learn procedure is in your Service Manual. If you don't have a factory manual, you need to buy one.

 

[YESZREE]

The Jeal cal.....was it custom-burned for your engine, bought off the shelf or bought used? If it was not custom-burned, do you know for what package of mods for which it's designed to work?

The TPS learn procedure is in your Service Manual. If you don't have a factory manual, you need to buy one. [/B][/QUOTE]

I believe that the Jeal cal was developed for my engine. It was part of a packege that included Jeal stainless headers, cold air intake, 3" B&B tri flow exhaust, and 4:09 gears. As I know it, these are the only mods. There were some information given to me by the previous owner that the intake was ported and polished but I do not know if this is accurate. The fellow that I bought the car from was the second owner. The original owner had the work done then sold the car as a result of ( as the story goes ) a very dangerous encounter with a few fellas" from Jersey that were collecting on a debt and he needed money fast!!!

The second owner brought the car home, covered it up, and there it sat until I bought it. When I picked it up, he was very unaware of what he had. I mean he new that it was a ZR-1 but that was the end of his knowledge about the car. He liked it because it sounded good had low miles and was very clean.

That's the story on this car. It is very beautiful with the Quasar Blue and saddle interior. The car looks as though it has been cleaned with a Q-tip in every concievable area.

I have replaced the idle air solenoid and cleaned the housing out.
I've checked all codes and didn't find any issues. What I really need is someone that is comfortable with the LT5 to look at the car. But I do not know of anyone here in Tampa that impresses me.

Any ideas?

Thanks, Ed





I have heard that this porting issue has been recieved with mixed reactions which rob low end performance. Do you have any thing to subtanciate this?

 

[Hib Halverson]

Ok.
At this point we can assume the Jeal cal. is proper for the engine and, for now, rule-out a cal issue.

You've cleaned the IAC and the TB's not dirty, so we can set those aside for now.

I'd run the IAC learn procedure in the Manual and I'd also check the TPS adjustment. How to do that is also in the book. Check, again, for any small vacuum leaks. Also, I'd run the fuel pressure tests in the book, looking for leaking injectors. I'd have a scan test done and look at the fuel trim values at idle. If you see.

This idle stability problem...has it been present as long as you've had the car or did it develop at some time after your bought it?

Lastly, in two different posts you've listed a varying amt. of mods. The number of types of mods are important to this diagnosis, I think.

Recently you said:

It was part of a packege that included Jeal stainless headers, cold air intake, 3" B&B tri flow exhaust, and 4:09 gears. As I know it, these are the only mods. There were some information given to me by the previous owner that the intake was ported and polished but I do not know if this is accurate.

Before that you said:

My car has 20k miles and some mods. Porting and polishing of the heads and intake, degreed cams, jeal chip and headers, followed by B&B 3" SS exhaust and 4:09 gears and ram air with K&N.

The difference is the ported heads and intake and the reset cams. Can you repost the list of mods as accurately as possible

 

[YESZREE]

Ok.

Lastly, in two different posts you've listed a varying amt. of mods. The number of types of mods are important to this diagnosis, I think.

Recently you said:


Before that you said:
[/b]

The difference is the ported heads and intake and the reset cams. Can you repost the list of mods as accurately as possible [/B]

I will need to remove the intake to determine if the porting and polishing is an accurate statement. The previous ownerhas stated that ,but, he certainly was parroting what was told to him.
The mods that CAN be substanciated are 4:09 gear,cold air induction,jeal headers,3" B&B Tri Flow exhaust,Jeal Cal.

However, the idle situation has been since I have owned the car, which is only about 6 weeks.

I will perform the testing which you have perscribed and get back to you. I thank you very much for your continued assistance.