rough idle

[sy2756]

My truck idles very rough and intermittently misfires at idle. After reading several posts about vacuum leaks and ignition problems I have tried the following:

1. Cap/rotor, no change
2. New wires and plugs, no change
3. Used the bike tube trick to find several vacuum leaks and replaced all of my vacuum lines, no change.
4. Corrected the problem with my fuel pressure bleeding down, no change.
5. Disconnected the tan wire to check my timing, timing looks good and the engine runs smoother with the wire disconnected.

Is it normal for the truck to run smoother with the bypass disconnected or do I have a problem with the EST?
I have a service manual but I am not sure where to go from here with this problem.

 

[thesyguy]

First, are you running the stock chip? If so, the rough idle is a normal condition. Even the GM technicians manual says that the rough idle is normal due to the lean idle condition required to pass emissions. There was a replacement chip for early Sy's that had rough idle, but I believe yours will have the later version based on the VIN.

Second, if you don't have Datamaster or some other scan tool, get one. This can give you the best diagnosis that you will get.

Mine has the ATR Pitbull chip and idles fairly well. I am upgrading to the ultimate chip soon to improve things even further.

Good Luck!

 

[dgoodhue]

Mine has the ATR Pitbull chip and idles fairly well. I am upgrading to the ultimate chip soon to improve things even further.

The ATR pitbull chip has the same exact fuel curve in vacuum and same timing curve as my 93 Typhoon prom (bbza), so any idle improvements are due to GM updates.

 

[sy2756]

I think I fixed my rough idle/misfire. I put in my third set of plugs, maybe I damaged them somehow although they look fine. Now my Idle is great in the drive range but surges in park.

I have a couple of more questions if someone could answer them:

1. IAC counts should be about 20. Is this in park or drive? Mine are about 20 in park and 0 in park.

2. I found another vacuum leak through the throttle body shaft via the bike tube trick. It seems to leak very badly but if you spray starting fluid or use propane it seems to have no effect on the idle. Anyone have any experience with this? Is this leak a problem?