Unstable Idle Problem

**S3TPI** · 09-13-2015

I have a TPI 305 installed in an old GMC truck. It runs great, except for park or neutral, the idle speed varies 100 RPM. In Drive, it is rock steady. I am using a 7727 ECM with an AUJP 305 bin. I have tried changing the idle speed, advance at idle, and VE at idle, with no effect. The target idle speed is now 750, but varies from 700 to 800. I searched for similar issues, but did not see a clear fix. The MAP is around 40, and IAC steps around 30. These change slightly with RPM. Advance is steady at 26. Does anyone have any suggestions?
Thanks,
Gary

**sturgillbd** · 09-15-2015

A datalog of the engine running will be needed. Surging idle can be caused by many things including vacuum leaks and improper fuel delivery. AUJP in its original form was a 5.7 liter bin. AXXB was a 5.0 liter bin. Is the 305 stock or has it had a cam change etc?

**S3TPI** · 09-16-2015

The engine is totally stock with supposedly a mild "260" cam. Yes, I'm aware that AUJP is a 350 bin, but I found the AUJP 305 bin in the old Moates files and figured it would work. The engine size and injector constants were correct for a 305, and, as I said, it runs great except for this bit of idle surge. I have blocked or capped all the vacuum hoses, and swapped MAP sensors, and it did not change. I can download the AXXB bin and try that, but I have never posted a log file here, so I am not sure how to do it. I know how to record engine data with Tunerpro, is it as easy as saving that and copying it to this forum?

**S3TPI** · 09-18-2015

I made a brief recording session of the idle today. I included this as a log file attachment, I do not know if this is the best way to post these. If not, please let me know the proper way to do it.
Thanks,
Gary

**84Elky** · 09-20-2015

It's always best export the log to an Excel file and post that so the true log file does not have to be decoded.

**S3TPI** · 09-20-2015

Originally Posted by 84Elky

It's always best export the log to an Excel file and post that so the true log file does not have to be decoded.

OK, I'll give that a try. Thanks for the info!

**sturgillbd** · 09-21-2015

You seem to be using an older version of TunerPro RT. Have you tried the later versions that create XDL log files?

**S3TPI** · 09-24-2015

I have found that my laptop does not have Excel installed, so I will have to work with that issue. Either find a way to install it, or copy the log file to my desktop computer, which does have Excel. Yes, I am using the older version of Tunerpro because I am more familiar with it. I did try the newer Tunerpro, and did not really like the ALDL Dash display very much either. I will try the log conversion as soon as I can.

Thanks for the replies.

**S3TPI** · 09-24-2015

My plan worked. I got the file from my laptop to desktop, here it is.

**S3TPI** · 09-27-2015

Since I haven't received any other suggestions, I changed the idle deadband setting from 37 RPM to 25 (as low as can be entered), and that helped a bit. There are still slight variations in idle speed, but not as much as before. Apparently this gives the IAC tighter control over the idle speed. I know this may be a band-aid, but it helped. I also rechecked every hose for vacuum leaks, and found none. The only other thing to do might be an intake smoke test, but I really don't have any way to do that. The intake was recently installed on the motor, with all new gaskets and O-rings, so that should not be a problem.

**sturgillbd** · 09-28-2015

I just now looked at your excel log. Do you have the park/neutral wire on pin D11 connected? If not, try grounding this wire and see if the idle becomes more stable. A lot of the ECMs need to have a Park/Neutral wire connected to have a stable idle.

**84Elky** · 09-30-2015

Originally Posted by S3TPI

My plan worked. I got the file from my laptop to desktop, here it is.

Any chance you can post your BIN? Many variables not in the log that could have a bearing on the issue. Also, do you have VSS installed?

**S3TPI** · 10-01-2015

Originally Posted by sturgillbd

I just now looked at your excel log. Do you have the park/neutral wire on pin D11 connected? If not, try grounding this wire and see if the idle becomes more stable. A lot of the ECMs need to have a Park/Neutral wire connected to have a stable idle.

Thanks for looking at my log. I do not have the park/neutral wire connected to anything, since the steering column did not have a switch on it. I did connect the park/neutral wire to ground while testing with the engine running, and it did not seem to do anything. I may give that another try, and take a close look at the RPM this time.

**S3TPI** · 10-01-2015

Originally Posted by 84Elky

Any chance you can post your BIN? Many variables not in the log that could have a bearing on the issue. Also, do you have VSS installed?

Hi, I would be glad to post the BIN here, let me see if I can figure out how to do that. I do have a VSS installed, I bought one from JTR and it seems to work fine. Vehicle speedo very closely matches the speed indicated by Tunerpro while driving.