Y'all...I'm sure you've seen my monster thread I posted a couple weeks ago about a bad TPS, etc. Well, it morphed into some complex stuff and I need some guidance. I'm a little defeated and need some C4 forum love right now

As the story goes, i've been restoring my 89 and have done immense amount of work (mechanical/cosmetic) to get her cleaned up and back on the road after a tough life of abuse from the PO. Back in Feb is when I officially got her back on the road. At the time she ran a little rough because the PO put a LT4 hot cam in it and never had the car re-tuned. I did notice the car sometimes would like to idle at 1000 then settle to 8 when coasting in neutral or at a light. Fast forward. PCM built a tune for me and it completely changed the car for the better, ran like a champ except I noticed some sluggish throttle response. Checked the TPS volts and they were super low and WOT was only giving me like 1.75v. Went through 4, that's right, 4 new TPSs that were failures out of the box. I eventually had to purchase an original from a forum member here that had a nice original. That TPS was perfect and read proper volts. However, during all of this process, the car started to develop this odd idle issue where it simply would not idle under 13-1500. No matter how many times I reset the IAC and TPS, it would bounce back to 13-1500. Took IAC out and it was pretty gross and I cleaned it up; installed with no change. I had some rewards at the parts store, so I threw a new IAC at it for like $20, no change.

I then turned to the MAF. Unplugging it would drop the revs right down to 800 and idle perfect. Rich, but perfect. The MAF was a new Bosch unit I put on the car when I first got it because it literally shut down on me twice while driving and the new MAF cured that. The unit was still in warranty so I dropped it off at the store and they supplied me with a new Spectra unit. I installed that tonight with an exact result of a 1500 idle. Oddly, tonight when disconnecting MAF, it only dropped revs to 1000. Trust me when I say i've investigated vacuum leaks. There are 0 vacuum leaks in that car.

The only real variable that has changed with this car is the temps we're in right now. I'm in NC and we've had some 90s/high 90s for like 2-3 weeks now. I know ECMs when failing tend to be more temp sensitive. I once read a quick test is to stick the ECM in the fridge for like an hour and see if that makes a difference. At this point i'm a bit lost and would love to have some feedback. Always, thank you all greatly

 

Have you tried completely blocking airflow into the throttle body? If you had an analyzer it would be interesting to know what the IAC counts are.

 

….. Start with basics … set the timing and the minimum idle speed per the instructions in the Factory Service Manual … you might want to set the min idle a little higher (600-700rpm) because of the cam … Lubricate the throttle body shaft where it passes through the TB with some silicone or Teflon based lube … unplug/disconnect then re-plug/re-connect EVERY electrical connector from the ECM out to the engine … this cleans the contacts … as mentioned above , get a scanner and check IAC counts and idle air flow … IAC should be somewhere around 10 to 30 at a normal idle at operating temperature … less than 10 means the throttle blades are open too far , taking idle control away from the IAC … more than 30 and the IAC is trying hard to control the idle because the throttle blades aren't open enough … idle air flow should be approx. 8-11 grams per second at normal idle speed at operating temps …..

 

I've got an ALDL to USB coming so I can link up TunerPro to my laptop. This whole thing is just past bizarre. Even with set screw completely out, revs only drop to 1100. That just sounds like something is commanding it to idle high.

Believe me, all of that has been done; about 15 times now. As soon as my cable comes in, I can pull some data and see what the ECM is actually doing. At this point I cant help but feel this is not going to be as simple as a turn or two of a set screw or a low functioning IAC. You can do crazy things like back the set screw completely out and loosen the TPS to read as little voltage as possible and the car will still race on to 1500. Something is commanding the car to run that high which is leading me to the ECM or PROM. I really don't see how a sensor could command anything that would cause the car to race like this

 

..... Vacuum leak ! … or plainly put , an air leak into the intake tract …..

 

 

..... Have you looked at the throttle blades with the idle screw backed out to see if there is any light coming around them ? Is the throttle shaft tight where it passes thru the TB ? … TB off engine of course … what intake manifold ? TPI has numerous opportunities for air leaks … smoke test ? … any air leaks between the MAF and TB ? … brake booster itself ? …..

 

 

 

Pick up a vac. gauge plug into a plenum port to be sure the problem is not a leak somewhere.

 

Y'all, she's fixed. Long story short, I got the ALDL cable and got TunerPro RT going and I found that my multimeter for whatever reason was not giving me an accurate reading when I was back-probing the TPS. With the set screw out completely, I was still showing 1.3v. I was running out of daylight so I eyeballed the set crew at 5 turns from blade contact and set the TPS at 0.54 per the ECM output on screen. Idled right at 750. I still need to go through the min idle reset process, but what a lesson learned. I verified the multimeter worked just fine, shows proper volts on other equipment. This leads me to believe I was either getting poor contact from on back probe or was picking up some resistance somehow in the readout. So from this point out, no more multimeter FSM procedures, I'm going straight to scan data...

Thank you all for all of your help, it's seriously appreciated