I installed an LS6 intake manifold the other day and ended up with a surging idle. I spent all of yesterday trying to troubleshoot it but finally fixed it today. The cause of the problem was due to dirty electrical contacts at the PCM. You could see where water was working its way into the blue connector. There was soap residue there so I'm thinking that I probably blasted some water in there by accident when I was washing my car.

Allow me to go through my symptoms and troubleshooting steps. Hopefully, this will help someone in the future if they experience the same problems.

After installing my LS6 manifold, my idle would surge about 75 rpm's only while in gear. In park, idle was fine. After a highway drive, it would idle high (1100 rpm's) when I would come to a stop. It threw a code for high idle (P0507) which I cleared but would either come back or be pending after another highway drive.

I initially thought it was a vacuum leak but I sprayed propane all over the place looking for a leak but never found one. I even disconnected every hose and plugged every nipple on the manifold but it still surged. The MAP sensor in the back of the manifold was a little loose but I held it down by hand to make it seal tighter and my car was still surging. I did ziptie it down though just to make sure it wouldn't come loose and leak. The last thing I did was loosen all of the manifold bolts and re-torque them in the proper sequence. After doing all of that, the idle was still surging so I pretty much ruled out a vacuum leak.

After doing some searching here I saw that some people had issues with their IAC valves and/or TPS. I logged my car with EFILive and monitored the IAC function and TPS readings. The TPS voltage was fine (from .6v to 4.6v) and the TPS % would read 0.4% at idle and 100% at WOT. The IAC function however was a little odd. In park, IAC counts were 0 and in gear the counts would fluctuate from 0 to 35. I started thinking the IAC was bad so I swapped it out with another one I had sitting here. No luck. Same behavior. So I swapped TPS sensors thinking that maybe that had something to do with it. Nope...still surged. I tried doing a re-learn on the TPS but that didn't work either. I took it for a test drive anyway. I floored it from a red light and the TCS kicked in. I kept my foot in it anyway and accelerated. However, my car was now short shifting instead of going all the way to redline and the torque converter wouldn't lock up! I did another run from a dead stop but this time with the ASR off. Again, it short shifted but threw a code for TPS high circuit voltage (P0123). I went home and datalogged the TPS function with the car off. The TPS voltage was now showing 4.8v at WOT. The TPS couldn't possibly be bad so I got under the hood and kind of reversed my TB bump stop grind mod. I noticed that the flat part of the throttle that hits the bump stop was bent slightly. I straightened it out and added some material to the bump stop by JB welding a small piece of metal plate to it. This brought the TPS voltages back into a normal range (.6v @ 0% and 4.5v @ 100%). I test drove the car but it was still surging at idle in gear!

After doing a little more in depth searching I found that some people had TPS grounding issues and/or dirty electrical contacts at the TPS or PCM. I pulled the PCM and that's when I saw the evidence of water intrusion. I cleaned the pins on the PCM with 800 grit sandpaper and used a safety pin to scrape the insides of the female connectors on the harness. If you try this, don't forget to disconnect the battery! I buttoned it all back up and voila! All fixed...

Let's just hope it stays fixed!

 

Fantastic write-up, and thanks for explaining your logic process. Glad you got it fixed!

 

Holly C$%p, this sounds exactly like what my car is doing, EXACTLY. And there is evidence of water in my PCM connector as well. I'm going to clean it really well and see what happens. Thanks for the write up. I've tried everything you listed and more, exept cleaning the damn connections.

-William

 

Don't forget to put some dielectric grease around the connectors to keep water out. You probably already know this, but don't put it ON the metal contact points.

My car started surging again not too long ago and I was like, WTF! There wasn't any water getting into the PCM connector. Turns out the hose that goes from the PCV valve to the TB was all cracked and dry rotted causing a vacuum leak.

 

Another product that may help... Deoxit D5 (www.deoxit.com) Its a contact cleaner that also improves electrical contact. No I do not work for this company. I work for a Mazda dealer as a tech, its something we've used for the 7 years I've been here. Was recommended by Mazda North America Technical Assistance, works awesome! We've always used it for similar concerns, High resistance codes for MIL or Air Bag light issues. Just spray it on and reconnect terminals, problem gone.

 

Sweet! I was looking for something like that. I see they list Radio Shack as one of their retail distributors but I didn't see it there. Maybe I'll check Guitar Center. It would have been easier to use deoxit instead of sandpaper.