Erratic idle speed, erratic a/f ratios at cruise (86 GLHS)

[Marcus86GLHS]

having a continued prob w/ my 86S hoping to get guidance.

car began throwing TPS code earlier in year. replaced the TPS. yet i still throw the code each time the car is cold started. once the car warms up, it doesnt throw the code until the next cold start.

it wont hold a stable idle speed even after full hot. idle jumps from 800-1,500 rpm. very erratic idling, never did this before, and the a/f ratios jump as the idle moves.

at steady state cruise, w/o moving the throttle, a/f ratios will look fine bouncing 14.5-15.0, but even though the throttle is not being moved suddenly go full lean (off the scale), then an instant later, go to 13.0 or richer, then an instant later return to normal. this cylce of normal to full lean to way rich and back to normal will repeat at steady cruise 55 mph driving. this a/f ratio jumping is exactly what is does during the erratic idle, leading me to believe this is an electrical problem.

checked ground wires: i have zero ohms from fuel rail to neg batt post, zero ohms from fuel rail to firewall, zero ohms neg batt post to engine block.

voltage checks w/ key on:
3.5 volts going into the TPS at the 6-way connector
.9 volts coming out of the TPS at the 6-way connector
as i rotate the throttle, TPS voltage rises to 3.5



is the 3.5 volts into the TPS too low? my manual says 5 volts goes to the TPS, could that be the problem? battery voltage is 13+.

any other advise as to why i am throwing TPS code when cold, getting erratic idle, and getting erratic a/f ratios at steady state cruise?

thanks.

[Chad T]

Mark, you tried cleaning the battery contacts or anything like that? Did the issue just show up out of the blue? Any work on the car/cleaning just before you noticed it? As we talked about earlier I'm getting 4.94 at the TPS power wire for anybody else that's reading the thread..

[boboli]

There should be 1 wire that has nearly 5 volts at the tps all the time the key is on. All the sensors, at least that I remember, on these cars get a constant 5 volts from the lm. On my car the tps was brand new and I couldn't get rid of the tps code. I tried testing it and it seemed to have a bad spot in it. As I rotated the tps it wouldn't read a smooth increase in voltage. I finally threw a junk yard one I had laying around on, and its been working great ever since. I think that the tps should read from .7 volts up to nearly 4 volts, so if yours is showing .9 to 3.5 that is about right. The other wire is the ground which will read opposite of the wire the lm reads, high to low voltage. Maybe your tps is bad. Mini mopar resources has info on lm pin locations @ the lm if you want to check your wires to the lm. Also, a lot of people advised me that they got water in the car sitting on the lm that was causing problems. Wasn't my problem but maybe check to make sure the lm doesn't have water on it and the connectors aren't corroded.

[Marcus86GLHS]

thanks guys.

i have been told that 3.5 volts going into the TPS is my problem, so i will focus my troubleshooting on the orange wire w/ white tracer and it's ground.

i am not sure why this prob popped up this year since i havent done anything to the car underhood for over a year. i do not use water at all on this car especially underhood.

no mice either Chad....unless you shipped me one.

:-)

[boboli]

I meant that l bodies tend to have poor seals in the cowl so if you park outside or drive in the rain, water can get into the car, passenger compartment, and then it some how ends up on the terminals to the lm. That is why the lms came from the factory with a plastic bag over them. Doesn't happen to me in the desert, but that's what others said.
Good luck, I had to put 2 tps and 2 computers and rewire the tps and ais all the way to the lm in my car to finally get it to run right. I am not planning to change anything for a while since I don't need any more problems!

[Gaboon]

Just a shot in the dark here but check for a vacume leak.

[4 l-bodies]

Marcus,
Yeah you need 5 volts at the TPS. Something definitely not right there. Great advice on the LM connections. L-bodies are notorious for water getting to the LM and no baggie around the LM and for mice entering the fresh air duct. Been down both of those roads once too often. Check voltage at the logic module pin #1 blue connector if your getting 3.5 volts at the TPS. I'll bet your problem is there. Those pins are available through various electrical stores.
Good to meet you, and your buddy Hemi at the show. Good to put a face with a name.
Todd

[Marcus86GLHS]

TODD it was a ton of fun talking to you as SDAC, and seeing your Shelby Charger in person was a real treat for me........ congrats on the trophy.

i am going out to diagnose this low TPS input voltage this morning.

thanks again for the help folks.

[Marcus86GLHS]

fixed.

prob was a poor connection at the blue logic module connector, position #1 as TODD correctly predicted....the blue connector was fully seated in place and all pins looked clean but there must have been a small amount of oxidation on the pin....i cleaned the pin and now have the full 5.0 volts to the TPS and the idle speed is now correct, i am sure once i test drive it the car will be fine.

its surprising to me how "sensitive" these sensors are, just that small voltage drop of 1.5v that i was loosing across that one pin caused so many problems.