I have a 1996 LT-4 which seems to have issues with high idle and searching idle with incorrect air fuel mixture, runs very rich at idle, The issue seems to occur after driving or trailering the car on open trailer in the rain (parked in rain does not seem to cause this) I have changed the mass airflow sensor (MAF) and manifold absolute pressure sensor (MAP), cleaned "all" electric connections with circuit board cleaner. Battery is good and fully charged.

Car runs perfect at wide open throttle, then you idle down and it hunts all over from 1,500 to 2,000 rpm's

NO codes and no check engine lights.

Magically, with no reason, it will start up and idle perfect until next time it is drivin' in the rain.

Any clues or idea's as to where to look are appreciated.

1996 LT-4 convetible
-motor stock
-long tube headers
-no cats
-no A.I.R. pump
-no EGR system
-computer programmed to eliminate those items
-stock throttle body
-stock injectors
-stock ignition system
-primarily an autocross car

 

I have since replaced the idle air control valve and water temperature sensor in water pump, replaced water temp sensor electrical connector, replace ground strap from frame to bellhousing, cleaned positive and negative connections from neg. battery to frame and pos. connections to large fuses near battery. Replaced three ground wire end terminals from wire loom to bellhousing. I have also removed and cleaned the throttle body.

Drove car in light rain and it started acting up within 4 miles, then park in garage for 3 days and it is fine once again.

I sprayed water from a bottle on all plug wires and coil area (not opti) and see no arcing in dark garage. Plugs are new as are wires.

Still no codes when this happens, that i what baffles me. Like I said, when dry, it runs great.

 

Tom, does it ever exhibit this problem when not yet warmed up (i.e., when it's running in open-loop mode)?

 

YES! when it has had moisture around it. (drive in rain or trailer in rain [open trailer]) doesn't seem to matter if it's a fresh start up (cold) or been running awhile. Few days in garage and it's fine (dry)

 

The only other thing I can think of off the top of my head would be if there's something wrong with the opti's vent system, such that it's somehow getting water into the opti. Maybe the vacuum hose is split or disconnected? I'd expect if that were messing up in that manner, though, then the tach signal would be off and you'd see the needle bouncing around. If the tach seems to be ready correctly while it hunts at higher RPM, then the PCM knows how fast the engine engine is spinning but somehow thinks it should be spinning that fast (as opposed to not knowing how fast the engine is spinning). Okay, what if the VSS in the transmission. When mine failed altogether, the engine kind of did the opposite: it would tend to die as I rolled to a stop and I had to blip the throttle to keep it running. But what if water is getting in there when the car is moving and causing it to send a false speed signal to the PCM, and the engine is trying to manage RPM as if the car were rolling (Throttle Follower IAC Offset vs MPH in Jet DST)? I'd think in this case you'd see this on the speedometer: it would read some non-zero MPH when the car is sitting still. I'm grasping at straws.

Addendum:

• Looking at a bunch of Idle Control Constant Parameters in the Jet DST calibrations for a 96, there are a ton of them. Almost all of them seem dependent on RPM (tach signal, which should show up on the dash tach) or the IAC (which you just replaced). However, there is one called "TPS IAC Offset Enable %TPS Hysteresis" that is calibrated to a percent of TPS. If your TPS is giving bad voltages then maybe that is getting thrown off.
• There's also a table called "TPS IAC Offset Enable %TPS Threshold vs RPM," which again is referenced to the TPS.
• There's another table called "IAC Offset for A/C Anticipate vs A/C Pressure." I can't recall if you still have A/C hooked up in your car, but if there's a refrigerant pressure switch that's giving the PCM bad readings due to moisture-based shorting, maybe the PCM is trying to compensate for A/C pressure that isn't really there. Maybe you could see that on Pin 12 of the blue PCM connector (Connector #4, red/blk wire), but I'm not sure what voltages would tell you whether it's reading properly or not with the A/C turned off. On the topic of A/C controls, there also an A/C Clutch Status signal on the black PCM connector (pin 21, dark grn wire), and the compressor clutch seems like a place where water could jack up the readings going to the PCM.

So in summary, check out the Opti vent system, VSS, TPS, and the various A/C signals going to the PCM.

 

WOW! I just replaced power steering pump (for other reasons) and the OPTI vent is right there, it IS connected at both ends, and seemed pliable and fine but I will investigate. A/C is still on car but hasn't worked due to refrigerant leak so it is un-plugged at compressor.(and not used) Tach works as it should and reads accurate, water temp is accurate, speedometer is accurate. I may replace the TPS and see what happens. VSS is an interesting option.

I may try to spray some water (mist from spray bottle) around any of those while everything is fine and see if it changes the idle (VSS, TPS, and [dread] Opti vent)

Thanks Matt!

 

On the A/C, I'm specifically thinking about the pressure switch(es) in the refrigerant lines. If that is getting wet and sending an inaccurate signal to the PCM - especially if it's telling it that the pressures indicate the A/C is running and putting a load on the engine when it really isn't - then maybe that could cause an idle hunt.

 

Is it ever a good thing to spray a little brake cleaner into opti vent to clean it out.? I have read that somewhere

 

I can't imagine that it is.

 

I tried spraying mist of water (with engine running normally) hit TPS sensor and harness with no change, A/C low pressure switch and high pressure switch and wire connectors with no change, all wire connections for MAF, and MAP, water temp. IAC, around throttle body and around the PCM mounted up by firewall above battery. One would think if something became wet while running normal, then started acting up as described earlier, you may have found the problem area. Kept running like it should, never found problem area. I did not get to VSS will do that after I get it back on jackstands, that makes some since because it is highly likely to get wet down under. I'll keep trying.

 

Replaced opti with new Petris model including the wire harness and both opti vent tubes. New coil, new ignition control module, new plug wires, plugs, coil wire. Still has same problem, runs great dry, hardly runs when exposed to moisture. so no change. The struggle continues.

 

Random thought....how about the connections to the ECM....maybe give the connections a few tugs and light mist spray. Another thought, on my '96 Formula WS6, I believe there's and air temp(?) sensor in the ram air box...I'm guessing you should have the same sensor in your air cleaner/duct assembly, maybe take a look at that! I absolutely do not know for sure if this sensor failing or shorting out would cause drive ability issues but worth a look. So, to clarify, it doesn't have to be driven in direct rain....as long as there's heavy moisture in the air it runs like this!? Is the engine ingesting water through the air cleaner assembly somehow (very small amount) causing the computer to compensate from a O2 sensor reading or other sensor/sender reading....!? Very interesting, I'll do some searching, I'd like to know since I have a '96 LT1👍

 

I've done the ECM connections, wiggled and such, sprayed with electrical contact cleaner as well, soaked with water, no results. I've also replaced the open air cleaner lid with stock, no results. I have NOT tried the Incoming air temp sensor, but will.