I have a 94 Corvette LT1 with automatic trans with approximately 160,000 miles.

First question - The LT1 runs great except when the car is run for a while & when I come to a stop, the idle is erratic where it goes up a little over 1,000 rpms & then back down to normal. It keeps doing this over & over but won’t do it if I put it back in park or neutral.
The engine temperature range sometimes has to be a little higher than normal for this to occur.

Second question…. the temperature gauges do not read the same. The digital gauge seems to be accurate but the mechanical gauge usually reads 20-40 degrees higher & in some cases is all over the place when it gets warmed up. Where is the temperature-sending unit located for the mechanical gauge & is it the same sending unit for the digital read out? Could this temperature sending unit/gauge problem be related to my first question of erratic idle?

Any ideas?

 

"open loop" operation, O2 sensor data not used by ECM while engine cold.

"closed loop" operation, ECM uses O2 sensor feedback to adjust AFR.
Dump your error codes and see if an O2 sensor(s) has gone bad.

The lack of engine "load" may be enough to allow idle to rise slightly and settle down.

this is normal; cooler temp when moving as air passes through radiator removing more heat. Hotter when sitting at stop light. Hotter on hotter days, cooler on cooler days.

PCM sending unit sensor located on front of engine

gauge sensor located on driver side below exhaust manifold.

Probably not, the ECM/PCM does not use CST temp data for calculations of AFR during idle.

I would suggest you check for error codes as a first step in your diagnosis.

For more detail, check out this writeup regarding temp. sensors:
http://www.theherd.com/articles/lt4_temp.html

 

Makes sense.

It's been a while since I pulled the codes but the last time I did nothing came up.

Thanks for the information

 

My 94 with 61,000 miles has been doing the same thing (idle jumps up to 1000 after hitting 205 degrees and then keeps jumping up and down) for about 8 months now. First I had both O2 sensors replaced (five months ago), no help. Two weeks ago, I had the water pump changed along with the plugs and wires, and they replaced both the front temp sensor and pigtail and the passenger side sensor and pigtail, no help. Last weekend I took off the throttle body and cleaned it and the IAC, no help. Without fail, as soon as I get to 205 degrees (give or take 1), the idle, while in drive, jumps to 1000 and then jumps up and down. If I'm not paying attention at a light, the car will lurch forward a foot or so .

I remember when I first got the car I moved the IAT sensor out of the plastic tube to the air filter housing. I moved the IAT back to the plastic tube about a year ago...probably has nothing to do with this. A few years ago I also had a respected corvette mechanic here in Dallas put in a fan switch in the driver's side front of the block to turn the fans on at 200 degrees.

I usually have my digital dash readout on "coolant temp", but out of curiosity, I put it on "volts", and when I hit 205 degrees yesterday, the volts dropped from 14.4 to around 13.6 or thereabouts. The idle problem started right away but the volts eventually returned to the 14.4 range. To me that seems like an issue. It's obviously something heat sensitive that sends a message that screws something up, I just am not smart/knowledgable enough to figure it out.

 

If you have ceramic-coated headers, you may need to switch to heated O2 sensors. Runs fine in open-loop, then takes a crap because it wants to switch between loops at idle.

 

Kind of funny (well, not really)....but that's exactly what happens to mine.

It learches forward but if you pop it into neutral....it doesn't seem to affect the idle as bad. It will still jump up & down but not as bad.

It seems that the concerns is saying O2 sensors & that very well may be. My car is in the shop for body repair but when I get it out....I am going to try the O2 Sensors & see if that helps.

 

Changing the O2 sensors didn't help mine. I think I'm going to start changing one thing at a time and run it to see if what I changed helps. First thing I'm thinking of changing, since it'll be free, is unhooking the 200 degree fan switch I had put in. The 200-205 degree range is the magic area where the idle has problems, and I can't help but think the fans turning on might somehow be the cause or starts the problem.

On a side note, when I took the throttle body off to clean it, I took the IAC out and cleaned it, but should I also have taken the whole bottom section of the TB off (the passage that uses about 6 hex screws to hold it to the bottom) and cleaned it? It looked to me like it was the passage for the coolant, but I bypassed that a long time ago. The rubber gasket on the top of the TB was warped also (under the plate), but I didn't think it mattered...in fact, I don't even know why there is a plate with a gasket there to begin with!

I have not had any codes lately.

 

Well,

I havent had a fan switch installed & I never removed or messed with the throttle body.

I do know from experience when I was a mechanic from a long time ago, that sometimes an O2 sensor (although may be new) can be bad. I guess that can be true with almost anything installed. I don't know how many electronic fuel pumps, starters, alternators, sensors I have changed & they have been bad out of the box.

It's not common, but it happens.

Just an idea.

 

...especially O2 sensors. I don't think these things are manufactured to the greatest tolerances, or tested correctly before shipped to retailer...but I have seen plenty of faulty "new" O2 sensors...

 

Do you think one brand is better than the others? I think I put the Bosch ones in. Damn little things aren't cheap.

How would I know which one might be "bad"? Back to pulling codes again?

 

I remember one time when I was a service writer here in California; a car failed the smog test. After a diagnosis, the O2 sensor was bad. The mechanic put in 2 different O2 sensors because the new one failed.

This wasn't the only case.... but it does make you wonder about quality & control issues with these new parts. I know the handling aspects are also another consideration too.

 

Thought I had it figured out. I checked the 200 degree fan switch and found that is was cracked and broked around the edges so that the connector was just barely staying connected. I disconnected it and set it aside. Ran the car to get it warm and at 205ish nothing, no stumble, no blipping idle. I thought, cool, I'll just get another fan switch and everything is back to normal. Nope.

Drove it to work today and while stuck in traffic, the temp goes to about 215 and, blip, blip there goes the idle again. The temp went all the way to 229 when the fans finally came on. This time the idle blip even did it when I shifted to neutral.

I did pull the codes on Sunday before I disconnected the 200 degree switch and got both problem codes for the fans (primary and secondary). I also got a code for the "vehicle speed sensor". Ironically, I'm having VSS problems in a 92 Honda right now also.