98 Z-28 coolant temp sensor question

**lsruthles** · 12-07-2011, 06:02 PM

is the coolant temp sensor (three prong) on my ls1 just a temp sending unit to the dash gauge, or does it also send info to the computer? Having problems tracking down my cold idle problem

**mike171562** · 12-08-2011, 06:39 AM

on my 98 TA, its a three prong, one pin went to the computer and the other to the guage, I know because I broke it once and all the pins werent making contact, if i moved it just right, the guage would work but the motor ran like shit, if i moved it a little again the motor ran fine, but the guage wouldnt work.

**lsruthles** · 12-08-2011, 02:13 PM

thats all i needed to know, thanks alot. i will post if this fixes my cold idle. if not i'll be back for more answers.

**pajeff02** · 12-08-2011, 02:57 PM

Do you have access to a scanner?

**lsruthles** · 12-09-2011, 02:29 PM

all ive got is a code reader, and no access to a scanner. been working on the car for over a year, that slow because of money, and have fixed everything ive found or codes brought up. no more codes now, but still has serious cold idle issues. had to order the temp sensor from out of state, wont get it for a week, so wont know if that was the prob till then. any other ideas are more than welcome and greatly appreciated!

**pajeff02** · 12-09-2011, 02:48 PM

I wouldn't think that a cold idle problem would be a temp sensor, unless it is prematurely sending the PCM into closed loop.

**lsruthles** · 12-09-2011, 02:54 PM

maf, or maybe map sensor? i havent done anything with either of those.

**pajeff02** · 12-09-2011, 04:11 PM

Describe your cold idle issue.

**lsruthles** · 12-09-2011, 04:47 PM

car is from lousiana, first time in weather below 50 degrees, it started doing as follows, and has not improved, the effects are more exaggerated the colder it gets, but runs great after coming up to operating temp. : starts and immediately revs to 2500-3200 rpm for a second then drops to 600 or stall depending on how cold temp is. fluctuates between these rpm constantly every couple seconds until it warms up. if i feather the throttle after start up, i can only hold it constant at about 1800-2000 or it will miss and dog out. when it misses it sounds like its backfiring through the intake manifold. it's got 208,000 miles on it, i bought it with 189,000, have done quite a bit of small work to it, and it runs great when warm, and uses no oil, gets 25 mpg, so fixing the cold idle prob would be outstanding!!

**pajeff02** · 12-09-2011, 04:53 PM

I would also check for a dirty throttle body or a malfunctioning or dirty Idle Air Control (IAC) valve.

**lsruthles** · 12-09-2011, 05:11 PM

sorry, not used to quick responses. Ive replaced the IAC motor, the IAT sensor, all the tubing as well as the PCV valve, water pump (because the bearing was going out, therefore causing the knock sensor to have a reading) all the O2 sensors (Plugged not universal, i made that mistake at first) cleaned the MAF with CRC MAF cleaner, and have cleaned the intake with Sea Foam's Deep Throttle several times, all to no effect.

**pajeff02** · 12-09-2011, 05:22 PM

Hmmm. I know that Zapper had idle issues that were caused by a malfunctioning temp sensor, but that was on an LT motor. I would normally suspect a vacuum leak, but the fact that it idles fine after warming up sort of rules that out. Your car has an EGR valve, right?

**lsruthles** · 12-09-2011, 05:33 PM

yeah, its still got everything stock, when i get the idle figured out, im going to frost for a tune with EGR and AIR delete, thanks to reading all you guys input on everything, kinda pointless till i figure this out though!

**pajeff02** · 12-09-2011, 05:41 PM

If the EGR is malfunctioning that can definitely affect idle. Might not be a bad idea to pull it off and inspect it as they tend to fail with age.

**lsruthles** · 12-09-2011, 05:52 PM

makes sense. about three months ago i removed it and flushed it out with carb cleaner, but i dont know how to tell if its bad or not. the spring controlled valve didnt seem loose, and when i shook it, it didnt rattle or seem loose in any way, but thats probably just the basic way to analyze something like that. if it was bad though, wouldnt it throw a code at least?

**pajeff02** · 12-09-2011, 06:31 PM

Hard to tell. You just want to make sure it is not dumping exhaust into a cold engine.

**lsruthles** · 12-09-2011, 06:43 PM

might be ignorant here, but isnt that what the PCV, and AIR system do when the engine is cold? Wait, thats not right, the AIR system adds outside air for smoother throttle response, and the PCV system is just vacuum control for the idle, i think. if the EGR valve was bad, that would dump oxygen depleted air into the system, therefore making the system starve for oxygen, which could explain the rpm deviation! another good idea, im definitely looking into that also, thanks jeff, makes a lot of sense when you think about it!

**pajeff02** · 12-10-2011, 04:09 AM

The secondary air system pumps filtered air into the exhaust manifolds on cold startup. This allows the catalytic converters to come up to temperature quicker and reduces emissions when the engine is cold. There are two check valves, one at the end of each stand pipe, that keep the air flowing in only one directions. These are known for going bad and you will generally get a code through the SES light.

Positive Crankcase Ventilation (PCV) works all the time and vacuums out blowby gases out from the internal cavities of the engine. On our cars, these systems are known for sucking oil into the intake and a catch can set up or conversion to an LS6 style PCV system are the recommended remedies.

**ldurham** · 12-13-2011, 01:04 PM

My 2000 z28 had a funky idle problem like that. It turned out being one of the wires going to my IAC was grounded on top of the right valve cover. Check that bundle of wires back there for any rubbing. Hope you figure it out!

**mike171562** · 12-14-2011, 01:49 PM

It sounds like your IAC motor is not working right. you can try taking your old one and installing it with the spindle pushed in and leave it unplugged. if it idles too high, take it out and pull the spindle out a little until you get the idle right.

I had to run mine like that for a while when the tune for my IAC was all jacked up.