Emissions Test Failure - '98 Chevy PU

Do you have the "check engine" light on?

Perhaps you should get one of these:

then you can reset the fault.

Many auto parts stores will do this for you for free.

Bet your radiator has been replaced!

Rad. too efficient or pump rpm has been increased with a smaller than stock pump pulley.....or larger drive pulley on crank.

We used to restrict the water flow through the radiator with a 2 inch washer..(or equal size to the rad. hose ID) choking down the infeed into the radiator from pump outlet.

Washer hole size can be adjusted/trial and error/... to make the thermostat and rad. heat gain happy.

Late 1940's.

MR. GUY

everything in cooling system is original or equivalent' thanks

Are you aware that the trouble code must be reset?

Assuming that it has been reset, but reappears just before you take the truck in for the test: The best first step is to measure the actual operating temperature with a thermometer of known accuracy. Then you will know if the engine temp is really low, or if the ecu just thinks it is low. If the actual temperature is correct, and you have tested the sender then it would seem that the ecu is at fault. Perhaps there is another component between the sender and ecu, but I doubt it.

nothing in between, I am told by the inspector that unhooking the bat and driving 100 miles will reset the code if problem is fixed.

When in DOUBT.... Reset the computer, changing parts wont cure the problem, disconnecting the battery will only give the computer a chance to cool down.....

are you sure you replaced the one for the comp you said you have your 5 v ref sig but do you have a sig to the comp check the sig at the comp if you get the proper sig replace the ecm but it is likely you have a bad wire keep it simple you have all ready replaced to many things when i see this code it usually means the coolant/(ECT) temp is bad and i sell the sensor and the t-stat just to be sure.by the way after 15 years i have never seen a radiator cool to much but i have seen plenty of t-stats get stuck open just enough to cause this problem. if you need a pin out for the ecm i need a little more info...98 Chevy 4+4 or no sport or no

also the code will reset it self if you fixed the problem after a few drive cycles.In NY if you reset the comp and then go for inspection you will fail because your monitors are not set so DON'T CLEAR IT FIX IT

yes I'm sure about the sensor, it may be the return wire as you said, I'll check it out. thanks

A code reader as Bricktop stated, may be a cheaper way for you to solve your problem. The better readers will allow you to read the engine temperature signal that your sensor is sending to your ecm. If the temp is too low, there could be very few things that should cause it. Number one is the thermostat. Are you sure that the thermostat is the correct temperature setting as required? If you are able to see the temperature reading, and the thermostat is the correct one, you could try blocking some airflow to the radiator to see if you can raise the sensor's reading. If that works, it would get you through inspection.

The sensors are changing resistance in relation to the temperature. We used to test temperature sending units (pre computer sensors) by placing them in a Pyrex beaker on a hot plate, and read the resistance to the threaded section of it as the temperature rose. If you are able to find the related resistance of the sensor at different temperatures, you might be able to decide where the problem is.

Snap-on used to sell a portable gauge tester. It was a series of variable resistors that you could set as needed to any desired resistance. It also came with a chart that told what resistance a 1978 Ford F150 fuel gauge should have when the gauge reads exactly 1/4, 1/2, or 3/4 full. Same with temp and oil gauges. Handy item to have when diagnosing questionable gauges.

If you were very clever, you would be able to attach your own variable resistor across the sensor leads to bring the sensor's reading right where you want it.

That is probably the wrong way to go. There is more than likely a simple solution to your problem. I would have to suspect the thermostat of being too cold a design. Good luck. Keep us informed.

I got the truck passed. I talked to a GM dealer mechanic about it and here is what he told me and what I did. He said that unhooking the battery did not always clear the codes out and I needed to do that with a code reader , he said I bet it will pass if you try that. I went to O Reilly auto parts store . They gave me a 30 second course on how to use it and I did and cleared the code and drove it through a drive cycle and took it in yesterday. It passed with flying colors. Explain that one to me. I have read a dozen places and heard it from a dozen people, including the "inspector" that unhooking your battery for five minutes did the job but this one mechanic told me to not rely on that and it turns out he was right. Pretty weird but very true. thought I'd post this up and maybe it will help someone else with an unsolvable emissions test problem. Thanks for all the replies. Jack

Glad your problem has been resolved. It appears that the computers are now retaining memories with batteries disconnected. Like it or not we are being forced to buy scanners. On the heavy duty trucks, the trouble codes are stored forever, or until cleared by factory authorized personnel. The overheat code, for example, is stored along with the engine hours it happened at, forever.

Why not join this group and help add to the pool of knowledge.

Explain that one to me. I have read a dozen places and heard it from a dozen people, including the "inspector" that unhooking your battery for five minutes did the job but this one mechanic told me to not rely on that and it turns out he was right.

Clearly, CR4 is the right place to come to get the right information. You will note that 3 out of the first four answers are correct. Ever since the advent of OBDII (1996) the default has been that you must manually reset codes. (This was true with many OBD I cars too.) Just as your cell phone does not loose it memory for contact numbers when the battery dies, your car's computer does not loose its memory when the battery is disconnected. Imagine what a pain in the arse it would be if every time you disconnected the battery for five minutes (a good precaution for many repairs) your ecu memory disappeared and you had to go through a drive cycle to get the engine running right. The EPA would disapprove of such a system.

Sadly, the automotive mechanic world is full of old wives' tales.

"Ever since the advent of OBDII (1996) the default has been that you must manually reset codes. (This was true with many OBD I cars too.) Just as your cell phone does not loose it memory for contact numbers when the battery dies, your car's computer does not loose its memory when the battery is disconnected."

Man I wish you'd have said that ten posts ago. As I stated I had thought I had reset the codes by unhooking the battery because that is what I was being told to do by everyone including the guy who inspected my truck in the first place, you'd sure think he'd know. But I thank you for your replys anyway. Like most things I've learned, being rather dense at times, I learned it the hard way. I have my own code reader now even though I can borrow O"reillys for free. Won't have to take this long ride again. Manually reset codes///manually reset codes/// manually reset codes/// Got it, thanks.

Woop, wrong thread.

You failed to state the jurisdiction of the state test; so hard to make full sense of your question; or advise as to corrective action...state tests do not render "trouble codes," for example; and it's unlikely such a fault would cause a state tail pipe emissions failure.

Please restate, along with test conditions and jurisdiction.