Just Build It

Hello all and happy holidays. I am having some strange issues with my formula. It runs just as normal once it is up to temp but it will backfire when its cold. It seems to get even worse now that the outdoor temp is in the 30s. I have scanned it and got no codes. My thoughts are the coolant temp sensor. If it is going bad will it set the check engine light?

Injuneer

I practical terms, the coolant temp sensor will only set a code if is totally fails - internal short (indicating more than 284°F = DTC 14) or internal open (indicating lower than -59°F = DTC 15). Or short or open in the wiring harness.

Do you have a scanner that can read the coolant temp directly from the PCM? That would tell you if the sensor is giving the PCM good data.

Or check the accuracy of the sensor, and the reference voltage supplied by the PCM (scroll down past the resistance table):

4th Gen LT1 F-body Tech Articles

Just Build It

When you say it won't set a code would it be one that turns on the check engine light or one that doesn't? In answer to your question about reading the temp with a scanner I do but only if my friend has time to bring his scanner over. I read the link posted. If I ohm the CTS how close to those numbers does it have to be? Or does it need to be exact? Thanks for the help.

GaryDoug

The readings don't have to match the chart exactly. Maybe 10% variation would be a good start. The goal is to tell if the sensor is way off.

Injuneer

Both DTC 14 and DTC 15 will turn on the SES light when they set.

Volume 2 of the factory service manual has all this info - free download:

Courtesy of GaryDoug

https://www.mediafire.com/?40mfgeoe4ctti

Just Build It

So I got a chance to do some testing on the car today. I ohmed across the sensor and got 5.8 on the 200k scale. If I remember correctly thats 5800 ohms. With a meat thermometer I measured the a\f in the radiator and got 40 degrees. The tech article says 7280 for 41 degrees and 5670 for 50 degrees. Thats a bit more than 10% off do you guys think its time for a new sensor? BTW the car had not been run for a few days at the time.[/QUOTE]

GaryDoug

The meat thermometer probably has 10 degrees error. The sensor is ok for accuracy at 50, but I would try it at boiling temp (212F) in an old pot on the cook range.

Just Build It

I will give that a try. Not sure why I didn't think of that So if it turns out to not be the CTS do any of you camaro gurus know what I should be checking next?

Just Build It

I would think the iat sensor could be next in line.

Injuneer

The IAT sensor has very little to do with how the engine runs, unless your MAF fails, and the PCM defaults to speed-density mode. You would have a code for the MAF sensor if that happened.

Did you verify you have 5 volts at the CLT sensor harness connector, connector unplugged from the sensor? And just to be sure, your are checking the CLT sensor in the water pump housing?

Next suspects would be the O2 sensors, or some component of the engine throwing the A/F ratio significantly rich or lean. When the engine is cold, the PCM operates in "open loop". That means the PCM ignores the O2 sensors, because they are cold and not working. The PCM uses the "learned" long term fuel term fuel corrections, limiting correction to a range of -15% to +25%.

When the coolant/engine warms up, the PCM switches to "closed loop", where it uses the O2 sensor readings to make further corrections to the A/F ratio. Now the PCM can cut fuel by more the 15%, or supply added fuel in excess of 25%.

Next step is the free download of Scan9495, get a cable, and data log the PCM from a cold start through full warmup, when the engine starts running better. GaryDoug is the author of Scan9495, and can help with the install. Gary and I can review the data log for you. The software produces a .csv format report that can be manipulated with Excel.

https://www.camaroz28.com/forums/com...95-lt1-874306/

Chimera96

OP

You have two extremely qualified guys, injuneer and garydoug, offering very qualified advice. Having the ability to data log greatly assists in finding "why" a problem occurs that is not self evident.

I recently chased a stumble, backfire through intake, not run in open loop without nursing the gas until closed loop, motor would fall on its face when hitting the gas (not always though) then catch up and rev to redline...No codes ever

I started by checking fuel pressure even monitoring gauge while stumble occurred. good. Hooked up scan tool and the most glaring data was STFT were pegged at 25. Often this is due to unmetered air from something like vacuum leak or exhaust leak pre 02. Check all vac hoses and sprayed starter fluid around intake. Nothing found

DVM test for MAP (while applying vacuum), temp, TPS sensors. all good. scan showed MAF in spec. Swapped a spare MAF in. same issue

Swapped to my good spare opti....no change.

I swapped the ICM. FIXED

ICM failure symptoms are typically a no start, engine shuts off once at operating temp and will not re-start. My car always started and re-started and did run better once warmed up

I had no idea a bad ICM could make a motor run so bad but still run and affect STFT, thus LTFT, so much.

Not saying this is your problem and not advising just throwing parts at it but if you hit the wall after checking everything else...something to consider.

Just Build It

I may try changing the ICM as suggested. Not just throwing money at it and blindly hoping but just because I have a known good one and they are known to fail. Also,its not very nice out there and the garage is full of another project that isn't mobile just yet. I have been reading alot on this forum since I purchased the car and found it to be a valuable resource. Much of the info was learned from the two very qualified guys mentioned. I am almost jealous of the amount of info they have to offer. When the weather gets better I will be looking into this issue a lot deeper. Here's to January thaw. I can't wait.

Just Build It

Almost forgot to mention,yes I am checking the sensor in the WP.

Chimera96

if you have a known good spare, swap it out to eliminate if the current one is good or bad. Compared to other parts it is very easy to R&R

If you have not DVM that sensor, do that first. There is a ohm per temp chart on shoebox site to compare your readings to

Just Build It

Will do. I know this is off topic but I saw your signature. I would be curious to know how you hung a hyd. clutch peddle in that impala. But thats a conversation for another day. Sadly I no longer own mine.