Rough running LT1 when cold

[rockF16]

Howdy all,

My '94 Z28 has recently started running rough when the engine is cold. Specifically, it idels fine, but it sputters and often backfires when the drive train is under load during accelearation or going uphill. Once the car is warmed up to normal operating temperatures, it runs fine. I just changed the spark plugs and EGR valve, but didn't notice any appreciable improvement. The car does have 190k miles on it, so an engine rebuild is in my future, but I'd rather hold off on that for a while if possible. Any ideas or suggestions would be much appreciated.

Thanks.

[Injuneer]

Could be related to the coolant temperature sensor. If the PCM isn't getting the correct coolant temp when the engine is cold, it may not be richening the mixture up enough for cold running.

[rockF16]

Could be related to the coolant temperature sensor. If the PCM isn't getting the correct coolant temp when the engine is cold, it may not be richening the mixture up enough for cold running.

Is the coolant temperature sensor disasscociated from the sensor that feeds the temperature guage on the dash? The gauge appears to be working correctly, so if the same sensor feeds both the gauge and the PCM, it seems like that wouldn't be the culprit, unless the PCM is getting bad information because of a faulty link. Also, I don't have an SES light...would a faulty temperature sensor trip it?

[Injuneer]

The 1-wire sensor for the gauge is in the driver's side head. The 2-wire sensor for the PCM is mounted in the water pump housing. Not connected in any way.

You will only get an SES light/DTC code for an open circuit (extreme low temp) or a short circuit (extreme high temp). If the sensor is faulty, it can provide the incorrect resistance and not set the light/code. I found my son's Saturn was notorius for developing a crack in the plastic tip of the sensor, and providing a very narrow range of resistance, that was always giving the PCM the wrong temperature. Hard to start cold, stumbled until the engine warmed up, never really ran right even with the engine warmed up.

Is the car difficult to start when cold?

Easiest way to check the sensor would be with a scanner, to see if the PCM temp reading matches the coolant gauge temp, or even ambient temperature before a cold start, where the car has been sitting overnight. Or you can check the resistance of the sensor to see if it is close to what you think the temperature is. There is a table of resistance vs. temperature in my online scanner guide:

http://members.aol.com/InjuneerZZ/ScanMast.htm

There can be problems beside the coolant temp sensor, but its one thing to check.

[rockF16]

Thanks, I'll pull the sensor and measure the resistance using the hot water technique. I hope that's the culprit. I thought another plausible source for the problem could be a bad seal somewhere...a gasket that doesn't seal properly until it expands after being warmed up. Cheers.

[rockF16]

and yes, the car starts like a champ when it's cold. It doesn't even idle rough; the only time it had trouble is when intiating acceleration or climbing from a stop (or slow speed). Then it sputters and backfires.

[Injuneer]

If it starts OK, its probably not the coolant temp sensor, unless its staying too low. If you pull it out, the coolant runs out. Try and measure it with the sensor in place, if possible.

[Joe 1320]

Cool air is more dense, and unless the rest of the sensors are up to snuff, could result in a lean condition which will give you the symptoms you describe. It could be anything or everything in combination. Watching sensors in real time with a scanner would be helpful to pinpoint what is happening. Is the backfire though the intake or the exhaust?

[rockF16]

It's tough for me to tell exactly where the backfire is coming from. Sitting in the drivers seat, it sounds like it's coming from somewhere in the engine compartment, but again it's tough to tell.