Thread: loping cold start

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I have a 1993 Chevy S10 with a 4.3 TBI injection. About a month ago it started loping during cold start for the first few seconds. It had a code for the IAC and the Knock sensor so I replaced them but nothing changed. I replaced cap, rotor and wires and it made it much better (for a few day) Checked codes again and I still have one for the IAC. All the wiring looks good and all my reference voltages to the sensors are correct. This morning I had to rev the engine for about 2 minutes before it would idle smoothly and it was about 70 degrees outside. Anybody have the problem before?

Replace the MAP sensor. I had the same issue on a 98 Silverado.

My guess is that the mass airflow sensor is dirty or perhaps, defective. The MAF’s “job” it to constantly measure the amount of air entering the intake manifold. This tells the fuel management computer how much gas to inject into the cylinders for proper combustion. Mass airflow sensors use hot-wire-type sensors – which is a short piece of wire kept at a constant (warmer than ambient air) temperature. Current is applied to the wire to maintain its temperature so the current necessary to maintain the temperature is regulated in direct proportion to the “mass” of cool air flowing past it. It’s a little bitty wire and the response rate (to the change of flow) is nearly instantaneous.

Even under the best of conditions, crud builds up and “bakes” on the wire, which effectively insulates it from the cooler air. This “confuses” the fuel management computer as it sees a value for air mass that's out of range and it throws an error code. There’s a “cleaning cycle” that’s akin to a “self-cleaning oven” which heats the wire to a very high temperature. This cycle usually occurs on a programmed basis just after you turn the ignition key off and it is supposed to burn off the wooly buggers.

If the cleaning cycle fails to remove all of the contaminants, the manufacturer (and the dealer) recommends MAF sensor replacement. I'd try cleaning using to use aerosol carb cleaner, or you can also buy MAF Sensor Cleaner. Remove the MAF sensor and be real careful not to touch the wires. I’ve been very successful in spraying the wires and seeing evidence of crud. I run a reusable, washable filter (K&N) and not only does it help a tic on the performance and mileage area, it’s a finer filter and will help keep MAF wires clean longer.

Regards,
Glenn

Originally Posted by glennsexton

My guess is that the mass airflow sensor is dirty or perhaps, defective. The MAF’s “job” it to constantly measure the amount of air entering the intake manifold. This tells the fuel management computer how much gas to inject into the cylinders for proper combustion. Mass airflow sensors use hot-wire-type sensors – which is a short piece of wire kept at a constant (warmer than ambient air) temperature. Current is applied to the wire to maintain its temperature so the current necessary to maintain the temperature is regulated in direct proportion to the “mass” of cool air flowing past it. It’s a little bitty wire and the response rate (to the change of flow) is nearly instantaneous.

Even under the best of conditions, crud builds up and “bakes” on the wire, which effectively insulates it from the cooler air. This “confuses” the fuel management computer as it sees a value for air mass that's out of range and it throws an error code. There’s a “cleaning cycle” that’s akin to a “self-cleaning oven” which heats the wire to a very high temperature. This cycle usually occurs on a programmed basis just after you turn the ignition key off and it is supposed to burn off the wooly buggers.

If the cleaning cycle fails to remove all of the contaminants, the manufacturer (and the dealer) recommends MAF sensor replacement. I'd try cleaning using to use aerosol carb cleaner, or you can also buy MAF Sensor Cleaner. Remove the MAF sensor and be real careful not to touch the wires. I’ve been very successful in spraying the wires and seeing evidence of crud. I run a reusable, washable filter (K&N) and not only does it help a tic on the performance and mileage area, it’s a finer filter and will help keep MAF wires clean longer.

Regards,
Glenn

This engine does not have a MAF sensor. It uses a MAP sensor to measure manifold pressure instead. But yes, if it did have a MAF, I would have looked there 1st.

Guess I will try to check the MAP sensor later today

Might want to clear any (if any) codes.Then the first code is the fault and the rest are reflecting what the first one is doing.Might want to consider the temp sensor.The EGR does create bad idle conditions,but when carbon build up is present,it should be consistent no matter what temps.Thinking out loud that is true until what ever carbon was holding it open was cleared.To clean that is alittle labor on your part and a help section from your local parts store screen gasket for only $8.00.

Originally Posted by 1gary

Might want to clear any (if any) codes.Then the first code is the fault and the rest are reflecting what the first one is doing.Might want to consider the temp sensor.The EGR does create bad idle conditions,but when carbon build up is present,it should be consistent no matter what temps.Thinking out loud that is true until what ever carbon was holding it open was cleared.To clean that is alittle labor on your part and a help section from your local parts store screen gasket for only $8.00.

Tried that, the only code that comes back is for the IAC. Tested the MAP, and it reads good on the scan tool. Checked vacuum and noticed that at idle the needle bounced between 20 and 23psi very rapid. Checked compression and my lowest is 205 and highest is 215 on an engine with 202K miles on it! Going to decarbon the engine now and see how that goes

Might try disconnecting the battery for an hour or so to clear adaptive memory.
When a code has been set the ecm substitutes a value so the thing will run and it gets stuck in the memory even after the sensor or such has been changed. Other than that the only thing that comes to mind is make sure your egr is not getting a vacume signal when the engine is cold,unless it has the digital egr.Also we have replaced a lot of injectors on throttle body systems.

A sharp bounce sometimes indicates a NON stock cam.........Did you mean Vac. or PSI?????

Originally Posted by cffisher

A sharp bounce sometimes indicates a NON stock cam.........Did you mean Vac. or PSI?????

Oops! My mistake. 20 to 23"Hg. After I decarboned the engine it regained some lost power which is good. This morning it only loped for about 2 seconds before idling normal. I did find a vacuum leak late yesterday at the throttle shaft; that could be why I keep getting a code 35 for the IAC. Does anyone make bushings for that? I can't find an aftermarket rebuild kit that has them (just gaskets and such). Also, this engine has never been out, never torn apart, all stock; owned it since it was new.