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Open loop vs closed loop backfiring

(1990 GMC Vandura 1500)
in Huntington, NY on February 26, 2011
My Vandura runs perfectly when first started running in Open Loop and doesn't miss at idle or at high RPM. Once the van reaches operating temp and goes into closed loop, it hesitates and backfires when pressing more on the pedal. A very slow increase in RPM will not cause it to backfire or miss. This is all in park position and when it is in drive, I can't get it out of the driveway without backfiring or stalling under a load.

I added Techron to it last year and the problem went away after 2 hours of running in the drive with some idling and accelarating. This problem din't occur again for the whole summer until last fall and Techron didn't work this time.

There is no engine light or any indication there is a problem. What is causing this?

Displaying 1 answers & 2 comments
Popular Answer
Hi Brad, Are there any Service Engine Soon/Check Engine Light on? If so a scan tool is needed to retrieve stored codes in the computer but even if the light isn't on a professional grade scan tool allows key engine sensor data to be read without doing intrusive testing at each sensor. I would be interested to see that fuel pressure and volume meet specification and what long and short term fuel trim read, what air flow into the engine reads, what O2 sensor values are and engine load figures are. Also it would be worth while to check the obvious check for vacuum leaks and ignition problems.
on March 06, 2011
Thanks Patrick,

I have been away so this is the first time to answer. There are no check engine lights on or any lights. My friend has a good scan tool and I will follow up on your suggestions when he has the time to get over this month. I'll report my findings then.
on March 31, 2011
Problem solved!
Sometimes it pays to check the simple things first, even though you might think you have a more complicated one. Inspected the distributor cap and there was extreme corrosion on the aluminum to rotor contacts inside the cap. A simple scraping with a knife and it ran without a single miss. I did replace the cap and rotor anyway.
In open loop, the timing and mixture was richer and did not require as strong a spark, and ran perfectly down the road. Once the engine reached operating temps running leaner with the timing advanced, it wouldn't handle the load and was not drivable.
Didn't need to use the scanner. Thanks, GMC VAN/Brad
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