The car was ok this morning, but when I started it in the afternoon, it has a difficult idle, was doing some backfires and not running well at all.

There is no code.

The strange thing is that it runs OK when I ground the test terminal and go in field service mode. I can drive it no problems.

Any ideas?

 

the best information I can give you right now is to hook it to a scannor and be sure all your reference are where they are supposed to be. I would pay special attention to the temp reading the blms and the 02. I hightly doubt you have a engine problem sense it does not back fire when in service mode. The only thing that is strange is the back fire usaly the only thing that will cause a back fire is a miss or timiming issue, You might want to unhook your timeing set connector hook up a timing light and see what the timing is maybe some how the dist turned just a thought

 

I fixed the car only a few days after my post, but I though I could give an update in case it could help other forum members.

Solution: MAF sensor.

Details: Friend gave me an OTC scanner, but it did not want to communicate to the car. I ordered the shop manual, but I did not wanted to wait several days for the manual to arrive. I started searching the net. I found the voltage values of each PINs at the ECM with key an ON and RUN. I took a digital voltmeter and measure the voltage at the pins one by one. The values for all the pins were fine with the exception of the MAF. The MAF value was OK with key ON, but not OK with key at Run. I searched the web again and found someone how had posted the CODE 34 manual pages. Reading it I found that one of the symptoms was rough idle. The test procedure from the manual consist in disconnecting the MAF and start the car. If car goes fine, problem is the MAF. I told the wife that I was going to the garage for 5 minutes; disconnected the MAF, started the car and it was running ok. Great! Ordered a MAF and problem solve.

Couple of things to notes:
MAF can be bad without code.
When the test terminal is grounded, it bypasses the MAF value. Amongs other things.

A former GM mechanics told me that grounding the test terminal is a kind of towing in mode; to bring the car home. So if your car ever experience weird problems, ground the test terminal before calling the towing. If I recall, he said that computer will bypass the sensors and take default value.