Datamaster Readings

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Apr 21, 2005 | 10:44 AM

KayCee

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From: Crockett CA

Datamaster Readings

1995 LT1 A4 completely stock. 126,000 miles.I am at wits end trying to track down the cause of a slightly shuddering idle. Have swapped out the ECT, IAC, set of injectors, and all TB gaskets. Thoroughly cleaned the TB. Prior to the beginning of this problem. Changed the water pump, Opti-Spark, wires and plugs. Car runs like a scalded dog, it just won't idle as it should. I now have Datamaster and the Lterms are 155 and 149.The spark advance is fluctuating from 18 to 21 degress, with a corresponding slight RPM fluctuation from 575 to 650. According to the Helms the Lterms is a lean condition.Also, the knock counts are at 843. Should there be any knock counts? There is no audible knocking. Anybody got any clues?

Apr 21, 2005 | 12:24 PM

kaiser

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From: des plaines, il

There are several possibilities- among them:
1. Vacuum leak
2. Replacement injectors were wrong size
3. Need new O2 sensors

Running lean and knock counts go together.

Apr 21, 2005 | 01:26 PM

KayCee

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From: Crockett CA

Thanks. I did replace the 02 sensors when I swapped out my A4 last summer. The left bank threw a code. If the 02 sensors have went south, wouldn't the ECM throw a code, or are the sensors capable of slightly failing enough to cause idling problems without throwing a code? The injectors are ACCEL 26 pounders.

Apr 22, 2005 | 07:54 AM

kaiser

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From: des plaines, il

If the injectors & O2 sensors are new - and the ignition is working OK - I'd go back to looking for vacuum leaks. Then I'd take a hard look at the MAF sensor. On an engine that old I'd also do a compression and leak-down test to see what shape the valves & pistons are in.

Apr 22, 2005 | 11:31 AM

KayCee

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From: Crockett CA

I'm not sure whether the ignition is working OK at idle, though the Opti-Spark, wires and plugs are less than 9 months old. The Datamaster readings show the spark advance fluctuating from 18 -21 degrees as the rpm fluctuates up and down by 75 rpm at idle . I can't tell by the readings whether the ignition is causing this fluctuation or the ECM is attempting to correct the idle and is changing the timing. In other words, is it the ignition or the ECM causing the spark advance to move at idle. Can the ignition cause a lean fuel condition?

Apr 23, 2005 | 01:18 AM

Redeasysport

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From: Myrtle Beach SC

St. Jude Donor '05-'06

Bad O2's will not always set a code. They will play havoc with you A/F ratio and BLM's. They should be close to the same around 800. If they aren't replace the one that is not around 800.

Optis fail whenever they like not based on milage.

BLM's (Long Term)should be around 128 looks like yours are not I would be looking at the fuel delivery system closely.When did you change your filter last? Have you done a pressure test?

Apr 23, 2005 | 10:15 AM

DOCTOR J

Burning Brakes

Joined: May 1999

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From: Greenwich, CT

Quote:

Can the ignition cause a lean fuel condition?

Not exactly. An ignition misfire will pump unburned air & fuel into the exhaust;
seeing that, the O2 sensors will interpret the unburned air as a 'lean' condition
and attempt to increase the fuel component. That misfire would also manifest
itself as a rough idle.

The best way to diagnose this is with an exhaust gas analyzer or a WB O2 sensor.

Or it could be that some of your new injectors are already fouled, and it really is a
lean condition. I'd probably put a timing light on the plugs and try to find a misfire
situation first before tearing into anything else, or take it to a shop with analyzer
equipment.

HTH