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Lean condition P0171 & P0174, on my DTS


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The Story:

About two months ago I got a Service Engine Soon light on my 2004 DTS. After checking the error codes, it appeared that I was getting a lean condition (P0171 & P0174) that was affecting both engine banks. I cleared it a few times, but it would come back, usually after a long coast-down. While checking things under the hood, I discovered that the intake bolts were all loose. After tightening them and the clamp to the intake plenum, I figured all was well; no SES light for a week or so. Then I started getting the SES light again.

This was a good excuse to add to my “garage toy” collection, I purchased the Auto Enginuity OBD II Scan Tool ($228), with the additional enhanced GM package (+$249). While I have a great desktop computer, I was laptop challenged. My next purchase was a Toshiba laptop (the recommended brand) with lots of bell and whistles, and a 16” screen; a great deal at $649. As I have a number of cars that I can use this scanner on, I believe it will justify the purchase. (The new laptop also came in handy on my vacation.)

Using my new “toy”, I verified the error codes and spent some time getting familiar with the scanner. This scanner has many impressive features, among them I can have it scan several sensors at once, and view the live data over different charts at the same time. I need to spend more time testing, as this scanner can provide incredible amounts of information, more than I understand at this time.

I briefly tested the GM enhanced package. Was able to start and stop the AC, and a few other things. Again I will need to devote much more time getting familiar with this very useful scanner.

Back to diagnosing:

I scanned, I worked the throttle, and compared the readouts. I believed that the lean problem was obviously from excess air getting in, but from where? I had already tightened the intake manifold bolts and the clamp to the plenum. I decided to pull the intake manifold to visually check the base gaskets.

As I started to lift the intake manifold off, I spotted the problem; the intake plenum was torn nearly two-thirds of the way around. I had heard of this issue before, and had tested for it by carefully spraying some throttle body cleaner all over it, but it made no engine speed change so I thought it was OK; I was wrong.

I replaced the intake plenum, cleaned the intake manifold mating surfaces, and decided to leave the old gaskets in place: as they appeared to be in fine shape. The car started right up, and after a week of my usual commute driving, no SES or error codes. Problem solved.

Query: :unsure:

I have puzzled over the Intake plenum failure. It is a stationary seal, how does it tear itself? Upon reflection I remembered the loose intake bolts. With the intake manifold loose, I wonder if the intake manifold is “pushed/slips” back and forth (acceleration/deceleration), and the constant stress eventually cracks the intake plenum seal?


Drive'em like you own 'em. - ....................04 DTS............................


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I believe that Logan stated that EGR gases settle in the bottom fold as that carbon gunk we see behind the TB, and eventually causes the failure, he noted that most always the failure is on the bottom, really glad you found this and surprised that a 2004 would need one soo soon

Pre-1995 - DTC codes OBD1  >>

1996 and newer - DTC codes OBD2 >> https://www.obd-codes.com/trouble_codes/gm/obd_codes.htm

How to check for codes Caddyinfo How To Technical Archive >> http://www.caddyinfo.com/wordpress/cadillac-how-to-faq/

Cadillac History & Specifications Year by Year  http://www.motorera.com/cadillac/index.htm

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