CEL with EGR codes using MP2

Mike Puckett

SCCoA Member
Last summer I swapped out my S-model sc with an MP2 that I got and started having intermittent check engine light issues. I always got EGR codes whenever I checked for codes yet the EGR system seemed to be working fine, valve opened and closed Ok and I tried several of the gray sensor blocks to no avail. I took it apart 3 times to find the problem but came up blank each time. I had no performance or vacuum/boost issues and the car ran great. I swapped the S-model back on during the off season and now the CEL is gone and I get no more codes. Performance is about equal with the S-model being just a tiny bit faster and quicker in the 1/4 mi. So, now I'm curious, why the CEL and EGR codes with the MP2? I'm using the same MP upper intake and 85mm throttle body. Anybody else have this experience or have any ideas?
 
Which codes if you remember??

Also most likely there isn't enough flow through the EGR valve, in turn not causing enough of a drop for the Flow sensor (DPFE??) to say there is enough flow..

Aftermarket headers?? And are you sure the passage on the MP2 for the EGR is open?? :p
 
As I recall, I got either a 31 or a 34 I believe they were. I can't find my code book to confirm them. It's around here somewhere. If I reset the eec overnight it could generate either code but not both at the same time. It never affected the way it ran and it passed emissions with flying colors. Last saturday I did have the CEL flash on/off twice during a couple of passes down the dragstrip. I've also found that it's running .1 sec quicker and 1 mph faster with the S-model than the MP2. The engine was tuned with the S-model. It's got shortie headers and like I said I had it all apart 3 times rechecking everything. Nothing else was changed, just the blower.
 
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You probably had a vacuum leak issue with the MPII. EGR will not cause intermittant CEL's. On a 1990 there is no monitoring of flow which is why people can delete their EGR and just leave the sensor hanging out behind the motor.
 
Maybe my EGR codes are coincidental then. I'll reset the computer again and run it for awhile and see if a code comes back. Could one of the vacuum hoses on the back of the upper intake being loose cause a CEL? I always had my usual 18" of vacuum and I could get 14-15psi of boost. Come to think of it, it is idling better. Before the idle rpm would fluctuate up and down from 1000-1600. Now it holds from 900-1100.
 
Maybe my EGR codes are coincidental then. I'll reset the computer again and run it for awhile and see if a code comes back. Could one of the vacuum hoses on the back of the upper intake being loose cause a CEL? I always had my usual 18" of vacuum and I could get 14-15psi of boost. Come to think of it, it is idling better. Before the idle rpm would fluctuate up and down from 1000-1600. Now it holds from 900-1100.

Yes, that is a very clear indication of a vacuum leak. You won't see that kind of a leak on the vacuum gauge, it's simply not accurate enough. Just about any time you get intermittent CEL's it means you have a fuel mixture problem. Most other codes will set and stay on for the duration of the run cycle but an AFR code will set when adaptive reaches limits for any given rpm/load and will reset as soon as parameters come into range - hence the on and off.
 
Whenever I've had boost leaks in the past I've gotten this high idle condition but only a CEL at idle and not for long periods of time. I did have a sealing problem with the MP2 at the back inlet opening at first so maybe it's still not flat. I'm thinking I'm going to stick with the S-model. Will the internals from the MP2 fit the S-model?
 
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