I've started to get a regularly occuring check engine light. However the car runs perfectly.
When I check the diagnastic code its always 71. That suppose to be an EGR system malfunction. I've check all the EGR components, EGR valve cleaned and tested, BVSV tested, EGR modulator cleaned and test, EGR gas temp. senor tested and all ok. I pull the EFI fuse to reset the system and the check engine light will stay out for several diagnostic code detection like driving intervals...well not stickly but similar.
Ray I have a factory shop manual and basically followed the procedures for testing the emissions controls.
The EGR valve was removed and tested on the bench. Negative presure from a vacuum source was applied the EGR valve dashpot and the flow through the valve was checked.
Differential presure was applied to modulator and modulator switch action was verified.
The BVSV was tested with with a hot water bath and checking operation at 35 deg. C and 54 deg.
The EGR temp sensor was also bench tested with water bath.
Good job on the checks! Try activating the EGR while the engine is idling to see if the engine starts to run roughly. If there is no change, the EGR is not opening sufficiently. You can also try temporarily disconnecting vacuum hose from the EGR valve and plugging the vacuum hose to see if the trouble code stays away.
Yes, the EGR valve has always check out as you descibed. If you pump down on the EGR vacuum hose, the engine will stumble when idling. Just to be sure I took the EGR assembly off the car and cleaned it thoughly and inspeced the operation. It looks good and operates smothly on the bench but I suppose it could be binding or something at temperature.
By the way, I said hot water bath, I should correct that and say I actually used motor oil. I have a little hot plate and an electronic themocouple temperature sensor. Both the EGR temperature sensor and BVSV where within the factory manual specifications.
So are you saying that if the EGR is closed (in the temporary disconnect case) that the ECM won't see a fault? That's interesting.... I thought it was monitoring the EGR temp sensor for a specific response vs. engine temperature, vehicle speed, and or some such thing or other.
By the way, I'm now turning my attention to the vacuum port on the throttle body. I can't remember if I checked that out now so I'm going back to that this weekend and see what's going on there. I know its a bit of a long shot but its at least due dilegence.
I don't think the ECM will see a fault if the EGR is closed and temporarily disconnected. Vehicles with OBD II have an EGR position sensor that checks to see if the EGR is actually opening when called for, but I don't think yours has one. I think the system uses the EGR temp sensor, but it takes a while to throw a code.
I admire the methodical way you are going about diagnosis. Fortunately (or unfortunately, depending on your point of view) you have not left me with any ideas as to the cause of the problem. I would check the "supply" vacuum for the EGR modulator, perhaps using a vacuum gauge on a t-fitting to see if it is getting sufficient vacuum for the modulator to work.
Ray~ I check out the throttle body E, R, and P ports for vacuum supply and they are all OK. Vacuum source appears to be good and checks out through the BVSV (after warm) to the modulator. I'm starting to turn my attention back to the modulator. That's a really simple valve and I've had it apart for cleaning and last time I looked appeared to be OK. I'm going to look at it a bit closer next.
This turned out to be the EGR BVSV valve afterall. I had check this valve out before and put it back on the engine. I then removed the EGR itself and cleaned it. When testing the engine I had assumed that cleaning the EGR valve was not the problem because the 71 code came back.
Apprently the nipples on the BVSV valve where old and brittle and one of them broke after reinstalling the hoses. The real problem was the sticky EGR valve which was cleaned but in the interim the BVSV had failed.
So I'd have to say if you follow the manual carefully and check your work you can fix this problem.
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