At 07:49 AM 6/2/01 -0400, you wrote:
>My 91 5.2 4WD Dakota was running rich, and rough. The computer
>spit a Bad O2 Sensor code. I replaced the O2 sensor with no change.
>I took the truck in and it was found that the EGR valve was being told to
>open by the computer. The tech. says it is a common problem for Dakotas.
>They plugged the vacuum line to the EGR and now the error code is gone
>and the truck is running great.
>Question: What tells the EGR valve to open?
Basically, the EGR valve opens during light throttle and warm engine cruising
and channels the exhaust gases back into the engine's inlet air. Generally,
EGR occupies about 10% of the total air fuel mixture volume in order to quench
the combustion chamber temp. and keep it below 2500F.
Oversimplify, the PCM is probably programmed to allow vacuum to reach the
EGR valve only after the coolant temperature sensor indicates the engine is
warm AND when the throttle position sensor says rpm is above idle. This is
similar to a switches-in-series circuit. If both conditions are met, the
computer
sends a signal to a vacuum switch, opening it and permitting engine vacuum to
operate the EGR valve.
>And not that mine is plugged, what kind of problems (if any) should I look
>for in the future.
If there isn't enough EGR, NOx emissions increase, but the only
driveability problems are surging at cruise and a complaint of spark
knock.
If there is too much EGR, or EGR at the wrong time, your clues will be
poor engine performance. The symptoms include:
* poor idle
* stalling, especially when starting after cold soak
* hesitation, stumble and rough running during warm-up
* tip-in hesitation or stumble
* surge at cruise, even with warm engine
* poor acceleration
* low engine vacuum.
Keep in mind that other things can cause these symptoms as well.
HTH, Bob.
This archive was generated by hypermail 2b29 : Fri Jun 20 2003 - 12:01:43 EDT