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'87, L98 - I'm getting MAT too high code, MAF sensor voltage too high/too low, EGR code, and Spark control codes. All popped at once. Seems like classic EGR valve stuck wide open. How can I check or unstick it w/ out removing the entire Plenum?
To test operation of the EGR Valve, apply vacuum to the valve (MityVac) then use a small mirror (if necessary) to see if it opens. Mine was dry rotted inside the diaphragm, resulting in a Code 32 on my ECM.
If the EGR Valve fails to open, replace the valve. If you don't have a vacuum pump handy, simply connect a tube between the Fuel Pump Regulator's vacuum fitting on the right side of the plenum to the EGR Valve vacuum fitting and start the engine.
I'm not looking to reprogram the computer and delete, I'm actually trying to sell the car. If anything the EGR would be stuck open, not closed. the valve looks pretty new, and I think it is less than a year old. I can try a mirror but even then I might just try disconnecting the intake from the exhaust and plugging it to see if it is sucking up air. This should work right?
If the EGR valve is stuck open, they usually don't start and it certainly won't idle - the exhaust at low RPMS wipes out combustion. Multiple Codes are usually a failed ECM or a fault in a shared Circuit such as a ground (for example, the TPS, CTS AND MAT all share the same ground and if it were to open at the ECM or the ECM's grounding point, you'll get 3 Codes indicating an open circuit for each device).
Now let's take your MAT Code - The ECM sends a 5 volt reference to this device and depending on the Resistance in the sensor, that voltage drops. The Sensor's resistance decreases as the Sensor warms up, so the ECM sees less voltage as air temp increases. Your fault code is telling you that the ECM is seeing an unusually high voltage or low temperature. A scan would interpret that temperature for you (it'll probably show something like -32 degrees), but the first thing to look for would be a faulty connection.
Moving on to your EGR code. EGR is only commanded at Cruise and a sensor is heated in the EGR tube which then grounds a reference from the ECM. The ECM knows that the commanded event took place, because the voltage on this Circuit disappears as it finds a path to ground. Under most circumstances, that leaves 2 things to look at - the Sensor Connection and if that's good, whether or not the EGR valve is getting vacuum and if the EGR valve can move when it does.
Frankly, I'm thinking your ECM is probably the culprit - it's may have lost a power/driver circuit, but you might just check ECM grounds first - they should be on the Bellhousing, so jack it up and see if anything is loose. Otherwise, get a hold of a scanner so that you can see what each sensor is showing and then post that info.
ok, I tested the EGR sensor and it read 12V w/ ignition and 14 after startup. Funny thing is, this time the car idled pretty well and was resposive on throttle. Each scan I took said different codes. anything from 25, 33, 42, 43.
The live data was really wierd.
-Coolant was constant 124.7,
-Engine RPM went from 75 then every second added about 1000 rpms till it
hit 6000 then started over,
-Throttle pos sensor was 0 at startup then 2.94 after lap around the
block but did not change when I reved after back at the house.
-O2 was changing between 0 and 17 mV, MAT was -22 or -40,
-knock sensor went up by about 10 a second,
-MAF always read 0 (changed it out w/ my other and still read 0, Idle air
pos was 0steps till I drove it then went to 150 after got home,
-Injector pulse was 1.8 at startup and then 585.5 after back home
This seems like computer stuff to me, but my 87 reads normal w/ it running off the same comp.
So I followed my flow chart in the service manual to the Manifold air Temp code and got to the bad connection, bad ECM. Then I tested for ground on the terminal that grounds the sensor from the ECM harness and it tested good ground w/ the multimeter.
Am I testing right? could both ECM's be bad and work well in my 87 but be terrible in the 86?
Last edited by rithsleeper; 03-14-2009 at 07:31 PM.