This van runs great when first started and continues to run fine after it’s warmed up and will do so until it’s turned off and restarted. When first started vacuum at idle is steady at 21” and 10” at 55mph cruising speed. I can continue driving for an hour or so and it will continue to run fine. It’s only when I shut it down and restart after 20 minutes or so that I have a rough running condition. After parking for 10 to 20 minutes and restarting idle is still steady at 21” vacuum but the van hesitates on acceleration and runs rough at cruise with a 5” vacuum. If I continue to drive I get low vacuum trouble codes for the MAP and Oxygen sensors. I’ve replaced the oxygen sensor and performed a diagnostic on the MAP sensor which checks out ok. I’ve checked for vacuum leaks – none found. I’ve replaced the spark plugs, cap and rotor. Does anyone have a suggestion on what to look at next?
When testing the MAP sensor, are you doing this while the engine is hot?
I tested the MAP sensor with the KOEO with a volt ohm meter and vacuum gauge, all reading were within spec as I increased vacuum. I then connected my volt ohm meter to the MAP signal wire and had a vacuum gauge connected to the intake manifold and watched the response as I was driving the van. Once again the MAP sensor responded as in the KOEO test as the engine vacuum reading changed while accelerating or decelerating.
The engine was at operating temperture during the second test.
Here’s from the factory manual on how the MAP sensor is tested.
Back probe terminals A & C on the MAP sensor connector.
With KOEO the voltage should be around 5.0 volts.
Back probe terminals B & A on the MAP sensor connector.
Voltage should be around 0.5 volts.
Record that voltage reading.
Start the engine.
Verify that the voltage is greater than 1.5 volts over the recorded reading while idling.
Drive the vehicle.
Verify that the voltage is greater than 4.5 volts at WOT over the recorded reading.
This is the same test I used, the results were all within specs
@TimG20 Can you please post the exact fault codes you have?
Is your van by chance OBD2 compliant?
I’m asking because some vehicles were compliant 1 or 2 years before the 1996 model year
Once you post the codes, I’ll try to send you a link to an official GM website, which lists the criteria for running a code. And it also lists the criteria for setting the code
Was the MAP sensor an “out of range” code?
Was the oxygen sensor code a low activity code?
But again, please post the exact codes
It’s OBD-1 and the two codes are:
33 map sensor signal out of range, high
44 oxygen sensor lean exhaust
I never get these codes on the initial start and driving of the van up to operating tempertures. Only after the second start when it hesitates on acceleration and runs rough do these codes apprear.
@TimG20 are you sure you don’t have some kind of a lean condition, which is a contributing factor for both of these codes
I know you’ve checked for vacuum leaks, but there are other causes of lean condition
I have a feeling that when it’s running rough, that is your lean condition which is causing the codes
I suspect the MAP sensor and the O2 sensor are just the messengers, not the cause