Question-------------------------
Followup To
Question -
Sometimes in Park, the idle goes up and down on a cyclical basis - it seems the computer senses the rpm dropping and brings it back up. Also starting out first thing it sometimes lunges forward when applying brake at first stop sign. Have not been able to find a vacuum leak, recently had fuel pump and filter replaced, and TB cleaned, egr de-carboned. Tech says it is running a little rich. No check engine light or stored codes. Runs fine on highway and around town, around 120K miles. Run amsoil synthetic oil in it. Any ideas?
Answer -
Hello Randy,
Check for vacuum leaking at the brake booster. Especially if hitting the brakes seems to make a big difference.
I would unplug the vacuum hose from the booster and plug the hose, and see what it does, keeping in mind that you won't have POWER brakes during the test.
Van
Well, the Astro uses the hydraboost power brake assist system, which borrows pressure from the power steering pump to help brake - not using the traditional vaccum brake booster. So this would eliminate the vac booster leak as a potential problem. I have had my mechanic friend look for vac leaks before and he has not found any. How can you measure your vac to see if it is OK? What about replacing the Oxygen sensor - I heard these wear out - is it located on exhaust manifold - can you get to it by taking off the engine cover inside the van?
AnswerHello Randy,
That hydroboost sure would eliminate that vacuum leak possibility.
You can check the vacuum with a regular old vacuum guage connected anywhere on the intake manifold. You can also read what the MAP sensor sends with a scan tool. MAP is vacuum.
Fuel pressure is also nice to know. With a new pump and filter, you could expect it to be fine, but there is a fuel pressure regulator that also affects it. I like 12 to 13 PSI at the throttle body, with the engine running.
As for the oxygen sensor, they do sometimes have problems, but generally are pretty long lasting. One thing that will hurt them is using silicone sealer in the intake gasket system that is not oxygen sensor safe silicone.
But with no codes being stored, yet the mechanic suggesting it is running rich, the oxygen sensor would sure be suspect. If something else is making it run rich, the oxygen sensor, if working correctly, should set a code.
Removing that doghouse cover could help you get to an oxygen sensor, but I'm not sure on that. Usually you can get to them under the vehicle, but I would remove that cover just to look at that part of the engine anyway...check hoses, etc.
The sensors aren't usually in the exhaust manifold, but close behind it, in the pipe betweet the manifold and the catalytic converter.(Don't hold me to that. Sure as I say it, you could have one in the manifold).
There could be more than one O2 sensor, especially on a newer V6, one for each bank, and possibly one behind the converter. That third one is to indicate weather the converter is working, and shouldn't affect engine operation.
A 95, I think you should only have one in the pipe ahead of the cat.
Good luck,
Van
